Oxford Physics: Soft and Biological Matter

Julia Yeomans FRS

Professor of Physics
Pauline Chan Fellow in Physics, St Hilda's College

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Swimming at low Reynolds number

Bacteria and other swimming microorganisms are examples of active systems, relying on processes that provide their own energy. Hence they represent a novel state of matter, existing and operating out of thermodynamic equilibrium. Other examples of active systems range from transport in living cells to driven granular matter and strongly perturbed fluids. Active systems have many common features, for example pattern formation, instabilities leading to chaotic behaviour, and defect formation. Novel theoretical tools, based on hydrodynamics and non-equilibrium statistical physics, are needed to identify which properties of active systems are generic, and to understand and predict their behaviour.

Microorganisms have evolved to swim in complex environments. Often they move in confined spaces, and in fluids crowded by inert particles such as colloids and biopolymers. Our aim is to understand how their motion and swimming strategies are affected by details of their surroundings, and how the environment is, in turn, perturbed by the motion of the swimmers.

As the swimmer moves past particles (dotted) follow
loops (red) to their final positions on the blue line.
Tracers close to the swimmer are entrained.
E-coli, swimmers that run and tumble

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Collective behaviour of active systems

Active systems are collections of particles that produce their own energy, such as bacterial suspensions and living cells. Hydrodynamic instabilities are inherent to active fluids, and active suspensions spontaneously generate complex flow patterns at length scales much larger than the actual constituents. These are characterised by strong variations in vorticity. Such turbulent-like patterns have been observed in experiments on mixtures of actin or microtubules and motor proteins designed to isolate the important ingredients leading to cellular motility. Very similar structured flows have been observed, at larger length scales, in dense suspensions of swimming E-coli. We are studying continuum theories to help isolate the generic features of flow in active systems and to understand their relation to cell motility.

Flow in an active nematic: the shading represents the vorticity field and the lines are streamlines of the flow

Velocity probability distribution function for a tracer in a suspension of swimmers for increasing number of swimmers, N (log-log scale). The power law persists to surprisingly large N. This is because swimmers are self propelled. This means that no net force can act on them and hence their velocity field is dipolar, decaying as 1/r2. Because of the strength of the divergence at small r the central limit theorem does not hold and the usual crossover to a Gaussian for increasing N does not occur.

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Liquid crystal colloids

Colloidal suspensions share many of the properties of molecular liquids. However the colloids are typically a few microns in size, allowing individual particles to be observed by techniques such as conformal microscopy. Hence colloids have proved useful in studying a wide range of liquid physics, such as crystallization, glass formation, phase transitions, and interface fluctuations on a single particle level.

The majority of research has focused on spherical colloids as a model system for simple liquids. However, both natural and fabricated anisotropic colloids are now available: these provide model systems mirroring the behaviour of more complicated liquids. An example is the fd virus which has a contour length of 0.88 microns and a diameter of 6.6nm. Because they are long and thin the viruses form liquid crystal phases, such as the nematic.

With Dirk Aarts' group in chemistry we are carrying out a joint experimental and theoretical study of the nematic phase of the fd virus in structured microchannels. We have excellent agreement between simulation and experiment for liquid crystal configurations in microscopic wedges. The next step is to study the hydrodynamics of the nematics and of their interfaces with an isotropic phase.

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Wetting on micropatterned surfaces

If a hydrophobic surface is patterned with micron scale posts it can become superhydrophobic. The surface is strongly water repellent, and drops slide off very easily. There are many examples where nature has exploited superhydrophobic designs, for example, on the surfaces of leave to aid the run-off of rainwater. We are using analytic and numerical approaches to understand the pinning and dynamics of drops on micro-patterned surfaces.

Simulations (above) and experiments (left) showing a fluid spreading through a lattice of triangular microposts.
Anisotropic imbibition through slanted microposts
Click here to view a computer simulation of a drop on a superhydrophobic surface

Click here to read about pancake bouncing on a superhydrophobic surface.

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