Dynamics of Three Phase Contact Line Motion in Liquid-Liquid-Solid Systems - Impact of Surface Roughness

Capillary driven liquid-liquid displacement in a system with two immiscible liquids of comparable viscosity was investigated by means of optical high speed video microscopy. New insights into the impact of substrate wettability and roughness on contact line dynamics in liquid-liquid systems are provided. Such systems are involved in many industrial processes, such as flotation, where the more efficient and the quicker the displacement, the smaller are the cost and waste associated with the process. Independently of substrates chemistry and topography, qualitatively different dynamics, in two distinct velocity regimes, were found. Hydrodynamic models apply to the fast stage of initial spreading, while non-hydrodynamic dissipation dominates contact line motion in a final stage at low speed, where the molecular kinetic theory (MKT) successfully captured the dynamics. The MKT model parameter values showed no systematic dependence on substrate wettability or roughness. This unexpected result is interpreted in terms of local contact line pinning and discussed in relation to surface roughness and solid-fluid affinities.