Adhesion Of Water Droplets Research Articles

Molecularly grafted PVDF membranes with in-air superamphiphilicity and underwater superoleophobicity for oil/water separation

Microporous polyvinylidene fluoride (PVDF) based membranes are widely used in separation and purification applications, including oil/water separation. However, due to hydrophobic nature, these membranes are prone to high fouling and wetting by low surface tension liquids and substances, thereby limiting its full potential in many important applications such as oil/water separation. Herein, we present a simple and straight forward method to modify and produce in-air superamphiphilic and under-water superoleophobic PVDF membranes by molecular grafting using three different amino silanes. The molecular grafting imparted novel properties due to the combined effect of chemistry, nanostructuring, and roughness. After grafting, the membranes exhibited in-air superamphiphilicity in both dry and wet conditions. In water, the membrane does not allow the attachment of oil droplets, making it superoleophobic. However, when placed under oil, the membrane showed superhydrophobicity but still allowed the adhesion of water droplets. After thorough characterization for structure, modification, and physicochemical properties, the membranes were tested for oil/water mixture separation. The functionalized membranes showed > 99% oil rejection and flux recovery. These results showed great promise for molecular grafting of membranes for separation and purification applications.

Organophilic nano-alumina for superhydrophobic epoxy coatings

Bilayer coatings with high hydrophobic characteristics were successfully prepared by depositing alumina nanoparticles functionalized with stearic acid as top layer paint onto epoxy –based bottom coatings. More stable alumina suspensions were obtained with reflux in 2-propanol than in toluene. The dispersions of functionalized alumina NP (2 wt% and 3 wt%) in 2-propanol were applied by the spray-coating method on a stainless steel substrate previously coated with two different partially cured epoxy -based coatings: DGEBA and Novolac Type II. Results show that DGEBA-based coatings with functionalized alumina NPs displayed superhydrophobic characteristics with very little interaction with water droplets, typical of the Cassie state, due to the low surface energy promoted by the functionalized alumina NPs. The alumina-modified Novolac Type II coating presented WCA in the range of 140–145°, with strong adhesion of water droplets to the surface, characteristic of the Wenzel state, which is related to the other fillers present in the commercial resin. Based on these results, it was possible to develop superhydrophobic coating by using a fast, cost effective and environmentally-friendly approach which can be scalable to relatively great surfaces, involving alumina functionalized with fatty acids and epoxy resins with controlled curing process.

Phase-Change Slippery Liquid-Infused Porous Surfaces with Thermo-Responsive Wetting and Shedding States.

Slippery liquid-infused porous surfaces (SLIPSs) prepared with phase invariant materials (e.g., Krytox GPL oil) have been increasingly researched as low-adhesion engineered functional surfaces in the last decade. However, phase change materials (PCMs) have been scarcely adopted, although they are potential candidates because of their inherent lubricant characteristics as well as temperature-dependent phases empowering unique thermo-responsive switchable wettability. Here, paraffin wax (an organic PCM) has been applied on a hydrophobized nanoporous copper substrate to realize the phase-change SLIPSs (PC-SLIPSs) fabricated via spin-coating followed by thermal annealing, which overcomes earlier limitations encountered on the PC-SLIPSs. Advantages of these PC-SLIPSs are the prompting of a low-adhesion Wenzel state as opposed to the earlier completely pinned Wenzel state in the solid phase and the optimized slippery state without excess of PCM in the liquid phase. Further, in order to characterize the intimate interactions between liquid droplets and the different phases of the PC-SLIPSs, that is, solid, mush, and liquid phases, the contact line dynamics have been comprehensively investigated, unveiling the water droplet adhesion and depinning phenomenon as the function of the thermo-responsive wetting states. Lastly, the PC-SLIPSs have also been tested for water vapor condensation, demonstrating the feasibility of dropwise condensation and the shift of the droplet size distribution in both the solid and liquid phases. The results suggest that such engineered surfaces have great potential to prompt and tune dropwise condensation via thermo-responsive switchable wettability for heat transfer and water harvesting applications.

Ten fatty acyl-CoA reductase family genes were essential for the survival of the destructive rice pest, Nilaparvata lugens.

Fatty alcohols are the precursors of sex pheromone components, wax esters and hydrocarbons in insects. Fatty acyl-CoA reductases (FARs) are important enzymes required for the reduction of fatty alcohol and thereby contribute to the production of cuticular hydrocarbon (CHC). Based on bioinformatics analyses we identified 17 FAR genes in the brown planthopper, Nilaparvata lugens. RNA interference against these genes demonstrated that ten NlFAR genes were essential for the survival of N. lugens. For instance, knockdown of NlFAR5, 6, 11 or 15 was lethal and caused a slender body shape, while the old cuticles of the respective animals remained attached to the abdomen or failed to split open from the nota. Knockdown of NlFAR9 resulted in a phenotype, with a smooth body surface and a decrease in CHC amounts. Similarly, CHC deficiency in N. lugens resulted in increased adhesion of water droplets and secreted honeydew to the insect surface and the inability of N. lugens to survive in paddy fields with varying humidity. Knockdown of NlFAR1, 4, 5, 6, 8, 9, 11 and 13 additionally resulted in female adult infertility. The present study illustrates the structural and functional differences of FAR family genes and provides potential targets for RNA interference-based rice planthopper management. © 2020 Society of Chemical Industry.

Impact and Adhesion of Surfactant-Amended Water Droplets on Leaf Surfaces Related to Roughness

Highlights The relationship between impacting droplet deposition and leaf surface roughness was examined. Complete deposition occurred on smooth leaves with roughness heights shorter than 1.5 micrometers. Droplet deposition on rough leaves was best achieved at low initial travel speeds and high surfactant concentrations. This research provides understanding of increasing droplet adhesion on plants for pesticide spray applications. Abstract. Understanding the effects of the microscopic-scale surface roughness of targeted leaves on spray droplet retention and spread can help develop optimal strategies to improve pesticide application efficiency. Improved spray deposition would minimize the amounts of chemicals to be applied and reduce off-target losses and production costs while protecting the environment. A 3D optical surface profiler was used to measure the arithmetic mean roughness height (Sa), providing a reliable metric for microscopic-scale leaf surface roughness. The relationship between leaf surface roughness and droplet deposition was examined by correlating droplet adhesion with the Sa of six leaf types for spray solutions at five surfactant concentrations (0.0% to 0.75%) and eight initial droplet horizontal travel speeds (0.5 to 4.5 m s-1). Leaf roughness and wettability, in terms of Sa and contact angle (θc), ranged from smooth and easy-to-wet to rough and difficult-to-wet (37° 1.6 μm), deposition decreased with increasing Sa and initial droplet horizontal flight velocity, while it increased as surfactant concentration increased. Thus, spray deposition on rough leaf surfaces could be greatly improved with slow initial droplet flight speeds and high surfactant concentrations. In contrast, greater deposition on smoother leaf surfaces could be achieved at any initial droplet flight velocity and with low concentrations of non-ionic surfactant, or possibly none.

Reduced adhesion of sparkling water droplets

This paper is associated with a video winner of a 2018 APS/DFD Gallery of Fluid Motion Award for work presented at the DFD Gallery of Fluid Motion. The original video is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2018.GFM.V0097

Improving the anti-icing performance of superhydrophobic surfaces by nucleation inhibitor

ABSTRACTTo investigate the effect of heterogeneous nucleation sites on the anti-icing performance of superhydrophobic surfaces, the superhydrophobic POSS/PDMS coatings were prepared and covered by PVA through spraying coating method. The surface morphology was characterized by SEM, and the anti-icing performance of surfaces was characterized through contact angle, freezing temperature of surface water droplet and ice adhesion. The results show that with the optimum match of the particle distance and PVA spraying times, the PVA-POSS/PDMS coating has better anti-icing performance than superhydrophobic POSS/PDMS coating: the freezing temperature of droplet on PVA-POSS/PDMS coating surface is −22.55°C, approximately 2°C lower than POSS/PDMS coating surface. This work confirms the adverse effect of heterogeneous nucleation sites and successfully improves the anti-icing performance of superhydrophobic surface by covering the heterogeneous nucleation sites with nucleation inhibitor. The synergism of nucleation inhibitor and the micro-nano structures is proposed, which will be conducive to the design of high-performance anti-icing materials.

Fabrication of superhydrophobic surfaces with tunable water droplet adhesion force by a two-step method

Abstract In this paper, we present a convenient approach to prepare hierarchical structured superhydrophobic coatings with tunable adhesion force, composed of micro-size glass beads, nano-size SiO2 particles and epoxy resin. Surfaces of two types with different roughness were fabricated, one type is only with single-scale roughness demonstrating lotus effect with low sliding angle, the other type is hierarchically micro-nano-structured roughness exhibiting petal effect with high adhesion force. The surface roughness is pivotal for controlling the wetting behavior and regulating the contact angle including the contact angle hysteresis. Varying the density of micro-size glass beads could adjust the roughness of the surface, which means the adhesion force of the prepared surface could be easily controlled based on the proposed method. Through variation of glass beads’ amount, the surface could be designed to pin the water droplet with different adhesion force when the surface turned upside down. The surface wettability, surface morphology, adhesion force of the prepared samples are investigated and mechanism of the Cassie-to-Wenzel state transition are discussed in detail. Furthermore, the convenient method provides a possibility for controlling surface morphology, composition and corresponding surface adhesion which could be applied to various substrates such as tile, wood, steel and fabric.

The fatty acid elongase gene family in the brown planthopper, Nilaparvata lugens

The cuticular hydrocarbon (CHC) biosynthetic pathways branches off from the synthesis of fatty acids. Fatty acid elongases (ELOs) are enzymes catalyzing the synthesis of long-chain fatty acids and thereby contribute to the diversification of CHCs. Based on bioinformatics analyses we identified 20 ELO genes in the brown planthopper, Nilaparvata lugens. RNA interference against these genes demonstrated that 9 NlELO genes were essential for the survival of N. lugens nymphs and adults. Indeed, knockdown of NlELOs 1, 3, 4, 7, 8, 9, 10, 12 and 18 caused lethal phenotypes with a thin and wizened body and reduced lipids in the fat body. Surface analysis by scanning electron microscopy and CHC quantification indicated that knockdown of NlELOs 2, 3, 8 and 16 additionally resulted in a smooth body surface and a decrease in CHC amounts. Therefore, we speculate that long-chain CHCs are needed for CHC attachment to the cuticle surface. CHC deficiency, in turn, resulted in increased adhesion of water droplets and secreted honeydew to the animal surface and the inability of N. lugens to survive in paddy fields with varying humidity. Our present study provides an initial comprehensive analysis of ELO gene functions in an insect, and may serve to better understand the biology of CHCs.

Reversible transition between adhesive and antiadhesive performances by stretching/recovery on superhydrophobic TPU/CNTs composite membrane surface

Reversible and facile transition between adhesive and antiadhesive performances on superhydrophobic surface is desired but still challenging. In this work, a composite membrane has been fabricated by filtering multi-walled carbon nanotubes (MWCNTs) suspensions on thermoplastic polyurethane (TPU) nonwovens. This surface exhibits superhydrophobic properties because of the hierarchical roughness. The water droplet adhesion behaviors on the surface depend crucially on the stretching or recovery state of the composite membranes. Upon stretching, the groove among fibers is so wide that more and more hydrophilic defects (TPU fibers) have been exposed to water droplets, resulting in the adhesive surface with higher sliding angle; on the contrary, the CNTs network covers the whole membrane after recovery, leading to the Cassie state and antiadhesive performance with enhanced mobility of droplet on the surface. Our result opens a new avenue for tailoring the water droplets adhesion behaviors on superhydrophobic surface.

Facile preparation of diverse alumina surface structures by anodization and superhydrophobic surfaces with tunable water droplet adhesion

Hierarchical alumina superhydrophobic surfaces with a series of different morphologies were obtained by anodization method, and then modified with stearic acid (STA). The effects of anodization parameters including time and voltage on surface morphology and the wettability of surface were investigated in detail. The diverse alumina surface architectures including nanoporous, nanowire clusters and nanorods were obtained by the anodization parameter modulation via a self-assembly process. The results demonstrated that these structures were able to achieve to superhydrophobic. More interestingly, the superhydrophobic alumina surfaces shown a high contrast water droplet adhesion, which conferred with a higher water contact angle of ∼156° and a sliding angle as low as ∼2.1° as well as a strongly sticky water adhesion. However, this paper provides a simple and effective approach to fabricate the superhydrophobic surfaces with tunable water adhesion, which has an important for designing advanced functional materials for application.

Parahydrophobic and Nanostructured Poly(3,4-ethylenedioxypyrrole) and Poly(3,4-propylenedioxypyrrole) Films with Hyperbranched Alkyl Chains.

Here, we control the surface hydrophobicity and the adhesion of water droplets by electrodeposition of poly(3,4-ethylenedioxypyrrole) (PEDOP) and poly(3,4-propylenedioxypyrrole) (PProDOP) with branched alkyl chains placed preferentially on the bridge to favor the formation of nanofibers. Branched alkyl chains of various sizes from very short (C3) to hyperbranched (C18) are studied because they have lower surface hydrophobicity than long alkyl or fluoroalkyl chains (preferable for parahydrophobic properties). The electrodeposition is much more favored with the PEDOP derivatives because the ProDOP films are more soluble. However, the formation of nanoparticles is favored with the PEDOP polymers in contrast to the formation of fibers, resembling the wax nanoclusters observed on lotus leaves, with the ProDOP polymers. With both these PEDOP and PProDOP derivatives, it is possible to reach parahydrophobic properties characterized by a sticking behavior toward water droplets. This kind of surfaces could be used in the future in water harvesting systems, for example.

Magnetically Responsive Superhydrophobic Surface: In Situ Reversible Switching of Water Droplet Wettability and Adhesion for Droplet Manipulation.

A smart, magnetically responsive superhydrophobic surface was facilely prepared by combining spray coating and magnetic-field-directed self-assembly. The surface comprised a dense array of magnetorheological elastomer micropillars (MREMPs). Benefitting from the magnetic field-stiffening effect of the MREMPs, the surface exhibited reversible switching of the wettability and adhesion that was responsive to an on/off magnetic field. The wettability and adhesion properties of the surfaces with MREMPs were investigated under different magnetic fields. The results revealed that the adhesion force and sliding behaviors of these surfaces were strongly dependent on the intensity of the applied magnetic field and the mixing ratio of poly(dimethylsiloxane) (PDMS), iron particles, and solvent (in solution) used for preparation of the magnetically responsive superhydrophobic surfaces. The adhesion transition was attributed to the tunable mechanical properties of the MREMPs, which was easily controlled by an external magnetic field. It was also demonstrated that the magnetically responsive superhydrophobic surface can be used as a "mechanical hand" for no-loss liquid droplet transportation. This magnetically responsive superhydrophobic surface not only provides a novel interface for microfluidic control and droplet transportation, but also opens up new avenues for achieving smart liquid-repellent skin, programmable fluid collection and transport, and smart microfluidic devices.

Electrodeposited Zn for Water-Repellent Coatings

We show that mildly alkaline electrolytes can be used to produce Zn coatings that improve the water repellent properties of stainless steel. Optimal Zn deposits were prepared under potentiostatic conditions from electrolytes that contained ZnCl2, NH4Cl, and a surfactant (polyethyleneimine). After deposition, the Zn electrodeposit was capped with stearic acid to prevent oxidation and to provide a lower surface energy. The capped electrodeposits display an impressive degree of water repellency, including extremely poor water droplet adhesion. We discuss the range of deposition parameters (electrolyte composition, pH, and applied potential) that produce the best water-repellent electrodeposits.

Creation of wettability contrast patterns on metallic surfaces via pen drawn masks

Micropatterned surfaces with wettability contrast have attracted considerable attention due to potential applications in 2D microfluidics, bioassays, and water harvesting. A simple method to develop wettability contrast patterns on metallic surfaces by using a commercial marker is described. A marker-drawn ink pattern on a copper surface displays chemical resistance to an aqueous solution of sodium bicarbonate and ammonium persulfate, thereby enabling selective nanowire growth in areas where ink is absent. Subsequent ink removal by an organic solvent followed by fluorocarbon film deposition yields a stable hydrophobic/super-hydrophobic patterned copper surface. Using this approach, hydrophobic dot and line patterns were constructed. The adhesion force of water droplets to the dots was controlled by adjusting pattern size, thus enabling controlled droplet transfer between two surfaces. Anisotropy of water droplet adhesion to line patterns can serve as a basis for directional control of water droplet motion. This general approach has also been employed to generate wettability contrast on aluminum surfaces, thereby demonstrating versatility. Due to its simplicity, low cost, and virtual independence of solid surface material, ink marker pens can be employed to create wettability patterns for a variety of applications, in fields as diverse as biomedicine and energy.

Foolproof Method for Fast and Reversible Switching of Water-Droplet Adhesion by Magnetic Gradients.

Reversible switching of water-droplet adhesion on solid surfaces is of great significance for smart devices, such as microfluidics. In this work, we designed a foolproof method for fast and reversible magnet-controlled switching of water-droplet adhesion surfaces by doping iron powders in soft poly(dimethylsiloxane). The water adhesion is adjusted by magnetic field-induced structure changes, avoiding complex chemical or physical surface design. The regulation process is so convenient that only tens of milliseconds are needed. The on-site responsive mechanism extends its use to unusual curved surfaces. Moreover, the excellent reversibility and stability make the film an ideal candidate for real-time applications.

Water Droplet Adhesion on Hydrophobic Surfaces: Influence of Droplet Size and Inclination Angle of Surface on Adhesion Force

Adhesion of various size sessile droplets on the hydrophobic surfaces is considered, and the moment generated about the locus of the droplet meniscus is determined for several inclination angles of hydrophobic surface. An experiment is designed to examine the influence of inclination of hydrophobic surface on the water droplet behavior. The flow field generated inside the droplet is simulated to predict the flow acceleration and its effects on adhesion force. Simulations are repeated for different inclination angles of hydrophobic surface. The flow predictions are validated through the experimental data. It is found that the moment about the locus of droplet meniscus increases with increasing inclination angle, which is more pronounced for the large volume water droplets, such as ∀ = 45 μL; however, further increase of inclination angle lowers the moment because of significant change of the location of the line of action of the total force during the excessive body deformation of the droplet. The flow field developed inside the droplet forms a circulation cell, and the orientation and size of the circulation cell change with droplet volume, which becomes significant at high inclination angles. The flow acceleration inside the droplet does not have significant contribution to the overall force generated on the droplet during the inclination of the hydrophobic surface. The shear force generated at the wetted surface of the droplet plays in significant role on the adhesion force.

Occurrence of Near – Petal Effect on the Leaf Surface of Annona squamosa

In this paper, we report the presence of densely distributed nanopillar structures with grooved edges on the adaxial leaf surface of Annona squamosa. The hierarchical roughness contributes to the observed surface hydrophobicity with strong water droplet adhesion which approaches the petal effect. The contact angle of the droplet on the leaf surface was found to be 138°, and the droplet remained attached to the surface even when tilted at 180°. The occurrence of petal effect has mostly been confined to flower petals. Thus, the discovery of a similar wetting property on a leaf surface is potentially useful not only in the design of biomimetic surfaces but also in advancing our knowledge on biological surfaces.

Water-resistant surfaces using zinc oxide structured nanorod arrays with switchable wetting property

Abstract This study presents an experimental approach for fabricating super-hydrophobic coatings based on a dual roughness structure composed of zinc oxide nanorod arrays coated with a sputtered zinc oxide nano-layer. The ZnO nanorod arrays were grown by means of a low-temperature electrochemical deposition technique (75 °C) on FTO substrates. The ZnO nanorods show a (002) orientation along the c-axis, and have a hexagonal structure, with an average length of 710 nm, and average width of 156 nm. On the other hand, the crystallite size of the top-coating sputtered ZnO layer is of 30 nm. The as-deposited ZnO nanorod arrays exhibited a hydrophobic behavior, with a surface water contact angle of 108°, whereas the dual-scale roughness ZnO nanorods coated with sputtered ZnO exhibited a super-hydrophobic behavior, with a surface water contact angle of 157° and a high water droplet adhesion. The photo-catalytic activity of the samples was investigated against the degradation of methylene blue under UV-A irradiation (365 nm). The ZnO nanorod arrays showed good photocatalytic activity whereas the superhydrophobic ZnO nanorod arrays top-coated with sputtered ZnO showed minimal activity regarding the degradation of methylene blue. The superhydrophobic films exhibited high sensitivity to UV light, with a UV-induced switching behavior from super-hydrophobic to super-hydrophilic after only 30 min of UV exposure.

ZnO nanowires grown directly on zinc foils by thermal oxidation in air: Wetting and water adhesion properties

Large scale ZnO nanowire arrays were grown directly on zinc foils using the thermal oxidation in air method. The X-ray diffraction and reflectance investigations confirm that the as-grown nanowires properties are typical for ZnO having a hexagonal wurtzite crystalline structure and band-gap values between 3.2 and 3.3eV. The scanning electron microscopy images prove that the density and the dimensions (diameter and length) of the ZnO nanowires can be tuned by controlling the oxidation temperature. Wettability studies reveal in the case of Zn foils a hydrophilic behavior with high water droplet adhesion which is transformed into a superhydrophobic one with low water droplet adhesion after the foils' surfaces are covered with ZnO nanowires. Obtaining functional surfaces with such interesting wetting properties using a simple, inexpensive and highly reproducible thermal oxidation in air technique is very attractive for anticorrosion coatings and self-cleaning applications.