The trapping mechanism of microdrops in wells of surface energy

Aqueous microdrops are tiny containers that allow chemical reactions, cell studies, or drug screening at high throughput within very small volumes. The evolution of individual drops can be monitored over time only if they are immobilized on a microfluidic chip, for instance using gradients of confinement. In this configuration, drops are squeezed into a microfluidic channel with height smaller than the diameter of the drop, thereby increasing their surface area. When they meet a spot with locally increased channel height they can partially relax their surface energy. This energy difference leads to a trapping force able to oppose the drag force applied by a flow in the continuous phase and keeps the drop immobilized. However, the mechanisms that lead to the removal of the drop from its trap are poorly understood. In this project we use the simulation software Surface Evolver to simulate equilibria positions of drops at rest. We then apply a pressure drag from the continuous phase, and find the critical pressure gradient at which the drop is removed from its trap. The aim is to find how the critical pressure gradient is influenced by the trap geometry (size, depth, shape).

In this project, you will learn how to use Surface Evolver to simulate confined microdrops under flow and how to read out critical parameters. You will then compare these findings to experimental values found by our collaborators with the final objective of derivating general laws and describe the physical mechanisms of trapped microdrops in wells of surface energy.

Figure 1: a) Surface Evolver simulation of a drop in a well of surface energy at rest. b-c) A pressure gradient is applied and gradually increased, deforming the drop. d) The drop is finally removed from its trap when the pressure gradient exceeds its critical value.

Supervisors: Hervé Elettro (STI-IGM-LFMI) and Michael Kessler (STI-IMX-SMaL).

Keywords: Microfluidics, two phase flow, surface tension, numerical simulations