18 research outputs found
Pressure is not a state function for generic active fluids
Pressure is the mechanical force per unit area that a confined system exerts
on its container. In thermal equilibrium, it depends only on bulk properties
(density, temperature, etc.) through an equation of state. Here we show that in
a wide class of active systems the pressure depends on the precise interactions
between the active particles and the confining walls. In general, therefore,
active fluids have no equation of state, their mechanical pressures exhibit
anomalous properties that defy the familiar thermodynamic reasoning that holds
in equilibrium. The pressure remains a function of state, however, in some
specific and well-studied active models that tacitly restrict the character of
the particle-wall and/or particle-particle interactions.Comment: 8 pages + 9 SI pages, Nature Physics (2015
Scalar <i>Ï</i><sup>4</sup> field theory for active-particle phase separation
Recent theories predict phase separation among orientationally disordered
active particles whose propulsion speed decreases rapidly enough with density.
Coarse-grained models of this process show time-reversal symmetry (detailed
balance) to be restored for uniform states, but broken by gradient terms; hence
detailed-balance violation is strongly coupled to interfacial phenomena. To
explore the subtle generic physics resulting from such coupling we here
introduce `Active Model B'. This is a scalar field theory (or
phase-field model) that minimally violates detailed balance via a leading-order
square-gradient term. We find that this additional term has modest effects on
coarsening dynamics, but alters the static phase diagram by creating a jump in
(thermodynamic) pressure across flat interfaces. Both results are surprising,
since interfacial phenomena are always strongly implicated in coarsening
dynamics but are, in detailed-balance systems, irrelevant for phase equilibria.Comment: 15 pages, 7 figure