240 research outputs found
Phase-field model of long-time glass-like relaxation in binary fluid mixtures
We present a new phase-field model for binary fluids exhibiting typical
signatures of self-glassiness, such as long-time relaxation, ageing and
long-term dynamical arrest. The present model allows the cost of building an
interface to become locally zero, while preserving global positivity of the
overall surface tension. An important consequence of this property, which we
prove analytically, is the emergence of compact configurations of fluid
density. Owing to their finite-size support, these "compactons" can be
arbitrarily superposed, thereby providing a direct link between the ruggedness
of the free-energy landscape and morphological complexity in configurational
space. The analytical picture is supported by numerical simulations of the
proposed phase-field equation.Comment: 5 Pages, 6 Figure
Cooperativity flows and Shear-Bandings: a statistical field theory approach
Cooperativity effects have been proposed to explain the non-local rheology in
the dynamics of soft jammed systems. Based on the analysis of the free-energy
model proposed by L. Bocquet, A. Colin \& A. Ajdari ({\em Phys. Rev. Lett.}
{\bf 103}, 036001 (2009)), we show that cooperativity effects resulting from
the non-local nature of the fluidity (inverse viscosity), are intimately
related to the emergence of shear-banding configurations. This connection
materializes through the onset of inhomogeneous compact solutions (compactons),
wherein the fluidity is confined to finite-support subregions of the flow and
strictly zero elsewhere. Compactons coexistence with regions of zero fluidity
("non-flowing vacuum") is shown to be stabilized by the presence of mechanical
noise, which ultimately shapes up the equilibrium distribution of the fluidity
field, the latter acting as an order parameter for the flow-noflow transitions
occurring in the material.Comment: 33 pages, 10 figure
On the impact of controlled wall roughness shape on the flow of a soft-material
We explore the impact of geometrical corrugations on the near-wall flow
properties of a soft-material driven in a confined rough microchannel. By means
of numerical simulations, we perform a quantitative analysis of the relation
between the flow rate and the wall stress for a number of
setups, by changing both the roughness values as well as the roughness shape.
Roughness suppresses the flow, with the existence of a characteristic value of
at which flow sets in. Just above the onset of flow, we
quantitatively analyze the relation between and . While for
smooth walls a linear dependency is observed, steeper behaviours are found to
set in by increasing wall roughness. The variation of the steepness, in turn,
depends on the shape of the wall roughness, wherein gentle steepness changes
are promoted by a variable space localization of the roughness
Direct evidence of plastic events and dynamic heterogeneities in soft-glasses
By using fluid-kinetic simulations of confined and concentrated emulsion
droplets, we investigate the nature of space non-homogeneity in soft-glassy
dynamics and provide quantitative measurements of the statistical features of
plastic events in the proximity of the yield-stress threshold. Above the yield
stress, our results show the existence of a finite stress correlation scale,
which can be mapped directly onto the {\it cooperativity scale}, recently
introduced in the literature to capture non-local effects in the soft-glassy
dynamics. In this regime, the emergence of a separate boundary (wall) rheology
with higher fluidity than the bulk, is highlighted in terms of near-wall
spontaneous segregation of plastic events. Near the yield stress, where the
cooperative scale cannot be estimated with sufficient accuracy, the system
shows a clear increase of the stress correlation scale, whereas plastic events
exhibit intermittent clustering in time, with no preferential spatial location.
A quantitative measurement of the space-time correlation associated with the
motion of the interface of the droplets is key to spot the long-range amorphous
order at the yield stress threshold
Mesoscopic lattice Boltzmann modeling of soft-glassy systems: theory and simulations
A multi-component lattice Boltzmann model recently introduced (R. Benzi et
al. Phys. Rev. Lett 102, 026002 (2009)) to describe some dynamical behaviors of
soft-flowing materials is theoretically analyzed. Equilibrium and transport
properties are derived within the framework of a continuum free-energy
formulation, and checked against numerical simulations. Due to the competition
between short-range inter-species repulsion and mid-range intra-species
attraction, the model is shown to give rise to a very rich configurational
dynamics of the density field, exhibiting numerous features of soft-flowing
materials, such as long-time relaxation due to caging effects, enhanced
viscosity and structural arrest, ageing under moderate shear and shear-thinning
flow above a critical shear threshold.Comment: 25 pages, 17 figures, submitted to Journal of chemical physics
Internal dynamics and activated processes in Soft-Glassy materials
Plastic rearrangements play a crucial role in the characterization of
soft-glassy materials, such as emulsions and foams. Based on numerical
simulations of soft-glassy systems, we study the dynamics of plastic
rearrangements at the hydrodynamic scales where thermal fluctuations can be
neglected. Plastic rearrangements require an energy input, which can be either
provided by external sources, or made available through time evolution in the
coarsening dynamics, in which the total interfacial area decreases as a
consequence of the slow evolution of the dispersed phase from smaller to large
droplets/bubbles. We first demonstrate that our hydrodynamic model can
quantitatively reproduce such coarsening dynamics. Then, considering
periodically oscillating strains, we characterize the number of plastic
rearrangements as a function of the external energy-supply, and show that they
can be regarded as activated processes induced by a suitable "noise" effect.
Here we use the word noise in a broad sense, referring to the internal
non-equilibrium dynamics triggered by spatial random heterogeneities and
coarsening. Finally, by exploring the interplay between the internal
characteristic time-scale of the coarsening dynamics and the external
time-scale associated with the imposed oscillating strain, we show that the
system exhibits the phenomenon of stochastic resonance, thereby providing
further credit to the mechanical activation scenario.Comment: 21 Pages, 9 figure
Stochastic resonance in soft-glassy materials
Flow in soft-glasses occurs via a sequence of reversible elastic deformations and local irreversible plastic rearrangements. Yield events in the material cause kicks adding up to an effectively thermal noise, an intuition that has inspired the development of phenomenological models aiming at explaining the main features of soft-glassy rheology. In this letter, we provide a specific scenario for such mechanical activation, based on a general paradigm of non-equilibrium statistical mechanics, namely {\it stochastic resonance}. By using mesoscopic simulations of emulsion droplets subject to an oscillatory strain, we characterize the response of the system and highlight a resonance-like behavior in the plastic rearrangements. This confirms that the synchronization of the system response to an external time-dependent load is triggered by the mechanical noise resulting from disordered configurations (polydispersity)
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