Tools and Functions of Reconfigurable Colloidal AssemblyMichael J. Solomon*

University of Michigan, Ann Arbor, Michigan 48109, United States

ABSTRACT: We review work in reconfigurable colloidalassembly, a field in which rapid, back-and-forth transitionsbetween the equilibrium states of colloidal self-assembly areaccomplished by dynamic manipulation of the size, shape, andinteraction potential of colloids, as well as the magnitude anddirection of the fields applied to them. It is distinguished fromthe study of colloidal phase transitions by the centrality ofthermodynamic variables and colloidal properties that are timeswitchable; by the applicability of these changes to generatetransitions in assembled colloids that may be spatiallylocalized; and by its incorporation of the effects of generalizedpotentials due to, for example, applied electric and magneticfields. By drawing upon current progress in the field, wepropose a matrix classification of reconfigurable colloidal systems based on the tool used and function performed byreconfiguration. The classification distinguishes between the multiple means by which reconfigurable assembly can beaccomplished (i.e., the tools of reconfiguration) and the different kinds of structural transitions that can be achieved by it (i.e., thefunctions of reconfiguration). In the first case, the tools of reconfiguration can be broadly classed as (i) those that control thecolloidal contribution to the system entropyas through volumetric and/or shape changes of the particles; (ii) those thatcontrol the internal energy of the colloidsas through manipulation of colloidal interaction potentials; and (iii) those thatcontrol the spatially resolved potential energy that is imposed on the colloidsas through the introduction of field-inducedphoretic mechanisms that yield colloidal displacement and accumulation. In the second case, the functions of reconfigurationinclude reversible: (i) transformation between different phasesincluding fluid, cluster, gel, and crystal structures; (ii)manipulation of the spacing between colloids in crystals and clusters; and (iii) translation, rotation, or shape-change of finite-sizeobjects self-assembled from colloids. With this classification in hand, we correlate the current limits on the spatiotemporal scalesfor reconfigurable colloidal assembly and identify a set of future research challenges.

■ INTRODUCTION

Colloidal particles self-assemble into ordered and disorderedstructures as a consequence of Brownian motion andinterparticle interactions.1,2 If gelation and glass transitionsare avoided or suppressed, colloidal systems explore theensemble of available configurations; in these situations, freeenergy is minimized and thermodynamic equilibrium isachieved. Excluded volume interactions, which depend onshape, yield crystalline phases with complex symmetry in bothtwo and three dimensions.3 Patchy interactionsthoselocalized to specific volumetric or surface regions of thecolloidcan produce new kinds of structures, including openlattices.4 Combining shape anisotropy with attractive inter-actions produces additional unit cell symmetries.5 Finally,programmable bondingas mediated for example by DNAisan effective tool for structural design.6 The explosion in phasespredicted by theory and simulation7−9with many now beingidentified in experimentsis remarkable.The structures into which colloidal particles assemble are

useful for new materials. Crystalline arrays of dielectricstructures produce structural color of variable wavelength andpolarization state;10,11 metallodielectric structures control thestrength and frequency of plasmonic responses.12 Certain

structures such as diamond,13 gyroid,14 and the icosahedralquasicrystal15 can yield a photonic band gap. Ordered arrays ofcolloids can be processed to produce materials with controlledporosity that are useful for conductive electrodes16 andmembranes.17 Finally, colloids can be assembled into arraysfor materials with mechanical properties useful for actuation18

and acoustic control.19 Compared to other constituentsavailable for self-assembly at similar scalessuch as blockcopolymers, DNA, surfactants, and proteinscolloids displayadvantages that stem from the range of different compositionsand material functionalities that can be incorporated either intothe volume or onto the surface of the colloids. Control ofvolumetric composition yields a versatile range of particledielectric, conductive, magnetic, and elastic properties. Controlof surface functionality generates tunable interactions that arewell modeled by statistical thermodynamics. These positives arebalanced against challenges such as the limited number ofcrystal structures that have been assembled from colloids todate, the slow kinetics of self-assembly because colloidal

Received: October 29, 2017Revised: February 1, 2018Published: February 3, 2018

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© XXXX American Chemical Society A DOI: 10.1021/acs.langmuir.7b03748Langmuir XXXX, XXX, XXX−XXX

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diffusion is retarded relative to molecules, and the tendency forself-assembled structures to be defect prone.Materials applications of self-assembled colloids can require

crystalline structures that are either static or dynamic. Staticstructuressuch as an iridescent coating or a porouselectrodeare first fabricated by self-assembly; a subsequentstepsuch as drying, sintering, or solvent photopolymeriza-tionpermanently fixes the structure in the state needed for itsapplication. By comparison, dynamic structures are not fixed;critical features such as building block symmetry, crystalstructure, and/or lattice spacing can be controlled in time.Dynamic structures are instrumental to applicationse.g.sensing, transduction, and actuationin which a time-depend-ent optical, structural, or mechanical response is sought. Thepurpose of this review is to examine the means by which thesedynamic colloidal structures have been achieved and thefunctions that such dynamic control yields. This kind ofdynamism is called reconfigurability. Reconfigurabilityas wedefine more specifically in this reviewcan be combined withequilibrium self-assembly in colloidal systems to generatereversible transitions between colloidal states with differentfunctions.There is broad interest in generating this kind of dynamic

control of colloidal structures; methods used to produce it have

varied considerably to date. Simultaneously, usage of the termreconfigurable colloidal assembly has grown. Here we classifythe distinctive features of reconfigurable colloidal assembly andidentify a set of research challenges that can motivate futurework in this field. The conception of reconfigurable colloidalassembly proposed here combines earlier descriptions20−22

under a single theme described as the reversible transition of acolloidal system between states with different function, asgenerated by any of a set of tools that modulatethermodynamic variables and colloidal properties of the systemin time.The organization of this paper is the following. After defining

reconfigurable colloidal assembly, we identify the tools thathave been used to achieve reconfiguration as well as the specifickinds of reconfigurable transitions that have been produced.The union of these tools and functions yields a matrixclassification of work in the field. Next we approachreconfigurable colloidal assembly from the point of view ofthe characteristic time scales of the dynamic response. Weidentify current limits, which depend on the tool used andfunction performed. In the final two sections, we identifyapplication areas in which the tools and functions ofreconfigurable colloidal assembly have been applied and discussavenues for future work in the field.

Figure 1. Classification of reconfigurable colloidal assembly by tool and function. (a) Ligand-induced particle shape change generates reconfigurationfrom simple cubic (SC) to face centered cubic (FCC) via rhombohedral structures.22 (b) Volumetric swelling of polymer colloids shifts the crystallattice spacing.152 (c) No entry yet for the bottom left matrix element. (d) Temperature-induced annealing of surface grafted complementary anddisplacing DNA strands yields reversible melting of a CsCl colloidal crystal.47 (e) Microparticle spacing at an interface depends on the effects ofsurfactant concentration on liquid crystal ordering.63 (f) The length of Janus ellipsoid chains is modulated by AC electric fields.5 (g) Homogeneousellipsoids undergo a order−disorder transition by light-assisted electrophoretic deposition.153 (h) Electrophoretic deposition and compressionchanges the lattice spacing of colloidal crystals.133 (i) Tunable optical trapping rotates a self-assembled colloidal object.96 Scale bars for e and g are 5μm, f is 20 μm, h is 100 μm, and i is 3 μm. Images reproduced with permission from the references as indicated. Reproduced with permission fromAAAS, RSC, AIP Publishing, APS, NAS, Wiley, and Springer Nature from indicated references.

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■ RECONFIGURABLE COLLOIDAL ASSEMBLYDEFINED

The following features characterize reconfigurable colloidalassembly:(1) It involves reversible, back-and-forth transitions between two

dif ferent equilibrium states of a colloidal system by themanipulation in time of thermodynamic variables or colloidalproperties. The thermodynamic variables include the typicalones such as temperature and (osmotic) pressure but alsogeneralized variables such as applied electric and magneticfields. The colloidal properties include particle size and shape aswell as the potential interactions between the colloids. Thefocus on reversible, back-and-forth transitions distinguishesreconfiguration from the broader study of phase transitions andself-assembly. Colloidsunlike other materials such as smallmolecules, surfactants, and polymersoften undergo phasetransitions that are, in practice, not reversible, even though theyoccur at equilibrium. For example, adding salt to a colloidalcrystal may change its structure from bcc to fcc23 by modulatingthe Debye length of the screened Coulombic repulsion;however, switching back and forth between the two phases isstymied by the irreversibility of the salt addition. Becausethermodynamic variables and particle properties cannotcommonly be reversibly manipulated to change colloidalassembly structure and function, advances in synthesis (e.g.,shape mutable colloids), interactions (e.g., colloids withtemperature-dependent potentials due to binding of comple-mentary DNA strands), and applied fields (e.g., tunableosmotic pressure as generated by electrophoretic deposition)have been applied to reconfigure the structure of colloidalassemblies.(2) It is def ined operationally: reconf igurable colloidal assembly

refers to a functional change in microstructure or property of theassembly. An example functional change is a phase transition ofthe colloidssuch as a change from fluid to crystal or betweentwo different crystal structures. A change in lattice spacing of acrystal, which would yield a change in iridescent structuralcolor, is also a kind of reconfiguration. Finally, changing theshape or position of an object assembled from colloids isreconfiguration; such shape morphing operations are founda-tional to microscale force generation, locomotion, and capture.The emphasis on functional change means that reconfigurationdoes not include changes in colloidal microstructure that aresmall, such as, for example, as introduced by fluctuationsaround a particular state point. The functional change in thecolloids can be with respect either to a macroscopic propertysuch as an elastic modulus−or to a microscopic variablesuchas an order parameter of a crystal phase.(3) Reconf igurable colloidal assembly includes structural

transitions that are in the thermodynamic limit, as well as thosewhich are spatially localized and involving only small numbers ofparticles. Large numbers of colloids can undergo reconfigura-tion, as in the case of a bulk phase transition that occurs in thethermodynamic limit. In addition, the self-assembly of clusters,thin layers, and even objects of colloidal particles is an aim ofthe field, because of the usefulness of these small-numberensembles for microscale function. Although such localizedregions may contain too few colloids to be in thethermodynamic limit, reconfiguration between states of theseensembles may still be described by a change in orderparameter. The terminology of reconfigurable colloidalassembly thus identifies a recent research direction in colloids

that seeks to develop dynamic, reversible function in smallcollections of particles; these new functions complement themany already existing uses for colloids based on bulk self-assembly.The literature has applied a broad range of tools to

accomplish colloidal reconfiguration. As illustrated in thecolumns of Figure 1, these tools manipulate the colloidalentropy, the interaction potential among the colloids, or thepotential energy landscape that the colloids experience in thesystem. The tools have then been applied to perform a varietyof functions. As shown in the rows of Figure 1, functionsinclude generating structural transitions between phases,changing the configuration of particles within particular phases,and manipulating the size, shape, and location of objects thatare themselves self-assembled from colloids. Classifying theseinstances based on the tool used and the function performedorganizes efforts in the field in a way that can assist its futuredevelopment.

■ TOOLS FOR RECONFIGURATION OFSELF-ASSEMBLED COLLOIDAL STRUCTURES

The use of the term reconfigurable with reference to colloidalassembly is recent;20 however, interest in switching thestructural characteristics of phases generated by colloidal self-assembly is much older.24 At its most basic level, configura-tional change in colloidal systems is a kinetic process that canoccur through mechanisms such as spinodal decomposition ornucleation and growth as well as transport processes such asdiffusion and convection.25 Such kinetics have been extensivelystudied in colloidal systems.1 As discussed previously,reconfigurable self-assembly of colloids is distinguished fromthe study of phase transitions and their kinetics by the back-and-forth reversibility that reconfigurability achieves. That is, inaddition to stepping from one thermodynamic state point toanother, the system can then step back again. To generate theback-and-forth step, the thermodynamic variable or colloidalproperty that drives the change must itself be free to vary intime. Thus, equilibrium phase transitions of colloids are notnecessarily instances of reconfigurable colloidal assembly,because many transitions of this kind proceed only in onedirection. The thermodynamic variables or colloidal propertiesthat drive reconfiguration can be classed as tools that affectcolloidal entropy, potential interactions, or phoretic motion;each of these tools is discussed in the sections following.

Entropy Controlled Reconfiguration. To illustrate thedistinction between colloidal reconfiguration and phasetransitions, consider colloidal particles confined in a fixedvolume and initially prepared as a homogeneous liquid phase atequilibrium. If the state variables are fixed, there is nothermodynamic driving force to support kinetic processes;the system is already at its free energy minimum. However, atfixed temperature and number density, the system may stilltransform into a different phaseif the colloidal properties thatcontrol its thermodynamic state are manipulated. For instance,a change in the volume and/or shape of the particle could drivethe system into a different phase. A volumetric change of theparticle could be induced, for example, by swelling a polymersphere through a change in the dielectric constant of thesolvent in which the particle is dispersed.26 A change in particleshapeas accomplished from a cube to a sphere27could alsobe induced. These changes in the colloidal building block sizeand shape change the number of configurations available to thesystem; the system’s phase boundaries shift in response to this

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entropy change. If the shift places the number density at whichthe system was prepared in a new region of the phase diagram,a phase transition will occur. If the change in particle shape orvolume is reversible, the system can transform back and forthbetween these two states. The capability to make this back-and-forth transition is the critical feature of reconfigurable colloidalassembly; in this case, the reconfigurability arises throughmanipulation of the colloidal contribution to the systementropy.Colloidal volumetric change by reversible swelling/deswel-

ling was an early demonstration of entropy-driven reconfigura-tion;24,28 this expansion/contraction yields control of the latticespacing of close-packed colloidal crystals. In a step up incomplexity, shape-shifting colloids29,30 increase the practicalityof reconfiguring between two different crystal phases.22 Shape-shifting can be performed by differential swelling of dissimilarmaterials incorporated into a single colloidal building block orby reconfiguration of bond distances between the (atom-like)constituents of colloidal molecules.31 The effect of shape-shifting on self-assembly has been demonstrated by simulationfor cases such as the reconfiguration of spheres into pentagonalprisms and of rods into rhombi.32 In each case, a reversiblechange in building block size and shape yields changes in self-assembly through their effect on the entropic contribution tothe free energy.Interaction Potential Controlled Reconfiguration. The

potential interactions between colloids represent a tool forreconfiguration when the strength, magnitude, and functionalitycan be reversibly controlled. While reconfiguration of colloidvolume controls the system’s free energy through its effect onentropy, reconfiguration of potential interactions exerts controlthrough its effect on the internal energy contribution tocolloidal free energy. For example, charged colloids interactthrough a screened Coulombic potential that is parametrized bythe Bjerrum length, the surface potential, and the Debyelength.25 Phase boundaries between fluid and crystal phasesdepend sensitively on these three parameters.33 Each can becontrolled by manipulation of environmental and solventconditions. The Bjerrum length depends on the solventdielectric constant, the surface potential is often a function ofsolution pH, and the Debye length depends on the electrolyteconcentration in solution. In reconfigurable colloidal assemblythese shifts in potential interactions must be both controllableand reversible. Solvent conditions such as dielectric constant,pH, and electrolyte concentration are reversed with difficulty,typically by means of a slow step such as solvent exchange ormembrane permeation.34 It is more common to generatesingle-instance step changes in such properties; however, suchstep changes are not reconfigurable.Instead, reconfiguration of colloidal interaction potentials by

reversible manipulation of environmental or solvent conditionshas been accomplished by, for example, manipulation of thedepletion interaction. The depletion interaction is generated bysmall nonadsorbing particles or polymers; their addition affectsthe system free volume to produce an effective attractionbetween larger colloids.35,36 The strength and range of theattraction is a function of the number density of the depletantand its size relative to the colloid. Although the depletioninteraction arises due to the effect of colloidal configurations onthe depletant entropy, its role in colloidal self-assembly istypically incorporated through the effect of the depletant sizeand number density on an effective potential of mean force

between colloids.37 That is, the physical properties of thedepletant control the interaction potential of the colloids.Because certain polymers, such as poly-N-isopropylacryla-

mide (poly-NIPAM), have strongly temperature-dependentsize, these thermoreversible polymers can be used toreconfigure the depletion interaction potential between colloidsin ways that impact their self-assembly.38−41 These polymersare often the same as those used to generate particle volumetricchanges in the entropy-driven reconfiguration, as discussed inthe previous section. In this case, however, it is the interactionpotential between colloids that is affected by their size change,rather than the colloid itself. Surfactant micelles can alsofunction as depletants;42 their formation can be controlled bytemperature. Temperature can also affect the potentialinteractions between colloids through other mechanismsforexample, by changing the conformation of grafted layers thatstabilize colloids,43,44 by controlling the adsorption of thin,small-molecule layers that mediate capillary bridging betweenparticles,45 and by affecting preferential wetting in criticalmixtures.46

Bonds between colloidal pairs can be produced by thehybridization of complementary DNA strands that have beengrafted to each colloidal surface.6 These programmable, highlyspecific bonds can also produce reconfigurable potentialinteractions between colloids in a number of ways. First, thetemperature-dependent, on−off nature of DNA hybridizationcan be applied to transition through the fluid−crystal boundaryby means of thermal cycling.47 If the DNA strand sequence,surface density, as well as grafted molecule length and flexibilityare designed to prevent irreversible aggregation, then crystalsincluding fcc, bcc, CuAu, AlB2, and diamondhave been self-assembled in nano- or microsized colloidal systems and with aone or two particle basis.48−50 The DNA bond can beprogrammed to melt at temperatures ∼30 °C and greater,depending on solvent composition,51 because of the temper-ature dependence of DNA hybridization.Second, the colloid pair bond produced by surface-grafted,

complementary DNA can be reconfigured by the addition ofsmall DNA strands to the solution. If the DNA sequences ofthe three strands are designed as a system, DNA stranddisplacement reactions can create colloidal bonds of variablelength. DNA colloids have been reconfigured between twocrystal structures by this method. For example, reconfigurationbetween CsCl crystal structures with different colloidalsubstitution was accomplished when the DNA sequenceswere programmed to recognize the different colloidal species.47

In addition to exploiting DNA displacement reactions, theDNA hairpin state,52−54 mixed layers of complementaryDNA,55 DNA intercalation,56 and solvent dielectric constant51

have all been used to control the potential interactions betweencolloids.The rich phase behavior and defect physics of liquid crystal

solvents have been used to reconfigure potential interactionsamong the colloids. Liquid crystal solvents align in the vicinityof colloids to accommodate an anchoring condition at thecolloidal surface. Homeotropic anchoring at a colloidal surfacein a nematic liquid crystal leads to an equatorial defect loop(i.e., a Saturn ring defect) around the particle57 or a pointdefect at the particle pole (i.e., a hedgehog defect).58 Thequadrupolar and dipolar symmetry of the liquid crystalorientation, respectively, generates an anisotropic interactionpotential that has been exploited to produce chains, clusters,sheets, and networked gels.59−61 Defect entanglement states are

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controlled by the relative positions of multiple particles.60

Because of the temperature62 and chemical63 sensitivity of theliquid crystal defect states around the colloid, these anisotropicpotential interactions are tunable. The temperature sensitivityof the reconfigurable assemblies leads to switchable functionalproperties, such as elasticity and rheology.59

Therefore, temperature can affect colloidal assembly bymediating the strength and range of potential interactions, inaddition to its role in the temperature/entropy conjugate pair.This role of temperature can have multiple physical origins, asshown in Figure 2. Temperature change mediates the strength

and range of interaction potentials through its effect oncolloidal properties such as surface-grafted polymer and ligandconformation (for depletion and thermoreversible interactions,Figure 2a), DNA hybridization (for DNA strands grafted tocolloidal surfaces, Figure 2b), or liquid crystal defect states (forthe induction of dipolar and quadrapolar interactions, Figure2c).The other common way to control potential interactions for

reconfigurable colloidal assembly is by application of fields; arecent review is available in ref 21. Electric and magnetic fieldsinduce dipolar interactions between polarizable and magneti-cally susceptible particles, respectively. These interactions areanisotropic; they depend on the orientation of the colloid pairrelative to the direction of the applied field. For uniform fieldsand particles with isotropic polarizability, the induced force andtorque align the pair in the direction of the applied field. Inconcentrated systems, these associations lead to chaining if theinteractions are strong.64 Weaker interactions can reconfigurethe unit cell of colloidal crystals.65,66 If the polarizability of theparticles is anisotropiceither because of their nonsphericalshape or their variable particle compositionthen theorientation of the colloidal pair relative to the field directionis offset. These interactions yield staggered chain,67 chain-link,5

sheet,68 and tubular69 structures as well as crystal−crystal phasetransitions.70 Such field-induced interactions drive reconfigura-tion if their energy significantly modifies the strength or rangeof the field-free interaction potential. For colloidal self-assembly, the magnitude of the latter is ∼ kBT, the energyscale of the Brownian motion that drives the system to exploreits ensemble of configurations.For electric fields applied to dielectric spheres, the dipolar

interactions generate new phases such as body centeredtetragonal for dipolar interaction strengths of about 5−10times thermal energy.66 Likewise, single ellipsoids orient in thefield directions when the dipolar interaction strength exceedsthermal energy; phases such as sheets and tubes occur when theinteraction strength is about ten times thermal energy.69

The conditions needed to generate dipolar interactions ofthis strength depend on the dielectric properties of the particleand medium, the solution electrolyte concentration, and thefrequency and voltage of the applied electric field. In nonpolarsolvents, polymeric spheres ∼1 μm in radius reconfigure toform strings and crystals at electric fields ∼0.1 V/μm and ∼0.5V/μm, respectively (∼1 MHz).71 The differential polarizabilityof metallodielectric Janus particles leads to chaining andassembly at the lower field strengths of ∼0.025 V/μm and0.04 V/μm, respectively.72 Polarizability also depends onparticle shape. Aqueous polymeric ellipsoids align and assembleat ∼0.025 V/μm and 0.16 V/μm, respectively (0.16 MHz);69

the required electric field strength to align and assemble isreduced to ∼0.01 V/μm and ∼0.02 V/μm at 10 kHz.73

Colloidal scale ferromagnetism can also be applied togenerate dipolar interactions. The ferromagnetism can beintroduced by depositing magnetic materials on colloidalsurfaces74,75 or by suspending paramagnetic or diamagneticparticles in ferrofluids.76,77 Even mild externally appliedmagnetic fields generate dipolar interactions that are strongerthan thermal energy and therefore capable of generating chains,clusters, and crystals.78 For reconfiguration, the challengeinstead is to maintain reversibility when the external field isreleased; the magnetic interactions can be so strong as to bringparticle pairs into irreversible contact. Strategies such as fieldrotation,79,80 surface repulsion,81 and agitation74 have beenused to switch structures or restore equilibrium after fieldremoval. Magnetic and electric fields have been appliedorthogonally to generate two modalities for control and self-assembly.82

Phoretically-Controlled Reconfiguration. The thirdmethod to generate reconfigurability is to exploit field-inducedcolloidal motion in a bounded system. This motion can begenerated by means of transport mechanisms such assedimentation, electrophoresis, or diffusiophoresis. Such trans-port mechanisms produce motion of individual colloids byexerting forces either directly on the colloid (e.g., sedimenta-tion, radiation pressure, dielectrophoresis)25 or on a thininterfacial layer that surrounds it (e.g., electrophoresis,diffusiophoresis).83,84 In a closed system, this colloidal motiongenerates spatial gradients in number density as the colloidsmove toward and concentrate at a system boundary. The forcedue to the applied field is locally balanced by a gradient inosmotic pressure; at equilibrium a spatially varying concen-tration is established which satisfies the following:

ρ∇Π =

rF rr

1( )

( ) ( )(1)

Figure 2. The multiple roles of temperature on colloidal interactionpotential reconfiguration. The strength and range of attractionsbetween colloids can be manipulated with temperature through itseffect on the following: (a) the configuration of polymeric oroligomeric species that are either dissolved in solution or grafted to thecolloidal surface; (b) hybridization of surface-grafted complementaryDNA, which determines the binding state of the DNA pairs; (c)solvent phase transitions, such as in liquid crystal solvents.

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Here, F(r) is the force applied to a colloid at position vector r.ρ(r) and Π(r) are the number density and osmotic pressure atthat position. Π(r) is a function of colloid number density andpotential interactions. The spatial dependence of these variableshas been made explicit to emphasize that the conditions forlocal equilibrium vary with position vector r.Physically, the total force ρ(r)F(r) exerted by the applied

field per unit volume is balanced by a gradient in chemicalpotential (i.e., osmotic pressure). The osmotic pressure is afunction of number density through the equation of state.Equation 1 can be solved at each spatial position r; the numberof colloids in the system is a parameter of this solution. If thefield strength is tuned such that local equilibrium is maintainedand a sufficiently large local osmotic pressure is generated, thenphase transitions can be induced due to the local densification.Different self-assembly states can then be achieved if theapplied field is varied in time.85 This phoretically controlled (ortransport-controlled) mechanism for reconfiguration is distinctfrom both entropy and interaction potential controlledreconfiguration. That is, the mechanism can be applied toswitch regions of the system between different self-assemblystates, even for fixed colloidal shape and potential interactions.If the applied field is uniform, the colloid number density

necessary to satisfy local equilibrium varies in one-dimension,along an axis parallel to the applied field. Sedimentation,86,87

centrifugation,88 radiation pressure,89,90 and electrophoresis91

act in this way. On the other hand, if the applied field isnonuniform, then the spatial variation in colloidal density willbe more complex. Dielectrophoresis92−95 and light are twoways to generate such forces for colloidal transport. In theformer cases, field gradients have been used to generate largevolumes (∼5 mm3) of crystalline phase.93 In the latter case, thefield-induced potential energy localizes particles at equilibriumin particular regions due to either optical trapping,89,96 light-induced electrophoresis,97 or thermally induced phoreticmotion.98 In these cases, the force in eq 1 is specified by thespatially varying potential energy generated by the electric fieldor light. Reconfiguration by dynamic change of systemboundariesthat is, by manipulating the degree of confine-mentis also of this class, because of the change’s effect on theosmotic pressure.34,99

The dispersing medium itself can furthermore generate thefield that drives assembly. For example, liquid crystal defectscreate potential energy landscapes that can drive spatiallyresolved self-assembly in their vicinity; annealing out theseliquid crystal defectsas through a temperature changecanthus lead to reconfiguration.100,101 Because of the change inpotential energy that results from manipulating defect structure,this is an example of phoretically induced reconfiguration.Furthermore, this kind of reconfiguration is distinct from theeffects that liquid crystal alignment and anchoring conditionshave on colloidal potential interactions, as introduced in theprevious section and Figure 2c.This contrast is further illustrated in Figure 3a,b. Liquid

crystal solvents can be used for reconfigurable colloidalassembly through their effect on potential interactions (as in,when colloids attract to each other through dipolar orquadrupolar interactions through combinations of defect states,as shown in Figure 3a for linked defect states in that loopsaround particles in a cluster60) or through their ability to attractcolloids to particular parts of the system (as in, when colloidsare attracted to point or line defects in liquid crystals, as shownin Figure 3b for a large-scale quadrupolar interaction of a

micropost that attracts and orders colloids around it102).Recently, these two kinds of reconfiguration by liquid crystalshave been combined. For example, defect sites attract andconcentrate particlesa phoretic mechanism; these proximatecolloids then interact through hedgehog and Saturn ringdefectsan interaction potential mechanismto producechains at the defect site.101

Applied fields can similarly generate two distinct kinds ofreconfiguration, as shown in Figure 3c,d. In the firstmechanism−interaction potential controlled reconfigurationstrong fields of spatially uniform potential energy induceanisotropic interactions (e.g., dipolar forces) between colloidalpairs (cf. Figure 3c, here showing a tubular structure assembledfrom interacting ellipsoids103). AC fields are a convenient wayto provide such strong fields of spatially uniform potentialenergy. Reconfiguration results because of a field-inducedchange in the system’s internal energy, as mediated throughchanges in potential interactions. In the second mechanismphoretically controlled reconfigurationweak fields (∼kBT)that produce a spatially varying potential energy field generate aspatially varying osmotic pressure; the osmotic pressure is self-consistently determined by the field strength, equation of state,number density, and system boundaries (cf. Figure 3d, hereshowing phase transitions generated after application of adielectrophoretic field104). That is, in the phoretic reconfigura-tion the field acts on individual colloids independent of anymodulation of the interaction potential between colloids. (Asfor liquid crystals, although the phoretic and potentialinteraction mechanisms for reconfiguration are distinct, theycan be usefully combined. For example, strong AC fields cangenerate both dipolar and dielectrophoretic interactions.72,94,105

Figure 3. The same control variable can be designed to generate eitherinteraction potential or phoretic reconfiguration of colloidal systems.For example, liquid crystals can have a dual role. (a) Defect statessuch as Saturn rings60yield a temperature-dependent potential ofmean force between colloids that produces self-assembly (scale bar is 5μm). (b) The system geometry may produce large-scale defect statesthat drive phoretic motion and self-assembly, such as the quadrupolegenerated around the ∼100 μm micropost, as shown.102 Also, ACfields also have a dual role. (c) When of sufficient strength, theygenerate dipolar interactions between colloids.69 (d) Spatial gradientsin AC electric field amplitude generate dielectrophoretic motion thatconcentrates colloids for self-assembly.115 Images reproduced withpermission from the references as indicated. Reproduced withpermission from AAAS, NAS, AIP Publishing, and Nature Springerfrom indicated references.

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The former is interaction potential reconfiguration; the latter isphoretic reconfiguration.)Mechanisms of phoretic motion, such as diffusiophoresis,

have recently been generated autonomously. An example is theself-diffusiophoretic motion of platinum coated Janus par-ticles.106,107 The platinum half of the Janus particle catalyzes thedecomposition of a fuel, such as hydrogen peroxide, whichdrives active motion. The collisional dynamics of these particlesat finite concentration can lead to segregation.108 When suchactive motion is triggered by stimuli, such as light,109

reconfiguration between different segregation states can beachieved, in ways that are analogous to reconfiguration by otherfield-induced phoretic mechanisms. Although this kind ofreconfiguration is clearly similar to the subject of this review, itdiffers from reconfigurable colloidal assembly because localequilibrium, as described by thermodynamic variables andgeneralized potentials, is absent.The previous discussion points out the multiple usages of the

term reconfigurable colloidal assembly in the literature and thevariety of tools that are available. The dictionary definition ofreconfigurability is a system’s capacity to rearrange either itssettings or its elements. Likewise, in colloidal self-assembly, twolevels of reconfigurability are available. The first level ofreconfiguration involves the back-and-forth adjustment ofsettings (i.e., “knobs”)these settings being the independentparameters that define the statistical mechanics of the system.As just discussed, the settings that can be changed involve thesize and shape of building blocks, the strength and range ofpotential interactions, and the spatial variability of a systempotential energy as specified by an applied field. Rearrangingthese settings manipulates the system’s free energy. As thesetools of reconfiguration are applied, the elements of thesystemthe colloids themselvescan then rearrange tominimize the free energy. Reconfiguration in this way canshift the system across a phase boundarysuch as in an order/disorder or a crystal/crystal transition. It can also shift thelattice spacing of a unit cell whose symmetry is otherwiseconserved. Finally, it can rearrange the shape or position of anobject self-assembled from colloids.Figure 1 introduced a matrix representation of these two

determinants of reconfigurable colloidal assembly. Changingshape, interaction potential, or phoretic strength represents atool for reconfigurable colloidal assembly. Any of these toolscan yield any of three different functional classes of colloidalreconfiguration. These classes are a change in phase, a changein lattice spacing, or a change in the shape and position of anobject that has been self-assembled from the colloids. Thismanner of describing this research area illustrates the diversityof ways in which reconfiguration has been generated, as well asgaps in what has so far been accomplished. In the next section,we describe examples of the three functions of reconfigurablecolloidal assembly.

■ FUNCTIONS OF RECONFIGURABLE COLLOIDALASSEMBLY

In the previous section, we identified the different categories oftools that can be applied to generate structural transitions inreconfigurable colloidal assembly. The tools can be broadlyclassified as entropy (e.g., shape shifting colloids), interactionpotential (e.g., field-induced dipolar interactions), or phoreti-cally (e.g., electrodeposition) controlled. To complete theclassification, we describe the range of structural transitions thatthe reconfiguration tools have generated (cf. Figure 4). The

first function is reconfiguration across a thermodynamic stateboundary, examples of which are shown in Figure 4a,b.Colloidal systems display fluid, cluster, gel, and crystal phases,and reconfiguration between any of these phases is possible.For example, Figure 4a shows a field-induced order disordertransition in colloidal spherocylinders.70 Figure 4b shows howmagnetic particles can be toggled between free particles andchain structures.74 The second function is reconfiguration ofthe particle positions with a particular phase. An example is thereconfiguration of the lattice spacing of a crystal phase. Anotherexample would be the reconfiguration of the hydrodynamicradius of clusters by varying the interparticle spacing of particlesin the cluster.53 A final example would be reconfiguration of thestructural color of colloidal crystals in droplets throughmanipulation of the colloidal separation34 (Figure 4c,d). Thethird function is the reconfiguration of the spatial boundaries ofa colloidal phase, so as to generate self-assembled objects whoseshape, size, and position can be reversibly modulated. Forexample, as should in Figure 4e−j, micropatterned electrodesact as sources or sinks for clusters of colloids (panels e−h),95 orcolloids are organized into specific shapes by light-assisted

Figure 4. Different reconfiguration functions realized by application ofthe three types of tools. Phase reconfiguration: (a) Laser diffractionshows fluid/glass to a crystal phase transition of colloidal rods by ACelectric fields;70 (b) fluid to chain reconfiguration by magnetic fieldswithcing.74 Lattice spacing reconfiguration: (c) spacing of satellitenanoparticles in a cluster, as controlled by DNA conformation;53 (d)osmotic pressure changes the lattice constantand thereby thestructural colorof colloidal crystals confined in vesicle droplets.34

Shape and location reconfiguration: (e−h) colloids applied topatterned electrodes reconfigure their location on the grid in responseto the frequency of the applied electric field;95 liquid-induced phoreticreconfiguration produces colloidal crystals of mutable shape (i, j).97

Images reproduced with permission from the references as indicated.Scale bars in b, i, j are 20 μm; e, g are 50 μm; and f, h are 200 μm.Reproduced with permission from AAAS, ACS, RSC, and SpringerNature from indicated references.

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electrophoresis (panels i−j)97). I next discuss these threefunctions of reconfiguration.Thermodynamic State Reconfiguration. Many of the

tools identified in the previous section have been used toproduce reconfiguration between fluid and crystal phases. Fromthe perspective of applications, this structural transition is acommon target because of the large difference between thefunctional properties of disordered fluids and ordered crystals.Iridescence is an example of a functional property that has beenswitched on and off as a result of reconfiguration between fluidand crystal phases.110

Each of the reconfiguration toolsbased on entropy,interaction potential, and phoretic mechanismshas beenapplied to generate this kind of thermodynamic transition. Forexample, temperature has been used to reversibly swell andshrink the volume of colloids, causing them to reconfigurebetween disordered liquid and ordered crystal phases.111

The phase diagram of colloids interacting through dipolarforces is rich.66 Reconfiguration among fluid, close-packed,body centered orthrorombic and body centered tetragonalphases has been reported for charged spheres71,112 when highfrequency AC electric fields are applied to induce dipolarinteractions. Ellipsoids polarized by means of high frequencyAC electric fields have furthermore been actuated between fluidand ordered microtubule phases69 (cf. Figure 3c). Temper-ature-mediated binding of complementary DNA yieldsreconfiguration of disordered colloids into crystal phases withthe symmetry of ionic crystals, including CsCl and CuAu.113

Examples of phoretically induced fluid to crystal structuraltransitions include dielectrophoretic concentration,92,114−116

electrophoretic depositionas induced either by appliedelectric fields85 or light97and radiation pressure. These fieldsdisplace colloids in the field direction, thereby generating large,near wall regions of high colloid concentration in whichordered phases are in local equilibrium. Other methods, such asoptical trapping89 and light-activated AC electroosmotictrapping,117 have been used to generate microscopic wellswith individual colloids attracted to each well. Reconfigurationoccurs as the trap pattern is switched on and off.Colloids have been reconfigured between the fluid phase

characterized by individual colloids with random, disorderedstructureand cluster phases. Cluster phases organize colloidsinto aggregates or crystallites of particular size, shape, andinternal structure.118 The tools of ligand reconfiguration, DNAhybridization,53 and DNA toehold-mediated displacementreactions have been applied for fluid−cluster reconfiguration.Spheres with attractive Janus faces or patches are also wellsuited to this kind of reconfiguration because the prescribedregion of the patch limits the number of bonds in which thecolloid can participate.119−121 This geometry leads to self-limiting assembly. The reversibility between the self-limited,cluster state and a fluid of individual colloids has beengenerated through magnetic fields77,122 as well as by changes insolvent properties.46

If the interaction potential between colloids is anisotropic,then low-dimensional structures such as chains can form. Suchchaining leads to the self-assembly of fibers, networks, and gels.Reversible actuation between these phaseswhich canpotentially support stressesand other, ergodic phases59,74which cannotgenerates materials with reconfigurable me-chanical properties. A mechanism for chaining is to introducespecific kinds of Janus functionality onto particles that aresufficiently anisometric,123 such as ellipsoids5 and rods.124

Confinement,79 nanocapillary interactions,45 and the combina-tion of shape and attractions125,126 can also limit thedimensionality of colloidal structures. Alternatively, induceddipolar and quadrapolar interactions can generate strings,chains, and networks.59,74,127,128 Chains themselves can bereconfigured into other phases, including crystals.63

Reconfiguration between two different crystal phases wasfirst demonstrated in the context of colloidal martensitictransitions through the application of AC electric fields.129 Thetransition was between close-packed and body-centeredtetragonal unit cells. AC electric fields have been used toactuate between plastic and liquid crystal phases of colloidalrods,70 as well as between two different crystal phases of Janusellipsoids.5 Temperature-dependent depletion interactions havebeen used to induce solid−solid phase transitions inmonolayers of colloidal superballs.41 Colloids coated withcomplementary DNA strands have been used to reconfigurebetween fcc and bcc phases by means of a fluid phaseintermediate.52 Displacement reactions with DNA-graftedlinkers have produced reconfiguration between two crystalphases of the same symmetry, but different colloidalsubstitution.47 Phases with different substitution have alsobeen formed as magnetic field strength is varied.78

Evaporation of solvent in a solution of ligand-functionalizednanocubes has been used to reversibly switch between fcc andsimple cubic structures;27 this reconfiguration is an example ofan entropy-induced transition in the sense that the conforma-tional change of the ligand modulates the shape and volume ofthe nanocube, thereby affecting the colloidal contribution to thesystem entropy.22 Simulation suggests further opportunities forshape-shifting colloids to reconfigure between multiple crystalphases.32,130

Lattice Spacing Reconfiguration. Reconfigurable self-assembly can function to tune the lattice parameters of acolloidal crystal or the separation distance between particles ina cluster. While state reconfiguration involves a transitionbetween equilibrium phases, lattice parameter reconfigurationmanipulates the separation distances of particles in the unit cell,while still conserving the unit cell symmetry. Functionalproperties of colloids depend sensitively on these separationdistances. For example, the wavelength of Bragg iridescence inbiomimetic structural color is a function of both the unit cellsymmetry and its lattice parameters.10 The Bragg peak shiftswith the lattice parameter.131 The lattice parameters andinterparticle spacing of ordered colloidal structures have beenreconfigured by means of DNA displacement reactions,54

electric and magnetic fields,5,80,132,133 temperature,24 osmoticpressure,34 and analyte concentration.28

Interaction potential tools for reconfigurationincludingDNA displacement reactions54,56 and solvent effects on ligandconformation and depletion interactions42,51,134have alsobeen used to switch bond separation distances in clusters andcolumnar chains. Bond distance reconfiguration is central to theproposed use of colloidal clusters for information storage.135

The spectral response of a self-assembled cluster can bereconfigured by manipulating the plasmon coupling betweenparticles in the cluster through interaction potential controlledchanges in particle separation distance.53,134

Self-Assembled Object Reconfiguration. To this point,we have focused on how reconfiguration can be used to controlthe microscopic features of self-assembled colloidal crystalsproperties such as the symmetry of the unit cell and its latticeparameters. However, another important feature of these

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crystals is their size, shape, orientation, and domain structure.Phoretic mechanisms, which apply forces to colloids that varyin space and time, can be used to reconfigure the shape,orientation, position, and patterning of colloidal crystals. Forexample, illuminating an ITO-coated electrode with UV or bluelight in an electrochemical cell yields sensitive control of ioniccurrents at the point of illumination. These ionic currents aresufficiently large to drive colloidal crystallization by electro-phoretic deposition97 or electrohydrodynamic flow.136 Theposition and shape of the illuminated region controls the spatialextent of the crystal. Electrothermal hydrodynamic forcesasinduced by IR illuminationhave similarly been used to createand reposition ∼50 μm colloidal crystal regions.137 Electrodepatterning can be used to localize colloids either on or off thepattern, depending on the properties of the electric field andthe colloids.95 Synchronized optical traps have been used torotate small colloidal crystals.96 Colloidal structures formedthrough interactions with liquid crystal defects can reconfigurein response to translation,138 rotation,139 or deformation140 ofthe defect field. Liquid crystal defects created by substratepatterning can respond to electric140 and light138,139 fields. Thereconfigured colloidal structures are one- or two-dimensionalarrays of particles decorated on the defects.

■ DYNAMICAL RESPONSE OF COLLOIDALRECONFIGURATION

An important characteristic of a reconfigurable colloidal systemis its dynamic response. What is the delay between triggeringreconfiguration and the adoption of the new functional state?The answer to this question constrains applications of colloidalreconfiguration. Moreover, it reveals fundamental informationabout the physical processes that are foundational to both thetools and functions of reconfiguration. Here we use the matrixclassification of Figure 1 to illustrate how the highly variableresponse times for colloidal reconfiguration can be rationalizedbased on adding the time constants for the use of the tool andthe performance of the function. Figure 5 illustrates thecomponents of the dynamic response; it distinguishes betweenthe time to trigger the reconfiguration tool and the time for thetool to act on the colloidal system. The latter time constant canhave two contributionsthe time constant for the structure toswitch states and the time constant for the colloids to traverseand accumulate in particular parts of the system.The time to activate the reconfiguration toolτtoolvaries

considerably among the entropy, interaction potential, andphoretic mechanisms. Broadly, these three mechanisms aretriggered by changes in either environmental conditions orelectromagnetic field strength. The former mechanism is muchslower than the latter. The trigger for entropy mechanisms istypically thermal or mass diffusion induced by changes inenvironmental conditions. For example, a temperature changethat propagates across the system through thermal diffusion(i.e., conduction) will swell colloids comprised of thermorever-sible polymers. Both environmental and field triggers arerepresented in the interaction potential and phoretic reconfigu-ration tools. For example, DNA hybridization is triggered bythermal diffusion, electric and/or magnetic fields induce dipolarinteractions, and electrophoretic deposition requires iontransport to set up the steady electric field that drives colloidsto the electrode.Once the tool has been activated by propagation of the

temperature, solute, or electromagnetic wave, the functionalchange in colloidal structure occurs according to its character-

istic dynamics. Depending on the three functionsphasechange, lattice spacing shift, or assembled object displace-mentstructural reconfiguration can require time to (i)generate a phase change or lattice position change thatpropagates though the system or (ii) accumulate colloids atvarious positions in the system so that the thermodynamic statetransition can be generated. The first time scale, τswitch, isdetermined by the kinetics of nucleation and growth, spinodaldecomposition, melting or dynamics of normal growth, orpolymorphic or martensitic transitions.1 The balance of fieldsand drag that determines transport properties sets the secondtime scale, τacc.The total reconfiguration time is therefore τreconf ig = τtool +

τswitch + τacc. As we will see, the reconfiguration time constantcompares favorably with the requirements of a variety ofapplications for reconfigurable colloidal assembly. Likewise, theconstraints of a particular application on τreconf ig inform theselection of particular tools and functions compatible with thespecified application dynamics.For example, consider a display or fabric of interest for

reconfiguration with a thickness ∼0.1 mm. The value of τtoolvaries considerably by the tool selected. For liquids, thermaland small-molecule diffusivities are approximately 10−1 mm2/sand 10−3 mm2/s, respectively. Therefore, the temperature-induced reconfiguration requires τtool ∼ 10−1 s to propagateacross the device. Reconfiguration triggered by a small-molecule solute requires larger times (τtool ∼ 101 s). On theother hand, at these dimensions, electromagnetic fieldssuchas those used to trigger interaction potential and phoreticmechanismspropagate nearly instantaneously.If the functional reconfiguration involves a change in phase,

additional time is required for the dynamics of phase change.For example, in one-dimensional, thermally activatedgrowth141as might occur in boundary nucleation85,142thistime requires the following:

Figure 5. Dynamics of colloidal reconfiguration. The total time toreconfigure a colloidal system, τreconf ig (top, purple surface), is the sumof τtool, τacc, and τswitch (see text for definitions). τtool grows as the squareof the characteristic dimension of the device and is independent of theparticle size (bottom green surface). τaccwhich arises in phoreticmechanismsgrows linearly with the device dimension and isindependent of particle size. The sum of τtool and τacc is the middlegreen surface. τswitch grows linearly with the smallest device dimensionand as the square of the colloidal size. How each characteristic timescales with particle size and device dimension is apparent from thecross section of each plane.

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τϕ

μ∼ − − Δ−⎛

⎝⎜⎜

⎛⎝⎜

⎞⎠⎟⎞⎠⎟⎟h a

D k T( )1 expswitch

switch

s B

1

(2)

Here hswitch is the characteristic dimension of the region whosephase will be reconfigured. Ds is the (volume-fraction-dependent) short-time self-diffusivity of the colloid, a is thecolloid radius, and kBT is the thermal energy. Δμ is thechemical potential difference between the two phases. Forchemical potential differences that are large relative to thermalenergy, the thermally activated growth rate saturates at avelocity, Ugrowth ∼ Ds(ϕ)/a.

142,143 Therefore, to induce a phasechange throughout a 100 μm device, τswitch ∼ 20 s for 0.1 μmcolloids. This time increases to ∼2400 s for colloids of radius1.0 μm. The characteristic growth velocity sets a lower boundon τswitch that can be applied both to different phase transitions(e.g., fluid−fluid, fluid−crystal, crystal−crystal) as well as to thelattice-spacing reconfiguration.Approaches to estimate τswitch, of course, will vary with both

the kinetic mechanism of the phase transition and the structureproduced. For example, heterogeneous nucleation and growthcan lead to Avrami kinetics, and diffusionless, martensitictransitions between crystal phases can occur very rapidly.112

Reconfiguration among fluid, cluster, and chain states may alsooccur rapidly because of the small number of participatingcolloids.5,82,101,144

Accumulation time, τacc, is required when the function ofreconfiguration (such as a phase change) first requires a changein the number density profile of colloids across the device. Forexample, phoretically driven reconfiguration might transform asystem from a volume fraction that is uniformly lowwith afluid structureinto one in which all the particles have beenconcentrated into a local region of high densitywith a crystalstructure. Mass conservation requires that the region of highdensity pervade only a small portion of the device volume; allcolloids need to be swept into this volume before phasereconfiguration is complete. The time to accomplish thisdensification is the accumulation time. An expression for thistime, from a simple model of one-dimensional convection is25

τϕ

ϕϕ

= −⎛⎝⎜⎜

⎞⎠⎟⎟h

U K( )1acc

acc

i

acc

i0 (3)

Here τacc is the time to fill a height hacc to volume fraction ϕaccgiven an initial volume fraction ϕi. U0 is the convective velocityof a single colloid; K(ϕ) is a factor accounting for theretardation of dynamics due to crowding. Its value depends onboth the colloid number density and the convectivemechanism. Convection in this case is by one of the manymechanisms for microscopic colloidal transporti.e. sedimen-tation, electrophoresis, dielectrophoresis, radiation pressure,etc. Although U0 varies considerably by mechanism and solvent,characteristic values range between ∼0.5 and ∼5.0 μm/s.Consider an application in which the colloid density isphoretically manipulated from a state of uniformly low density(ϕ0 ∼ 0.05) to one in which a high density of colloids (ϕacc ∼0.5) is localized in a small region of the device. For a devicewith smallest dimension ∼0.1 mm, such accumulation timeswould fall between ∼101 and 102 s. The accumulation timevaries linearly with the smallest dimension of the device.The three characteristic time constantsτtool, τswitch, and

τaccsum to give the total reconfiguration time, τreconf ig, asillustrated in Figure 5. Note that one or more of the three time

constants might not contribute significantly for somecombinations of tool and function. For example, usingdielectrophoresis to reconfigure a dense suspension of colloidsbetween a liquid and crystal phase yields τreconf ig ∼ τswitch,because neither τtool nor τacc are significant in this case.The parameter space illustrated in Figure 5 constraints the

choice of reconfiguration tool if a specific reconfigurationfunction must be achieved in a particular time. Broadly, for 0.1μm colloids, reconfiguration is never faster than about 60 s.However, many combinations of reconfiguration tools andfunction require times as long as 6000 s, especially for colloidso f l a r g e r a d i i (∼ 1 . 0 μm) . L i t e r a t u r e me a s -ures34,55,85,97,110,112,143,145,146 of the dynamic response ofreconfiguration broadly confirm these approximate limits (cf.Figure 5). The expressions for τswitch and τacc point toparameters of the colloidal system or device that can bemanipulated to design for a particular target reconfigurationtime.

■ APPLICATIONS OF RECONFIGURABLE COLLOIDALASSEMBLY TO FUNCTIONAL MATERIALS

The tools and functions of colloidal reconfiguration have been appliedto manipulate the properties of materials produced by self-assembly.Properties that have been targeted for reconfiguration includeiridescence and structural color, plasmonic response, conductivity, aswell as rheological and mechanical properties, including the capacityfor self-healing. Future concepts that have been explored for colloidalreconfiguration include information storage and microrobotics.

Because the size of colloids matches the visible wavelengths of light,colloidal crystals can produce structural color. Because this color isgenerated by physical dielectric properties, rather than chemicalabsorption, it is both iridescent and resistant to fading throughmechanisms such as photobleaching. Through its effect on the Braggcondition for optical diffraction, lattice-parameter reconfiguration hasbeen used to tune the characteristic wavelength of the structural colorresponse34,81,147 (as e.g. generated when a macroscopic elongationalstrain affinely deforms the microstructure, cf. Figure 6a131). The fluid-crystal phase reconfiguration has furthermore been used as an on−offswitch for structural color.110

If the colloidal building blocks are smaller, then reconfigurable self-assembly can be used to produce optical metamaterials throughcontrol of plasmonic response. Localized surface plasmon resonance inmetallic nanocolloids produces strong absorption at visible wave-lengths; dielectrophoresis has been used to reversibly concentrate andorient such gold nanorods near an electrode.148 The concentration andorientation produced by dielectrophoresis generates a polarization-dependent optical response in which a central electrode’s shadowaround which the rods are concentratedchanges in response to anelectric field applied along a particular axis (as shown in Figure 6b,right).

Conductive colloids can produce reconfigurable wires andinterconnects potentially useful for applications involving mobileelectronic components, such as neuromorphic engineering, rapidprototyping of circuit networks, or self-repairing electronics.Dielectrophoresis has been applied to self-assemble silicon nanowiresinto bundles that are then switched between two interconnect states149

(cf. Figure 6c, in which the electrodes are yellow; the connectionamong them switches according to the position of a self-assemblednanowire).

Reconfiguration between fluid and gel phases produces materialswith tunable elasticity. Weak elastic networks are an importantconstituent of complex fluids used in consumer products, agriculturalformulations, and pharmaceuticals. In these products, gel networksmaintain homogeneity of microscale particles and droplets, therebyensuring dosage uniformity. Sensitive, temperature-dependent elas-ticity has been produced by reconfiguration of potential interactionsbetween colloids dispersed in liquid crystal solvents. This reversible

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transition is shown in Figure 6d,59 for the particular case of colloidsdispersed in a liquid crystal solvent. The colloids form a gel phasedependent on the temperature-dependent phase of the liquid crystal.Self-healing can be an outcome of colloidal reconfiguration. For

example, magnetic nanoparticles coated with thin fatty acid layers bindabove a critical temperature due to nanocapillary interactions of thefluidized layer.45 Magnetic fields drive assembly of the particles intochains. The nanocapillary interactions preserve the network structureeven if the field is removed. If damaged, the networks can bereconstituted by renewed application of a magnetic field (cf. Figure6eshowing pre- and poststates of the chains upon rupture andhealing).Reconfiguration between two phases can be used for actuation and

force transduction. For example, Janus ellipsoids self-assemble intofibers with a four-particle unit cell. The unit cell elongates upon theapplication of an AC electric field. Fibers comprised of a sequence ofthese unit cells therefore deform when the AC field is switched on.The fibers snap back into their equilibrium conformation when thefield is switched off. This on/off process is shown in Figure 6f, inwhich the four image frames show field off, on, on, and off states,respectively. The unit cells of the unstretched and stretched phases arealso shown, at right. The reconfiguration arises because the potentialenergy barrier of the ellipsoids to sliding is relatively low. Simulationestimates that forces of ∼5 pN magnitude can actuate this slidingtransition, which yields a ∼ 35% change in fiber length.5 Colloids withreconfigurable spacing, clustering, and assembled object shape havebeen proposed for rewritable information storage,90,135 antennas,150

and microrobotics.18,151

■ OUTLOOKThis review has highlighted the broad range of tools that havebeen discovered for reconfiguration of self-assembled colloidalstructures. A single tool, used in different ways, can oftenproduce different functions (as shown in Figure 3).Alternatively, individuals seeking a specific function frequently

have the flexibility to select from among different tools (asshown in Figure 2).A scientific gap that remains in the development and use of

colloidal reconfiguration is at the intersection of entropy-basedtools and assembled object reconfiguration (as shown in Figure1c). Entropy tools, which affect self-assembly by directmanipulation of particle shape and size, have strong advantagesfor reconfiguration.22 These advantages are (i) access toparticular equilibrium phases that are hard to produce becauseof kinetic limits. In this case the kinetically limited phase can bereached through an intermediate phase produced from a simpleshape. Particle shape change can then yield an accessiblepathway to the target phase. (ii) reconfiguration kinetics itselfcan be accelerated because entropy tools operate locally; thekinetics of crystal−crystal phase transitions can therefore avoidthe slow, collective dynamics that retard other phase transitionpathways, albeit these advantages, experimental demonstrationsof entropy-driven reconfiguration due to particle shapechange,27 are not as widespread as the possibilities suggestedby computer simulation.31,32

In a similar vein, the use of these tools to change the shapeand position of a self-assembled colloidal objectgateway toachieving more complex functionsis in an early stage ofdevelopment. Examples of complex functions to whichreconfiguration can contribute include those recently realizedon large scales by soft robotics, such as engulfing a target,grabbing and translating a target, and drawing targets intoproximity with each other. On the microscale, these functionsare biomimetic to phagocytosis, intercellular transport ofvesicular cargo, and bacterial quorum sensing. All these aimscan potentially be advanced by exploiting the hierarchy of scalespresent in objects self-assembled from colloids whose shapeand position can be reversibly controlled. By applying the toolsof reconfiguration, fine variations in object shape and positioncan be realized either autonomously, through local energytransformation, or at a distance, through application of fields.The phoretic tools that currently dominate the availablemethods for object transformation represent a kind of dynamicself-assembly. The intersection of dynamic self-assembly andreconfiguration combines energy input with reversibility,thereby yielding the potential for cyclic function, durability,and self-repair.The tools represented by reconfigurable colloidal assembly

are limited if the pathway between the target structures iskinetically inaccessible. Earlier we discussed how entropy toolsmay open new pathways between structures by passing throughintermediate phases. In addition, real-time monitoring ofcolloidal configurations could allow the identification offluctuations favorable to a particular pathway; controlalgorithms could then be applied to direct and amplify thesefluctuations to accelerate the otherwise slow dynamics ofreconfiguration.116

Finally, the tools of reconfigurable colloidal assembly arediverse and versatile; they are ready for integration into devicesand materials with switchable and tunable function. Figure 5shows that the time scales for reconfiguration are compatiblewith many such applications. Engineering science questions tonext address in pursuit of this aim include the following: (i) thequality of the mechanical, optical, or other functional responseachieved; (ii) the dependence of that quality on thereconfiguration tool and structure; and (iii) the degree towhich functional quality is conserved as device dimensions arescaled up into the ranges of greatest applications interest.

Figure 6. Applications of reconfigurable colloidal assembly. (a)Iridescence and structural color, demonstrated through lattice spacingreconfiguration.131 (b) Phoretically driven reconfiguration shifts thepolarization-dependent optical response of gold nanorods.148 (c)Nanowire bundlesself-assembled and manipulated by phoreticreconfigurationswitch between different connection architectures.149

(d) Sol−gel transitions generated by liquid crystal phases switch themechanical state of colloidal materials.59 (e) Magnetic-field inducedreconfiguration of strand networks applied to self-healing materials.45

(f) One-dimensional assemblies of reconfigurable length actuate forceson objects and boundaries.5 Scale bars in c, d, and f are 15 μm, 1 cm,and 3 μm, respectively. Images reproduced with permission from thereferences as indicated. Reproduced with permission from AAAS,ACS, IOP, AIP Publishing, and Springer Nature from indicatedreferences.

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Addressing these questions will assist in tool selection forspecific applications. For example, consider the design of anarchitectural coating that changes color on demand. Would thiskind of reconfiguration be better accomplished by adopting aphoretic methode.g. by performing electrophoretic deposi-tionor by deploying a change in the interaction potentiale.g. by inducing dipolar interactions? The choice depends onthe reconfiguration times and crystal quality achieved by thetwo methods at the scale of interest; these are questions thatcan be answered by future research that combines theory,simulation, and experiment.

■ AUTHOR INFORMATION

Corresponding Author*E-mail: mjsolo@umich.edu.

ORCIDMichael J. Solomon: 0000-0001-8312-257XNotesThe author declares no competing financial interest.Biography

Mike Solomon is Professor of Chemical Engineering at the Universityof Michigan. He received his Ph.D. at the University of California atBerkeley in 1996. After a postdoctoral appointment at the Universityof Melbourne, Australia, he joined the faculty at the University ofMichigan. His research interests are in colloidal assembly, gel rheology,and the biomechanics of bacterial biofilms.

■ ACKNOWLEDGMENTS

Work on this feature article about colloidal self-assembly forreconfigurable biomimetic structural color and mechanics wassupported by the US Department of Energy, Basic EnergySciences, under Grant DE-SC0013562 and the US ArmyResearch Office under Grant Award No. W911NF-10-1-0518,respectively. Opinions and conclusions expressed in this articleare the author’s; they do not necessarily reflect the views of thefunding agencies. I thank Ron Larson, Carlos Silvera Batista,Sepideh Razavi, and Ashis Mukhopadhyay for discussion of themanuscript.

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DOI: 10.1021/acs.langmuir.7b03748Langmuir XXXX, XXX, XXX−XXX

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