Contact line dissipation in short-time dynamic wetting

Abstract

Dynamic wetting of a solid surface is a process that is ubiquitous in Nature, and also of increasing technological importance. The underlying dissipative mechanisms are, however, still unclear. We present here short-time dynamic wetting experiments and numerical simulations, based on a phase field approach, of a droplet on a dry solid surface, where direct comparison of the two allows us to evaluate the different contributions from the numerics. We find that an important part of the dissipation may arise from a friction related to the motion of the contact line itself, and that this may be dominating both inertia and viscous friction in the flow adjacent to the contact line. A contact line friction factor appears in the theoretical formulation that can be distinguished and quantified, also in room temperature where other sources of dissipation are present. Water and glycerin-water mixtures on various surfaces have been investigated where we show the dependency of the friction factor on the nature of the surface, and the viscosity of the liquid.

Soft Substrates

“…This observation motivated the investigation of the contact line velocity. The friction parameter μ f 5 0.07 Pa s delivered the best fit for the contact line receding velocities on harder substrates (E 5 1.5 MPa and E 5 0.8 MPa) and is similar to the values found in recent works (Carlson et al, 2012). (12.1), shows the plots of contact line receding velocities of pure water and water-silica suspension drops evaporating on substrates with different Young's moduli.…”

Drop motion induced by vertical vibrations

“…The wettability properties are characterized by the equilibrium contact angle θ E , the advancing contact angle θ A and the receding contact angle θ R . We here briefly introduce the mathematical model with particular attention to the boundary conditions and we refer to [6,21,26,32,33] for a detailed derivation and some applications. The binary system constituted by two pure, immiscible fluids of density ρ A and ρ B , respectively, is described by a scalar function, the phase field x t , ,…”

Universality in dynamic wetting dominated by contact-line friction

“…By integrating experiments and axi-symmetric simulations based on the Cahn-Hilliard Navier Stokes equations [14,15] we estimate values for the friction factor (µ f ) that appears in the free energy formulation. Theoretically, the friction factor generates a local dissipation at the contact line through its boundary condition.…”

“…To obtain the experimentally observed spreading behavior, an additional dissipation at the contact line was necessary through a non-zero µ f [14]. µ f was determined by obtaining a direct agreement between simulations and experiments, enabling a direct measurement of µ f even in the presence of other contributions such as viscosity and inertia [15]. The values for µ f are reported in table I for all the surfaces and viscosities.…”