The three-phase contact line shape and eccentricity effect of anisotropic wetting on hydrophobic surfaces (original) (raw)
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Strongly Anisotropic Wetting on One-Dimensional Nanopatterned Surfaces
Nano Letters, 2008
This communication reports strongly anisotropic wetting behavior on one-dimensional nanopatterned surfaces. Contact angles, degree of anisotropy, and droplet distortion are measured on micro-and nanopatterned surfaces fabricated with interference lithography. Both the degree of anisotropy and the droplet distortion are extremely high as compared with previous reports because of the well-defined nanostructural morphology. The surface is manipulated to tune with the wetting from hydrophobic to hydrophilic while retaining the structural wetting anisotropy with a simple silica nanoparticle overcoat. The wetting mechanisms are discussed. Potential applications in microfluidic devices and evaporationinduced pattern formation are demonstrated.
A Theory for the Morphological Dependence of Wetting on a Physically Patterned Solid Surface
We present a theoretical model for predicting equilibrium wetting configurations of two-dimensional droplets on periodically grooved hydrophobic surfaces. The main advantage of our model is that it accounts for pinning/ depinning of the contact line at step edges, a feature that is not captured by the Cassie and Wenzel models. We also account for the effects of gravity (via the Bond number) on various wetting configurations that can occur. Using freeenergy minimization, we construct phase diagrams depicting the dependence of the wetting modes (including the number of surface grooves involved in the wetting configuration) and their corresponding contact angles on the geometrical parameters characterizing the patterned surface. In the limit of vanishing Bond number, the predicted wetting modes and contact angles become independent of drop size if the geometrical parameters are scaled with drop radius. Contact angles predicted by our continuum-level theoretical model are in good agreement with corresponding results from nanometer-scale molecular dynamics simulations. Our theoretical predictions are also in good agreement with experimentally measured contact angles of small drops, for which gravitational effects on interface deformation are negligible. We show that contact-line pinning is important for superhydrophobicity and that the contact angle is maximized when the droplet size is comparable to the length scale of the surface pattern.
Langmuir, 2012
Describing wetting of a liquid on a rough or structured surface is a challenge because of the wide range of involved length scales. Nano-and micrometersized textures cause pinning of the contact line, reflected in a hysteresis of the contact angle. To investigate contact angles at different length scales, we imaged water drops on arrays of 5 μm high poly(dimethylsiloxane) micropillars. The drops were imaged by laser scanning confocal microscopy (LSCM), which allowed us to quantitatively analyze the local and large-scale drop profile simultaneously. Deviations of the shape of drops from a sphere decay at two different length scales. Close to the pillars, the amplitude of deviations decays exponentially within 1−2 μm. The drop profile approached a sphere at a length scale 1 order of magnitude larger than the pillars' height. The height and position dependence of the contact angles can be understood from the interplay of pinning of the contact line, the principal curvatures set by the topography of the substrate, and the minimization of the air−water interfaces.
Wetting behavior on hybrid surfaces with hydrophobic and hydrophilic properties
Applied Surface Science, 2014
Hybrid surfaces consisting of a micropillar array of hydrophobic and hydrophilic sites were designed and fabricated to understand the effects of their unique surface morphology and chemistry on droplet condensation. Droplet impingement experiments have revealed that hybrid surfaces exhibit high contact angles, which is characteristic of purely hydrophobic surfaces. However, little is known about the wetting behavior of droplets that nucleate and grow on hybrid surfaces during condensation. In fact, condensed droplets display a distinct wetting behavior during the droplet growth phase which cannot be reproduced by simply impinging droplets on hybrid surfaces. In this study, hybrid surfaces with three different spacing ratios were subjected to condensation tests using an environmental scanning electron microscopy (ESEM) and a condensation cell under ambient conditions. For hybrid surfaces with spacing ratio below 2, droplets were observed to form on top and sides of the micropillars, where they grew, coalesced with adjacent droplets, and shed after reaching a given size. After shedding, the top surface remained partially dry, which allowed for immediate droplet growth. For hybrid surfaces with spacing ratio equal to 2, a different wetting behavior was observed, where droplets basically coalesced and formed a thin liquid film which was ultimately driven into the valleys of the microstructure. The liquid shedding process led to the renucleation of droplets primarily on top of the dry hydrophilic sites. To better understand the nature of droplet wetting on hybrid surfaces, a surface energy-based model was developed to predict the transition between the two observed wetting behaviors at different spacing ratios. The experimental and analytical results indicate that micropillar spacing ratio is the key factor for promoting different wetting behavior of condensed droplets on hybrid surfaces.
Anisotropy in the wetting of rough surfaces
Journal of Colloid and Interface Science, 2005
Surface roughness amplifies the water-repellency of hydrophobic materials. If the roughness geometry is, on average, isotropic then the shape of a sessile drop is almost spherical and the apparent contact angle of the drop on the rough surface is nearly uniform along the contact line. If the roughness geometry is not isotropic, e.g., parallel grooves, then the apparent contact angle is no longer uniform along the contact line. The apparent contact angles observed perpendicular and parallel to the direction of the grooves are different. A better understanding of this problem is critical in designing rough superhydrophobic surfaces. The primary objective of this work is to determine the mechanism of anisotropic wetting and to propose a methodology to quantify the apparent contact angles and the drop shape. We report a theoretical and an experimental study of wetting of surfaces with parallel groove geometry.
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2018
Highlights Microstructured surfaces are developed for various hydrophobicity values An experimental and analytical study has been conducted to investigate the influence of micronano-textured surfaces on the liquid wetting phenomena. The wetting phenomena with different microstructured surfaces as well as an unstructured surface have been evaluated Experimental results of droplet contact angle and surface energy are then compared with analytical calculations (thermodynamic model and capillary Laplace equation) Experimental results reveal that the average water contact angle increases by 34.5 % and 52.5 % for the square-grooved and v-grooved surface, respectively. The surface coating of the square-grooved surface has a larger average contact angle than the Vgrooved surface by 4.6 %.
Evaporation-Triggered Wetting Transition for Water Droplets upon Hydrophobic Microstructures
Physical Review Letters, 2010
When placed on rough hydrophobic surfaces, water droplets of diameter larger than a few millimeters can easily form pearls, as they are in the Cassie-Baxter state with air pockets trapped underneath the droplet. Intriguingly, a natural evaporating process can drive such a Fakir drop into a completely wetting (Wenzel) state. Our microscopic observations with simultaneous side and bottom views of evaporating droplets upon transparent hydrophobic microstructures elucidate the water-filling dynamics and the mechanism of this evaporation-triggered transition. For the present material the wetting transition occurs when the water droplet size decreases to a few hundreds of micrometers in radius. We present a general global energy argument which estimates the interfacial energies depending on the drop size and can account for the critical radius for the transition.
The influence of Wettability on the Droplet Impact onto Micro-Structured Surfaces
International Conference on Liquid Atomization and Spray Systems (ICLASS)
The flourishing of applications in need of self-cleaning mechanisms increased the search for water repellent hydrophobic surfaces with induced roughness. Disclosing the small-scale interface phenomena on the wetting behavior is essential to design efficient hydrophobic materials with defined topography. On the other hand, the spreading behavior concerning the formation of thin films on a surface is required to assure the quality of spray cooling and coatings. The contact angle undoubtedly plays an important role in the droplet impact, providing different outcomes. Moreover, an open question is, how surface topography can affect the impact process. Therefore, to evaluate these matters, different surface patterns were manufactured to assess the surface topography influence on the impact dynamic behavior. Additionally, the wettability of the micro-structured surfaces was flexibly influenced through plasma activation and plasma polymerization. The impact of distilled water and isopropanol droplets on the different surface patterns was captured from three perspectives providing high-quality images of the phenomena. Different surface morphologies can be obtained depending on the surface micro-structures and wettability, affecting spreading shape and evolution. The fluid penetration within the micro-structures is a key feature influencing not only the structures of the outcomes but also the transition between regimes.
Wetting characteristics of liquid drops at heterogeneous surfaces
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 1994
We!l-defined heterogeneous surfaces consisting of hydrophobic and hydrophilic regions were prepared on gold (a 2000 A gold film supported on an Si/SiO2/Ti substrate) by patterning self-assembled monolayers (SAMs), using an elastomer stamp. One surface was composed of alternating and parallel hydrophobic (2.5 ~) and hydrophilic (3 ~) strips, and the second surface consisted of alternating hydrophilic squares (3 ~ x 3 ~) separated by hydrophobic strips (2.5 ~). The wetting characteristics of these well-defined heterogeneous solid surfaces were examined by contact angle measurements. The contact angles for water drops, which varied in pH from 5.8 to 10.0, were measured with the strips both tangential to and normal to the three-phase contact line. The experimental contact angles are in good agreement with theory as calculated from the Cassie equation when the three-phase contact line is non-contorted (i.e. the three-phase contact line is situated along the hydrophobic strip). On the other hand, when the strips are normal to the drop edge, corrugation of the three-phase contact line affects the contact angle significantly. Contact angles, measured with the strips normal to the drop edge, were lower by 7-160 than those calculated from the Cassie equation. Analysis of these measurements, together with contact angle/drop size measurements for fully hydrophobic and hydrophilic surfaces, demonstrate the validity of a modified Cassie equation that includes a term describing the line tension contribution.