Research Article pubs.acs.org/journal/ascecg
Durable Superhydrophobic Particles Mimicking Leafhopper Surface: Superoleophilicity and Very Low Surface Energy Ramakrishna Sukamanchi, Dona Mathew, and Santhosh Kumar K.S.* Polymers and Special Chemicals Division, Vikram Sarabhai Space Centre, ISRO P.O., Thiruvananthapuram-22, Kerala 695022, India
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S Supporting Information *
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 150°. To find the rationale for this distinctive and unexpected behavior equivalent to the leafhopper, the surface energy of the 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 the coating is a close match with the surface energy of leafhopper powder coating (0.74 mN/m).13 Similar to the 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 150°. 255
DOI: 10.1021/acssuschemeng.6b01413 ACS Sustainable Chem. Eng. 2017, 5, 252−260
Research Article
ACS Sustainable Chemistry & Engineering
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 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 static contact angle (deg)
a
liquids
surface tension (mN/m)
water glycerol diiodomethane ethylene glycol
72.8 64.0 50.8 47.7
LH particle coating 167 160 161 157
± ± ± ±
roll-off angle (deg)
original leafhopper
LH particle coating
original leafhopper
165−172 NA 148−156 152−164
2−4 ∼8 ∼6 no roll-off (Wenzel state)
NAa NA NA NA
1 1 2 2
Not available.
256
DOI: 10.1021/acssuschemeng.6b01413 ACS Sustainable Chem. Eng. 2017, 5, 252−260
Research Article
ACS Sustainable Chemistry & Engineering
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.
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 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 toward 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 the 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 the SI). Further, after the boiling water test, the
can be considered as a reference only because carbon present in carbon coated copper grid also contributed for this enhanced carbon percent. 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 are twice (approximately) vis-à-vis the carbon content in pristine silica due to uniform grafting of stearate groups (Figure S2 of the 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 toward 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 the 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 polytetrafluoroethylene). Hence, the high repellency of LH particles toward low surface tension liquids is attributed to this morphology and very low surface energy. 257
DOI: 10.1021/acssuschemeng.6b01413 ACS Sustainable Chem. Eng. 2017, 5, 252−260
Research Article
ACS Sustainable Chemistry & Engineering
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.
particles were dried for 2−3 h at 100 °C and recoated 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. 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 h stirring in pH = 1 or 13 solutions (Video S3 of SI demonstrates the process). 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, 7, and 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 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 may be lost during these harsh exposures. 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-and-superoleophilicity, such materials can maintain the super-repellency 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 seawater related applications.58 In addition, these materials are capable to separate water from common oleophilic organic solvents.59 Toward 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 super-
hydrophobic and wetted only by kerosene. The LH material was tested in both solvent mixtures for still a higher duration of 24 h and we observed that, the repellency to water is maintained (Video S4 of SI demonstrates the process as a video).
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CONCLUSIONS The current research contribution is the first report that mimics original leafhopper (LH) surface properties via nonfluorinated one-pot approach. This artificial leafhopper surface is achieved by grafting octadecyl chains over the nanosilica surface. Similar to the original leafhopper surface, artificial LH particle coating exhibits very low surface energy (150° and a roll-off angle of
