Anomalous capillary flow of a molten metal – Ageing contact angle*

An anomalous behavior of dynamic contact angle and position of contact line has been recently observed in capillary flow experiments of molten alloys. The experimental setup consists of a combination of a wetting/non-wetting solid substrate with the molten alloy attracted by the wetting wall and simultaneously repelled by the non-wetting substrate. The dynamic receding contact angle experiences a non-monotonic evolution towards its equilibrium value, i.e, it drops below the equilibrium and then it overshoots. The receding contact line moves either gradually or abruptly, in different experiments.

We postulate a novel diffusive boundary condition for the phase field model where the quasi-equilibrium contact angle changes in time, as a result of slow chemical changes at interfaces, which affect interface energies and therefore – the quasi-equilibrium contact angle. We discover two distinct topological paths: one is characterized by a coalescence of liquid-solid contact domains, the other by a contact line separation. The selection of the two paths in the configurational space depends on both contact ageing parameters and viscosity of the melt.