Green algae scatter off sharp viscosity gradients

It is quite common to find neighbouring regions of different viscosity in a variety of places in nature, such as biofilms or the female reproductive tract. For this reason, microorganisms can change the way they swim in the presence of steep viscosity changes to move away or towards regions that are more favourable to them. Recently, many efforts have been made in an attempt to develop a theoretical framework to predict where microswimmers would concentrate based on their flow fields (pushers/pullers), but no extensive experimental investigation has been carried out to confirm predictions so far. In order to investigate this phenomenon we use a microfluidics device which allows to generate two adjacent regions of significantly different viscosity and uniform concentration of microorganisms throughout the chamber.
We then investigate how the behaviour of Chlamydomonas Reinhardtii (puller) compare in the two regions, as well as how they approach the viscosity interface and cross it. Lastly, we measure how the concentration profile of the swimmers changes over time in the microfluidics device, leading to results not immediately obvious from previously published theoretical frameworks.