Anisotropy in the wetting of rough surfaces (original) (raw)
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Effects of Hierarchical Surface Roughness on Droplet Contact Angle
Langmuir : the ACS journal of surfaces and colloids, 2015
Superhydrophobic surfaces often incorporate roughness on both micron and nanometer length scales, although a satisfactory understanding of the role of this hierarchical roughness in causing superhydrophobicity remains elusive. We present a two-dimensional thermodynamic model to describe wetting on hierarchically grooved surfaces by droplets for which the influence of gravity is negligible. By creating wetting phase diagrams for droplets on surfaces with both single-scale and hierarchical roughness, we find that hierarchical roughness leads to greatly expanded superhydrophobic domains in phase space over those for a single scale of roughness. Our results indicate that an important role of the nanoscale roughness is to increase the effective Young's angle of the microscale features, leading to smaller required aspect ratios (height to width) for the surface structures. We then show how this idea may be used to design a hierarchically rough surface with optimally high contact angles.
Effects of the Surface Roughness on Sliding Angles of Water Droplets on Superhydrophobic Surfaces
Various superhydrophobic films having different surface roughnesses were prepared, and the relationships between the sliding angle, the contact angle, and the surface structure were investigated. In the highly hydrophobic region, the sliding angles of water droplets decreased with increasing contact angles. Microstructural observation revealed that surface structures that can trap air are important for the preparation of low-sliding-angle surfaces. We have also derived an equation that describes the relationship between sliding angles and contact angles on superhydrophobic surfaces with roughness. The results calculated on the basis of this equation agreed well with the experimental ones. Moreover, we have successfully prepared a transparent superhydrophobic film whose sliding angle is ∼1° for a 7 mg water droplet. On this film, there was almost no resistance to the sliding of water droplets. The film obtained satisfies the requirements of superhydrophobicity, transparency, and a low water sliding angle.
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.
Superhydrophobicity: Localized Parameters And Gradient Surfaces
Contact Angle, Wettability and Adhesion, Volume 6
The use of Cassie and Baxter's equation and that of Wenzel have been subject to some criticism of late. It has been suggested that researchers use these equations without always considering the assumptions that have been made and sometimes apply them to cases that are not suitable. This debate has prompted a reconsideration of the derivation of these equations using the concept of parameters for the Wenzel roughness and Cassie-Baxter solid surface fractions that are local to the three-phase contact lines. In such circumstances, we show the roughness and Cassie-Baxter solid fractions depend not only on the substrate material, but also on which part of the substrate is being sampled by the three-phase contact lines of a given droplet. We show that this is not simply a theoretical debate, but is one which has direct consequences for experiments on surfaces where the roughness or spatial pattern varies across the surface. We use the approach to derive formulae for the contact angle observed on a double length scale surface under the assumption that the smallscale features on the peaks of larger scale features are either wetted or non-wetted. We also discuss the case of curved and re-entrant surface features and how these bring the Young's law contact angle into the formula for roughness and the condition for suspending droplets without penetration into the surface. To illustrate the use of local parameters, we consider the case of a variation in Cassie-Baxter fraction across a surface possessing a homogeneous hydrophobic surface chemistry and discuss the conditions (droplet volume, surface hydrophobicity, gradient in superhydrophobicity and contact angle hysteresis) under which a droplet may be set into motion. We show that different contact angles on each side of a droplet of water placed on such a surface can generate sufficient lateral force for the droplet to move towards the region of the surface with the lowest contact angle. Using an electrodeposited copper surface with a radial gradient in superhydrophobicity we exemplify these ideas by showing experimentally that droplets enter into selfactuated motion and accumulate in the centre of the surface where the wettability is higher. In principle, paths can be defined and water droplets can be collected by creating such gradients in superhydrophobicity through changes in the lateral topography of the surface.
Soft Matter, 2013
This paper experimentally evaluates the combined effects of eccentricity, relative spacing, and viewing directions on the wetting conditions and the three-phase contact line shapes of hydrophobic surfaces patterned with discrete micropillars. Different techniques to depict the tortuosity of the contact line between the water droplet and microstructured surfaces are presented. First, square micropillars with different values of normalized eccentricity, , and relative spacing, , were fabricated using double casting replication technique. Subsequently, the contact angles were measured along different viewing angles by gradually rotating the sample from to. The contact angle distribution was found as a periodic function of viewing angle whose period depends on the micropillar eccentricity. The results showed that anisotropy increases by increasing the micropillar eccentricity or decreasing the pillar relative spacing. However, the effect of changing the micropillar eccentricity was much more pronounced. Micropillars with and smaller showed maximum degrees of anisotropic wetting and droplet distortion corresponding to 7% and 15%, respectively. Using the measured droplet aspect ratio, corrugated shapes of the three-phase contact line of the micropillars were also reconstructed. Finally, a simple yet effective semi-analytical model, based on Fourier series curve-fitting of the experimental data, was developed to describe the equilibrium 3D shape of the droplet on anisotropic surfaces. Experimental and simulation results reveal that the degrees of anisotropic wetting and droplet distortion were directly proportional to the energy barriers of the system resulting from noncircular corrugated shape of the three-phase contact line. The obtained results may further shed light on the underlying mechanism influencing anisotropic wetting on micropatterned surfaces.
Drop impact and wettability: From hydrophilic to superhydrophobic surfaces
Physics of Fluids, 2012
ABSTRACT Experiments to understand the effect of surface wettability on impact characteristics of water drops onto solid dry surfaces were conducted. Various surfaces were used to cover a wide range of contact angles (advancing contact angle from 48 • to 166 • , and contact angle hysteresis from 5 • to 56 •). Several different impact conditions were analyzed (12 impact velocities on 9 different surfaces, among which 2 were superhy-drophobic). Results from impact tests with millimetric drops show that two different regimes can be identified: a moderate Weber number regime (30 < W e < 200), in which wettability affects both drop maximum spreading and spreading characteristic time; and a high Weber number regime (W e > 200), in which wettability effect is secondary, because capillary forces are overcome by inertial effects. In particular, results show the role of advancing contact angle and contact angle hysteresis as fundamental wetting parameters to allow understanding of different phases of drop spreading and beginning of recoiling. It is also shown that drop spreading on hy-drophilic and superhydrophobic surfaces occurs with different time scales. Finally, if the surface is superhydrophobic, eventual impalement, i.e., transition from Cassie to Wenzel wetting state, which might occur in the vicinity of the drop impact area, does not influence drop maximum spreading. C 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4757122\]
Microstructure Design for Artificial Superhydrophobic Surfaces
International Journal of Engineering and Advanced Technology, 2020
Superhydrophobic surfaces are the surfaces that do not allow the droplets of liquid to spread and wet it. Ideally, the droplets remain almost spherical in shape and with a very small angle of tilt, slide away from the surface. This occurs due to very high contact angle. A perfectly spherical droplet would make 1800 angle of contact, but practically this high contact angle is never possible for a stable droplet. The surfaces that make contact angle (CA)>90o are said to be hydrophobic surfaces. If CA is greater than 150o , the surface is known as superhydrphobic surface. This property of the surface is termed as superhydrophobicity. In this paper, the surface morphology to be engineered is studied, which is governed by certain principles. Theories of Thomas Young [1], Wenzel [2] and Cassie-Baxter [3] are reviewed and effect of micro and nano level of roughness, producing hierarchical structures is analyzed. Subsequently, the designing of such super hydrophobic surfaces is attempted.
Mapping micrometer-scale wetting properties of superhydrophobic surfaces
Proceedings of the National Academy of Sciences, 2019
There is a huge interest in developing superrepellent surfaces for antifouling and heat-transfer applications. To characterize the wetting properties of such surfaces, the most common approach is to place a millimetric-sized droplet and measure its contact angles. The adhesion and friction forces can then be inferred indirectly using Furmidge’s relation. While easy to implement, contact angle measurements are semiquantitative and cannot resolve wetting variations on a surface. Here, we attach a micrometric-sized droplet to an atomic force microscope cantilever to directly measure adhesion and friction forces with nanonewton force resolutions. We spatially map the micrometer-scale wetting properties of superhydrophobic surfaces and observe the time-resolved pinning–depinning dynamics as the droplet detaches from or moves across the surface.
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 %.