Wetting Of Rough Surfaces Research Articles

Wetting of rough surfaces in a phase field model

AbstractSurface wetting can be simulated using a phase field approach which describes the continuous liquid‐gas transition with the help of an order parameter. In this publication, wetting of non‐planar surfaces is investigated based on a phase field model by Diewald et al. [1, 2]. Different scenarios of droplets on rough surfaces are simulated. The static equilibrium for those scenarios is calculated using an Allen‐Cahn evolution equation. The influence of the surface morphology on the resulting contact angle is investigated while the width of the phase transition from liquid to gas is varied as a model parameter.

Dynamics of thin precursor film in wetting of nanopatterned surfaces

The spreading of a liquid droplet on flat surfaces is a well-understood phenomenon, but little is known about how liquids spread on a rough surface. When the surface roughness is of the nanoscopic length scale, the capillary forces dominate and the liquid droplet spreads by wetting the nanoscale textures that act as capillaries. Here, using a combination of advanced nanofabrication and liquid-phase transmission electron microscopy, we image the wetting of a surface patterned with a dense array of nanopillars of varying heights. Our real-time, high-speed observations reveal that water wets the surface in two stages: 1) an ultrathin precursor water film forms on the surface, and then 2) the capillary action by nanopillars pulls the water, increasing the overall thickness of water film. These direct nanoscale observations capture the previously elusive precursor film, which is a critical intermediate step in wetting of rough surfaces.

Gravitational Effect on the Advancing and Receding Angles of a Two-Dimensional Cassie-Baxter Droplet on a Textured Surface.

Advancing and receding angles are physical quantities frequently measured to characterize the wetting properties of a rough surface. Thermodynamically, the advancing and receding angles are often interpreted as the maximum and minimum contact angles that can be formed by a droplet without losing its stability. Despite intensive research on wetting of rough surfaces, the gravitational effect on these angles has been overlooked because most studies have considered droplets smaller than the capillary length. In this study, however, by combining theoretical and numerical modeling, we show that the shape of a droplet smaller than the capillary length can be substantially modified by gravity under advancing and receding conditions. First, based on the Laplace pressure equation, we predict the shape of a two-dimensional Cassie-Baxter droplet on a textured surface with gravity at each pinning point. Then, the stability of the droplet is tested by examining the interference between the liquid surface and neighboring pillars and analyzing the free energy change upon depinning. Interestingly, it turns out that the apparent contact angles under advancing and receding conditions are not affected by gravity, while the overall shape of a droplet and the position of the pinning point are affected by gravity. In addition, the advancing and receding of the droplet with continuously increasing or decreasing volume are analyzed, and it is shown that the gravitational effect plays a key role in the movement of the droplet tip. Also, the gravitational effect on the degree of the stability of the droplet upon the external effect such as vibration is discussed. Finally, the theoretical predictions were validated against line tension-based front tracking modeling (LTM) that seamlessly captures the attachment and detachment between the liquid surface and the solid substrate. Our findings provide a deeper understanding on the advancing and receding phenomena of a droplet and essential insight into designing devices that utilize the wettability of rough surfaces.

Wetting of Rough Surfaces by a Low Surface Tension Liquid

There exist textured surfaces that demonstrate large advancing contact angles when the expected behavior is complete wetting due to high Wenzel roughness. The roughness can represent an impediment to the motion of the contact line, leading to the possibility of contact line pinning and thus increased advancing contact angle. A set of fabricated textured surfaces with varying pillar diameters and pillar spacing were tested using hexadecane. Because of the low surface tension of hexadecane, the majority of the surfaces exhibited penetration of the liquid into the roughness, also known as the Wenzel state. Because of this penetration, the empirical pinning force framework we previously developed for wetting behavior on smooth surfaces and surfaces with texture where liquid does not penetrate the roughness was expanded to include the Wenzel state. For the surfaces that had Wenzel wetting, the receding contact angle tended to follow the predictions of the Wenzel equation, while the advancing contact angle tended to increase with increasing roughness when according to the Wenzel equation it would be expected to decrease. For surfaces where nonpenetrated Cassie wetting was observed, a constant high advancing contact angle and a receding contact angle that follows the trend predicted by the Cassie equation were observed.

Beyond Wenzel and Cassie-Baxter: second-order effects on the wetting of rough surfaces.

The Wenzel and Cassie-Baxter models are almost exclusively used to explain the contact angle dependence of the structure of rough and patterned solid surfaces. However, these two classical models do not always accurately predict the wetting properties of surfaces since they fail to capture the effect of many interactions occurring during wetting, including, for example, the effect of the disjoining pressure and of crystal microstructure, grains, and defects. We call such effects the second-order effects and present here a model showing how the disjoining pressure isotherm can affect wettability due to the formation of thin liquid films. We measure water contact angles on pairs of metallic surfaces with nominally the same Wenzel roughness obtained by abrasion and by chemical etching. These two methods of surface roughening result in different rough surface structure, thus leading to different values of the contact angle, which cannot be captured by the Wenzel- and Cassie-type models. The chemical and physical changes that occur on the stainless steel and aluminum alloy surfaces as a result of intergranular corrosion, along with selective intermetallic dissolution, lead to a surface roughness generated on the nano- and microscales.

Fine‐Tuning of Superhydrophobicity Based on Monolayers of Well‐defined Raspberry Nanoparticles with Variable Dual‐roughness Size and Ratio

Superhydrophobic surfaces have been extensively investigated for self‐cleaning, low‐adhesion, anti‐corrosion or reduced‐drag applications. Roughness and its characteristics, i.e., morphology, overall roughness and individual feature size, is an essential factor for superhydrophobicity. Several experimental methods and theoretical models strived to predict how the surface wettability is affected by the surface roughness. However, due to the difficulty of making practical surfaces with well‐defined roughness profiles, only limited and arbitrary experimental studies focused on practical superhydrophobic films. Here, the roughness factors which determine the wetting properties of films are reported, based on monolayers of well‐defined raspberry silica‐silica nanoparticles, exhibiting a wide‐range and systematic variation of individual features sizes and ratios (large over small features). The advancing water contact angle does not depend on the feature size or ratio, while the contact angle hysteresis (CAH) is strongly dependent on both. The minimum size and size ratio to reach superhydrophobicity were determined. These new insights into the wetting of rough surfaces can be used to direct the design of practical superhydrophobic materials for advanced applications such as solar panels, microelectronics or microfluidic devices.

Contact Angles in Imaginary Number Space: A Novel Tool for Probing the Remaining Mysteries of Ultrahydrophilicity and Superhydrophilicity.

ABSTRACTImaginary contact angles underlying hyperhydrophilicity and the Inverse Lotus Effect introduce a fundamental new development in the area of contact angles and wettability. Just as the Lotus Effect expanded hydrophobicity beyond the maximal contact angle of 119° on a smooth surface, the Inverse Lotus Effect expands hydrophilicity beyond the minimal contact angle of 0° on a smooth surface. Imaginary dynamic contact angles thus offer an exciting enhancement in tools and methodology for measuring the wettability on rough, highly hydrophilic surfaces. Contrary to current thinking, full or perfect wetting of rough surfaces is only little understood and cannot be predicted by classical equations. Therefore also the exact physical basis of imaginary dynamic contact angles remains to be elucidated. In this short treatise some aspects of the new field will be treated with examples derived from rough titanium surfaces employed in the medical field.

Superhydrophobic Coatings on Cellulose‐Based Materials: Fabrication, Properties, and Applications

Wettability of a solid surface by a liquid plays an important role in several phenomena and applications, for example in adhesion, printing, and self‐cleaning. In particular, wetting of rough surfaces has attracted great scientific interest in recent decades. Superhydrophobic surfaces, which possess extraordinary water repelling properties due to their low surface energy and specific nanometer‐ and micrometer‐scale roughness, are of particular interest due to the great variety of potential applications ranging from self‐cleaning surfaces to microfluidic devices. In recent years, the potential of superhydrophobic cellulose‐based materials in the function of smart devices and functional clothing has been recognized, and in the past few years cellulose‐based materials have established themselves among the most frequently used substrates for superhydrophobic coatings. In this Review, over 40 different approaches to fabricate superhydrophobic coatings on cellulose‐based materials are discussed in detail. In addition to the anti‐wetting properties of the coatings, particular attention is paid to coating durability and other incorporated functionalities such as gas permeability, transparency, UV‐shielding, photoactivity, and self‐healing properties. Potential applications for the superhydrophobic cellulose‐based materials range from water‐ and stain‐repellent, self‐cleaning and breathable clothing to cheap and disposable lab‐on‐a‐chip devices made from renewable sources with reduced material consumption.

Dynamics of contact line motion during the wetting of rough surfaces and correlation with topographical surface parameters

Dynamics of contact line motion and wettability is essential in many industrial applications such as liquid coating, lubrication, printing, painting, condensation, etc. However, the wettability of surfaces depends not only on liquid-solid chemical properties but also can be strongly affected by surface roughness. As a practical application of controlled wettability, we can mention the self-cleaning surfaces, protective clothing, microfluidics devices, electro wetting, etc. In this article, we experimentally investigate the spreading of droplets deposited onto rough surfaces. Anisotropic surfaces were prepared by abrasive polishing on the following materials: aluminium alloy AA7064, titanium alloy Ti-6Al-4V, steel AISI 8630, copper alloy UNS C17000, machinable glass ceramic, and poly-methylmethacrylate. Topographical 2D parameters were calculated according to the following standards, defining Geometrical Product Specifications (GPS): ISO 4287, ISO 12085, ISO 13565, ISO 12780, and ISO 12181. The influence of topographical parameters on wettability and spreading phenomenon has been evaluated by statistical covariance analysis. The following parameters have strong influence on fluid spreading on rough surfaces: R(mr) is the relative material ratio of the roughness profile, T(rc) is the microgeometric material ratio, P(mr) is the relative material ratio of the raw profile, K(r) is the mean slope of the roughness motifs, RON(t) is the peak to valley roundness deviation, and P(sk) is the Skewness of the raw profile. The physical meaning of selected parameters is discussed, and K(r) (the mean slope of the roughness motifs) is selected as the most important and physically meaningful parameter. It has been found that for all tested materials, fluid spreading shows increasing tendency when mean slope of the roughness motifs (K(r) ) increases.

Relativistic Wetting Effects for Sessile Drops

Relativistic wetting of flat and rough substrates is treated and the transformation of the contact angle is derived. Concepts of hydrophilicity and hydrophobicity are discussed from relativistic point of view. The sign of the spreading parameter demonstrates relativistic invariance, whereas the notions of hydrophilicity and hydrophobicity are not relativistically invariant and depend on the velocity of the reference frame. Relativistic wetting of rough surfaces is also discussed.

Wetting of rough surfaces: a homogenization approach

The contact angle of a drop in equilibrium on a solid is strongly affected by the roughness of the surface on which it rests. We study the roughness–induced enhancement of the hydrophobic or hydrophilic properties of a solid surface through homogenization theory. By relying on a variational formulation of the problem, we show that the macroscopic contact angle is associated with the solution of two cell problems, giving the minimal energy per unit macroscopic area for a transition layer between the rough solid surface and a liquid or vapour phase. Our results are valid for both chemically heterogeneous and homogeneous surfaces. In the latter case, a very transparent structure emerges from the variational approach: the classical laws of Wenzel and Cassie–Baxter give bounds for the optimal energy, and configurations of minimal energy are those leading to the smallest macroscopic contact angle in the hydrophobic case, to the largest one in the hydrophilic case.

Anisotropy in the wetting of rough surfaces

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.