Njord Seminar with Mazi Jalaal

Mazi Jalaal (UvA)Free surface flow of yield stress materials on small scales


A liquid droplet or filament 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. We experimentally show how the yield stress dictates the final shape of gently deposited yield stress drops and filaments on a surface.

We first present the results of a single drop and filament deposition on a pre-wetted surface for a range of yield stress and the deposition flow rate. We show how the final width of the filament or the radius of the droplet decreases as the yield stress values increase. In addition to the experiments, we present a counterpart model 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. We find a fair comparison between the experiments and the theory. However, discrepancies have also been observed. We will discuss the possible origin of these discrepancies.

Towards the end of the talk, we will present the dynamics and final shape of two viscoplastic filaments. We show when the distance between the filaments is smaller than a critical value, they merge. The final shape of these coalesced filaments is governed by multiple factors, mainly the competition between yield stress and surface tension. We will systematically explore this final shape as a function of control parameters and provide a theoretical counterpart to explain their main geometrical features.


You will find the complete schedule for Njord Seminar Series spring '22 here.

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Published Apr. 25, 2022 3:08 PM - Last modified Apr. 25, 2022 3:08 PM