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Tuesday 08 Dec 2020Passive & Active Soft Matter: From Viscoplastic Droplets to Glowing Algae

Mazi Jalaal - University of Cambridge Meeting ID: 977 9026 7260 Password: 018209 13:30-14:30

This talk covers two topics on the soft matter of polymeric materials and living marine organisms.

In the first part, we will talk about a class of soft matters called viscoplastic or yield stress materials. If not sufficiently stressed, such materials behave like an elastic solid, but once the stress exceeds a critical value (the yield stress), the material deforms like a viscous fluid. We will discuss the effect of the yield stress on spreading droplets. We use experiments, asymptotic solutions, and numerical simulations to explain the droplets' dynamics and final shape. 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.

In the second part, we will talk about our study on Dinoflagellate Bioluminescence. Bioluminescence (emission of light from living organisms) is a common form of communication in the ocean. Here we study the bioluminescence on a single-cell level, aiming to understand the response to mechanical stimulation. In our experiments, the cell (a dinoflagellate called P. Lunula) was immobilized via micro-pipette aspiration. We impose pressure on the cell via a submerged impinging jet. We show that the flow-induced stress on the cell membrane results in a local elastic deformation. As a result, a series of chemical signalling events occur that eventually yields to light production in sub-cellular compartments. Besides experiments, we propose a counterpart model based on linear ODEs that includes the membrane's viscoelasticity.

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