146 research outputs found
Hyperuniformity with no fine tuning in sheared sedimenting suspensions
Particle suspensions, present in many natural and industrial settings,
typically contain aggregates or other microstructures that can complicate
macroscopic flow behaviors and damage processing equipment. Recent work found
that applying uniform periodic shear near a critical transition can reduce
fluctuations in the particle concentration across all length scales, leading to
a hyperuniform state. However, this strategy for homogenization requires fine
tuning of the strain amplitude. Here we show that in a model of sedimenting
particles under periodic shear, there is a well-defined regime at low
sedimentation speed where hyperuniform scaling automatically occurs. Our
simulations and theoretical arguments show that the homogenization extends up
to a finite lengthscale that diverges as the sedimentation speed approaches
zero.Comment: 11 pages, 6 figure
Viscous to Inertial Crossover in Liquid Drop Coalescence
Using an electrical method and high-speed imaging we probe drop coalescence
down to 10 ns after the drops touch. By varying the liquid viscosity over two
decades, we conclude that at sufficiently low approach velocity where
deformation is not present, the drops coalesce with an unexpectedly late
crossover time between a regime dominated by viscous and one dominated by
inertial effects. We argue that the late crossover, not accounted for in the
theory, can be explained by an appropriate choice of length-scales present in
the flow geometry.Comment: 4 pages, 4 figure
Multiple transient memories in sheared suspensions: robustness, structure, and routes to plasticity
Multiple transient memories, originally discovered in charge-density-wave
conductors, are a remarkable and initially counterintuitive example of how a
system can store information about its driving. In this class of memories, a
system can learn multiple driving inputs, nearly all of which are eventually
forgotten despite their continual input. If sufficient noise is present, the
system regains plasticity so that it can continue to learn new memories
indefinitely. Recently, Keim & Nagel showed how multiple transient memories
could be generalized to a generic driven disordered system with noise, giving
as an example simulations of a simple model of a sheared non-Brownian
suspension. Here, we further explore simulation models of suspensions under
cyclic shear, focussing on three main themes: robustness, structure, and
overdriving. We show that multiple transient memories are a robust feature
independent of many details of the model. The steady-state spatial distribution
of the particles is sensitive to the driving algorithm; nonetheless, the memory
formation is independent of such a change in particle correlations. Finally, we
demonstrate that overdriving provides another means for controlling memory
formation and retention
Multiple transient memories in experiments on sheared non-Brownian suspensions
A system with multiple transient memories can remember a set of inputs but
subsequently forgets almost all of them, even as they are continually applied.
If noise is added, the system can store all memories indefinitely. The
phenomenon has recently been predicted for cyclically sheared non-Brownian
suspensions. Here we present experiments on such suspensions, finding behavior
consistent with multiple transient memories and showing how memories can be
stabilized by noise.Comment: 5 pages, 4 figure
Memory formation in matter
Memory formation in matter is a theme of broad intellectual relevance; it
sits at the interdisciplinary crossroads of physics, biology, chemistry, and
computer science. Memory connotes the ability to encode, access, and erase
signatures of past history in the state of a system. Once the system has
completely relaxed to thermal equilibrium, it is no longer able to recall
aspects of its evolution. Memory of initial conditions or previous training
protocols will be lost. Thus many forms of memory are intrinsically tied to
far-from-equilibrium behavior and to transient response to a perturbation. This
general behavior arises in diverse contexts in condensed matter physics and
materials: phase change memory, shape memory, echoes, memory effects in
glasses, return-point memory in disordered magnets, as well as related contexts
in computer science. Yet, as opposed to the situation in biology, there is
currently no common categorization and description of the memory behavior that
appears to be prevalent throughout condensed-matter systems. Here we focus on
material memories. We will describe the basic phenomenology of a few of the
known behaviors that can be understood as constituting a memory. We hope that
this will be a guide towards developing the unifying conceptual underpinnings
for a broad understanding of memory effects that appear in materials
Exact solutions for the wrinkle patterns of confined elastic shells
Thin elastic membranes form complex wrinkle patterns when put on substrates
of different shapes. Such patterns continue to receive attention across science
and engineering. This is due, in part, to the promise of lithography-free
micropatterning, but also to the observation that similar patterns arise in
biological systems from growth. The challenge is to explain the patterns in any
given setup, even when they fail to be robust. Building on the theoretical
foundation of [Tobasco, to appear in Arch. Ration. Mech. Anal.,
arXiv:1906.02153], we derive a complete and simple rule set for wrinkles in the
model system of a curved shell on a liquid bath. Our rules apply to shells
whose initial Gaussian curvatures are of one sign, such as cutouts of saddles
and spheres. They predict the surprising coexistence of orderly wrinkles
alongside disordered regions where the response appears stochastic, which we
confirm in experiment and simulation. They also unveil the role of the shell's
medial axis, a distinguished locus of points that we show is a basic driver in
pattern selection. Finally, they explain how the sign of the shell's initial
curvature dictates the presence or lack of disorder. Armed with our simple
rules, and the methodology underlying them, one can anticipate the creation of
designer wrinkle patterns.Comment: Extended text including Supplementary Information. Heavily revised to
focus the exposition and incorporate new results; title chang
Crumples as a generic stress-focusing instability in confined sheets
Thin elastic solids are easily deformed into a myriad of three-dimensional
shapes, which may contain sharp localized structures as in a crumpled candy
wrapper, or have smooth and diffuse features like the undulating edge of a
flower. Anticipating and controlling these morphologies is crucial to a variety
of applications involving textiles, synthetic skins, and inflatable structures.
Here we show that a "wrinkle-to-crumple" transition, previously observed in
specific settings, is a ubiquitous response for confined sheets. This unified
picture is borne out of a suite of model experiments on polymer films confined
to liquid interfaces with spherical, hyperbolic, and cylindrical geometries,
which are complemented by experiments on macroscopic membranes inflated with
gas. We use measurements across this wide range of geometries, boundary
conditions, and lengthscales to quantify several robust morphological features
of the crumpled phase, and we build an empirical phase diagram for crumple
formation that disentangles the competing effects of curvature and compression.
Our results suggest that crumples are a generic microstructure that emerge at
large curvatures due to a competition of elastic and substrate energies.Comment: 12 pages, 7 figure
Coalescence of bubbles and drops in an outer fluid
When two liquid drops touch, a microscopic connecting liquid bridge forms and rapidly grows as the two drops merge into one. Whereas coalescence has been thoroughly studied when drops coalesce in vacuum or air, many important situations involve coalescence in a dense surrounding fluid, such as oil coalescence in brine. Here we study the merging of gas bubbles and liquid drops in an external fluid. Our data indicate that the flows occur over much larger length scales in the outer fluid than inside the drops themselves. Thus, we find that the asymptotic early regime is always dominated by the viscosity of the drops, independent of the external fluid. A phase diagram showing the crossovers into the different possible late-time dynamics identifies a dimensionless number that signifies when the external viscosity can be important
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