Rough Hydrophobic Surfaces Research Articles

Spreading dynamics of a microgel particle-laden thermosensitive polymer drop along smooth and nanofiber surfaces

The research focuses on the influence of 300-μm microgel particles in an aqueous solution of a thermosensitive biopolymer on the spreading and deformation of 3.7-mm drops. The drops impact a smooth hydrophilic and a rough hydrophobic surface. A mass fraction of microgel particles varies in a range of 0–0.2. A universal physical model of the spreading of thermosensitive polymer drops laden with microgel particles along surfaces with significantly different roughness is proposed. It explains the strong inhomogeneity of the contact line stretching due to the deceleration of the continuous phase flow by microgel particles and the increased flow vorticity because of the addition of the surface roughness factor. The validity of the proposed physical model is proven by qualitative and quantitative assessments of the contact line deformation when spreading. An empirical expression for the maximum spreading factor is derived, taking into account the properties of liquids, wall roughness, and microgel particle concentration; it reliably predicts when Re≈110−3100, the surface roughness is 0.5–125 nm, Ca=4.5×10−7, and the number of microgel particles in drops is up to 100. The expression was successfully tested during the modeling of arbitrary surface roughness and the increased concentration of microgel particles relative to those considered in experiments during the formation of a biopolymer layer. When developing the method of additive manufacturing of a functional layer, a practical correlation was established between the volume content of microgel particles, acting as potential containers for living cells, in a drop and the area of the biopolymer layer.

Bubble Growth on Hydrophobic Rough Surfaces in the Shear Flow.

It is well known that bubbles will form on a hydrophobic rough surface immersed in water, which can create a surface covered with bubbles and leads to drag reduction. However, it is still not clear how bubbles grow on the surface under flow conditions. In this work, a rotating flow field is created using a parallel-plate setup of a rotational rheometer, and sample surfaces with different roughnesses and wettabilities are examined with different shear rates. The growth of bubbles is exclusively observed on the hydrophobic rough surface, and subsequent drag reduction is also detected simultaneously. The growth of bubbles is attributed to heterogeneous nucleation in the crevices under a local pressure reduction generated by the shear flow. A geometric model is established to describe the profile evolution of the trapped bubble in the crevice based on the contact angle and the pressure balance across the gas-liquid interface, which involves the variations of the Laplace pressure resulting from changes in the local liquid pressure. The growth of bubbles on the hydrophobic rough surface does not need a large decrease of the surrounding pressure or a high moving speed, which will have potential applications in drag reduction under the condition of a moderate shear rate.

Modeling of multiphase flow in low permeability porous media: Effect of wettability and pore structure properties

Multiphase flow in low permeability porous media is involved in numerous energy and environmental applications. However, a complete description of this process is challenging due to the limited modeling scale and the effects of complex pore structures and wettability. To address this issue, based on the digital rock of low permeability sandstone, a direct numerical simulation is performed considering the interphase drag and boundary slip to clarify the microscopic water-oil displacement process. In addition, a dual-porosity pore network model (PNM) is constructed to obtain the water-oil relative permeability of the sample. The displacement efficiency as a recovery process is assessed under different wetting and pore structure properties. Results show that microscopic displacement mechanisms explain the corresponding macroscopic relative permeability. The injected water breaks through the outlet earlier with a large mass flow, while thick oil films exist in rough hydrophobic surfaces and poorly connected pores. The variation of water-oil relative permeability is significant, and residual oil saturation is high in the oil-wet system. The flooding is extensive, and the residual oil is trapped in complex pore networks for hydrophilic pore surfaces; thus, water relative permeability is lower in the water-wet system. While the displacement efficiency is the worst in mixed-wetting systems for poor water connectivity. Microporosity negatively correlates with invading oil volume fraction due to strong capillary resistance, and a large microporosity corresponds to low residual oil saturation. This work provides insights into the water-oil flow from different modeling perspectives and helps to optimize the development plan for enhanced recovery.

Pore-Level Multiphase Simulations of Realistic Distillation Membranes for Water Desalination.

Membrane distillation (MD) is a thermally driven separation process that is operated below boiling point. Since the performance of MD modules is still comparatively low, current research aims to improve the understanding of the membrane structure and its underlying mechanisms at the pore level. Based on existing realistic 3D membrane geometries (up to 0.5 billion voxels with 39nm resolution) obtained from ptychographic X-ray computed tomography, the D3Q27 lattice Boltzmann (LB) method was used to investigate the interaction of the liquid and gaseous phase with the porous membrane material. In particular, the Shan and Chen multi-phase model was used to simulate multi-phase flow at the pore level. We investigated the liquid entry pressure of different membrane samples and analysed the influence of different micropillar structures on the Wenzel and Cassie-Baxter state of water droplets on rough hydrophobic surfaces. Moreover, we calculated the liquid entry pressure required for entering the membrane pores and extracted realistic water contact surfaces for different membrane samples. The influence of the micropillars and flow on the water-membrane contact surface was investigated. Finally, we determined the air-water interface within a partially saturated membrane, finding that the droplet size and distribution correlated with the porosity of the membrane.

Low-Temperature Rough-Surface Wafer Bonding with AlN/AlN Via Oxygen Plasma Activation

In this study, we demonstrate breaking through the main obstacle to achieve perfect wafer bonding: the requirement of the surface needs to be very flat and smooth. Strictly, the roughness must be less than 0.5 Å. We use sintered aluminum nitride as an example. AlN is a ceramic material with excellent dielectric and thermal properties and is usually used in the fields of microelectronics and energy. Despite CMP polishing, the high roughness (rms> 10 nm) of AlN wafers due to voids formed in sintering cannot meet the surface requirements for direct wafer bonding (rms < 5 Å). We present that bonding an AlN to another AlN with a high roughness surface can be achieved by design bonding processing. First, we used oxygen plasma to activate the hydrophobic rough surface to become a hydrophilic surface to introduce strong capillary action. Then, both surfaces of the bonded AlN/AlN pair were reacted with bonding species (OH-). The reaction was strengthened via electron transfer caused by clamping the bonded AlN/Al pair. Finally, low-temperature annealing (<150°C) was performed on the bonded AlN/AlN pair to synthesize Al2O3 at the bonding interface. After annealing at 150°C for 4 hours, the bonding interface of the AlN bonding pair was observed with scanning electron microscopy (SEM), as shown in Figure 1. The bonding interface showed that the two surfaces were perfectly fused without cracks and nothingness. TEM and XPS results also showed that there is a transition layer between the bonding surfaces, in which the nitrogen concentration decreases and the oxygen concentration increases significantly. It can be inferred that after annealing, an Al2O3 layer is formed by hydrolysis and dehydration, which acts as a bridge to firmly connect the two AlN wafers. Figure 1

Low-Temperature Rough-Surface Wafer Bonding with AlN/AlN Via Oxygen Plasma Activation

We demonstrate breaking through the main obstacle to achieve perfect wafer bonding: the requirement of the surface needs to be very flat and smooth. Strictly, the roughness must be less than 0.5 Å. We use sintered aluminum nitride as an example. AlN is a ceramic material with excellent dielectric and thermal properties and is usually used in the fields of microelectronics and energy. Despite CMP polishing, the high roughness (rms> 10 nm) of AlN wafers due to voids formed in sintering cannot meet the surface requirements for direct wafer bonding (rms < 5 Å). We present that bonding an AlN to another AlN with a high roughness surface can be achieved by design bonding processing. First, we used oxygen plasma to activate the hydrophobic rough surface to become a hydrophilic surface to introduce strong capillary action. Then, both surfaces of the bonded AlN/AlN pair were reacted with bonding species (OH-). The reaction was strengthened via electron transfer caused by clamping the bonded AlN/Al pair. Finally, low-temperature annealing (<150°C) was performed on the bonded AlN/AlN pair to synthesize Al2O3 at the bonding interface. After annealing at 150°C for 4 hours, the bonding interface of the AlN bonding pair was observed with scanning electron microscopy (SEM), as shown in Figure 1. There is a transition layer between the bonding surfaces, in which the nitrogen concentration decreases and the oxygen concentration increases significantly. Alumina layer is formed by hydrolysis and dehydration, which acts as a bridge to firmly connect the two AlN wafers.

Nanoscale Thin-Film Boiling Processes on Heterogeneous Surfaces

Acquiring rapid and efficient boiling processes has been the focus of industry as they have the potential to improve the energy efficiency and reduce the carbon emissions of production processes. Here, we report nanoscale thin-film boiling on different heterogeneous surfaces. Through nonequilibrium molecular dynamics simulation, we captured the triple-phase interface details, visualized the bubble nucleation, and recorded the internal fluid flow and thermal characteristics. It is found that nanoscale thin-film boiling without the occurrence of bubble nucleation shows excellent heat and mass transfer performance, which differs from macroscale boiling. In general, rough structures advance the onset time of stable boiling and improve the efficiency. The heat transfer coefficient and heat flux on a rough hydrophilic surface respectively reach to 7.43 × 104 kW/(m2·K) and 1.3 × 106 kW/m2 at a surface temperature of 500 K, which are 100-fold higher than those of micrometer-scale thin-film boiling. However, due to the resultant vapor film trapped between the liquid and the surface, the rough hydrophobic surface leads to heat transfer deterioration instead. It is revealed that the underlying mechanism of regulatory effects resulting from surface physicochemical properties is originated from the variation of interfacial thermal resistance. It is available to reduce the overall interfacial resistance and further improve the heat and mass transfer efficiency through increasing surface roughness, enhancing surface wettability, and increasing the area proportion of the hydrophilic region. This work provides guidelines to achieve rapid and efficient thin-liquid-film boiling and serves as a reference for the optimized design of surfaces utilized for high-heat flux removal through vaporization processes.

Construction of rough and porous surface of hydrophobic PTFE powder-embedded PVDF hollow fiber composite membrane for accelerated water mass transfer of membrane distillation

The construction of rough and porous hydrophobic membrane surface is expected to overcome the obstacles including low permeate water flux and membrane pore wetting which greatly restrict the development of membrane distillation (MD) technology. In this study, a rough and porous polytetrafluoroethylene (PTFE) powder-embedded polyvinylidene fluoride (PVDF) hydrophobic coating layer was compounded on the outer surface of PVDF hollow fiber support membrane by the dilute solution coating-phase inversion method. PVDF hollow fiber support membrane was fabricated by the dry-jet wet-spinning technique. PTFE powder was directly incorporated in PVDF dilute coating solution and embedded in the porous PVDF coating layer after the nonsolvent induced phase separation (NIPS). The variations of membrane morphology, surface chemical compositions, hydrophobicity and wetting resistance were investigated by scanning electron microscope (SEM), Attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR), dynamic water contact angle (WCA) and liquid entry pressure (LEP) analysis. Membrane separation performance including desalination and ionic dyes removal properties was evaluated by VMD experiment. Compared with PVDF hollow fiber support membrane, both the surface hydrophobicity and the water permeability of the PTFE powder-embedded PVDF hollow fiber composite membrane (HFC) had obvious improvement. The surface WCA and permeate water flux increased from 92.6° and 11.3 kg/m2.h for PVDF support membrane to 133.6° and 26.8 kg/m2.h for the PTFE powder-embedded HFC membrane meanwhile NaCl rejection can be maintained above 99.9% (3.5 wt% NaCl aqueous solution at 50 °C and permeate pressure at 31.3 kPa). The separation performance of HFC membrane can remain stable without obvious pore wetting during the continuous MD operation for 36 h. Different from the desalination, porous hollow fiber membrane would adsorb two different charged dyes, Congo red (CR) and methylene blue (MB) to a certain extent, resulting in the decrease of membrane flux and the change of permeate water quality. Finally, the MD separation mechanism of inorganic salt, anionic dye and cationic dye by PTFE powder-embedded HFC membrane was proposed.

Toward droplet dynamics simulation in polymer electrolyte membrane fuel cells: Three-dimensional numerical modeling of confined water droplets with dynamic contact angle and hysteresis

This work focuses on three-dimensional simulation of the dynamics of droplets with contact-angle hysteresis. In order to consistently model the dynamics of the contact line, a combination of the linear molecular kinetic theory and the hydrodynamic theory is implemented in the present numerical method. Without presetting the contact line and/or the contact angle, such simulations are generally prone to irregularities at the contact line, which are mainly due to the imposition of the pinning and unpinning mechanisms associated with the hysteresis phenomenon. An effective treatment for this issue is proposed based on a simple procedure for calculating the nodal contact angle within the framework of enriched finite element/level set method. The resulting method also benefits from a manipulated momentum conservation equation that incorporates the effect of the liquid mass conservation correction, which is essentially important for simulations with a rather long (physical) run-time. In this paper, the proposed numerical model is validated against the previously published experimental data addressing the configuration of a water droplet on a tilted rough hydrophobic surface. In this test, the effect of the contact-line pinning as the underlying mechanism for droplet hysteresis phenomenon is also studied. The model is further employed to simulate a liquid droplet confined in a channel in the presence of air flow.

Anti-icing fluids interaction with surfaces: Ice protection and wettability change

Some aircraft anti-icing fluids can dangerously weaken on hydrophobic surfaces. However, information about such fluids and surfaces was not full. Therefore, we considered the interaction of commercially available Newtonian and pseudo-plastic anti-icing fluids with super-hydrophilic and hydrophobic aluminum surfaces. In freezing rain simulations, no noticeable surface effect was observed on fluid endurance times at 10% ice coverage of the surfaces. The difference with previous works can be caused by fluid surface tensions, the contact angle hysteresis of test plates, and fluid viscosity (the last is irrelevant for Newtonian fluids). In further comparative studies, the roughness must also be considered because on rough hydrophobic surfaces the Newtonian fluid took longer to freeze once ice coverage surpassed 20% compared to smooth super-hydrophilic surfaces. Furthermore, the fluid physical adsorption in the surface texture leads to the drifting of receding contact angles of water on hydrophobic surfaces, thereby worsening their water-repelling. Thus, smooth hydrophobic surfaces are probably the preferred solution for ice mitigation systems contacting aircraft anti-icing fluids.

CONDENSATION PHASE CHANGE BEHAVIORS ON A ROUGH SURFACE CHARACTERIZED BY FRACTAL CANTOR

Understanding the fundamental mechanisms of vapor condensation on rough surfaces is crucial to a wide range of industrial applications. A hybrid thermal lattice Boltzmann model of the condensation heat transfer process on downward-facing rough surfaces characterized by the Cantor fractal is developed and numerically analyzed to investigate the condensation phase change behaviors on rough hydrophobic and hydrophilic surfaces. The dynamic behaviors of vapor condensation, including the evolutions of vapor–liquid interface, heat flux, condensate mass, and temperature distribution, on the hydrophilic and hydrophobic rough surfaces are presented and compared with corresponding smooth surfaces. The results indicate that the rough surface preferred a filmwise condensation under hydrophilic conditions but a hybrid dropwise–filmwise condensation under hydrophobic conditions. On the rough hydrophobic surface, the liquid film can rapidly adsorb droplets, maintaining a high-efficiency dropwise condensation. The absorption of droplets accelerates the liquid film growth and detachment process on the rough hydrophobic surface, which reduces the time-averaged thermal resistance of the filmwise region. These two behaviors together enhance condensation heat transfer on the downward-facing rough hydrophobic surface. Besides, stable dropwise condensation could also be formed on smooth hydrophilic surfaces and has better heat transfer performance than corresponding hydrophobic surfaces under the same heat transfer condition.

Facile and scalable synthesis of functional Janus nanosheets – A polyethoxysiloxane assisted surfactant-free high internal phase emulsion approach

HypothesisJanus nanosheets, which have two surfaces of different functionalities, exhibit unique interfacial properties. In this work, we propose a facile and scalable technique for preparation of silica-based Janus nanosheets, which is based on formation of high internal phase water-in-oil emulsions stabilized solely by alkyl-substituted polyethoxysiloxanes due to their hydrolysis-induced interfacial activity. ExperimentsJanus nanosheets are then obtained by crushing the silica foams converted from such emulsions. The morphology of Janus nanosheets is investigated by field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). The chemical structure of functional silica materials is characterized by Fourier transform infrared spectroscopy (FT-IR). The asymmetric structure of silica nanosheets is observed by confocal laser scanning microscopy. FindingsThe resulting nanosheets have a rough hydrophobic surface and a smooth hydrophilic one, and are capable of stabilizing Pickering oil-in-water emulsions. Remarkably, pH-responsiveness of emulsions can be attained using the nanosheets whose hydrophilic surface is substituted with amino groups. Fast oil–water separation is achieved by the Janus nanosheets, which has been demonstrated by the nanosheets with a polystyrene-coated hydrophobic surface. This work paves a new avenue for large-scale production of functional silica-based Janus nanosheets suitable for numerous promising applications.

Slippery surfaces: A decade of progress

Slippery surfaces have received great attention for more than a quarter-century. In particular, during the last decade, interest has increased exponentially, resulting in thousands of articles concerning three types of slippery surfaces: superhydrophobic, superoleophobic, and omniphobic. This review focuses on recent developments and significant findings in naturally inspired slippery surfaces. Superhydrophobicity can be characterized by water droplets beading on a surface at significantly high static contact angles and low contact-angle hystereses. Microscopically rough hydrophobic surfaces could entrap air in their pores, resulting in a portion of a submerged surface with an air–water interface, which is responsible for the slip effect and drag reduction. Suberhydrophobicity enhances the mobility of droplets on lotus leaves for self-cleaning purposes, the so-called lotus effect. Surface hydrophobicity can be advanced to repel low-surface-tension liquids, i.e., become superoleophobic. Another kind of slippery coating is the slippery liquid-infused porous surfaces (SLIPS), which are omniphobic coatings. Certain plants such as the carnivorous Nepenthes pitcher inspired SLIPS. Their interior surfaces have microstructural roughness, which can lock in place an infused lubricating liquid. The lubricant is then utilized as a repellent surface for other liquids or substances such as water, blood, crude oil, ice, insects, and bio-fouling. In this review, we discuss different slippery mechanisms in nature. We also cover recent advances in manufacturing, texturing, and controlling slippery surface at the micro- and nanoscales. We further discuss the performance, sustainability, and longevity of such surfaces under different environmental conditions. Very-recent techniques used to characterize the surfaces are also detailed.

Non-Fluorinated, Superhydrophobic Binder-Filler Coatings on Smooth Surfaces: Controlled Phase Separation of Particles to Enhance Mechanical Durability.

There has been a recent drive to develop non-fluorinated superhydrophobic coatings due to the toxicity, cost, and environmental impact of perfluorinated components. One of the main challenges in developing superhydrophobic coatings in general and non-fluorinated superhydrophobic coatings in particular is optimization of mechanical durability, as the rough asperities required for maintaining superhydrophobicity tend to be easily removed by abrasion. Although rough and self-similar hydrophobic surfaces composed of loosely adhered particles or highly porous structures tend to produce excellent superhydrophobicity, they have low inherent mechanical durability and their longevity under real conditions is compromised. To address this issue, this work investigates the addition of a polymeric matrix material (the binder) to hydrophobic nanoparticles (the filler) to produce spray-coated superhydrophobic surfaces with improved inherent mechanical durability. Hansen solubility parameters were used to tune the interactions between the binder, filler, and solvent used to deliver the coating. It was found that lowering the binder/filler miscibility and using a poor solvent mixture generates more surface roughness, thereby lowering the minimum filler load required to achieve superhydrophobicity. This leads to an overall more inherently durable system that remains hydrophobic for thousands of light abrasion cycles.

Forced oscillation dynamics of surface nanobubbles.

Surface nanobubbles have potential applications in the manipulation of nanoscale and biological materials, waste-water treatment, and surface cleaning. These spherically capped bubbles of gas can exist in stable diffusive equilibrium on chemically patterned or rough hydrophobic surfaces, under supersaturated conditions. Previous studies have investigated their long-term response to pressure variations, which is governed by the surrounding liquid's local supersaturation; however, not much is known about their short-term response to rapid pressure changes, i.e., their cavitation dynamics. Here, we present molecular dynamics simulations of a surface nanobubble subjected to an external oscillating pressure field. The surface nanobubble is found to oscillate with a pinned contact line, while still retaining a mostly spherical cap shape. The amplitude-frequency response is typical of an underdamped system, with a peak amplitude near the estimated natural frequency, despite the strong viscous effects at the nanoscale. This peak is enhanced by the surface nanobubble's high internal gas pressure, a result of the Laplace pressure. We find that accurately capturing the gas pressure, bubble volume, and pinned growth mode is important for estimating the natural frequency, and we propose a simple model for the surface nanobubble frequency response, with comparisons made to other common models for a spherical bubble, a constant contact angle surface bubble, and a bubble entrapped within a cylindrical micropore. This work reveals the initial stages of growth of cavitation nanobubbles on surfaces, common in heterogeneous nucleation, where classical models based on spherical bubble growth break down.

Numerical study of droplet evaporation on heated flat and micro-pillared hydrophobic surfaces by using the lattice Boltzmann method

The evaporation processes of sessile droplets on both heated flat and structured hydrophobic surfaces are numerically studied by using a thermal multi-phase lattice Boltzmann model. The contact-line dynamics and heat transfer regarding the evaporation of droplets are simulated and the effects of the heat conductivity, wettability, and roughness of the substrate on the droplet evaporation are investigated. The results indicate that the different evaporation modes of the droplets on the rough surfaces are attributed to the microscopic “stick–slip” motion of the contact line. The constant contact angle (CCA) mode of droplet evaporation on the rough hydrophobic surface can be regarded as a macroscopic average of the microscopic “stick–slip” processes with a short stick period and small change in apparent contact angle. The weaker stick effect of the surface with a higher hydrophobicity and smaller solid fraction makes the droplet evaporation more likely to occur in the CCA manner.

Liquid slippage on rough hydrophobic surfaces with and without entrapped bubbles

The process of liquid slip on rough-walled hydrophobic surfaces with and without entrapped gas bubbles is modeled. Here, starting with the Navier–Stokes equations, a set of partial differential equations (PDE) and boundary conditions for the general effective slip tensor of a rough hydrophobic surface are constructed by an asymptotic analysis. The intrinsic slip and surface roughness are considered as the characteristics of the surface. The solution is based on a weak variation form that fully recovers the set of PDE and Navier slip boundary. For the surface with entrapped bubbles, a semi-analytical model based on eigenfunction expansion is developed. In addition to the surface characteristics, the size and contact angle of the bubbles are considered in the semi-analytical solution. Both models are validated with the published data as well as direct numerical simulation. Based on the model results, we present correlations of effective slip length with surface characteristics and entrapped bubbles. We found that surface roughness reduces liquid slippage on a surface. However, if the asperities on a surface are filled with gas bubbles, the effective slip length can significantly increase as long as the bubble contact angle is small. Interestingly, bubbles with a larger contact angle could act inversely and change a hydrophobic surface with a large intrinsic slip to a no-slip or even a sticky surface. These results shed light on the controversy over the order of magnitude of the actual slip length of water flow in carbon-based nanotubes and nanochannels. The model results also help understand the anomalies of high water production and high amounts of hydraulic fracturing fluid leak-off observed in tight oil and shale gas reservoirs.

Characterization of the solvent specific evaporation from a fluoropolymer surface roughened by layered double oxide (LDO) particles

Abstract Solid/ liquid interaction strongly depends, among other things, on the chemical and physical properties of fluids, therefore, there are unexploited analytical possibilities in investigating the surface wetting and evaporation behavior. Strongly hydrophobic rough surface thin films were prepared by spray-coating a fluoropolymer film with incorporated layered double oxide (LDO) microparticles. We studied the evaporation of ethanol–water mixtures from the low energy (8.4 ± 2.6 mJ/m2) composite surfaces by simultaneous high-speed visible imaging, infrared imaging and weight loss monitoring. The wetting behavior changed from Cassie's wetting mode (pure water) to Wenzel's (pure ethanol) as a function of solvent composition. The vaporization process could be divided into three stages described by constant evaporate rates and the heat transfer coefficient between the studied layer and water is KT = 1768 W/(m2K). We have found strong correlations between parameters of the measured evaporation profiles and certain physical properties of the solvents. It is possible to estimate the viscosity, the boiling point, or the surface tension of a studied liquid merely from the total evaporation time. This novel solvent identification method can serve as a new application field for such low energy hybrid surfaces.

Hydrophobicity Evolution on Rough Surfaces.

Hydrophobicity is abundant in nature and obtainable in industrial applications by roughening hydrophobic surfaces and engineering micropatterns. Classical wetting theory explains how surface roughness can enhance water repellency, assuming a droplet to have a flat bottom on top of micropatterned surfaces. However, in reality, a droplet can partially penetrate into micropatterns to form a round-bottom shape. Here, we systematically investigate the evolution of evaporating droplets on micropatterned surfaces with X-ray microscopy combined with three-dimensional finite element analyses and propose a theory that explains the wetting transition with gradually increasing penetration depth. We show that the penetrated state with a round bottom is inevitable for a droplet smaller than the micropattern-dependent critical size. Our finding reveals a more complete picture of hydrophobicity involving the partially penetrated state and its role in the wetting state transition and can be applied to understand the stability of water repellency of rough hydrophobic surfaces.

Preparation of a rough hydrophobic surface on jute fibers via silica hydrosol modification and properties of fiber-reinforced polylactic acid composites

Scoured jute fiber was modified with silica hydrosol and then mixed with polylactic acid to form bio-based fiber-reinforced composites. The effects of modification methods on the morphology and hydrophobicity of the fibers, as well as the tensile strength and fracture structure of the composites were studied. Also, the influence of sodium hydroxide concentration, MTMOS concentration, and numbers of dipping steps on the tensile strength of the composites were examined. The results indicated that silica hydrosol condensation could be used to prepare a smooth hydrophobic surface on modified fibers, while a rough hydrophobic surface could be prepared when alkaline treatment was carried out onto the surface of incompletely condensed silica hydrosol film. Rough hydrophilic fibers revealed poor interfacial compatibility and adhesion strength. Smooth hydrophobic surfaces improved interfacial compatibility and dispersibility of the fibers in hydrophobic matrix, whereas a rough hydrophobic structure further improved the adhesion strength through mechanical interlock. The data illustrated that morphology of the silica hydrosol modified fiber surface depended on the alkaline treatment conditions and amounts of adsorbed silica hydrosol, while the effect of MTMOS precursor concentration was negligible.