Leiden University Richard Green Theoretical soft matter physics
I am doing my PhD at the Lorentz Institute at Leiden University, in the Soft Condensed Matter Theory Group of Vincenzo Vitelli. I took my first degree in Mathematics at the University of Cambridge many years ago and completed the Masters in Physics at Leiden University in 2014. Before returning to academia, I pursued a varied career in business, with roles in mainstream and quantitative finance in government, pharmaceuticals, banking and mining in the UK, Hong Kong, Poland and the Netherlands. Photo

Active matter

Geometry of threshold-less active flow in nematic microfluidics

Active flow in a toroid The liquid crystal in the display of your watch is static: it does not flow as long as time displayed does not change. This is because the long, cigar-shaped molecules of which it is composed are dead. What happens if we put live swimmers into such a medium? This can now be done: non-toxic liquid crystals have been made, and live, swimming bacteria put in them. Does this make the liquid crystal as a whole flow?

The answer to this question has long been known to depend on the configuration of the liquid crystal. In nematic liquid crystals, the molecules "like" (i.e., have lowest energy) if they line up with their axes parallel. In this uniform state, it's already known that the density of swimmers must exceed some non-zero critical density before the liquid crystal will flow.

However, liquid crystal display technology distorts liquid crystals into spatially non-uniform configurations. In some of these, the liquid crystal will start to flow as soon as any non-zero density of swimmers is placed in it; we call this "thresholdless" flow. In others, there continues to be critical threshold density that must be exceeded before flow occurs.

Until now, there has been no general criterion for determining which distortions lead to thresholdless flow, and which do not. In our work, we provide such a general criterion, which will make it possible to design microscopic "pumps" in which the work of pumping is done by bacteria, without human intervention, and without moving parts. We also calculate the flow that occurs for low density of swimmers in such thresholdless conditions.

Active flow in a Freedericksz cell

Paper

Richard Green, John Toner and Vincenzo Vitelli (2016)
The geometry of threshold-less active flow in nematic microfluidics