On the Dynamic Contact Angle of Capillary-Driven Microflows in Open Channels.

The true value of the contact angle between a liquid and a solid is a thorny problem in capillary microfluidics. The Lucas-Washburn-Rideal (LWR) law assumes a constant contact angle during fluid penetration. However, recent experimental studies have shown lower liquid velocities than those predicted by the LWR equation, which are attributed to a velocity-dependent dynamic contact angle that is larger than its static value.

On the dynamic contact angle of capillary-driven microflows in open channels.

The true value of the contact angle between a liquid and a solid is a thorny problem in capillary microfluidics. The Lucas-Washburn-Rideal (LWR) law assumes a constant contact angle during fluid penetration. However, recent experimental studies have shown lower liquid velocities than predicted by the LWR equation, which are attributed to a velocity-dependent dynamic contact angle that is larger than its static value.

Young's equation at the nanoscale.

In 1805, Thomas Young was the first to propose an equation to predict the value of the equilibrium contact angle of a liquid on a solid. Today, the force exerted by a liquid on a solid, such as a flat plate or fiber, is routinely used to assess this angle. Moreover, it has recently become possible to study wetting at the nanoscale using an atomic force microscope.

Toward a predictive theory of wetting dynamics.

The molecular kinetic theory (MKT) of dynamic wetting, first proposed nearly 50 years ago, has since been refined to account explicitly for the effects of viscosity and solid-liquid interactions. The MKT asserts that the systematic deviation of the dynamic contact angle from its equilibrium value quantitatively reflects local energy dissipation (friction) at the moving contact line as it traverses sites of solid-liquid interaction. Specifically, it predicts that the coefficient of contact-line friction ζ will be proportional to the viscosity of the liquid ηL and exponentially dependent upon the strength of solid-liquid interactions as measured by the equilibrium work of adhesion Wa(0).

Influence of solid-liquid interactions on dynamic wetting: a molecular dynamics study.

Large-scale molecular dynamics (MD) simulations of liquid drops spreading on a solid substrate have been carried out for a very wide range of solid-liquid interactions and equilibrium contact angles. The results for these systems are shown to be consistent with the molecular-kinetic theory (MKT) of dynamic wetting, which emphasizes the role of contact-line friction as the principal channel of energy dissipation. Several predictions have been confirmed.

Nonlocal hydrodynamic influence on the dynamic contact angle: slip models versus experiment.

Experiments reported by Blake [Phys. Fluids., 11, 1995 (1999)] suggest that the dynamic contact angle formed between the free surface of a liquid and a moving solid boundary at a fixed contact-line speed depends on the flow field and geometry near the moving contact line.

The physics of moving wetting lines.

Scientists tend to think in terms of their most familiar models. It is not accidental that the earliest descriptions of the moving wetting line and its associated dynamic contact angle were in terms of displaced equilibria (chemists), friction (physicists) and viscous bending of the liquid-vapour interface (engineers and mathematicians). Each of these approaches has progressed since its inception, but, while each reflects a different facet of the underlying physical mechanism, and each offers at least a semi-empirical route to its description, none is complete.

Wetting at high capillary numbers.

The coating of liquids onto solids is an important industrial process. A prerequisite for successful coating is that the liquid dynamically wet the surface of the solid. One of the limits to high-speed coating is the onset of dynamic wetting failure, which leads to air entrainment.