Mazi Jalaal (Univ Amsterdam) - Yield Stress Fluids under Tension

A liquid droplet spreads on a surface until it reaches equilibrium. In the case of yield stress (aka viscoplastic) fluids, the dynamics and the final static shape are determined by the surface tension, gravity, solid surface properties, and the liquid's rheological properties. Using experiments, simulations, and theory, we show how the yield stress dictates the final shape of gently deposited yield stress drops and filaments on a surface.

We show how the filament's final width or the droplet's radius decreases as the yield stress values increase. In addition to the experiments, we present numerical simulations of the problem, solving the momentum equations furnished by regularised Bingham rheological constitutive model. We also discuss a counterpart asymptotic solutions based on viscoplastic lubrication theory and find a power-law scaling for the final width of the filament as a function of the plastocapillarity number.

Learning from viscoplastic spreading, we will propose a new technique to externally control a spreading droplet's shape using temperature-dependent rheology. The latter relates to the problem of droplet solidification, where (using direct experimental observations) we show that at a slow-spreading regime, the contact line arrest determines the final radius of the droplet.