Nanoscale

PAPER

Cite this: Nanoscale, 2019, 11, 11774

Received 7th May 2019,Accepted 23rd May 2019

DOI: 10.1039/c9nr03882e

rsc.li/nanoscale

A hybrid bioinspired fiber trichome with specialwettability for water collection, friction reductionand self-cleaning†

Deke Li,a,c Zhentao Wang,a Daheng Wu,c,d Guocai Han*a and Zhiguang Guo *b,c

Inspired by biological surfaces, we designed a magnetic fiber trichome based on the surface properties of

caterpillars and earthworms. The caterpillar-inspired fiber trichome possesses a cooperative superhydro-

philic–superhydrophobic–slippery lubricant-infused porous surface with gradient wettability and shows

excellent fog harvesting behavior due to the driving force of the gradient wettability fiber similar to cater-

pillar spines. The earthworm-inspired fiber trichome exhibits excellent friction reduction and antiwear

properties under harsh oil-bathed friction conditions, and it moves rapidly in mud under magnetic stimu-

lation because of the self-lubricating transfer film formed between friction contact surfaces. In addition,

the earthworm-inspired fiber trichome also has continuous antifouling capacity in mud due to the self-

releasing lubricating layer that can be replenished after being consumed under solid friction. Therefore,

the caterpillar- and earthworm-inspired fiber trichomes extend the scope of potential applications, such

as self-driven water collection, self-floating oil spill cleanup, reducing friction and wear resistance, high-

efficiency antifouling, and transport of heavy loads, among others.

Introduction

Multifunctional bioinspired surfaces can be designed by com-bining the surface properties of multiple biological species.Among the various biologically inspired surfaces, cross-speciesbiological surfaces with different surface liquid-repellent pro-perties have aroused interest in both fundamental researchand engineering.1–5 Intriguing creatures, such as the Stenocarabeetle, spiders, nepenthes, and cacti, collect and transportwater from fog efficiently for subsistence in desert regions byusing their hierarchical structured superhydrophilic (SHH)surfaces, superhydrophobic (SHB) surfaces, and slippery lubri-cant-infused porous surfaces (SLIPS).6–9 Some animals, suchas earthworms, exhibit exceptional low-friction propertieswhen passing through mud due to their unique self-lubricat-ing behaviour, the mechanism of which relies on their sophis-

ticated surface texture with rough ripples that adjustablysecrete mucus to form a slippery layer in response to externalmechanical action. The stable self-lubricating film preventsthe earthworm from directly touching the soil to achieve thelow-friction performance.10,11

Bioinspired surfaces have been fabricated for water harvest-ing, which offers a possible solution to urgent water scarcityin desert and semiarid regions.12–14 The fog harvesting sur-faces can be classified as one-dimensional (1D), two-dimen-sional (2D), and three-dimensional (3D) materials based ontheir geometrical dimensions. Inspired by spider silk andcacti, several 1D bioinspired structures with superwettabilityhave been fabricated by different methods for fog harvesting.Biomimetic 2D surfaces inspired the Stenocara beetle thathave a special alternating hydrophobic/hydrophilic structuregradient showed excellent fog harvesting ability; the fog har-vesting rate is increased by the hydrophilic surfaces and thewater removal is accelerated by the hydrophobic surfaces.15

For 3D materials, the structure gradient has a directionalwetting driving force and Laplace pressure, which improvesthe fog harvesting ability. However, there are few studies ofwater harvesting using 3D biomimetic superwettable surfacescomposed of 1D wires.16 On the one hand, owing to the com-bination of chemical and physical property gradients, mostbioinspired surface structures have required complex fabrica-tion technologies. On the other hand, when a droplet is trans-ported along a limited SHB surface, the droplet shows specialwettability owing to the contact angle hysteresis. As a result,

†Electronic supplementary information (ESI) available. See DOI: 10.1039/c9nr03882e

aSchool of Materials Engineering, Lanzhou Institute of Technology, Lanzhou 730050,

People’s Republic of China. E-mail: hgclzu@163.com; Tel: +86 -931-2868471bMinistry of Education Key Laboratory for the Green Preparation and

Application of Functional Materials, Hubei University, Wuhan 430062,

People’s Republic of ChinacState Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics,

Chinese Academy of Sciences, Lanzhou 730000, People’s Republic of China.

E-mail: zguo@licp.cas.cn; Fax: +86-931-8277088; Tel: +86-931-4968105dUniversity of Chinese Academy of Sciences, Beijing 100049,

People’s Republic of China

11774 | Nanoscale, 2019, 11, 11774–11781 This journal is © The Royal Society of Chemistry 2019

many bioinspired surfaces show a low efficiency for collectingwater under gravity due to droplet re-evaporation. Hence, itremains a major challenge to construct a wetting fabricsurface that allows efficient rapid fog capture and fast removalof water simultaneously.

Self-lubricating low-friction materials have been developedbased on several innovative strategies that mimic animals’skins.17,18 However, earthworm-inspired surfaces can be ineffi-cient in contact with solid surfaces, showing high friction,poor wear resistance, small locomotion length, low load-carry-ing capacity and limited self-cleaning due to high friction andthe destruction of the self-replenished lubricating layer.

In the present work, we were inspired by a caterpillar thatharvests fog and transports tiny water droplets on its conicalspines, and by the low-friction mechanism of earthworms, inwhich they release lubricating fluid while they burrow throughsoil. We fabricated a fiber trichome with SHH–SHB–SLIPS gra-dient wettability inspired by the caterpillar trichome spinesused for self-driven fog collection. The caterpillar-inspiredfiber trichome is covered with rough Fe3O4 nanoparticles(NPs), allowing tiny fog-droplets to be captured by the super-hydrophilic tip from moist air. Then, the droplets are removedquickly along the SHB middle region and transported by theSLIPS root that is obtained by immersion in a lubricant.Therefore, the caterpillar-inspired fiber trichome propels thefog droplet in a capture–coalescence–removal–transport cycleof self-driven water harvesting. Experimentally, we demon-strate that the efficiency of fog collection can be improved byoptimizing the length of the special wettability fiber, whichcan be attributed to a synergistic effect among the surfaceenergy released from droplet condensation, the wettabilityforce, and the difference in Laplace pressure on the fibersurface with gradient wettability. Moreover, we design an oil-based friction environment to investigate the tribology andlubricating mechanism of the earthworm-inspired fiber tri-chome to examine the low-friction, antiwear and self-lubricat-ing properties. Lubricating liquids are stored stably in a softplastic tube as secretion droplets to create a droplet-embeddedfiber structure. As the external load increases gradually, theembedded lubricants are released in situ and cover the fibertrichome surface in a self-regulating manner. A fiber trichomeconsisting of bioinspired soft tubes achieved combined dis-continuous and continuous locomotion, which was similar toearthworm movement, in response to magnetic stimuli. Thus,hybrid wettable surfaces can be extended to wider appli-cations, such as wear-resistant, energy-saving microfluidicdevices, efficient locomotion in microrobots, heavy load carry-ing, and drug delivery.

ExperimentalMaterials

Short plush polyester fabric (pile height: 3.5 mm, surfacedensity: 300 g m−2) was obtained from a local textile market(Lanzhou, China). Iron(III) chloride hexahydrate (FeCl3·6H2O,

98%) and sodium acetate trihydrate (NaAc, 99%) were boughtfrom Xilong chemical Co., Ltd (Shantou, China). Polyethyleneglycol was obtained from Aladdin Chemical Co., Ltd. The pre-polymer of polydimethylsiloxane (PDMS; SYLGARD 184A) andthe corresponding curing agent (SYLGARD 184B) wereobtained from Dow Corning Corp. (USA). 1H,1H,2H,2H-Perfluorodecanethiol (97%) was purchased from Alfa Aesar. Apermanent magnet (surface magnetic density: 0.500 T, dia-meter: 50 mm and height: 30 mm,) was purchased from alocal hardware store. Perfluoropolyether (PFPE) lubricating oilwas obtained from DuPont Chemical Company (USA). Allother chemical reagents were analytical reagents, which wereused as received without further purification or treatment.

Fabrication of the caterpillar-inspired fiber trichome

The caterpillar-inspired fiber trichome with a surface exhibit-ing gradient wettability was prepared by spray coating andetching (Fig. 1). The silicone elastomer (1 g) was prepared bymixing PDMS with curing agent in a weight ratio of 10 : 1, andwas dissolved in n-hexane (3 mL) under stirring at 60 °C for30 min. The as-prepared SHB Fe3O4 NPs (1.5 g) (Fig. S1, ESI†)were dispersed in ethanol (30 mL), and then the PDMSpolymer was added to the solution, forming a homogeneousmixed solution. The mixed solution was uniformly sprayedonto the as-prepared fiber trichome substrate surfaces using

Fig. 1 Fabrication of the bioinspired fiber trichome with liquid-repellentbehavior. Schematic of the fabrication procedures of (a) the superhydro-phobic fiber trichome, (b) the caterpillar-inspired fiber trichome withSHH–SHB gradient wettability, and (c) the earthworm-inspired fiber tri-chome with low-friction wettability. (d) EDS spectra and the corres-ponding element distribution of the fiber trichome with multi-wettability.

Nanoscale Paper

This journal is © The Royal Society of Chemistry 2019 Nanoscale, 2019, 11, 11774–11781 | 11775

N2 gas at 0.2 MPa (Fig. 1a and S2†). To cure the SHB fiber andimprove the bonding strength of the coating, the sample con-taining Fe3O4 NPs was gently pressed, and then dried at 60 °Cfor 2 h. After spraying, the fiber tip was etched with air-plasmafor 2 min to obtain a SHH tip region, and stainless-steel meshsheet was used as a mask to protect the middle region of theSHB fibers. The fiber roots were modified with PFPE lubricat-ing oil for 10 min to obtain the SLIPS root. The root lengthwas 0.5 mm, and total length of the SHH tip and SHB fibermiddle region was 3 mm. The surfaces of the caterpillar-inspired fiber trichome showed the desired SHB–SHH–SLIPSgradient wetting performance (Fig. 1b).

Fabrication of the earthworm-inspired fiber trichome

Based on the caterpillar-inspired fiber trichome, PFPE lubricantwas injected into a soft plastic tube covered with holes to releasethe lubricant from the tube under external force to fabricate theearthworm-inspired fiber trichome with the SLIPS (Fig. 1c).

Characterization

The microstructure of the synthesized Fe3O4 was determinedwith a transmission electron microscope (TF20). The surfacemorphology of the Fe3O4 NPs was observed with a field emis-sion scanning electron microscope (FESEM; JSM-6701 F). Thesamples were pre-treated by Au sputtering. The behaviour ofwater on the specimens was characterized by a contact anglemeter system (JC2000D, Shanghai Zhongchen DigitalTechnology Apparatus Co., Ltd, China) under ambient con-ditions. Optical photographs were taken with a digital camera(DSC-HX200, Sony). The purity of the adsorbed oil wasmeasured by using a moisture titrator (831 KF, Metrohm). ACOD analyzer (DRB 200, HACH) was used to analyze theorganic content of the collected water according to the relevantpolicies (US EPA).

Results and discussionSelf-floating and selective oil adsorption of the caterpillar-inspired fiber trichome

The caterpillar-inspired fiber trichome showed reduced dragin water. The friction between the solid surface and viscous

liquids was decreased by stable, continuous air rings. The thinair film was formed by trapping air layers on the fiber array andflat regions.19,20 Therefore, the caterpillar-inspired fiber tri-chome also showed self-floating properties on the surface ofwater and moved quickly under magnetic stimulation becauseof the SHB region (Movie S1, ESI†). The trichome has potentialapplications for reducing drag in marine engineering and fluidtransportation applications to improve energy efficiency.21,22

The caterpillar-inspired fiber trichome was driven at anappropriate speed with a magnetic stimulus in mixtures ofwater and oil. The trichome adsorbed oil (Fig. 2), whereaswater gradually passed through the trichome. This resultshowed that the trichome selectively adsorbed oil (dyed red forclear observation) quickly when the trichome floated on thewater surface (Fig. 2a). Moreover, the trichome retained itshigh adsorption efficiency (above 99.80%) after 10 cycles(Fig. S3†), demonstrating that the caterpillar-inspired fiber tri-chome collected oil continuously from the water due to theintrinsic SHB properties of the SHB region.14,23 Furthermore,the caterpillar-inspired fiber trichome selectively collected oilsfrom organic solvents with high efficiency (Fig. 2b), and stillfloated on the solvent surface when the oil it was carrying washeavier than its own weight. This result indicated that the tri-chome is a promising candidate for repelling oil spills andprotecting the environment from large-scale marine oils spill,and that it has a large potential carrying capacity.24,25

Self-driven water harvesting of the caterpillar-inspired fibertrichome

Water collection by the caterpillar-inspired fiber trichome wastested in a home-made humidifier. Water collection involvedfog capture, water condensation, water removal, and watertransport (Fig. S4† and Fig. 3a–d and Movie S2†).26 First, fogwas extracted from the moist air by the SHH tips of thetapered fibers. The fog capture was improved by the SHH fibertip because it was more hydrophilic than the SHB fiber, allow-ing it to gather water vapor easily and form larger drops.27

Therefore, the SHH surface displayed outstanding fog capturewhen the fog flow was vertical to the fiber and had a high velo-city. The excellent fog capture was due to the increased contactarea between the SHH fiber tip and the fog, even though thepath of the fog flow deviated slightly from the fiber direction.

Fig. 2 Selective oil adsorption capacity of the self-floating SHB fiber trichome. Oil adsorption capacity of the fiber trichome for various (a) oil/waterand (b) oil/organic solvent mixtures.

Paper Nanoscale

11776 | Nanoscale, 2019, 11, 11774–11781 This journal is © The Royal Society of Chemistry 2019

The fog capture was increased when the fiber surface wascovered with water because the increased contact areaincreased the interactions between the water film and fog. Themajority of the fog flow reached the fiber surface, and thencoalesced and formed a water film, supplying water. The highfog deposition on the SHH fiber tip may make the trichomesuitable for fog harvesting if the water supply is improved.Furthermore, the fiber surface filled with Fe3O4 NPs increasedthe water supply capacity due to the rough hierarchical struc-ture and improved fog harvesting. The droplets were removedby the SHB fibers. Most microdroplets merged, acceleratingthe growth of water droplets and controlling the condensedwater, allowing the water droplets to leave the SHB surfacequickly. Finally, the droplets were transported into a softplastic tube through the SLIPS fiber root, and the collectedwater was pumped out of the tube using a syringe (Fig. S5†)for continuous water harvesting. Continuous water transpor-tation was achieved by using a hydrophilic plastic tube thatimproved the absorbing water ability.28,29

Fibers with solely SHB or superhydrophilic propertiesexhibited a low fog collection rate (FCR). The fog was noteasily captured by the hydrophobic surface, and the water dro-plets could not roll down the hydrophilic surface in theabsence of a hydrophobic region. Therefore, the FCR wasaffected by the lengths of SHH tip, SHB middle, and SLIPSroot (Fig. S6†).30 The FCR reached a maximum for the SHHtip, SHB middle and SLIPS root with lengths of 1, 2, and0.5 mm, respectively (Fig. 3e). Furthermore, a higher FCRvalue was obtained when the caterpillar-inspired fiber tri-chome was set at 90° to the fog flow due to minimal contactangle hysteresis, which is consistent with the flexible methodof water collection of the Stenocara beetle (Fig. 3f).31

Optimizing the length of the superhydrophilic fiber tip region

captures more fog, and the drops move faster along the SHBmiddle fiber region. Consequently, the alternating arrange-ment of the SHH–SHB–SLIPS fiber regions improves fogcapture, water condensation, water removal, and water trans-port. Compared with solely SHB or superhydrophilic fibers,fibers with gradient wettability had far higher FCR values andcould be used repeatedly without a reduction in performance.

The caterpillar-inspired fiber trichome continuously pro-pelled the fog droplets allowing high-efficiency water collectionvia capture, coalescence, removal, and transport, which wasattributed to a synergistic effect among the Laplace pressureinduced by the geometric curve gradients, the surface energyreleased during fog droplet coalescence, and the wettabilityforce generated by the wettability gradient.32,33 Consequently,the caterpillar-inspired fiber trichome exhibited excellentwater harvesting behavior owing the similarity of the structureof the 1D fiber spine to that of a caterpillar spine, the wettabil-ity of which varies gradually from the tip to the root. Inaddition, the caterpillar-inspired fiber trichome providesenough self-driving force to remove water drops condensed onthe tip of fiber to the root, resulting in continuous water har-vesting by the rough tip structure and the wetting gradient ofthe SHH–SHB–SLIPS fiber.

Tribology properties of the earthworm-inspired fiber trichome

The earthworm-inspired fiber trichome texture showed excellentfriction reduction and antiwear performance. The tribologicalproperties of the corresponding fiber fabric composites were esti-mated in sliding friction tests in dry, distilled water, and PFPElubricant conditions (Fig. S7 and Movies S3 and S4†).11,17,34

The friction results indicated that the micro- and nanoscalehierarchical rough structure of the fiber trichome affected thefriction-reducing behavior (Fig. 4a). The spherical Fe3O4 NPs

Fig. 3 Self-driven water harvesting behavior of the caterpillar-inspired fiber trichome. (a)–(d) Microscope images of water collection: (a) fogcapture, (b) water supply, (c) water removal, and (d) water transport. (e) Fog collection rate of the caterpillar-inspired fiber trichome as a function ofthe superhydrophobic fiber length. (f ) Fog collection rate of a single fiber with gradient wettability placed at different tilt-angles at different fogvelocities.

Nanoscale Paper

This journal is © The Royal Society of Chemistry 2019 Nanoscale, 2019, 11, 11774–11781 | 11777

functioned as lubricants, improving the tribological behaviorof the fabric composite due to their rolling motion comparedwith the pristine unfilled fabric composites (Fig. S8a†). Theresults indicate that pristine fiber fabric that was not filledwith Fe3O4 NPs suffered from serious wear due to the shearingeffect during sliding friction, resulting in easy yielding underfriction-induced shearing (Fig. 4b and S8b†). Hence, Fe3O4

NPs improved the wear-resistance of the fiber fabric samplesunder the test conditions and prevented severe wear damage.In addition, the fiber fabric composite displayed and main-tained the lowest friction coefficient when the applied loadwas increased.

The friction coefficient was decreased by a self-lubricatingoil film, which was formed in the oil-bathed friction medium.The friction coefficient under lubricating oil-bathed slidingconditions was smaller than that under dry sliding conditions,

which could be due to the lubricating action of the oil inter-layers between the friction pair and sample. In particular, theantifriction contribution to the wear resistance of the fiberfabric composite was large, which was attributed to shearingon a macroscopic scale. Hence, the combined action of theself-lubricating film and the rolling of the Fe3O4 NPs decreasedthe contact between the counterfaces, which lowered the fric-tion coefficient and enhanced the antiwear properties.35

Based on the friction test results, the tribological propertiesof the earthworm-inspired fiber trichome were further evalu-ated by examining the lubrication self-release mechanism(Fig. 5). First, the earthworm-inspired fiber trichome withstored lubricant showed slow friction reduction similar to thatof the fiber fabric composite under dry sliding because the oillubrication film formed on the fiber surface was severelydamaged during the dry friction testing. Then, the coefficient

Fig. 4 Tribological properties of the earthworm-inspired trichome: (a) friction coefficient and (b) wear rate of the fabric composite as a function ofthe applied load. The sliding speed was 280 rpm in the friction tests. FESEM images of the worn surfaces of fabric composites tested under differentfriction conditions: (c) dry conditions, (d) distilled water-bathed conditions, and (e) oil-bathed conditions. (f ), (g), and (h) FESEM images of the weardebris from the tests shown in (a), (b), and (c), respectively. The sliding speed and applied load in the friction tests were 280 rpm and 10 N, respect-ively. Schematics of the formation mechanism of the lubrication layer: (i) friction between sliding surfaces, (g) wear debris as a lubricant forming thesolid lubrication film, and (g) self-lubricating film under oil-bathed sliding conditions.

Paper Nanoscale

11778 | Nanoscale, 2019, 11, 11774–11781 This journal is © The Royal Society of Chemistry 2019

decreased gradually and returned to the stable state as theapplied load increased because of the lubricant layer formedby external mechanical stimulation. The stored lubricant wasreleased via the funneled deformation of the elastic plastictube, and the tube retained the lubricant oil due to the recov-ery of the dynamic resilience. Thus, the liquid release wasmediated by a mechanical stress trigger. In addition, the earth-worm-inspired trichome fibers functioned as channels for oilrelease under mechanical stress, which could be due to theintrinsic capillarity and swelling ability of fibers contributingto the fast oil transport. Moreover, the removal-release behav-ior of the earthworm-inspired fibers were tested successivelyfor 40 min without severe friction, under a constant appliedload of 5 MPa. Moreover, the friction coefficient decreased andbecame constant, similar to that of carpenterworm-inspiredfibers until the oil on the surface was removed. This resultindicated that replenishing the lubricating oil in the fiberstructure substantially improved the durability of the earth-worm-inspired fibers. Consequently, the lubricating mecha-nism of the fiber fabric composite also improved the tribologi-cal behavior, which is important in reducing the friction of theearthworm-inspired fiber trichome.

The results suggest that the self-releasing lubricating oillayer was restored by replenishing via the mechano-inducedmechanism during sliding friction and made a significant con-tribution to the tribological properties of the earthworm-inspired fiber trichome texture surface.11 Furthermore, theearthworm-inspired fiber trichome endured severe friction andexhibited excellent friction reduction and wear resistancebecause the lubricating film was regenerated when the oillayer was consumed under friction via the self-replenishinglubricating mechanism.

For the pristine fiber fabric (Fig. 4c and f), numerous fiberswere detached and the fabric was worn away, indicating thatsevere adhesive wear occurred during dry sliding wear.Consequently, the counterpart pin surface with a discontinu-ous transfer film was rough and had obvious scratches and ahigh wear rate (Fig. S9a†). In contrast, the worn surface of thefiber fabric composite was smooth and had few fatigue pits or

exposed debris during sliding against the counterpart surfaceunder the distilled water test conditions (Fig. 4d and g), indi-cating that the fatigue wear occurred on the sample surface. Inaddition, signs of scuffing were observed on the correspondingpin, indicating that a continuous transfer film was formed,except for a few scratch marks (Fig. S9b†). This result wasexplained by the good dispersion of superhydrophobic Fe3O4

NPs on the composite surface, which bear the majority of theapplied load therefore dispersing the stress, resulting in a highinterface bond strength between the fiber and NPs. Moreover,wear debris exposed on the worn surface was trapped in wearscars and acted as a third body in the contact area. The weardebris was transferred and formed a self-lubricating film layeron the friction pair surface, which protected the fibers fromplowing and cutting. However, the fabric composite showedthe best tribological properties under oil-bathed sliding con-ditions, and the worn surface of the fabric composite was rela-tively smooth without obvious wear scars or abrasion traces(Fig. 4e and h). This observation indicated that the solid self-lubricating film also protected the fabric composites fromsevere abrasion under the oil-bathed sliding conditions.

Moreover, the surface roughness of the counterpart pinsliding against the earthworm-inspired fibers was lower thanthat sliding against the pristine fiber fabric, indicating that thedamage to the counterpart pin was slight (Fig. S9c†). F, S andSi were found on the counterpart worn pin surface for slidingagainst the fiber fabric composite under oil-bathed conditions,which confirmed that the pin surface was protected fromsevere wear by the formation of a uniform continuous transferfilm during sliding (Fig. 4i–k and S9d–i†). Moreover, thecontact angles for the SHB region still exceeded 150° andthose for the superhydrophilic region remained at 0° after40 min dry abrasion tests (Fig. S10†), indicating excellenthydrophilic-SHB stability.

Self-cleaning properties of the earthworm-inspired fibertrichome

The earthworm-inspired fiber trichome containing lubricatingoil exhibited excellent antifouling properties after beingdragged through mud with magnetic stimulation (Fig. 6 andS11 and Movie S5†). These properties were attributed to theslippery action of the self-releasing lubricating oil layer formedon the trichome surface (Fig. 5 and Movie S6†).11 The trichomewas in contact with the soil, and then the oil was releasedunder mechanical shear. Hence, when the trichome wasdragged through the mud under magnetic stimulation, anintermediate oil–soil layer formed between the fiber trichomeand the mud. Moreover, the oil–soil layer was retained by thefiber capillarity. Consequently, the mud was removed withoutstaining the surfaces because interfacial sliding occurredbetween the soil and the intermediate oil–soil lubricatinglayer. However, the original fiber trichomes without lubricat-ing oil were badly contaminated with mud, where the oil layerwas not formed on the surface, and were covered with residualsoil under the same conditions; the earthworm-inspired tri-chome maintained its stain-free surface by sacrificing the

Fig. 5 Schematic of the mechanism of lubricating oil release in theearthworm-inspired fiber trichome under mechanical friction stimulus:(a) lubricating oil infusion, (b) lubricating oil transfer by capillary actionunder external mechanical stimulus, (c) lubricating film forming on thefiber trichome surface and being removed from the surface, and (d)regeneration of the lubricating film.

Nanoscale Paper

This journal is © The Royal Society of Chemistry 2019 Nanoscale, 2019, 11, 11774–11781 | 11779

lubricating layer. Few residual soil particles remained on theearthworm-inspired fiber surface due to its special slipperystructure. The results showed that the earthworm-inspiredfibers maintained their self-cleaning ability after 10 cycles ofthe stain resistance test, demonstrating the durability of theself-cleaning performance for potential applications (Fig. 6b).In addition, the results indicated that the earthworm-inspiredfibers showed excellent anti-fouling properties when they weremoved with a load in the mud under magnetic stimulation.

Conclusions

In summary, we designed a surface with integrated wettabilityby a facile synthetic strategy inspired by surfaces in caterpillarsand earthworms. The caterpillar-inspired fiber trichome with agradient wetting texture generates an asymmetrical drivingforce to capture fog, allowing water harvesting with highefficiency. The earthworm-inspired fiber trichome possesses aself-releasing oil lubricant layer, which can be regenerated byexternal force, and demonstrates excellent anti-friction, wear-resistance and self-cleaning properties in mud. Designsinspired by multiple biological species could be used to fabri-cate hybrid biomimetic surfaces with excellent liquid-repellentproperties. Multifunctional wettability offers opportunities forcreating biomimetic surfaces with potential applications incarrying heavy loads, droplet adhesion and bubble transportin industry, drug transport in medicine, soft robot locomotionin dry and wet environments, and oil spill cleanup in environ-mental pollution treatment.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported by financial support from the “Kaiwu”Innovation Team Support Project of Lanzhou Institute ofTechnology (No. 2018KW-05) and the National Nature ScienceFoundation of China (No. 51675513 and 51735013).

Notes and references

1 Y. Huang, B. B. Stogin, N. Sun, J. Wang, S. K. Yang andT. S. Wong, Adv. Mater., 2017, 29, 1604641.

2 M. Y. Cao, X. Jin, Y. Peng, C. M. Yu, K. Li, K. S. Liu andT. S. Wong, Adv. Mater., 2017, 29, 1606869.

3 Y. Li, L. L. He, X. F. Zhang, N. Zhang and D. L. Tian, Adv.Mater., 2017, 29, 1703802.

4 A. A. Ali, A. Haidar, O. Polonskyi, F. Faupe, H. Abdul-Khaliq, M. Veith and O. C. Aktas, Nanoscale, 2017, 9,14814–14819.

5 D. L. Tian, N. Zhang, X. Zheng, G. L. Hou, Y. Tian, Y. Du,L. Jiang and S. X. Dou, ACS Nano, 2016, 10, 6220–6226.

6 Y. J. Zhu, F. L. Song, H. J. Qian, H. Y. Wang, L. W. Mu andJ. H. Zhu, Chem. Eng. J., 2018, 338, 670–679.

7 C. H. Zhang, B. Zhang, H. Y. Ma, Z. Li, X. Xiao,Y. H. Zhang, X. Y. Cui, C. M. Yu, M. Y. Cao and L. Jiang,ACS Nano, 2018, 12, 2048–2055.

8 L. Guo and G. H. Tang, Int. J. Heat Mass Transfer, 2015, 84,198–202.

9 H. Bai, R. Z. Sun, J. Ju, X. Yao, Y. M. Zheng and L. Jiang,Small, 2011, 7, 3429–3433.

10 Z. F. Sun, Y. Yamauchi, F. Araoka, Y. S. Kim, J. Bergueiro,Y. Ishida, Y. Ebina, T. Sasaki, T. Hikima and T. Aida, Angew.Chem., 2018, 130, 15998–16002.

11 H. X. Zhao, Q. Q. Sun, X. Deng and J. X. Cui, Adv. Mater.,2018, 30, 1802141.

12 Y. Gao, J. Wang, W. Xia, X. F. Mou and Z. S. Cai, ACSSustainable Chem. Eng., 2018, 6, 7216–7220.

13 P. W. Wang, R. X. Bian, Q. A. Meng, H. Liu and L. Jiang,Adv. Mater., 2017, 29, 1703042.

14 Y. Tian, P. P. Zhu, X. Tang, C. M. Zhou, J. M. Wang,T. T. Kong, M. Xu and L. Q. Wang, Nat. Commun., 2017, 8,1080.

15 S. N. Zhang, J. Y. Huang, Z. Chen, S. Yang and Y. K. Lai,J. Mater. Chem. A, 2019, 7, 38–63.

16 A. M. R. Habib, R. R. Kumar, S. M. Islam, V. Arivazhagan,M. Salman, D. R. Yang and X. G. Yu, J. Mater. Chem. A,2018, 6, 22437–22464.

17 H. J. Lu, M. Zhang, Y. Y. Zhang, Q. Huang, T. Fukuda,Z. K. Wang and J. Y. Shen, Nat. Commun., 2018, 9, 3944.

18 B. Li, T. Du, B. Yu, J. van der Gucht and F. Zhou, Small,2018, 11, 3494–3501.

Fig. 6 Antifouling properties of the earthworm-inspired fiber trichomein mud (about 40% water). The dimensions of the fiber trichome were aradius of about 3.5 mm, length of 3 cm, and weight of 0.1 g. (a–c)Photographs of the earthworm-inspired fiber trichome moving throughmud. (d) Weight of the soil particles adhering to the trichome as a func-tion of the number of antifouling test cycles.

Paper Nanoscale

11780 | Nanoscale, 2019, 11, 11774–11781 This journal is © The Royal Society of Chemistry 2019

19 H. B. Hu, J. Wen, L. Y. Bao, L. B. Jia, D. Song, G. Pan,M. Scaraggi, D. Dini, Q. J. Xue and F. Zhou, Sci. Adv., 2017,3, 1603288.

20 Y. Y. Zhao, C. M. Yu, H. Lan, M. Y. Cao and L. Jiang, Adv.Funct. Mater., 2017, 27, 1701466.

21 Y. Lu, Surf. Coat. Technol., 2017, 331, 48–56.22 Y. Wang, X. W. Liu, H. F. Zhang and Z. P. Zhou, RSC Adv.,

2015, 5, 18909–18914.23 Y. H. Dou, D. L. Tian, Z. Q. Sun, Q. N. Liu, N. Zhang,

J. H. Kim, L. Jiang and S. X. Dou, ACS Nano, 2017, 11,2477–2485.

24 F. Z. Chen, Y. Lu, X. Liu, J. L. Song, G. J. He, M. K. Tiwari,C. J. Carmalt and L. P. Parkin, Adv. Funct. Mater., 2017, 27,1702926.

25 A. R. Siddiqui, R. Maurya and K. Balani, J. Mater. Chem. A,2017, 5, 2936–2946.

26 L. S. Zhong, R. C. Zhang, J. Li, Z. G. Guo and H. B. Zeng,Langmuir, 2012, 34, 15259–15267.

27 M. Y. Cao, J. Jie, K. Li, S. X. Dou, K. S. Liu and L. Jiang, Adv.Funct. Mater., 2017, 24, 3235–3240.

28 X. Heng, M. M. Xiang, Z. H. Lu and C. Luo, ACS Appl.Mater. Interfaces, 2014, 6, 8032–8041.

29 H. Zhou, M. X. Zhang, C. Li, C. L. Gao and Y. M. Zheng,Small, 2018, 14, 1801335.

30 D. L. Chen, J. Li, J. Y. Zhao, J. Guo, S. B. Zhang,T. A. Sherazi, N. Ambreen and S. H. Li, J. Colloid InterfaceSci., 2018, 530, 274–281.

31 T. Xu, Y. C. Lin, M. X. Zhang, W. W. Shi and Y. M. Zheng,ACS Nano, 2016, 10, 10681–10688.

32 A. Rawal, Langmuir, 2012, 28, 3285–3289.33 J. Ju, K. Xiao, X. Yao, H. Bai and L. Jiang, Adv. Mater., 2013,

25, 5937–5942.34 Q. S. Bai, J. X. Bai, X. P. Meng, C. C. Ji and Y. C. Liang,

Friction, 2016, 4, 165–175.35 J. Y. Yuan, Z. Z. Zhang, M. M. Yang, F. Guo, X. H. Men and

W. M. Liu, Tribol. Int., 2017, 115, 8–17.

Nanoscale Paper

This journal is © The Royal Society of Chemistry 2019 Nanoscale, 2019, 11, 11774–11781 | 11781