Yield stress (a.k.a Viscoplastic) fluids such as emulsions, suspensions, gels, or foams feature peculiar mechanical properties. Below the yield stress, they behave like soft elastic solids, but once sufficiently stressed, they flow like a (nonlinear) viscous liquid. Many problems in engineering and geophysics feature the free surface flow of a yield stress fluid. Although negligible at large scales (e.g. lava flows and landslides), capillary forces may become significant at small scales (e.g. coating polymeric materials and 3D printing). I refer to the phenomena in this regime as Plastocapillarity, which is the focus of this Talk.

I will present a series of problems involving droplets and bubbles, e.g., bubble motion in tubes and droplet spreading. Some of these problems, like the spreading of a droplet or a filament on a surface, allow for asymptotic solutions. Some others, like a bubble bursting on a surface, are too complicated (for us); hence we use direct numerical simulations to solve the governing equations.

In the end, I will discuss the general importance of plastocapillarity (when yield-stress fluids are driven by surface tension) in design and manufacturing at small scales.