Microfluidic devices involving small droplets or bubbles flowing along narrow channels are becoming increasingly important, not least in the context Covid-19 testing. However there is a need to increase throughput in such devices to facilitate rapid diagnostic testing. Despite the physical forces governing microfluidic flows (namely driving pressure, viscous drag and capillary suction) all being well characterised, the way in which trains of droplets or bubbles flow remains poorly understood. The reason is that droplets and bubbles stack into microfluidic channels with very particular topologies. Certain topologies arrange the droplets or bubbles in a symmetric fashion, meaning they can propagate through channels very quickly. Other topological arrangements are however asymmetric, and while these structures can propagate slowly, they tend to break up if driven too quickly. Worse still, an individual topological break up or topological rearrangement need not necessarily stabilize a flowing system. Instead it might just lead to a cascade of short-lived, topologically unstable states before the train of droplets or bubbles eventually settles into a topologically stable configuration. Moreover the topological rearrangement sequence which a given system selects and the eventual stable state which it finds might depend on the rate at which the system is driven. To this extent a microfluidic device behaves in a fashion analogous to a particle accelerator such as the large hadron collider: driving the system quickly enough can lead to new topological states and new decay sequences.
This project under the supervision of Dr Paul Grassia will focus on foam motion through narrow channels as a paradigm for droplet and bubble microfluidic flows. It will develop the mathematical models needed to describe foam bubbles moving rapidly along such channels, and implement these models in a computer simulation program. The simulation results will be used to design flowing microfluidic systems, optimising bubble size relative to channel size, whilst positioning bubbles both geometrically and topologically to maximise throughput, whilst simultaneously driving the foam towards a stable configuration. Effectively what the project will achieve therefore is to establish design principles for a 'foam hadron collider'.
The project results will find application not just in microfluidic medical diagnostics, but in other fields besides. For example carbon capture and storage systems can enclose sequestered carbon dioxide within foam bubbles, making CO2 less likely to escape from geological formations into which it is injected. Foams flowing through channels within porous soils can in addition be used to clean contaminated land and/or protect aquifers from contamination. Moreover in the chemical industry, multiphase microfluidic reactors can bring reactants carried by different droplets or bubbles into contact for precise periods of reaction time.
In addition to undertaking cutting edge research, students are also registered for the Postgraduate Certificate in Researcher Development (PGCert), which is a supplementary qualification that develops a student’s skills, networks and career prospects.
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