68 research outputs found
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
S-shaped flow curves of shear thickening suspensions: Direct observation of frictional rheology
We study the rheological behavior of concentrated granular suspensions of
simple spherical particles. Under controlled stress, the system exhibits an
S-shaped flow curve (stress vs. shear rate) with a negative slope in between
the low-viscosity Newtonian regime and the shear thickened regime. Under
controlled shear rate, a discontinuous transition between the two states is
observed. Stress visualization experiments with a novel fluorescent probe
suggest that friction is at the origin of shear thickening. Stress
visualization shows that the stress in the system remains homogeneous (no shear
banding) if a stress is imposed that is intermediate between the high and
low-stress branches. The S-shaped shear thickening is then due to the
discontinuous formation of a frictional force network between particles upon
increasing the stress.Comment: 5 pages + 6 figure
Universal scaling of flow curves: comparison between experiments and simulations
Yield stress materials form an interesting class of materials that behave
like solids at small stresses, but start to flow once a critical stress is
exceeded. It has already been reported both in experimental and simulation work
that flow curves of different yield stress materials can be scaled with the
distance to jamming or with the confining pressure. However, different scaling
exponents are found between experiments and simulations. In this paper we
identify sources of this discrepancy. We numerically relate the volume fraction
with the confining pressure and discuss the similarities and differences
between rotational and oscillatory measurements. Whereas simulations are
performed in the elastic response regime close to the jamming transition and
with very small amplitudes to calculate the scaling exponents, these conditions
are hardly possible to achieve experimentally. Measurements are often performed
far away from the critical volume fraction and at large amplitudes. We show
that these differences are the underlying reason for the different exponents
for rescaling flow curves
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