46 research outputs found
Asymmetric response of a jammed plastic bead raft
Fluctuation-dissipation relations have received significant attention as a
potential method for defining an effective temperature in nonequilibrium
systems. The successful development of an effective temperature would be an
important step in the application of statistical mechanics principles to
systems driven far from equilibrium. Many of the systems of interest are
sufficiently dense that they are close to the jamming transition, a point at
which interesting correlations develop. Here we study the response function in
a driven system of plastic beads as a function of the density in order to
elucidate the impact of the jamming transition on the use of
fluctuation-dissipation relations. The focus is on measuring the response
function for applied shear stress. We find that even when the amplitude of the
applied stress leads to a linear response in the strain, the time scale of the
response is dependent on the direction of the applied stress
Viscoelastic shear banding in foam
Shear banding is an important feature of flow in complex fluids. Essentially,
shear bands refer to the coexistence of flowing and non-flowing regions in
driven material. Understanding the possible sources of shear banding has
important implications for a wide range of flow applications. In this regard,
quasi-two dimensional flow offers a unique opportunity to study competing
factors that result in shear bands. One proposal is the competition between
intrinsic dissipation and an external source of dissipation. In this paper, we
report on the experimental observation of the transition between different
classes of shear-bands that have been predicted to exist in cylindrical
geometry as the result of this competition [R. J. Clancy, E. Janiaud, D.
Weaire, and S. Hutzlet, Eur. J. Phys. E, {\bf 21}, 123 (2006)]
Direct observation of twist mode in electroconvection in I52
I report on the direct observation of a uniform twist mode of the director
field in electroconvection in I52. Recent theoretical work suggests that such a
uniform twist mode of the director field is responsible for a number of
secondary bifurcations in both electroconvection and thermal convection in
nematics. I show here evidence that the proposed mechanisms are consistent with
being the source of the previously reported SO2 state of electroconvection in
I52. The same mechanisms also contribute to a tertiary Hopf bifurcation that I
observe in electroconvection in I52. There are quantitative differences between
the experiment and calculations that only include the twist mode. These
differences suggest that a complete description must include effects described
by the weak-electrolyte model of electroconvection
Impact of noise on domain growth in electroconvection
The growth and ordering of striped domains has recently received renewed
attention due in part to experimental studies in diblock copolymers and
electroconvection. One surprising result has been the relative slow dynamics
associated with the growth of striped domains. One potential source of the slow
dynamics is the pinning of defects in the periodic potential of the stripes. Of
interest is whether or not external noise will have a significant impact on the
domain ordering, perhaps by reducing the pinning and increasing the rate of
ordering. In contrast, we present experiments using electroconvection in which
we show that a particular type of external noise decreases the rate of domain
ordering
Impact of boundaries on velocity profiles in bubble rafts
Under conditions of sufficiently slow flow, foams, colloids, granular matter,
and various pastes have been observed to exhibit shear localization, i.e.
regions of flow coexisting with regions of solid-like behavior. The details of
such shear localization can vary depending on the system being studied. A
number of the systems of interest are confined so as to be quasi-two
dimensional, and an important issue in these systems is the role of the
confining boundaries. For foams, three basic systems have been studied with
very different boundary conditions: Hele-Shaw cells (bubbles confined between
two solid plates); bubble rafts (a single layer of bubbles freely floating on a
surface of water); and confined bubble rafts (bubbles confined between the
surface of water below and a glass plate on top). Often, it is assumed that the
impact of the boundaries is not significant in the ``quasi-static limit'', i.e.
when externally imposed rates of strain are sufficiently smaller than internal
kinematic relaxation times. In this paper, we directly test this assumption for
rates of strain ranging from to . This
corresponds to the quoted quasi-static limit in a number of previous
experiments. It is found that the top plate dramatically alters both the
velocity profile and the distribution of nonlinear rearrangements, even at
these slow rates of strain.Comment: New figures added, revised version accepted for publication in Phys.
Rev.