(132b) On Plastocapillarity

Many problems in engineering and geophysics involve the free-surface flow of yield stress fluids. Although capillary forces are negligible at large scales (e.g., in lava flows and landslides), they may become significant at small scales (e.g., in coating polymeric materials and 3D printing). We demonstrate how capillary forces can plastically deform yield stress (elasto-viscoplastic) materials on small scales. In particular, we consider the effect of yield stress on droplet dynamics. Experiments, asymptotic solutions, and numerical simulations are employed to explain the droplet dynamics and their final equilibrium shapes. Additionally, a new technique is proposed to externally control the shape of a spreading droplet using temperature-dependent rheology and solidification. Finally, we discuss the general importance of plastocapillarity, when yield-stress fluids are driven by surface tension, in design and manufacturing at small scales.