Spreading droplets of yield-stress fluids with and without gravity

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 study the dependence of the final droplet shape on yield stress and gravity. Droplets are deposited on a thin film of the same material, mimicking a fully wetting surface, and allowing to directly test scaling laws derived from the thin-film equation for viscoplastic fluids, in this limit. Microgravity conditions allow us to vary the two relevant dimensionless numbers independently, 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 final droplet shapes. Simulations using a viscoelastic 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.