15 research outputs found
Connecting Structural Relaxation with the Low Frequency Modes in a Hard-Sphere Colloidal Glass
Structural relaxation in hard-sphere colloidal glasses has been studied using
confocal microscopy. The motion of individual particles is followed over long
time scales to detect the rearranging regions in the system. We have used
normal mode analysis to understand the origin of the rearranging regions. The
low frequency modes, obtained over short time scales, show strong spatial
correlation with the rearrangements that happen on long time scales.Comment: Accepted in Phys. Rev. Let
Density of states of colloidal glasses
Glasses are structurally liquid-like, but mechanically solid-like. Most
attempts to understand glasses start from liquid state theory. Here we take the
opposite point of view, and use concepts from solid state physics. We determine
the vibrational modes of a colloidal glass experimentally, and find soft
low-frequency modes that are very different in nature from the usual acoustic
vibrations of ordinary solids. These modes extend over surprisingly large
length scales
Long-range strain correlations in sheared colloidal glasses
Glasses behave as solids on experimental time scales due to their slow
relaxation. Growing dynamic length scales due to cooperative motion of
particles are believed to be central to this slow response. For quiescent
glasses, however, the size of the cooperatively rearranging regions has never
been observed to exceed a few particle diameters, and the observation of
long-range correlations that are signatures of an elastic solid has remained
elusive. Here, we provide direct experimental evidence of long-range
correlations during the deformation of a dense colloidal glass. By imposing an
external stress, we force structural rearrangements that make the glass flow,
and we identify long-range correlations in the fluctuations of microscopic
strain, and elucidate their scaling and spatial symmetry. The applied shear
induces a transition from homogeneous to inhomogeneous flow at a critical shear
rate, and we investigate the role of strain correlations in this transition.Comment: 11 pages, 3 figures. Accepted for publication in PR
Emergent vortices in populations of colloidal rollers
Coherent vortical motion has been reported in a wide variety of populations
including living organisms (bacteria, fishes, human crowds) and synthetic
active matter (shaken grains, mixtures of biopolymers), yet a unified
description of the formation and structure of this pattern remains lacking.
Here we report the self-organization of motile colloids into a macroscopic
steadily rotating vortex. Combining physical experiments and numerical
simulations, we elucidate this collective behavior. We demonstrate that the
emergent-vortex structure lives on the verge of a phase separation, and single
out the very constituents responsible for this state of polar active matter.
Building on this observation, we establish a continuum theory and lay out a
strong foundation for the description of vortical collective motion in a broad
class of motile populations constrained by geometrical boundaries
Preventing transition to turbulence: a viscosity stratification does not always help
In channel flows a step on the route to turbulence is the formation of
streaks, often due to algebraic growth of disturbances. While a variation of
viscosity in the gradient direction often plays a large role in
laminar-turbulent transition in shear flows, we show that it has, surprisingly,
little effect on the algebraic growth. Non-uniform viscosity therefore may not
always work as a flow-control strategy for maintaining the flow as laminar.Comment: 9 pages, 8 figure
An attempt to categorize yield stress fluid behaviour
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Direct Observation of Percolation in the Yielding Transition of Colloidal Glasses
When strained beyond the linear regime, soft colloidal glasses yield to steady-state plastic flow in a way that is similar to the deformation of conventional amorphous solids. Because of the much larger size of the colloidal particles with respect to the atoms comprising an amorphous solid, colloidal glasses allow us to obtain microscopic insight into the nature of the yielding transition, as we illustrate here combining experiments, atomistic simulations, and mesoscopic modeling. Our results unanimously show growing clusters of nonaffine deformation percolating at yielding. In agreement with percolation theory, the spanning cluster is fractal with a fractal dimension df≃2, and the correlation length diverges upon approaching the critical yield strain. These results indicate that percolation of highly nonaffine particles is the hallmark of the yielding transition in disordered glassy systems.Peer reviewe