Matthew Dennison

Model elastic network

Active force-dipoles

Cells contain active proteins that can undergo conformational changes driven by catalytic chemical reactions. Experimental studies have shown that, compared to when their activity is suppressed, these proteins can enhance not only their own diffusion coefficients, but also those of passive tracer particles. Although the exact origins of this effect are unclear, various mechanisms have been suggested, e.g. force dipole interactions due to molecular motors which move along filaments and collective hydrodynamic effects caused by active proteins.

My research focuses on using computer simulations to study this enhancement of the particle diffusion coefficients, in order to understand how hydrodynamic flow-fields driven by the motion of active particles can affect the motion of passive particles and to elucidate the important control parameters for the effect.

Model elastic network

Enhancing particle diffusion

In order to investigate the diffusive dynamics in a system that contains active particles, a passive tracer particle and a solvent we use the Multi-Particle Collision Dynamics method (MPCD). In this method, a fluid is modelled as a large number of point-like particles, which interact with each other and with the active and passive particles. Active proteins are modelled as active dumbbell particles, which stochastically cycle between open and closed conformations and act as non-equilibrium fluctuating force dipoles.

Consistent with experiments, we find that the diffusion coefficients of both the passive tracer particle and the dumbbells themselves are enhanced when the dumbbells are active. Our findings show that it is the hydrodynamic interactions induced by the active dumbbells that give rise to this effect. Two key parameters are the density of dumbbells and the strength of the force dipole (i.e. the force with which the dumbbells change conformation). When more dumbbells are present, the effect is increased, in contrast to inactive dumbbells, which crowd the passive particle and reduce its diffusion coefficient. When the force dipole is stronger, the effect is also increased. While the observed effects are not large, their existence indicates that the conformational changes of active proteins could be important for transport in active systems.

Model elastic network