The spreading behaviour of capillary driven yield-stress drops

Surface tension driven spreading of viscoplastic drops after impact on a solid substrate is investigated experimentally and compared with purely shear-thinning drops. It is shown that the spreading rate is affected independently both by the degree of fluid shear-thinning and by the yield-stress magnitude. Moreover, spreading can be inhibited when the yield-stress becomes greater than a critical value, preventing drops from assuming a spherical cap shape with a static contact angle consistent with the Young–Laplace equation. This critical value can be related to a transition between capillary governed and viscoplastic governed behaviour, with the regime change characterised by the ratio of capillary pressure to yield-stress magnitude.