Spreading droplets of yield-stress fluids with and without gravity

TL;DR

This study explores how gravity influences the spreading of yield-stress fluid droplets through microgravity experiments, simulations, and scaling laws, revealing the separate effects of surface tension and yield stress on droplet shape.

Contribution

It provides new insights into the role of gravity and yield stress in droplet spreading, using microgravity experiments to independently vary key dimensionless parameters.

Findings

Gravity affects droplet shape depending on yield stress and surface tension.

Microgravity experiments allow independent variation of Bond and plastocapillary numbers.

Simulations show good agreement with experiments, highlighting elastic effects at high yield stress.

Abstract

We investigate the effect of gravity on the spreading of droplets of yield stress fluids, by performing both microgravity experiments (in a drop tower) and experiments under terrestrial gravity. We investigate the dependence of the final droplet shape on yield stress and gravity. Droplets are deposited on a thin film of the same material, allowing to directly test scaling laws derived from the thin-film equation for viscoplastic fluids. Microgravity conditions allow to vary independently the two relevant dimensionless numbers, the Bond number, B, and the plastocapillary number, J, and thus to disentangle the influence of surface tension from that of the yield stress on the droplet shapes. Simulations using a visco-elastic model with shear thinning complement the experiments and show good agreement regarding the droplet shapes. Possible deviations arising in the regime of non-negligible elastic effects and large plastocapillary numbers (large yield stress) are discussed.