16 research outputs found
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)]
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.
Reversible plasticity in amorphous materials
A fundamental assumption in our understanding of material rheology is that
when microscopic deformations are reversible, the material responds elastically
to external loads. Plasticity, i.e. dissipative and irreversible macroscopic
changes in a material, is assumed to be the consequence of irreversible
microscopic events. Here we show direct evidence for reversible plastic events
at the microscopic scale in both experiments and simulations of two-dimensional
foam. In the simulations, we demonstrate a link between reversible plastic
rearrangement events and pathways in the potential energy landscape of the
system. These findings represent a fundamental change in our understanding of
materials--microscopic reversibility does not necessarily imply elasticity.Comment: Revised pape
Homology and symmetry breaking in Rayleigh-Benard convection: Experiments and simulations
Algebraic topology (homology) is used to analyze the weakly turbulent state
of spiral defect chaos in both laboratory experiments and numerical simulations
of Rayleigh-Benard convection.The analysis reveals topological asymmetries that
arise when non-Boussinesq effects are present.Comment: 21 pages with 6 figure
Limits of the equivalence of time and ensemble averages in shear flows
In equilibrium systems, time and ensemble averages of physical quantities are
equivalent due to ergodic exploration of phase space. In driven systems, it is
unknown if a similar equivalence of time and ensemble averages exists. We
explore effective limits of such convergence in a sheared bubble raft using
averages of the bubble velocities. In independent experiments, averaging over
time leads to well converged velocity profiles. However, the time-averages from
independent experiments result in distinct velocity averages. Ensemble averages
are approximated by randomly selecting bubble velocities from independent
experiments. Increasingly better approximations of ensemble averages converge
toward a unique velocity profile. Therefore, the experiments establish that in
practical realizations of non-equilibrium systems, temporal averaging and
ensemble averaging can yield convergent (stationary) but distinct
distributions.Comment: accepted to PRL - final figure revision
Comparison of Low-Amplitude Oscillatory Shear in Experimental and Computational Studies of Model Foams
A fundamental difference between fluids and solids is their response to applied shear. Solids possess static shear moduli, while fluids do not. Complex fluids such as foams display an intermediate response to shear with nontrivial frequency-dependent shear moduli. In this paper, we conduct coordinated experiments and numerical simulations of model foams subjected to boundary-driven oscillatory planar shear. Our studies are performed on bubble rafts (experiments) and the bubble model (simulations) in two dimensions. We focus on the low amplitude flow regime in which T1 events, i.e., bubble rearrangement events where originally touching bubbles switch nearest neighbors, do not occur, yet the system transitions from solid- to liquidlike behavior as the driving frequency is increased. In both simulations and experiments, we observe two distinct flow regimes. At low frequencies ω, the velocity profile of the bubbles increases linearly with distance from the stationary wall, and there is a nonzero total phase shift between the moving boundary and interior bubbles. In this frequency regime, the total phase shift scales as a power law ∆~ωn with n ≈ 3. In contrast, for frequencies above a crossover frequency ω\u3eωp, the total phase shift ∆ scales linearly with the driving frequency. At even higher frequencies above a characteristic frequency ωnl\u3eωp, the velocity profile changes from linear to nonlinear. We fully characterize this transition from solid- to liquidlike flow behavior in both the simulations and experiments and find qualitative and quantitative agreements for the characteristic frequencies
Bubble kinematics in a sheared foam
We characterize the kinematics of bubbles in a sheared two-dimensional foam
using statistical measures. We consider the distributions of both bubble
velocities and displacements. The results are discussed in the context of the
expected behavior for a thermal system and simulations of the bubble model.
There is general agreement between the experiments and the simulation, but
notable differences in the velocity distributions point to interesting elements
of the sheared foam not captured by prevalent models