My biological physics research is on liquid/liquid phase separation in cells, and on the dynamics of proteins and RNA, in both cases inside cells.

Liquid droplets inside cells

We have known for a while that what looks very much like liquid/liquid phase separation occurs inside living cells, and that the  formation of liquid droplets inside cells is at least correlated with function. For example, the wonderfully named protein Dishevelled can form liquid droplets inside cells, at least when biologists increase its concentration in a cell. These droplets are dynamics, molecules leave and enter them, and two droplets can coalesce. And their formation is regulated.

Here is a simple simulation in which I turn on a chemical reaction about a third of the way through, and then accelerate this reaction two thirds of the way through. We have known since work of Glotzer and co-workers in the 1990s that a chemical reaction that cycles molecules through two states, can stop the growth of droplets, and you can see this in the movie above.

We still have very little idea of how many of these droplets contribute to the function of cells, or how cells regulate them.

Over the last ten years, this field has really taken off with droplets of many different types being observed. Although both in a simulation and in a microscope, the liquid-like tendency to flow is quite striking, it is not at all clear that cells/evolution are taking advantage of the ability to flow, it may be that it is more the cooperative, switch-like, nature of liquid/liquid phase separation that is of biological relevance. See here for a preprint where I discuss this.

Transport in cells

The complex viscous energy-consuming fluid inside cells is very far from uniform, and very far from thermodynamic equilibrium. In a Physical Review Letter (arXiv), I suggest that larger particles (about 100 nm across) inside cells may move around in cells at least partly by surfing these gradients. Motion of particles due to concentration gradient is called diffusiophoresis, which is quite topical at the moment.

Older work

Modelling diffusion inside cells of the protein whose absence causes Duchenne muscular dystrophy

I have collaborated with muscle cell biologists on modelling the dynamics of the protein Dystrophin inside live muscle cells. Dystrophin is a medically important protein, the absence of this protein causes the genetic disease Duchenne Muscular Dystrophy. This work was published in eLife in October 2015. This site hosts the app used there. There is a news release on this work.

I was co-chair of the CECAM workshop ‘The Self-Organised Cytoplasm‘ in July 2014 (report here).