34 research outputs found
Activity-dependent self-regulation of viscous length scales in biological systems
Cellular cortex, which is a highly viscous thin cytoplasmic layer just below
the cell membrane, controls the cell's mechanical properties, which can be
characterized by a hydrodynamic length scale . Cells actively regulate
via the activity of force generating molecules, such as myosin II. Here
we develop a general theory for such systems through coarse-grained
hydrodynamic approach including activity in the static description of the
system providing an experimentally accessible parameter and elucidate the
detailed mechanism of how a living system can actively self-regulate its
hydrodynamic length scale, controlling the rigidity of the system. Remarkably,
we find that , as a function of activity, behaves universally and roughly
inversely proportional to the activity of the system. Our theory rationalizes a
number of experimental findings on diverse systems and comparison of our theory
with existing experimental data show good agreement
Spinodals with Disorder: from Avalanches in Random Magnets to Glassy Dynamics
We revisit the phenomenon of spinodals in the presence of quenched disorder
and develop a complete theory for it. We focus on the spinodal of an Ising
model in a quenched random field (RFIM), which has applications in many areas
from materials to social science. By working at zero temperature in the
quasi-statically driven RFIM, thermal fluctuations are eliminated and one can
give a rigorous content to the notion of spinodal. We show that the latter is
due to the depinning and the subsequent expansion of rare droplets. We work out
the associated critical behavior, which, in any finite dimension, is very
different from the mean-field one: the characteristic length diverges
exponentially and the thermodynamic quantities display very mild
nonanalyticities much like in a Griffith phenomenon. From the recently
established connection between the spinodal of the RFIM and glassy dynamics,
our results also allow us to conclusively assess the physical content and the
status of the dynamical transition predicted by the mean-field theory of
glass-forming liquids.Comment: Published version: Total 7 pages including 2 pages of supplemental
materia
Comment on "Layering transition in confined molecular thin films: Nucleation and growth"
When fluid is confined between two molecularly smooth surfaces to a few
molecular diameters, it shows a large enhancement of its viscosity. From
experiments it seems clear that the fluid is squeezed out layer by layer. A
simple solution of the Stokes equation for quasi-two-dimensional confined flow,
with the assmption of layer-by-layer flow is found. The results presented here
correct those in Phys. Rev. B, 50, 5590 (1994), and show that both the
kinematic viscosity of the confined fluid and the coefficient of surface drag
can be obtained from the time dependence of the area squeezed out. Fitting our
solution to the available experimental data gives the value of viscosity which
is ~7 orders of magnitude higher than that in the bulk.Comment: 4 pages, 2 figure
How Do Glassy Domains Grow?
We construct the equations for the growth kinetics of a structural glass
within mode-coupling theory, through a non-stationary variant of the 3-density
correlator defined in Phys. Rev. Lett. 97}, 195701 (2006). We solve a schematic
form of the resulting equations to obtain the coarsening of the 3-point
correlator as a function of waiting time . For a quench
into the glass, we find that attains a peak value at
, providing a theoretical basis for the numerical
observations of Parisi [J. Phys. Chem. B 103, 4128 (1999)] and Kob and Barrat
[Phys. Rev. Lett. 78, 4581 (1997)]. The aging is not "simple": the
dependence cannot be attributed to an evolving effective temperature.Comment: 6 pages, 5 figure