Probing Molecular, Nanoscale and Adhesive Forces Related to Fiber-Fiber Bonding and Optimized Surface Interactions2009 International Conference on Nanotechnology for theForest Products Industry, June 23–26, 2009, Edmonton, AB, Canada

Agne Swerin, Birgit Brandner, Viveca Wallqvist and Martin Wåhlander

YKI, Institute for Surface Chemistry, Stockholm

Outline

• New knowledge from investigations on nano / micro scaleinteraction in experimental systems of relevance to forestproducts using AFM colloidal probe microscopy– Cellulose fiber-fiber joint strength linking to paper properties

and chemical mapping of bonded areas– Interaction between non-polar surfaces in water with focus

on pitch collector systems and influence of surfaceroughness on long-range capillary forces

– Interaction forces between superhydrophobic surfaces

Adhesional, frictional and bonding properties in fiber-fiber joints

Use AFM colloidal probe, confocal Raman/AFM/SNOM and XPS as well as new model cellulose surface to explain connection between properties in fiber joints (micro level) with paper (macro/ensemble level)

Unbleached / bleachedcellulosic fibers

Principle of SNOM

AFM colloidal probe• Friction and adhesion

Raman - Confocal Raman• Properties in bonding sites

XPS - Chemical imaging

SNOM - Optical imaging

Research project

Development of a noveltechnique to measure adhesion in fiber-fiber joints

AFM colloidal probe adhesionforce measurement

Chemical and topographicalimaging of thermo mechanical pulp fiber surfaces

Combination of confocal Ramanmicroscopy and AFM

Sub-projects

Techniques

Goal

Understanding of influence of - extractives- chemical environment- fiber- etc

on strength of fiber-fiber joints

Introduction

Force measurements using AFM colloidal probe

• Cellulose fiber instead of sharp tip• Cellulose fiber mounted on stage• A liquid cell makes it possible to measureinteractions in liquids

- Cellulosic fibers from regenerated cellulose (Lyocell process) with a diameter of 25 µm

- Fibers were washed for 30 minutes in a 0.001 M sodium bicarbonate solution (NaHCO3) and rinsed with deionised water until the pH value reached 7

- Pieces of fibers (65-300 µm) were glued onto the edge of a used AFM chip (substrate) and on a cantilever, respectively

- The fibers were used in crossed configuration, seefigure on the left (schematics on top, photo from experiment at the bottom)

- Methodology for mounting similar to fiber-fiberfriction measurements using AFM colloidal probe technique Mizuno, Wallqvist et al., Friction measurement between polyester fibres using the fibre probe SPM. Australian Journal Of Chemistry 2006, 59, (6), 390-393.

Crossed configuration of fibres Crossed configuration of fibres (10x)(10x)

Sample and sample preparation

Force measurements and results

- The force curves between these crossed fibers were recorded at ten different spots, both in air and in water.- In each spot at least 20 and up to 30 individual force curves were recorded. - The adhesion force was calculated from those curves and normalized,

which means divided by the effective radius Reff (6.25 µm) of the fibers

Measurements in air

- The figure on the right shows up to 28 individual measurements in 6 spots.

- Very good reproducibility within one series measured in one spot.

- Between the different series, only the measurements in spot 6 deviates significantly from the others.

- Averaging the values for the adhesion forces in all spots results in a value of (146 ± 13) mN/m

- Environmental conditions:Room temperature: 20.5oCRelative humidity: 20-30 %

Feiler, Rutland et al., Effect of Capillary Condensation on Friction Force and Adhesion Langmuir, 2007, 23 (2), 517

Effect of capillary forces similar to interaction betweencellulose spheres in work by Feiler et al.

Measurements in water

- Deionised water (MilliQ) was used as ambient medium.

- The figure on the right shows up to 22 individual measurements in 5 spots.

- Varying reproducibility within one series measuredin one spot varies; it is not as good as for the measurements in air.

- Averaging the adhesion forces over all spots results in a value of (0.56 ± 0.17) mN/m

Measurements agree with literature: Torgnysdotter, Wågberg et al. (2004 and 2007)

Overview - Confocal Raman/AFM/SNOM

Confocal Ramanchemical imaging, crystallinity, orientation…lateral resolution: ~200 nm

SNOM (Scanning Near-field Optical Microscopy)refractive index profile, topography...lateral resolution: ~ 50nm

AFM (Atomic Force Microscopy)topography, elasticity, friction, adhesion, surface forces…lateral resolution: ~1.5 nm

Combined Confocal Raman Microscopy and AFM on TMP FibersFresh TMP Fibers

Cellulose + LigninExtractives

AFM Phase AFM Topography

Extracted TMP Fibers

Cellulose + LigninCellulose

AFM Phase AFM Topography

- Repeat measurements with different fibers- refined Lyocell fibers- softwood / hardwood fibers

- Effect of relative humidity- Conduct measurements in a chemical environment with pH8

to simulate conditions for fine paper - Investigate influence of chemical additives on adhesion

- High resolution topography and chemical mapping of bondedareas by combined Raman and AFM

Future Plans

- Bright field image of coated paper with marked areas where the AFM (blue) and the Raman image were recorded

- Better overlap of the 2 areas can be achieved with cantilevers having the tip on the front; the cantilever used had the tip approximately in the middle of the blue square

Raman image (green: latex, red CaCO3)

Left: AFM phase, right: AFM topography

Simultaneous measurement of Raman/AFM

Outline

• New knowledge from investigations on nano / micro scaleinteraction in experimental systems of relevance to forestproducts using AFM colloidal probe microscopy– Cellulose fiber-fiber joint strength linking to paper properties

and chemical mapping of bonded areas– Interaction between non-polar surfaces in water with focus

on pitch collector systems and influence of surfaceroughness on long-range capillary forces

– Interaction forces between superhydrophobic surfaces

Structure and properties of talcTalc is described by the chemical

formula Mg3Si4O10(OH)2

Pure talc consists of a magnesium hydroxide layer (MgO•H2O) between two silicate layers (SiO2), forming a three layer structure

Adjacent layers are connected by weak London-van der Waals forces, giving pure talc a platy structure

The low energy silicate layers on talc surfaces make talc slightly hydrophobic

Talc edges are hydrophilic

Surface profile of a cleavable talc sample

The properties of pitchDeposits of wood resincomponents present in pulp

Resins and fatty acidsSteryl esters and sterolsTerpenoids including terpenes and

polyisoprenesWaxes

Interactions between surfacesThere are three main types of colloidal forces:

van der Waals interactions (electrostatic origins)Electrical double-layer interactions (charged interfaces in polar solvents)Steric interactions (polymers)

The hydrophobic long-range attraction

Attractive vdW ~2-5 nmRepulsive double-layer forces ~10 nm

Attractive forces up to 100 nmOne theory is formation of spontaneouscavities between surfaces

Luckham (2004)

Forces between two hydrophobic modelsurfaces in water

Non hydrophobic surfaces

Hydrophobic surfaces (in water)

The AFM colloidal probe method

A colloidal probe (r=2-15 μm) is attached to the calibrated cantilever

The piezo crystal is moving the sampleup and down towards the probe

When there is a force acting betweenthe probe and the surface, the cantilever is deflected

The measurement can be performed in air or in a liquid

Hydrophobic long-range attraction with talcsurfaces?

The force profiles with talc are similar to those obtained with two model hydrophobic surfacesThe interaction is long-range, even if talc is only slightly hydrophobic (contact angle with water

~75-86°)No connection between salt concentration and force profile has been observedAttractive and adhesive force values are not stable over time within the same measurement

(same contact positions)

0 5 10 15 20 25 30 35 40 45 50-10

-8

-6

-4

-2

0

2

Separation (nm)

Forc

e F/

R (m

N/m

)

AFM approach dataTheoretical VdW

Forc

e/ra

dius

(mN

/m)

Separation (nm)

Theories and conclusions

Steps sometimes present in approach and/or retraction curves indicate microbubbles present on the talc surface

Outline

• New knowledge from investigations on nano / micro scaleinteraction in experimental systems of relevance to forestproducts using AFM colloidal probe microscopy– Cellulose fiber-fiber joint strength linking to paper properties

and chemical mapping of bonded areas– Interaction between non-polar surfaces in water with focus

on pitch collector systems and influence of surfaceroughness on long-range capillary forces

– Interaction forces between superhydrophobic surfaces

The Lotus leaf effect

determined by:

Superhydrophobicity = extreme water repellencyself-cleaning effect

• Surface free energy (hydrophobicity, chemical composition) • Surface roughness

10 um

Hydrophobicity can be induced even with intrinsically hydrophilic materials!

Barthlott, W.; Neinhuis, C., Planta 1997, 202, (1), 1-8

Possible applications for superhydrophobicity

• Water repellent surfaces• Protection against condensation of ice and water vapor?• Stain repellency through self-cleaning• Microfluidics (control of drop motions)?• Entertainment• Antifouling• Antireflex treatment?…have to meet product specifications• Mechanical properties• Adhesion and friction• Optical properties• Taste and odor, etc.…and be able to apply in a viable process

Growing research field

Superhydrophobicity interesting in paper, packaging, building materials, etc. but important to find suitable applicationsDevelop prototypes and demonstrators for relevant cases

Number articles

1

10

100

1000

10000

2000 2001 2002 2003 2004 2005 2006 2007

superhydrophob*nanostruct*

Number patents

1

10

100

1000

2000 2001 2002 2003 2004 2005 2006 2007

superhydrophob*nanostruct*

Israelachvili, J.; Pashley, R.The Hydrophobic Interaction Is Long-Range, Decaying Exponentially With Distance. Nature 1982, 300, (5890), 341

Christenson, H. K.; Claesson, P. M.,Cavitation And The Interaction Between Macroscopic Hydrophobic Surfaces. Science 1988, 239, (4838), 390

Wetting behaviour

a) Young’s equation

b) Wenzel model

c) Cassie-Baxter model

d) Intermediate cases

LV

SLSV

γγγ

θ−

=0cos

1)1(coscoscoscos

0 −+

=+=

θϕθϕθϕθ

SL

LVLVSLSLCB

0coscos θθ fW R=F

actualf A

AR =

Conditions of superhydrophobicand self-cleaning surfaces

Contact angle above 150o

Combination of roughness and surface chemistry

Low sliding angle and CA hysteresis (zero degrees)

Stabilized liquid-vapor interface (Cassie-Baxter)Stable properties over a period of time

SH mica, 159° SH PE-coated paperboard, 154°

SH silicasubstrate, 159°

Petal effect

A Superhydrophobic State with High Adhesive Force

Feng et al. (2008), Langmuir 24, 4114-4119

In the literature…

Vast amount of material science publications and reviews, e.g. inAugust 2008 issue of MRS Bulletin

Many publications end with..

160°

Different methods of preparation

A. Superhydrophobic surface in two steps• Create a suitable roughness• Give hydrophobicity to the surface

B. Superhydrophobic surface in one stepMethods to simultaneously give suitable roughness and hydrophobicity to the surface

Hydrophobic particles in different matrices, e.g.Fluoropolymer particles in polyuretane or other polymers Hydrophobized inorganic particles in silicone or

fluorobased matricesPlasma polymerized layers of fluorocarbonsPlasma sputtered Teflon-like layers (on organic materials)

Nanotechnology applied to paper and packaging

Lotus paper made at YKI by treatment of copy paper, illustratingwater repellency and self-cleaning through nanotechnologyChallenge to meet demands on durability, adhesion, friction,food contact, in paper machine and packaging applications

Also need to investigate fundamental aspects

YKI Example – Copy Paper

Untreated Superhydrophobic

Air layer gives a mirror-like surface

The air layer effect can be used as an indication of degree and evenness of superhydrophobicity

Measures for superhydrophobicity

Contact angle (CA)Should be over 150°

Rolling angleGood secondary indicator, especially of

contact angle hysteresis (associated with self-cleaning)

Stain sizeReflects the penetration of liquid into the

surface treatment, usually associated with a gradual decrease in CA

Modified Cobb testAn indicator of resistance to hydrostatic

pressureFriction test

For mechanical stability

One measure isnot enough!

Solvent based SH formulation

Has served as a convenient reference for water borne formulations

Produced e.g. from methylated silica nanoparticles treated with a fluoropolymer

Advantage - can render almost any substrate superhydrophobic

Disadvantages- cannot be applied to all ranges of products with traditional coating techniques

- expensive- environmental and sustainability issues

Water-borne, one-step superhydrophobic coating

One-step coating procedure to produce required hydrophobicity and roughness to achieve a contactangle of 150°Both macro and microscale roughness important (scale bar 20 μm)

Paper board as packaging material: Barrier requirements

Water vapour - WVTRGas/Oxygen - OTRWater - CobbGrease

Barrier functionality and packaging surfaces

To meet requirements in terms of barrierproperties papers are coated with polymer films

- Extrusion- or dispersion-coated

Renewable Functional Barriers

Long-term and sustainable production of high quality foodpackaging for excellent moisture, water and gas barrier properties

New processing - building on earlier raw material focused projects (proteins, starches)Dispersion coating, Extrusion coatingControlled phase separation

Surface modificationSuperhydrophobic coatingPlasma, inkjet, electrospinning

Active packagingMoisture responsiveOxygen scavenging, anti-microbial

Industrial partnersSixteen companies – chemical suppliers, paper and packaging,

converters, machinery suppliersR&D partners

Eight partners – universities, research institutes and consultants

Recently started!

Four-year project

SH treatment in packaging barriers?

Sample Coat weightg/m2

WVTR*g/(m2 day)

Moisturecontent(w/w %)

Untreated board 0 566 5.8

Hydrophobic 2.0** 554

Solvent SH 3.7** 522

Water borne SH 15 519 5.7

* Tappi Standard T448 om-97, 23°C and 55 % RH**Probably evenly distributed in the board

a

untreated board 117° hydrophobic 132° solvent SH 154° water borne SH 145°

b c d

Swerin, Wåhlander: XIVth Fundamental Research Symposium,‘Advances in Pulp and Paper Research’, Oxford 2009 (in Press)

Earlier work on forces between SH surfaces

Sudden development of a negative (attractive) forceat about 0.4 µm relative displacement correspondsto cavitation

Conclude due to confinedwater and not because of pre-existing nanobubbles

Singh et al. (2006), Superhydrophobicity: Drying transition of confined water, Nature 442:7102, 526

Force measurements using AFM colloidal probe

• SH treated probe insteadof sharp tip• Stable SH surface• A liquid cell makes it possible to measureinteractions in liquids

Schematic force curves

Bumps on the road…Flaked off SH substrates due to high water flow in AFM liquid cell

Potential contaminantsSample preparation – dispersed and soluble gassesStiffer cantileversUse of multiple cantilevers to spanthe long range interactionApparent and real probe radii

Mica substrate

Loose SH coating

AFM canti lever

Video capture fromAFM liquid cell

Superhydrophobicity in a higher meaning?

Development of robust SH surfaces for AFM

Silica particles in polymer matrixon silicon wafers

Calcination at1450 °C

Silanation with trifunctionalfluorosilane

Similar for silicaprobes but lowercalcinationtemperature

Swerin and Wåhlander (submitted); Wåhlander, MSc thesis, KTH and YKI (2008)

Fundamentals of superhydrophobicity

• Colloidal probe measurements made in a liquid cell using a Veeco - Multimode AFM with NanoscopeIIIa controller, with a Nanoscope Extender and a PicoForceTM from Digital Instruments• Development of robust superhydrophobic surface and robust probe in order to make AFM forceinteraction measurements show an extremely large interaction distance of around 300 nanometers• Superhydrophobic surface made on a silicon wafer with calcinated silica particles silanized for hydrophobicity. Similar treatment of AFM probes

Swerin and Wåhlander (submitted); Wåhlander, MSc thesis, KTH and YKI (2008)

Jump-in at 300 nm

Very long-range due to air/vapor cavities

Jump-in at 300 nmAdhesion at 5000 nm

Apply cavity model to accountfor capillary forces

F - attractive forceR - sphere radiusγlv - interfacial energy between liquid and vaporθ - contact angle of meniscus substance on surface substanceD - surface separationd - radius related to the size of the capillary condensateRecently applied to explain long-range interaction in hydrophobic systems (Wallqvist,

Swerin, et al., Langmuir, Article ASAP, DOI: 10.1021/la900759e) but could not explain the very long-range cavities seen here on retract but only on approach

Approach

Retract

⎥⎦⎤

⎢⎣⎡ −π=

dD

RF

lv 1cos4 θγ

* Israelachvili, Intermolecular and Surface Forces. 2nd Ed., Academic Press, London 1992

*

Approach and retract curves between SH surface and probe

SH surface and SH probe experience very large interaction distances and large interaction forces

Swerin and Wåhlander (submitted); Wåhlander, MSc thesis, KTH and YKI (2008)

Interaction forces in non-polar and hydrophobic systems• Attractive forces between hydrophobic surfaces in water are too

long-ranged to be explained by van der Waals’ interactions

0 10 20 30 40 50

-4

-2

0

2

Separation (nm)

F/R

(mN

/m)

Exp’l data for non-polarsurfaces (talc/pitch)Jump-in at 30 nm

Calculated vdW force – ~5 nm

Wallqvist et al. (2007), Colloids Surf A

Mechanisms behind long-range interaction betweenhydrophobic or non-polar surfaces

Approach• One likely explanation- Cavitation or bridgingbubbles

• Other possible mechanisms- Water structural effects- Contamination- Hydrodynamic force- Electrostatic fluctuations

Retract

Effect of surfactant on forces between SH surfaces

Surfactant (SDS) removedmuch of long-rangeinteraction dependingon concentration10 % of cmc – 0.82 mM50 % of cmc – 4.1 mM122% of cmc – 10 mM

Long-range interaction restored after rinsing offthe surfactant

Degassing strong effect

Swerin, Wåhlander: XIVth Fundamental ResearchSymposium,‘Advances in Pulp and Paper Research’,Oxford 2009 (in Press)

High variability in adhesion and attraction distance betweenmeasurements – consistent with dynamic systems involving formation and break-up of air/vapor cavities and capillaries

n.b. Diagrams also contain measurements that could not be evaluated

Swerin and Wåhlander (submitted); Wåhlander, MSc thesis, KTH and YKI (2008)

Fundamentals of SH interactions – influence of surfactant

Schematic illustrations of surfactants remaining in the liquid-vapor interface of a rough SH surface. Some remaining surfactants would be difficult to completely remove, due to the stabilized interface

Close-up of air-capillary between SH probe sphere and SH surface stabilized by surfactants

Swerin and Wåhlander (submitted); Wåhlander, MSc thesis, KTH and YKI (2008)

Research groupPer Claesson – Professor Surface Chemistry at KTHRobert Corkery – solvent and water-borne SHAndrew Fogden – solvent and water-borne SH. Now with ANU, CanberraPetra Hansson – PhD student on forces/roughness/hydrophobic/SH systemsKenth Johansson – plasma coatings, surface modificationIsabel Mira – barrier coatings, electrospinning, printed functionalityMikael Sundin – lab assistance coatings, WVTR, surface analysesEsben Thormann – postdoc at KTH on AFM long-range interactionsJouko Vyörykkä – solvent and water-borne SH, now with Dow Europe. First

discussions of AFM on SH surfacesViveca Wallqvist – former PhD student on forces/non-polar surfaces. First

discussions and tests of AFM on SH surfacesMartin Wåhlander – former MSc student YKI / KTH. Now with ABB Corporate

R&DPatrick Gane, Cathy Ridgway and Joachim Schoelkopf at Omya

FundingVINNOVA, Swedish Government Agency for Innovation SystemsRISE Holding AB (Research Institutes of Sweden)Bo Rydin Foundation for Scientific ResearchTroëdsson Foundation (grant for combined AFM/Raman instrument)Industrial companies in mineral pigments, chemical supply, pulp-, paper and packaging

Conclusions

• AFM colloidal probe microscopy continues to prove useful in determining fundamental interactions in industrially relevant experimental systems

• Examples given;– Fiber-fiber joint strength using cellulosic fibers

mounted in a crossed configuration– Interaction between non-polar surfaces in water to

explain mechanism of talc as a pitch control additive

– Long-range interaction in superhydrophobicsystems and influence of surfactants

• Examples serve to probe nano-scale interactions related to surface chemistry and structural features