Durable Superhydrophobic Particles Mimicking Leafhopper Surface

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Durable Superhydrophobic Particles Mimicking Leafhopper Surface: Superoleophilicity and Very Low Surface Energy Ramakrishna Sukamanchi, Dona Mathew, and Santhosh Kumar KS ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01413 • Publication Date (Web): 24 Oct 2016 Downloaded from http://pubs.acs.org on October 25, 2016

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ACS Sustainable Chemistry & Engineering

Durable Superhydrophobic Particles Mimicking Leafhopper Surface: Superoleophilicity and Very Low Surface Energy Ramakrishna Sukamanchi, Dona Mathew, Santhosh Kumar K.S.* Polymers and Special Chemicals Division, Vikram Sarabhai Space Centre, ISRO P.O., Thiruvananthapuram-22, Kerala, India. *E-mail: [email protected]

ABSTRACT

The powder coating on brochosomes of leafhopper can repel water, diiodomethane (DM) and ethylene glycol (EG) droplets with contact angle >150°. This is attributed to the very low surface energy as low as < 1.0 mN/m and their porous honeycomb structure. In this work, grafted silica nanoparticles with properties similar to leafhopper (LH) particle coating is synthesized in a single step by catalytic grafting of stearic acid on non-porous silica nanoparticles. The particles repel water, diiodomethane and ethylene glycol with contact angle 167 °, 161 ° and 157 ° respectively. The surface energy is also found as < 1.0 mN/m. The particle coating shows sliding angle < 10 ° with these probe liquids. The superhydrophobic particles are resistant to pH =1, 13

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and boiling water conditions and preserves super-repelling nature to reference liquids after the treatments. In a mixture of hexane/water or kerosene/water, the particles completely wet the oleophilic counterpart and repel water layer due to superoleophilic-superhydrophobic nature.

KEYWORDS Stearic acid; one-step method; pH tolerance; boiling water resistance; superoleophilicity

INTRODUCTION Significance of superhydrophobic (SH) materials have enormously increased during past two decades due to their versatility and promising applications like anti-wetting, anti-corrosion, antibacterial and self-cleaning. Presence of waxy nanostructures on the surface (two-tier roughness) endows superhydrophobicity to lotus leaf. 1-2 During the course of mimicking this plant surface, methods like electro spinning,3-4 plasma treatment,5 lithography,6-7 sol-gel,8 chemical vapor deposition,9 and layer-by-layer assembly10 were evolved. These attempts resulted in the development of superhydrophobic, superoleophobic and superamphiphobic (repels both water and oils) materials. Recently, a new class of advanced superhydrophobic particles have been revealed on the brochosomes of leafhopper insects (diverse family of cicadellidae) i.e. they repel water, diiodomethane (DM) and ethylene glycol (EG) with contact angle > 150 ° but not oils. 1112

The leafhopper (LH) surface possesses static contact angle of 165-172 ° for water, 152-164 °

for ethylene glycol and 148-156 ° for diiodomethane with surface energy as low as 0.74 mNm-1. 13

By electron imaging the surface, it was evident that the integuments of leafhopper were coated

with porous honeycomb structured brochosomes and this special surface texture brought superior


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and selective liquid repellency to the powder coating. Chemically, the surfaces are accommodated with large amount of acids, esters and proteins. 13 Avoiding multi-steps and fluorochemicals (bio-accumulating) in the synthesis are a welcome environment-benign strategy.

14-16

Previously, SH (but not LH) surfaces had been attempted,

which concentrate on the functionalization of nanoparticles in multiple steps.

17

Preparation of

SH surface using expensive fluorinated molecules and dangerous reactants were well explored. 18-23

We reported the use of fast and effective aromatic and long chain isocyanates for the

preparation of SH materials but those reactant molecules are lung irritant and hazardous to human body.

20, 24

Though many of these attempts led to SH materials, multiple steps or

perfluoro molecules are invariably used. Hence, there is a strong need to find greener ways for realization of SH surfaces. Previously, environment friendly lignin or tannic acid based foams displaying superhydrophobicity were prepared by using health hazardous isocyanate or perfluorinated compounds.

14-16

However, these SH foams could not show repellency towards

low surface tension liquids. Stearic acid (octadecanoic acid, pKa= 4.75), a plant-derived long chain hydrocarbon, is an exciting green candidate for preparing SH materials.

25

It is environment friendly and highly

abundant in neem seeds (~ 24.0 %),26 and kokum (~ 56.4 %). The role of this long chain acid to obtain SH coatings was demonstrated elsewhere. 27-34 Gao et al prepared an SH coating by a twostep modification on silica nanoparticles where stearic acid (physically attached) and perfluorooctyl triethoxysilane units were grafted.

27

Superhydrophobic wood surfaces were

prepared with physical attachment of stearic acid molecules over TiO2 pre-coted CaCO3 micronano composites.

35

In another report, self-cleanable superhydrophobic zinc rods were prepared

by chemical modification using stearic acid, where stearate grafts were grown on zinc seeds but


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no repellency towards diiodomethane and ethylene glycol was achieved.

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36

Other studies also

emphasized the key role of stearic acid in achieving superhydrophobicity on surface of various materials like CaCO3 micro-nano powder, ZnO micro-nanoparticles, carbon nanotubes, silver nanoparticles, copper foils, alumina surfaces etc.

28-34

Silva et al prepared a non-fluorinated

hydrophobic cotton fabric using nano silica grafted with hexadecyltriethoxysilane, stearic acid or triethoxyoctylsilane.

37

Another recent study shows the major role of stearic acid to achieve

superhydrophobicity on zinc nanoparticles. Here, the introduction of stearic acid lowers the surface energy of zinc nanoparticles to display SH properties.

38

A recent study examines the

durability of SH coating with ice where the SH surface was prepared by grafting different alkyl terminated chains such as stearic acid/fluoro alkyl silane/octadecyl trimethoxysilane.

39

Though

these coatings maintained icing and deicing properties up to ~30 cycles, adhesion of ice with this coating surface increased gradually in further tests.

Most notably, none of these works reported super-repellency to low surface tension liquids like diiodomethane or ethylene glycol as seen in leafhopper brochosome surface. It is fascinating to achieve particles with super-repellency to water, diiodomethane and ethylene glycol equivalent to leafhopper surface. It is further interesting, if no fluorochemical or silane molecules are employed for mimicking leafhopper brochosome coating. However, in current literature on SH materials, repellency with low surface tension liquids such as DM and EG remains an elusive goal from a non-fluorinated material. To the best of our knowledge, no artificial LH materials has so far been reported.


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The present work reports the preparation of a superhydrophobic particle by grafting stearic acid on silica nanoparticles in a single step.

The surface properties are equivalent to the

superhydrophobic leafhopper brochosome particle coating. The synthesis of LH material followed by its super-repellency towards probe liquids-water, DM and EG are described. Its super-repellency towards wide pH range and durability in extreme acidic/basic/boiling water conditions are studied. Preferential superoleophilicity of LH particles is also demonstrated.

EXPERIMENTAL Materials Silica nanoparticles (20-40 nm, Sigma-Aldrich, Germany) were dried at 120 °C for 6 h prior to use. Stearic acid (98%, Otto, India), conc. H2SO4 (98%, Merck, India), toluene (99.5%, Thermofisher Scientific India Pvt. Ltd), distilled water, hexane (≥95%, Merck, India), ethylene glycol (99.5%, Thermofisher Scientific India Pvt. Ltd.), glycerol (≥99.5%, Merck, India), diiodomethane (98%, Spectrochem, India), sudan III (S.D. Fine Chem. Pvt. Ltd., India), kerosene (Commercial grade, Thiruvananthapuram, India), N-methyl-2-pyrrolidone (99.5%, SRL, India), N,N-dimethyl formamide (99.5%, SRL, India), 1,2- dichloro ethane (99.8%, Merck, India), 1,4dioxane (99.5%, Merck, India), xylene (97%, Qualigens, India) were used as procured without additional purification process. Instrumentations FTIR spectra of materials were recorded with Perkin Elmer Spectrum GX-A FTIR spectrometer in the wavenumber range of 4000-400 cm-1. Elemental analysis was carried out by Thermo Finnigan EA 1112 Series Flash Elemental Analyser. Raman spectrum of modified nanoparticle


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was performed by WITec alpha 300 R Confocal Raman Microscope (532 nm, 5 no. with 1 s integration with a grating of 600 lines/mm). The pH of acidic and basic solutions were measured by using Systronics 362 pH meter. Mettler Toledo M P70 melting point apparatus was used for determining the melting features of LH particles. Specific surface area of pristine and modified nanoparticles were determined by Brunauer-Emmett-Teller (BET) isotherm method using Quantachrome NOVA 1200e surface area analyzer at the temperature of liquid nitrogen. The grafting yield was determined by thermogravimetric analysis (TGA) on TA instruments 2960 from 30 ° to 900 °C in nitrogen atmosphere at a heating rate of 10 °C/min. The wetting nature of the surface was measured from static contact angles with Data Physics contact angle instrument OCA-15EC. Drop sizes of 5 µl were placed (five different locations) on the surface and readings were recorded using SCA 20 software based on Laplace-Young fitting method. The surface morphology of grafted nanoparticle was captured by Field Emission-Scanning Electron Microscopy on Carl Zeiss Supra 55 Field Emission Scanning Electron Microscope (FESEM) instrument. Surface morphology of pristine silica and LH particles was studied on FEI Tecnai G2 F30 high resolution S-TWIN Transmission Electron Microscope instrument where carbon coated copper grid was used for collecting particles. Synthesis of LH particles Synthesis of stearic acid grafted silica nanoparticles was carried out as follows. About 1.0 g of silica nanoparticles, 17 ml of toluene, 5.0 g of stearic acid and 0.2 ml of conc. H2SO4 were mixed together and ultrasonicated for about 90 min. at 60 °C. This process leads to the formation of colloid of silica nanoparticles in toluene. The formation of colloidal solution ensures segregation of silica nanoparticles and avoids further aggregation during the reaction. The mixture was transferred into an RB flask fitted with a water condenser. The reaction was conducted at 100 °C


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(about 16 h) with constant stirring. The mixture was filtered and the solid particles were washed several times with hexane to remove unreacted stearic acid. The catalyst was removed by continuous washing with water till neutral pH is achieved. The final product (LH particles) was obtained by drying under vacuum at 80 °C for 24 h. The LH particles are brown in color. This coloration is ascribed to the interaction of stearate chain with conc. H2SO4 which may lead to partial decomposition of fatty acid chain. Previously, brown colour formation and partial decomposition of fatty acids on interaction with H2SO4 of conc. > 97.3 % was reported.

40

To

investigate the colour change further, we conducted two more experiments without changing the original reaction conditions used for the synthesis of LH particles i.e. i) reaction between silica nanoparticles and conc. H2SO4 alone (without incorporating stearic acid) and ii) reaction between stearic acid and conc. H2SO4 alone (without adding silica nanoparticles). A brown coloured mixture was obtained in the latter case due to the interaction of stearic acid with conc. H2SO4 whereas first reaction mixture remained colourless. For the preparation of LH coating, about 2.0 wt. % of LH particles were dispersed in toluene and then coated on a glass substrate by drop-cast method. The coating was kept at room temperature for 10-15 min. and the final LH coating was obtained by drying at 80 °C for 6 h in an air oven.

RESULTS AND DISCUSSION

About the grafting chemistry of this synthetic route, different surface reactions are possible on the silica nanoparticle. Since sulphuric acid is a good dehydrating agent, it can condense hydroxyl groups on silica surface to form -Si-O-Si- bond. Meanwhile, silanol groups are capable to react with sulphuric acid to form -Si-OSO3H that is known as a very good catalyst for


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esterification reaction and other rearrangement reactions.

41-42

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These competitive reactions will

lead to –Si-O-Si- structure in majority and chemical grafting of stearate groups to silica surface but in a lesser extent. An advantage is that, due to the conversion of Si-OH groups into –Si-O-Sibond on silica surface along with long chain alkyl grafting via esterification can improve the durability of coating due to the less content of surface hydroxyl groups (vide infra). The schematic representation of reaction between silanol groups of silica nanoparticles with H2SO4 and stearic acid is shown in Figure 1.

Figure 1. (a) Schematic representation of reaction between silica nanoparticles and H2SO4/stearic acid (b) Raman spectrum of LH particles at a range of 1100-1900 cm-1 (c) Thermogravimetric profiles of bare silica and LH particles (Image of leafhopper is reproduced with permission from Springer publications, Functional Surfaces in Biology, Vol. 1, 2009)


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Grafting of long hydrocarbon chain on silica nanoparticles was confirmed by FT-IR analysis (Figure S1 of the supporting information (SI)). The vibrational frequencies of –CH groups were noted at 2921 and 2849 cm-1 respectively (low intense) which confirmed the presence of hydrocarbon chain on silica surface. Peak corresponds to ester linkage was noted at 1780 cm-1 in FT-IR (Figure S1 of SI). Further, Raman spectroscopy of LH particles ensured the presence of CH2, CH3, asymmetric str. of COO- and C=O str. bonds at 1400 cm-1, 1450 cm-1, 1570-1590 cm-1 and 1720 cm-1 respectively.

43-46

A carbon percentage of 4.7 wt.% from elemental analysis

further validated the grafting of stearate groups. The surface modification over silica nanoparticles resulted in the reduction of surface area from 134 m2/g to 84 m2/g. Thermogravimetric analysis of LH particles assessed the yield as about 15 wt. % which corresponds to a grafting density of 1.03 molecules/nm2 (from Barendson equation).

Actual Coverage density (α) = [10 6 Pc/ (1200 nc -PcM1) × S BET] (µmol/m2)

(1)

Grafting density (N) = (α×NA)/10 24 (molecules/nm2)

(2)

where Pc is the % of carbon content in the grafted particle, nc is the no. of carbons in grafted molecule, M1= molecular weight of grafted group, SBET is the surface area of bare silica nanoparticles and NA is the Avogadro’s number. In addition, to prove that the grafting involves chemical bonding, the possibility of melting (determination of melting point) of LH particles was studied. No melting point was observed till


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150 °C which ensured the absence of unreacted or physically bonded stearate (melting of stearic acid occurs at 55-61 °C) groups over the silica surface.

The alkyl chain modification over silica surface changed the surface property from superhydrophilic to superhydrophobic. The LH material tested with water showed an excellent repellency with static water contact angle as 167±1° which is similar to the water repellency of leafhopper (WCA = 165-172 °).

11-12

The water droplets were bounced and rolled- off from the

surface at a very low tilting angle ~ 2-4°. Subsequently, the coating was tested for pH tolerance (pH solutions from 1 to 13) where LH particle coating nicely repelled acidic and basic water droplets with more or less same tilting angle as seen in neutral water. The static WCAs with varying pH solutions were between 162-167°. This study points out the tolerance of LH particle coating towards highly acidic and alkaline solutions.

Subsequently, the present LH particle coating was tested with diiodomethane and ethylene glycol. Surprisingly, both of them showed high repellency with contact angle 161±1° and 157±2° respectively. Diiodomethane droplets were rolled-off with an angle of 6 ° (Cassie-Baxter state) (Figure 2) (Video S1 and S2 of SI demonstrates the repellency with water and diiodomethane respectively). Hence, the modified particle behaves like leafhopper particle coating and repels water, DM and EG with contact angle >150 °. To find the rationale for this distinctive and unexpected behavior equivalent to leafhopper, the surface energy of LH coating was determined. Surface energy as low as 0.06 mN/m was obtained by Owen-Wendlt-Rebel-Keven (OWRK) method and 0.75 mN/m by Harmonic mean method. The surface energy of coating has a close match with the surface energy of leafhopper powder coating (0.74 mN/m).

13

Similar to


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leafhopper surface, the LH particle coating got completely wetted by ethanol. 13 Additionally, the present LH coating was also evaluated using glycerol droplets (surface tension 64 mN/m), the surface showed roll-off properties with sliding angle ~ 8 °. It is observed that, when the surface was tested with liquids having surface tension < 45 mN/m, the surface was wetted. But, liquids with surface tension above 45 mN/m, present LH particle coating shows super-repellency (Table S1 of SI). To note, original leafhopper surface could not roll-off any of these liquids though contact angle is >150 °.


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Figure 2. (a) pH tolerance of LH particle surface (b) Optical images of droplets of varying pH solutions on LH particle surface (inset: static water contact angle at pH=7) (c) static contact angle with (i) water (ii) diiodomethane (iii) ethylene glycol and (iv) glycerol (d) (i) bouncing and rolling of acidic (pH=1) solution from LH particle surface (methylene blue added for visualization) (ii) rolling of diiodomethane from LH particle surface.

The observed super-repellency of LH particles depends on surface morphology and roughness factors. We could not notice any morphology similar to leafhopper brochosomes (i.e. porous honeycomb) in FESEM images (Figure 3a). The images show a coverage over spherical silica particles due to grafting. There is no much shape change also. Transmission Electron Microscopy images of both unfunctionalised silica and LH nanoparticles were captured as seen in Figure 3b. The LH particles are more segregated due to grafting of stearate groups. It is also visible that bare silica particles are more aggregated compared to LH particles. In addition, the size of LH particles after grafting is also in the nano range which enables the LH particles to exhibit super-repellancy to various liquids (vide supra). To evaluate the uniform grafting on LH particles, elemental mapping on three different areas of LH particle was carried out by TEM (Table 1). The percentage of carbon present in three different areas of LH particle is quantified as about 33±3 wt. %. This more or less uniform carbon content in LH particles ensures uniform grafting. However, these values can be considered as a reference only because carbon present in carbon coated copper grid also contributed for this enhanced carbon %. Though we used pristine silica nanoparticles with high purity (99.5% by trace metal analysis and no detection of carbon in CHN analysis), about 15 wt. % of carbon was detected in TEM which is attributed to the carbon present in the grid. It is also observed that, carbon contents in three different areas of LH particle


12

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are twice (approximately) vis-à-vis the carbon content in pristine silica due to uniform grafting of stearate groups (Figure S2 of SI). The aliphatic chains are flexible and spread over the surface of silica nanoparticles. This feature along with inorganic silica-organic interface creates roughness on particles (we tried to obtain AFM for roughness but could not succeed). In our previous work, melting SH coatings based on octadecyl groups were reported where octadecyl group was linked to the silica surface via urethane linkage instead of ester linkage as shown here. 24 In both cases, octadecyl groups were grafted but they behave differently towards low surface tension liquids. The urethane system was super-repellent only to water whereas LH particle coating repels low surface tension liquids also (DM, EG and glycerol). In urethane bonded octadecyl system, -NCO groups can undergo secondary reaction with urethane to form polar allophanate linkages. Hence, the polar urethane/allophanate bonds are distributed among the octadecyl chains, which pave easy access to low surface tension liquids and get wet. In LH particle coating, a monocarboxylic acid is used which cannot undergo secondary reaction, hence the result is the engulfing of polar ester groups by long alkyl chains i.e. polar groups are well seated under the umbrella of octadecyl

coverage

(like

coverage

of

fluorine

over

and

below

C-F

bond

in

polytetrafluroethylene). Hence, the high repellency of LH particles towards low surface tension liquids is attributed to this morphology and very low surface energy.

Previously, Rakitov et al found that the brochosome particles have very weak bonding with integuments of leafhopper due to very-low surface energy.

11-12

To evaluate the adhesion of

present LH particles, we tried to get a coating of LH particles on commercial adhesives such as epoxy, double-sided adhesive tapes, polycyanoacrylates and polyacrylates. The LH particle coating was sprayed over the adhesive surface, but due to very low surface energy, the particles


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could not effectively bond with all adhesives and failed to adhere on the adhesive surface. This also supports that the LH particles mimic exactly (in properties) that of brochosome particles seen in leafhopper species.

11

Table 1 describes the comparison of super-repellancy of LH

particles vis-à-vis original leafhopper brochosome particle coating. Additionally, the whole LH product was divided into three portions and coating was prepared from toluene to assess the uniformity of the grafting in terms of super-repellency towards various liquids. The liquids viz: water, glycerol, ethylene glycol and diiodomethane were repelled from all the three surfaces with contact angle > 150 ° (Table S3 of SI).

To study further, the LH particles were treated in boiling water at 70 °C for 7 h. The particles were intact and no wetting was observed (floated over water surface). Generally, most of SH coatings get wet by hot water, because, continuous contact of water droplets with SH material makes feeble interaction with surface groups and replace all the entrapped air present in the rough regimes.

47-48

In previous reports, a superhydrophobic cotton fabric was tested in boiling

water for 5 h and found as durable. However, perfluoro chains were employed as backbone for imparting the durability. 49 A complete repellency of present LH particles with boiling water are also observed. A glowing silver mirror is also preserved during the boiling water test due to the interface of air between liquid droplets and solid surface (Figure S3 of SI). Further, after the boiling water test, the particles were dried for 2-3 hours at 100 °C and re-coated to test the regeneration of properties. The surface displayed contact angle 160±1° for water, 155±2° for diiodomethane and 145±1 ° for ethylene glycol. Lowering of surface tension of water and elevation of surface energy of LH surface on boiling water treatment do not affect much on the original properties.


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Figure 3. (a) FESEM images of LH particles (b) TEM images of pristine silica and LH nanoparticles drop-cast in acetone solvent. More aggregation can be seen in pristine


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nanoparticles whereas LH particles got segregated due to grafting of stearate groups on silica surface (c) superhydrophobicity on LH particle coating before (left) and after (right) boiling water treatment (d) proposed graphical design of LH surface repellent to various low surface tension liquids.

Table 1. Wettability with various surface tension liquids on LH particle coating. Liquids

Surface tension (mN/m)

Static contact angle (°) LH particle coating

Original leafhopper

Roll-off angle (°) LH particle coating

Original leafhopper

Water

72.8

167 ±1

165-172

2-4

NA

Glycerol

64.0

160 ± 1

NA

~8

NA

Diiodomethane

50.8

161 ± 2

148-156

~6

NA

Ethylene glycol

47.7

157±2

152-164

no roll-off (Wenzel state)

NA

NA-not available

Durability of the LH particle was further assessed by testing in harsh acidic and basic conditions (pH =1 and 13). Most commonly, this kind of harsh treatments will destroy the superhydrophobic properties.

50-53

In the current work, the LH material maintain its highly

repellent nature even after 24 hours stirring in pH=1 or pH=13 solutions (Video S3 of SI demonstrates the process as a video). After these acid/base treatments, particles were dried at 100 °C for 2 h and coated over a glass substrate. Then, different pH solutions (pH=1, pH=7 and pH=13) were placed over the coating and determined the stability of the droplet for a duration of 5 h continuously. The different droplets maintained superhydrophobicity on both acidic and


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basic treated coatings (Figure 4). However, DM and EG droplets wetted the surface after these treatments. This clearly implies that the delicate surface feature of LH particles which may be lost during these harsh exposures.

Figure 4. (a) and (b) Water contact angle on acidic/basic treated LH coating as a function of time, (c) and (d) Repellency of acidic (pH=1), neutral (pH=7) and basic (pH=13) water droplets on acidic and basic treated LH particle coatings. Most of the superhydrophobic materials fail to display repellency with water when their surface regimes are blocked with other oleophilic liquids. However, if a material consists of dual nature like superhydrophobicity-cum-superoleophilicity, such materials can maintain the superrepellency with water even they are wetted with oleophilic liquids. 54-57 The SH materials which can show repellency with water in the presence of oil pollutants are highly beneficial for


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seawater related applications.

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In addition, these materials can able to separate water from

common oleophilic organic solvents.

59

Towards this, a mixture of water and hexane (2:1

volume ratio, hexane was colored with Sudan III dye for easy visualization) was taken and LH particles were poured into that. The particles repelled water layer meanwhile they were completely wetted by hexane layer (surface tension of hexane = 18 mN/m). We tried to disturb the stability of mixture by stirring but the same water-repellency and superoleophilicity was maintained throughout (Figure 5). Another solvent mixture, kerosene/water was also tested to see the selective superoleophilicity of LH particles. In this case also, LH particles are superhydrophobic and wetted only by kerosene. The LH material was tested in both solvent mixtures for still a higher duration of 24 h and observed that, the repellency to water is maintained (Video S4 of SI demonstrates the process as a video).

Figure 5. (a) LH particles in water-hexane mixture, (b) LH particles in water-kerosene mixture (c) Camera images of response of coconut oil (colorless; left) and water (blue; right) on LH surface. CONCLUSIONS


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The current research contribution is the first report that mimics original leafhopper (LH) surface properties via non-fluorinated one-pot approach. This artificial leafhopper surface is achieved by grafting octadecyl chains over nano silica surface. Similar to the original leafhopper surface, artificial LH particle coating exhibits very low surface energy (150° and a roll-off angle of

Durable Superhydrophobic Particles Mimicking Leafhopper Surface

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