IWNET12
IWNET12
Osmosis in non-equilibrium simulations with active solute particles
Thomas Lion1
1 The University of Edinburgh, UK
Abstract
Osmosis is a fundamental and important process, responsible for physical and biological phenomena ranging
from water desalination to the ow of sap in plants. In the classic osmosis experiment, in which two solutions of
diering concentration are separated by a semi-permeable membrane, an initial imbalance in the solvent chemical
potentials causes solvent to ow from low to high solute concentration. This leads to the well-known osmotic
pressure dierence at equilibrium. We present Molecular Dynamics (MD) simulations of a non-equilibrium
version of this experiment, in which the solute particles are active. This is achieved in two distinct ways:
by coupling the solute particles to a separate, higher temperature thermostat or by propelling them using an
external force, as a model for bacterial or colloidal swimmers. Our simulations allow us to dene a new, osmotic
eective temperature for the active solute particles. Furthermore, for active solutes, our simulations reveal
an interesting and counter-intuitive reverse osmotic phenomenon: the active solute particles can drive solvent
particles out of the more concentrated solution, in direct contrast to osmotic ow in non-active systems. Our
results could have interesting implications for the likely osmotic behaviour of suspensions of active particles
such as motile bacteria or colloidal "swimmers".
E-mail: T.Lion-2@sms.ed.ac.uk

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