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In several biological systems, such as bacterial cytoplasm,
cytoskeleton-motor complexes and cell nuclei, self-propulsion or
activity is found to fluidize a glassy state that exhibits
characteristic glassy features in the absence of activity. To develop a
theoretical understanding of this activity-induced non-equilibrium glass
transition, we have studied, using molecular dynamics and Brownian
dynamics simulations, the effects of activity in two model glass forming
liquids: the Kob-Andersen binary mixture in three dimensions and a
two-dimensional system of two kinds of dumbbells. Activity is introduced
by assuming that some of the particles experience a random active force.
Our studies differ from earlier studies of activity-induced fluidization
in that we consider the liquid-glass phase boundary in the
(temperature-activity) plane, rather than in the (density-activity)
plane (the temperature refers to that of a heat bath in contact with the
active system).
We find that the introduction of activity dramatically reduces the glass
transition temperature and the glass transition disappears beyond a
threshold value of the activity. Some of the effects of activity on the
dynamics in the liquid state are determined by an "active temperature"
that adds to the bath temperature. However, several properties of the
"active" supercooled liquid, obtained as the glass transition is
approached by reducing the activity at a low temperature, are found to
be qualitatively different from those of the "thermal" supercooled
liquid obtained as the glass transition is approached by lowering the
temperature at low activity. We present simple analytic arguments, based
on a heuristic Langevin description of the dynamics of a particle in the
cage formed by its neighbors, which provide a rationalization of some of
the features of the dynamics observed in our simulations.
This work was carried out in collaboration with R. Mandal, P. J. Bhuyan,
M. Rao and P. Chaudhuri. |