Final Report Summary - PHYSAPS (The Physics of Active Particle Suspensions)
Active matter is a relatively new field of physics. It studies active 'agents' that can utilise energy from the environment to grow, divide and move. This project concentrates on motility. Unlike the case of passive ‘agents’ like molecules, there is currently no general theoretical framework for making sense of the collective behaviour of active ‘agents’. As in other areas of science, progress requires high quality experimental data. A prerequisite for collecting such data is the availability of very well characterised experimental systems, so called ‘models’, with which to perform scientific investigations. The overarching goal of this project is to generate well-characterised model systems for experimenting with one class of active matter: suspensions of self-propelled micro-swimmers in liquids, and then use these systems to probe how micro-swimmers behave individually and collectively.
On the individual level, our work has helped establish that the propulsion mechanism of a class of synthetic micro-swimmers based on half coating plastic beads with metallic catalysts is very much more complicated than hitherto suspected; explaining how these synthetic particles manage to propel themselves is still an active field of research today.
On the other hand, perhaps surprisingly, we have been able to establish a living organism, swimming Escherichia coli bacteria, as a well characterised model system for studying active matter physics. Careful work has allowed us to 'tame' the various foibles of these cells, so that we now have a protocol whereby a sample of E. coli can be maintained to swim at constant speed for hours: clearly a precondition for careful experiments. We have also perfected a technique, ‘differential dynamic microscopy’, that allows us to capture the swimming behaviour of tens of thousands of cells simultaneously in different regions of a large sample, and follow their speed as a function of time.
Using this technique, we have been able to probe many fascinating behaviour. For example, we find that adding polymers – long-chain molecules – to water, which hugely increases the liquid’s viscosity, does not seem to affect the swimming speed of bacteria up to quite high concentrations. This overturns a 50-year-old belief, based on using impure polymers, that adding polymer actually increases the swimming speed of bacteria, and has implications for understanding how pathogens in the gut may invade its mucus lining.
In a separate development, we have constructed a strain of E. coli bacteria that swims only when illuminated with green light, and stops swimming immediately when the light is switched off. Using this new strain, we have been able to give experimental proof of one of the very few exact results in theoretical active matter physics, namely, that microswimmers should accumulate where they swim slowest. This apparently intuitively obvious result (think of shoppers on a busy high street) is in fact a startling one for physicists who are used to investigating passive agents, whose density distribution is not allowed to depend on their dynamics. We have also learnt how to use this strain of bacteria to 'paint' pictures in two dimensions - which may form the basis of a new method to self assemble useful structures on the sub-millimetre scale.
A more immediate practical application come from perhaps a surprising quarter. All mammalian life begins with a swimming sperm fertilising an egg. In this project, we have also learnt how to use differential dynamic microscopy to quantify the motility of sperm, and, with help from an ERC Proof of Concept grant, are now applying it to characterise animal fertility.
On the individual level, our work has helped establish that the propulsion mechanism of a class of synthetic micro-swimmers based on half coating plastic beads with metallic catalysts is very much more complicated than hitherto suspected; explaining how these synthetic particles manage to propel themselves is still an active field of research today.
On the other hand, perhaps surprisingly, we have been able to establish a living organism, swimming Escherichia coli bacteria, as a well characterised model system for studying active matter physics. Careful work has allowed us to 'tame' the various foibles of these cells, so that we now have a protocol whereby a sample of E. coli can be maintained to swim at constant speed for hours: clearly a precondition for careful experiments. We have also perfected a technique, ‘differential dynamic microscopy’, that allows us to capture the swimming behaviour of tens of thousands of cells simultaneously in different regions of a large sample, and follow their speed as a function of time.
Using this technique, we have been able to probe many fascinating behaviour. For example, we find that adding polymers – long-chain molecules – to water, which hugely increases the liquid’s viscosity, does not seem to affect the swimming speed of bacteria up to quite high concentrations. This overturns a 50-year-old belief, based on using impure polymers, that adding polymer actually increases the swimming speed of bacteria, and has implications for understanding how pathogens in the gut may invade its mucus lining.
In a separate development, we have constructed a strain of E. coli bacteria that swims only when illuminated with green light, and stops swimming immediately when the light is switched off. Using this new strain, we have been able to give experimental proof of one of the very few exact results in theoretical active matter physics, namely, that microswimmers should accumulate where they swim slowest. This apparently intuitively obvious result (think of shoppers on a busy high street) is in fact a startling one for physicists who are used to investigating passive agents, whose density distribution is not allowed to depend on their dynamics. We have also learnt how to use this strain of bacteria to 'paint' pictures in two dimensions - which may form the basis of a new method to self assemble useful structures on the sub-millimetre scale.
A more immediate practical application come from perhaps a surprising quarter. All mammalian life begins with a swimming sperm fertilising an egg. In this project, we have also learnt how to use differential dynamic microscopy to quantify the motility of sperm, and, with help from an ERC Proof of Concept grant, are now applying it to characterise animal fertility.