Theory of wetting and spreading (original) (raw)
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Reviews of modern physics, 2009
Wetting phenomena are ubiquitous in nature and technology. A solid substrate exposed to the environment is almost invariably covered by a layer of fluid material. In this review, the surface forces that lead to wetting are considered, and the equilibrium surface coverage of a substrate in contact with a drop of liquid. Depending on the nature of the surface forces involved, different scenarios for wetting phase transitions are possible; recent progress allows us to relate the critical exponents directly to the nature of the surface forces which lead to the different wetting scenarios. Thermal fluctuation effects, which can be greatly enhanced for wetting of geometrically or chemically structured substrates, and are much stronger in colloidal suspensions, modify the adsorption singularities. Macroscopic descriptions and microscopic theories have been developed to understand and predict wetting behavior relevant to microfluidics and nanofluidics applications. Then the dynamics of wetting is examined. A drop, placed on a substrate which it wets, spreads out to form a film. Conversely, a nonwetted substrate previously covered by a film dewets upon an appropriate change of system parameters. The hydrodynamics of both wetting and dewetting is influenced by the presence of the three-phase contact line separating "wet" regions from those that are either dry or covered by a microscopic film only. Recent theoretical, experimental, and numerical progress in the description of moving contact line dynamics are reviewed, and its relation to the thermodynamics of wetting is explored. In addition, recent progress on rough surfaces is surveyed. The anchoring of contact lines and contact angle hysteresis are explored resulting from surface inhomogeneities. Further, new ways to mold wetting characteristics according to technological constraints are discussed, for example, the use of patterned surfaces, surfactants, or complex fluids.
Spreading of a wetting film under the action of van der Waals forces
Physics of Fluids, 2000
The profiles of a spreading wetting film are computed using a variable grid implicit scheme. The form of Tanner's law is deduced from the scaling, and the dependence of its coefficient on ratio of the van der Waals to capillary length and on the inclination angle is determined.
Dynamics of wetting: from inertial spreading to viscous imbibition
Journal of Physics: Condensed Matter, 2009
We report the influence of the nature of boundaries on the dynamics of wetting. We review some work recently published and highlight new experimental observations. Our paper begins with the spreading of drops on substrates and demonstrates how the exponents of the spreading laws are affected either by the surface chemistry or by the droplet shape. We then discuss the imbibition of completely and partially wetting fluids into channels and over microtextured surfaces. Starting with the one-dimensional imbibition of completely wetting liquids in tubes and surface textures, we show that (i) shape variations of channels change the power-law response of the imbibition and (ii) the geometrical parameters of a surface roughness change the spreading behavior. For partially wetting fluids, we observe directionally dependent spreading: polygonal wetted domains can be obtained. We conclude with a tabular summary of our findings, allowing us to draw connections between the different systems investigated, and shed light on open questions that remain to be addressed.
Current Opinion in Colloid & Interface Science, 1997
Recent advancements in experimental studies of wetting phenomena have helped to bridge the gap between the progress made in theory and simulation over the past decade, and the experimental evidence or verification of the theoretical predictions. These developments include new measurements of the equilibrium thickness of precursor wetting films on solid and liquid substrates and at the liquid/gas interface, experimental studies of critical adsorption, as well as measurements of the dynamics of wetting and spreading and the nucleation of wetting layers in simple and complex systems. There have also been some recent results on dewetting of solid substrates by liquid films. Addresses ·'wan-N.
Kinetics of Wetting and Spreading of Droplets over Various Substrates
Langmuir : the ACS journal of surfaces and colloids, 2017
There has been a substantial increase in the number of publications in the field of wetting and spreading since 2010. This increase in the rate of publications can be attributed to the broader application of wetting phenomena in new areas. It is impossible to review such a huge number of publications; that is, some topics in the field of wetting and spreading are selected to be discussed below. These topics are as follows: (i) Contact angle hysteresis on smooth homogeneous solid surfaces via disjoining/conjoining pressure. It is shown that the hysteresis contact angles can be calculated via disjoining/conjoining pressure. The theory indicates that the equilibrium contact angle is closer to a static receding contact angle than to a static advancing contact angle. (ii) The wetting of deformable substrates, which is caused by surface forces action in the vicinity of the apparent three-phase contact line, leading to a deformation on the substrate. (iii) The kinetics of wetting and sprea...
Short-Time Dynamics of Partial Wetting
Physical Review Letters, 2008
When a liquid drop contacts a wettable surface, the liquid spreads over the solid to minimize the total surface energy. The first moments of spreading tend to be rapid. For example, a millimeter-sized water droplet will wet an area having the same diameter as the drop within a millisecond. For perfectly wetting systems, this spreading is inertially dominated. Here we identify that even in the presence of a contact line, the initial wetting is dominated by inertia rather than viscosity. We find that the spreading radius follows a power-law scaling in time where the exponent depends on the equilibrium contact angle. We propose a model, consistent with the experimental results, in which the surface spreading is regulated by the generation of capillary waves.
Thin films in wetting and spreading
Advances in Colloid and Interface Science, 2003
Thin films differ from bulk phases both from a thermodynamic point of view, i.e. the chemical potential of a molecule in a film depends on the film thickness, and in their dynamical response, because relaxation times may become large in these confined media. Examples of slow structural relaxation in thin films of liquid crystals, and their consequences on the wetting properties of the systems are presented first. Then, we illustrate the thermodynamic specificity of thin films when evaporation is considered. The consequences on the wetting dynamics of macroscopic evaporating droplets are presented in the last part of the paper. ᮊ
Wetting kinetics in forced spreading
2016
Under dynamic conditions, the dynamic contact angle (the angle that the liquid makes with the solid) of a liquid on a solid surface varies dramatically with substrate velocity from its equilibrium value. Experimental data of the dynamic contact angles for polydimethylsiloxane (PDMS or silicone oil) under air on a glass substrate coated with teflon, for water under PDMS, for solutions of the polymer polyethylene oxide (PEO) under air, for solutions of PEO under PDMS, were obtained to simulate and understand the systems of brine or brine containing a polymer displacing viscous crude. A variety of solid substrates were used other than above to displace oil with the object that the equilibrium contact angles ~ 90º. The method used was that of a plate immersed or withdrawn from a pool of liquid, and the machine (Cahn-Thermo) calculates for us the dynamic advancing and receding contact angles. The dynamic contact angles determine the basic driving forces such as capillary pressures. The data were correlated with a number of available models. In most cases, the models were developed further to fit the requirements of various cases. In general, it is necessary for the model to include fluid flow, interfacial phenomena, and rheology. Photography was used to verify cases of entrainment and instability. One object of the present work was to determine the contribution of the non-Newtonian nature of the PEO solution. For the PEO solution under oil (PDMS) no obvious signs are observed although solutions at high polymer concentrations, that is, high elasticity, show some anomalous effects. However, it is not possible to conclude that shear thinning effects will be absent in all cases since a criterion is established here that shows under what condition the above may not hold. I am thankful to Elmergib University, the Higher Education Ministry in Libya and Canadian Bureau for International Education (CBIE) for their support through the Libyan-North American Scholarship Program (LNASP). In the end, I could not express my deepest gratitude to my parents, Salma and in memory of Mohammad, and my beloved wife, Haifa, for their unconditional support, understanding, love, and encouragement. vi
Dynamic wetting and spreading and the role of topography
Journal of Physics: Condensed Matter, 2009
The spreading of a droplet of a liquid on a smooth solid surface is often described by the Hoffman-de Gennes law, which relates the edge speed, v e , to the dynamic and equilibrium contact angles θ and θ e by v e ∝θ(θ 2 -θ e 2 ). When the liquid wets the surface completely and the Figure 1 Changes in interfacial areas as the leading edge of a droplet advances by a small area, ∆A(x), across a) a rough surface and b) a composite surface of two types.