Spreading droplets of yield-stress fluids with and without gravity

Kurzfassung

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, ⁠I, 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.