1,598 research outputs found
Experimental study of the Fluctuation-Dissipation-Relation during an aging process
The validity of fluctuation dissipation relations in an aging system is
studied in a colloidal glass during the transition from a fluid-like to a
solid-like state. The evolution of the rheological and electrical properties is
analyzed in the range . It is found that at the beginning of the
transition the fluctuation dissipation relation is strongly violated in
electrical measurements. The amplitude and the persistence time of this
violation are decreasing functions of frequency. At the lowest frequencies of
the measuring range it persists for times which are about 5% of the time needed
to form the colloidal glass. This phenomenology is quite close to the recent
theoretical predictions done for the violation of the fluctuation dissipation
relation in glassy systems. In contrast in the rheological measurements no
violation of the fluctuation dissipation relation is observed. The reasons of
this large difference between the electrical and rheological measurements are
discussed.Comment: to be published on physica
Rheology of sedimenting particle pastes
We study the local and global rheology of non-Brownian suspensions in a
solvent that is not density-matched, leading to either creaming or
sedimentation of the particles. Both local and global measurements show that
the incomplete density matching leads to the appearance of a critical shear
rate above which the suspension is homogenized by the flow, and below which
sedimentation or creaming happens. We show that the value of the critical shear
rate and its dependence on the experimental parameters are governed by a simple
competition between the viscous and gravitational forces, and present a simple
scaling model that agrees with the experimental results from different types of
experiments (local and global) in different setups and systems
Normal stresses in semiflexible polymer hydrogels
Biopolymer gels such as fibrin and collagen networks are known to develop
tensile axial stress when subject to torsion. This negative normal stress is
opposite to the classical Poynting effect observed for most elastic solids
including synthetic polymer gels, where torsion provokes a positive normal
stress. As recently shown, this anomalous behavior in fibrin gels depends on
the open, porous network structure of biopolymer gels, which facilitates
interstitial fluid flow during shear and can be described by a phenomenological
two-fluid model with viscous coupling between network and solvent. Here we
extend this model and develop a microscopic model for the individual diagonal
components of the stress tensor that determine the axial response of
semi-flexible polymer hydrogels. This microscopic model predicts that the
magnitude of these stress components depends inversely on the characteristic
strain for the onset of nonlinear shear stress, which we confirm experimentally
by shear rheometry on fibrin gels. Moreover, our model predicts a transient
behavior of the normal stress, which is in excellent agreement with the full
time-dependent normal stress we measure.Comment: 12 pages, 8 figure
Finite size effects in nonequilibrium wetting
Models with a nonequilibrium wetting transition display a transition also in
finite systems. This is different from nonequilibrium phase transitions into an
absorbing state, where the stationary state is the absorbing one for any value
of the control parameter in a finite system. In this paper, we study what kind
of transition takes place in finite systems of nonequilibrium wetting models.
By solving exactly a microscopic model with three and four sites and performing
numerical simulations we show that the phase transition taking place in a
finite system is characterized by the average interface height performing a
random walk at criticality and does not discriminate between the bounded-KPZ
classes and the bounded-EW class. We also study the finite size scaling of the
bKPZ universality classes, showing that it presents peculiar features in
comparison with other universality classes of nonequilibrium phase transitions.Comment: 14 pages, 6figures, major change
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