— REPORTÉ (Covid) — 2/10/20 – M. Jalaal – Viscoplastic droplets

Séminaire IUSTI – 2 oct. 2020 – 11h salle 250

Viscoplastic droplets

Mazi Jalaal – DAMTP, Cambridge, UK

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