Abstract

Viscoplastic or yield stress materials can behave like solids or fluids. Such materials, if not sufficiently stressed, behave like an elastic solid, but once the stress exceeds a critical value (the yield stress), the material deforms like a viscous fluid (typically with a nonlinear viscosity).

We will discuss the effect of the yield stress on spreading droplets. We use experiments, asymptotic solutions, and numerical simulations to explain the dynamics and final shape of the droplets. Later, we will show how one can externally control the shape of a spreading droplet, using temperature. For that, we will first present the rheological properties of a thermo-responsive material that undergoes sol(Newtonian)-gel(yield stress) transition upon heating. Then, we show the final diameter of a thermo-responsive droplet can be controlled by simply changing the surface temperature. In the same part, we introduce an experimental method based on optical coherence tomography to identify the solidified region inside a droplet.

Eventually, we will briefly discuss the other applications of viscoplastic droplets.