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Microflow in Complex Environments

Final Report Summary - MICE (Microflow in Complex Environments)

The research carried out under the grant MiCE focused on simple, complex and active fluids concentrating on mesoscopic length scales in the range 50 nm to 1 mm. The properties of fluids on such scales control or contribute to a huge range of physical phenomena and systems, and our research activities are relevant inter alia to microfluidic engineering, flow in porous media, oil recovery, soil drainage, smart fabrics, microreactors and bacterial motility. Moreover they represent a fascinating interplay between equilibrium or non-equilibrium statistical physics, low Reynolds number hydrodynamics, rheology, and biological physics. I shall focus on two highlights of our work, the identification of a new way in which droplets can bounce, and evidence for the relevance of topological defects in biology.

A dense suspension of bacteria shows turbulent-like behaviour. The velocity field is continuously changing, with swirls and jets forming and decaying. Very similar flow fields are seen in other such active systems, on widely varying length and time scales, from suspensions of microtubules
and molecular motors, to agitated granular matter, schools of fish, and flocks of birds. Normally turbulence is a consequence of inertia, absent at low Reynolds numbers, and such ‘active turbulence’ needs a different explanation. We have argued that the formation and decay of motile topological defects underlies the turbulent patterns in active materials.

The removal of cells from a tissue occurs regularly. Not only are damaged or dying cells removed, but the process of cell extrusion can prevent regions from becoming overcrowded. This is particularly important during developmental processes when tissues and organs are being formed, and also relevant to diseases such as cancer, when tumors grow uncontrollably. Despite the importance of cell extrusion in development and aging, as well as the pathological importance in cancer progression, the cues that flag a cell for removal are poorly understood.

In collaboration with colleagues in Paris and Singapore we have studied single-layers of epithelial cells grown in the lab and, by comparing to numerical models, we have found that a major factor driving cell death and removal is the physical arrangement of cells in the surrounding cell layer. In particular, the appearance of topological defects in the cellular patterns of epithelial layers promotes cell death and elimination from the tissues. The reasons for this are not completely understood, but our results suggest that the additional pressure at the topological defects leads to chemical signalling which initiates the cell death.

In collaboration with Professor Zuankai Wang's group at the City University of Hong Kong we showed that millimetric water droplets can bounce off a superhydrophobic surface, in a pancake shape (see The bouncing occurs because fluid is pushed between the posts by the inertia of the falling drop. The posts are water-repellent, so the liquid is slowed and then pushed back out again. This acts as a spring so the drop takes off before it has time to retract across the surface. For there to be enough energy to lift the drop the distance between the posts must be at least 0.1mm. This is much larger than conventional superhydrophobic surfaces the reason why pancake bouncing has not been seen before. The time for bouncing is much smaller on pancake substrates than on conventional surfaces, and the novel surfaces may find uses in condensers, for dropwise heat removal, or to resist rain or ice.

As a result of this work undergraduate students at the University of Roskilde (Denmark) carried out a project supervised Professor Tina Hensher studying a scaled up version of pancake bouncing: a balloon impacting a bed of nails. We found close similarities between the behaviour of the balloon and the water droplets (see "Pancake bouncing balloons").