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.

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Wetting and Spreading

This paper presents results of an experimental investigation of a single drop impact onto a dry, partially wettable substrate and its numerical simulation Particularly, the drop spreading diameter and the dynamic contact angle are measured at different time instants after impact Two surfaces, wax (low wettability) and glass (high wettability), are used to study the effect of surface wettability (static contact angle) on the impact dynamics It is shown that existing empirical models for the dynamic contact angle (eg, Hoffman–Voinov–Tanner law) do not predict well the change of the dynamic contact angle, especially at high capillary numbers In addition to the experimental investigations, the drop impact was studied numerically, focusing primarily on the contact angle treatment The singularity in the neighborhood of the moving contact line is removed from the computational domain and replaced by a local force with some dependence on the instantaneous advancing/receding contact-line velocity The predicted time dependence of the drop spreading diameter and of the dynamic contact angle agrees well with the experimental data for both the advancing and receding phases of the impact process

Dynamic contact angle of spreading droplets: Experiments and simulations

Bacterial infection of biomaterial surfaces is an important problem in the biomedical and health industries. The design of materials resistant to infections necessitates an understanding of the forces driving bacterial adhesion. Escherichia coli cells were immobilized onto the tip of a standard atomic force microscope (AFM) cantilever, and force measurements were performed by approaching the modified cantilever onto mica, hydrophilic glass, hydrophobic glass, polystyrene, and Teflon. Consistent with prior qualitative observations, we show that bacterial adhesion is indeed enhanced by the surface hydrophobicity of the substrate. The forces of interaction measured with the AFM are compared to those of model predictions based on an extended-DLVO approach. In this model, short-range acid−base and steric interactions are included with the conventional van der Waals attraction and electrostatic components. The theoretical predictions agree well with experimental data for E. coli D21f2, a strain whose outer surf...

Adhesion Forces between E. coli Bacteria and Biomaterial Surfaces

Staphylococcus aureus is a key bacterium commonly found in numerous infections. S. aureus infections are difficult to treat due to their biofilm formation and documented antibiotic resistance. While selenium has been used for a wide range of applications including anticancer applications, the effects of selenium nanoparticles on microorganisms remain largely unknown to date. The objective of this in vitro study was thus to examine the growth of S. aureus in the presence of selenium nanoparticles. Results of this study provided the first evidence of strongly inhibited growth of S. aureus in the presence of selenium nanoparticles after 3, 4, and 5 hours at 7.8, 15.5, and 31 μg/mL. The percentage of live bacteria also decreased in the presence of selenium nanoparticles. Therefore, this study suggests that selenium nanoparticles may be used to effectively prevent and treat S. aureus infections and thus should be further studied for such applications.

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Jill E. Seebergh

Papers

Evidence of a hairy layer at the surface of polystyrene latex particles

Dynamic wetting in the low capillary number regime

A comparison of force and optical techniques for the measurement of dynamic contact angles

The effect of organic cosolvent on the aggregation stability of an aqueous polystyrene latex dispersion