140 research outputs found
Filamentous phages as building blocks for reconfigurable and hierarchical self-assembly
Filamentous bacteriophages such as fd-like viruses are monodisperse rod-like
colloids that have well defined properties: diameter, length, rigidity, charge
and chirality. Engineering those viruses leads to a library of colloidal rods
which can be used as building blocks for reconfigurable and hierarchical
self-assembly. Their condensation in aqueous solution \th{with additive
polymers which act as depletants to induce} attraction between the rods leads
to a myriad of fluid-like micronic structures ranging from isotropic/nematic
droplets, colloid membranes, achiral membrane seeds, twisted ribbons,
-wall, pores, colloidal skyrmions, M\"obius anchors, scallop membranes to
membrane rafts. Those structures and the way they shape shift not only shed
light on the role of entropy, chiral frustration and topology in soft matter
but it also mimics many structures encountered in different fields of science.
On one hand, filamentous phages being an experimental realization of colloidal
hard rods, their condensation mediated by depletion interactions constitutes a
blueprint for self-assembly of rod-like particles and provides fundamental
foundation for bio- or material oriented applications. On the other hand, the
chiral properties of the viruses restrict the generalities of some results but
vastly broaden the self-assembly possibilities
Influence of boundary conditions on yielding in a soft glassy material
The yielding behavior of a sheared Laponite suspension is investigated within
a 1 mm gap under two different boundary conditions. No-slip conditions, ensured
by using rough walls, lead to shear localization as already reported in various
soft glassy materials. When apparent wall slip is allowed using a smooth
geometry, the sample is shown to break up into macroscopic solid pieces that
get slowly eroded by the surrounding fluidized material up to the point where
the whole sample is fluid. Such a drastic effect of boundary conditions on
yielding suggests the existence of some macroscopic characteristic length that
could be connected to cooperativity effects in jammed materials under shear.Comment: 4 pages, 5 figure
The yielding dynamics of a colloidal gel
Attractive colloidal gels display a solid-to-fluid transition as shear
stresses above the yield stress are applied. This shear-induced transition is
involved in virtually any application of colloidal gels. It is also crucial for
controlling material properties. Still, in spite of its ubiquity, the yielding
transition is far from understood, mainly because rheological measurements are
spatially averaged over the whole sample. Here, the instrumentation of creep
and oscillatory shear experiments with high-frequency ultrasound opens new
routes to observing the local dynamics of opaque attractive colloidal gels. The
transition proceeds from the cell walls and heterogeneously fluidizes the whole
sample with a characteristic time whose variations with applied stress suggest
the existence of an energy barrier linked to the gelation process. The present
results provide new test grounds for computer simulations and theoretical
calculations in the attempt to better understand the yielding transition. The
versatility of the technique should also allow extensive mesoscopic studies of
rupture mechanisms in soft solids ranging from crystals to glassy materials.Comment: 8 pages, 5 figure
Shear-induced fragmentation of Laponite suspensions
Simultaneous rheological and velocity profile measurements are performed in a
smooth Couette geometry on Laponite suspensions seeded with glass microspheres
and undergoing the shear-induced solid-to-fluid (or yielding) transition. Under
these slippery boundary conditions, a rich temporal behaviour is uncovered, in
which shear localization is observed at short times, that rapidly gives way to
a highly heterogeneous flow characterized by intermittent switching from
plug-like flow to linear velocity profiles. Such a temporal behaviour is linked
to the fragmentation of the initially solid sample into blocks separated by
fluidized regions. These solid pieces get progressively eroded over time scales
ranging from a few minutes to several hours depending on the applied shear rate
. The steady-state is characterized by a homogeneous flow with
almost negligible wall slip. The characteristic time scale for erosion is shown
to diverge below some critical shear rate and to scale as
with above
. A tentative model for erosion is discussed together with
open questions raised by the present results.Comment: 19 pages, 13 figures, submitted to Soft Matte
Local Oscillatory Rheology from Echography
Local Oscillatory Rheology from Echography (LORE) consists in a traditional
rheology experiment synchronized with high-frequency ultrasonic imaging which
gives access to the local material response to oscillatory shear. Besides
classical global rheological quantities, this method provides quantitative
time-resolved information on the local displacement across the entire gap of
the rheometer. From the local displacement response, we compute and decompose
the local strain in its Fourier components and measure the spatially-resolved
viscoelastic moduli. After benchmarking our method on homogeneous Newtonian
fluids and soft solids, we demonstrate that this technique is well suited to
characterize spatially heterogeneous samples, wall slip, and the emergence of
nonlinearity under large amplitude oscillatory stress in soft materials.Comment: 10 pages, 5 figures, submitted to Phys. Rev. Applie
Interplay between Spinodal Decomposition and Glass Formation in Proteins Exhibiting Short-Range Attractions
We investigate the competition between spinodal decomposition and dynamical
arrest using aqueous solutions of the globular protein lysozyme as a model
system for colloids with short-range attractions. We show that quenches below a
temperature Ta lead to gel formation as a result of a local arrest of the
proteindense phase during spinodal decomposition. The rheological properties of
these gels allow us to use centrifugation experiments to determine the local
densities of both phases and to precisely locate the gel boundary and the
attractive glass line close to and within the unstable region of the phase
diagram
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