43 research outputs found
Glassy swirls of active dumbbells
The dynamics of a dense binary mixture of soft dumbbells, each subject to an
active propulsion force and thermal fluctuations, shows a sudden arrest, first
to a translational then to a rotational glass, as one reduces temperature
or the self-propulsion force . Is the temperature-induced glass different
from the activity-induced glass? To address this question, we monitor the
dynamics along an iso-relaxation-time contour in the plane. We find
dramatic differences both in the fragility and in the nature of dynamical
heterogeneity which characterise the onset of glass formation - the
activity-induced glass exhibits large swirls or vortices, whose scale is set by
activity, and appears to diverge as one approaches the glass transition. This
large collective swirling movement should have implications for collective cell
migration in epithelial layers.Comment: 13 pages, 11 figure
Logarithmic connections on principal bundles over normal varieties
Let be a normal complex algebraic variety with a reduced Weil divisor
. Let be a complex linear algebraic group. We formalize the notion of a
logarithmic connection on a Zariski locally trivial principal -bundle over
, which is singular along . The existence of a logarithmic connection on
the frame bundle associated to a vector bundle over is equivalent to the
existence of a covariant derivative on the vector bundle. A torus equivariant
principal bundle over a toric variety admits an integrable logarithmic
connection singular along the boundary divisor. We consider the notion of
residue of a logarithmic connection on a vector bundle over a toric variety,
and show that for a toric vector bundle, the residue encodes the equivariant
structure of the vector bundle.Comment: 30 page
Activity controls fragility: A Random First Order Transition Theory for an active glass
How does nonequilibrium activity modify the approach to a glass? This is an
important question, since many experiments reveal the near-glassy nature of the
cell interior, remodelled by activity. However, different simulations of dense
assemblies of active particles, parametrised by a self-propulsion force, ,
and persistence time, , appear to make contradictory predictions about
the influence of activity on characteristic features of glass, such as
fragility. This calls for a broad conceptual framework to understand active
glasses; here we extend the Random First-Order Transition (RFOT) theory to a
dense assembly of self-propelled particles. We compute the active contribution
to the configurational entropy using an effective medium approach - that of a
single particle in a caging-potential. This simple active extension of RFOT
provides excellent quantitative fits to existing simulation results. We find
that whereas always inhibits glassiness, the effect of is more
subtle and depends on the microscopic details of activity. In doing so, the
theory automatically resolves the apparent contradiction between the simulation
models. The theory also makes several testable predictions, which we verify by
both existing and new simulation data, and should be viewed as a step towards a
more rigorous analytical treatment of active glass
Active fluidization in dense glassy systems
Dense soft glasses show strong collective caging behavior at sufficiently low
temperatures. Using molecular dynamics simulations of a model glass former, we
show that the incorporation of activity or self-propulsion, f0, can induce cage
breaking and fluidization, resulting in a disappearance of the glassy phase
beyond a critical f0 . The diffusion coefficient crosses over from being
strongly to weakly temperature dependent as f0 is increased. In addition, we
demonstrate that activity induces a crossover from a fragile to a strong glass
and a tendency for clustering of active particles. Our results are of direct
relevance to the collective dynamics of dense active colloidal glasses and to
recent experiments on tagged particle diffusion in living cells.Comment: 8 pages, 9 figure