245 research outputs found
Aggregates relaxation in a jamming colloidal suspension after shear cessation
The reversible aggregates formation in a shear thickening, concentrated
colloidal suspension is investigated through speckle visibility spectroscopy, a
dynamic light scattering technique recently introduced [P.K. Dixon and D.J.
Durian, Phys. Rev. Lett. 90, 184302 (2003)]. Formation of particles aggregates
is observed in the jamming regime, and their relaxation after shear cessation
is monitored as a function of the applied shear stress. The aggregates
relaxation time increases when a larger stress is applied. Several phenomena
have been proposed to interpret this behavior: an increase of the aggregates
size and volume fraction, or a closer packing of the particles in the
aggregates.Comment: 7 pages, 7 figures; added figures included in the pdf versio
Averaging rheological quantities in descriptions of soft glassy materials
Many mean-field models have been introduced to describe the mechanical
behavior of glassy materials. They often rely on averages performed over
distributions of elements or states. We here underline that averaging is a more
intricate procedure in mechanics than in more classical situations such as
phase transitions in magnetic systems. This leads us to modify the predictions
of the recently proposed SGR model for soft glassy materials, for which we
suggest that the viscosity should diverge at the glass transition temperature
with an exponential form .Comment: 4 pages, Latex, 1 eps figur
Slow dynamics, aging, and glassy rheology in soft and living matter
We explore the origins of slow dynamics, aging and glassy rheology in soft
and living matter. Non-diffusive slow dynamics and aging in materials
characterised by crowding of the constituents can be explained in terms of
structural rearrangement or remodelling events that occur within the jammed
state. In this context, we introduce the jamming phase diagram proposed by Liu
and Nagel to understand the ergodic-nonergodic transition in these systems, and
discuss recent theoretical attempts to explain the unusual,
faster-than-exponential dynamical structure factors observed in jammed soft
materials. We next focus on the anomalous rheology (flow and deformation
behaviour) ubiquitous in soft matter characterised by metastability and
structural disorder, and refer to the Soft Glassy Rheology (SGR) model that
quantifies the mechanical response of these systems and predicts aging under
suitable conditions. As part of a survey of experimental work related to these
issues, we present x-ray photon correlation spectroscopy (XPCS) results of the
aging of laponite clay suspensions following rejuvenation. We conclude by
exploring the scientific literature for recent theoretical advances in the
understanding of these models and for experimental investigations aimed at
testing their predictions.Comment: 22 pages, 5 postscript figures; invited review aricle, to appear in
special issue on soft matter in Solid State Communication
Shear-banding in a lyotropic lamellar phase, Part 1: Time-averaged velocity profiles
Using velocity profile measurements based on dynamic light scattering and
coupled to structural and rheological measurements in a Couette cell, we
present evidences for a shear-banding scenario in the shear flow of the onion
texture of a lyotropic lamellar phase. Time-averaged measurements clearly show
the presence of structural shear-banding in the vicinity of a shear-induced
transition, associated to the nucleation and growth of a highly sheared band in
the flow. Our experiments also reveal the presence of slip at the walls of the
Couette cell. Using a simple mechanical approach, we demonstrate that our data
confirms the classical assumption of the shear-banding picture, in which the
interface between bands lies at a given stress . We also outline
the presence of large temporal fluctuations of the flow field, which are the
subject of the second part of this paper [Salmon {\it et al.}, submitted to
Phys. Rev. E]
Stress overshoot in a simple yield stress fluid: an extensive study combining rheology and velocimetry
We report a large amount of experimental data on the stress overshoot
phenomenon which takes place during start-up shear flows in a simple yield
stress fluid, namely a carbopol microgel. A combination of classical
rheological measurements and ultrasonic velocimetry makes it possible to get
physical insights on the transient dynamics of both the stress and
the velocity field across the gap of a rough cylindrical Couette cell during
the start-up of shear under an applied shear rate . (i) At small
strains (), increases linearly and the microgel
undergoes homogeneous deformation. (ii) At a time , the stress reaches a
maximum value which corresponds to the failure of the microgel and
to the nucleation of a thin lubrication layer at the moving wall. (iii) The
microgel then experiences a strong elastic recoil and enters a regime of total
wall slip while the stress slowly decreases. (iv) Total wall slip gives way to
a transient shear-banding phenomenon, which occurs on timescales much longer
than that of the stress overshoot and has been described elsewhere [Divoux
\textit{et al., Phys. Rev. Lett.}, 2010, \textbf{104}, 208301]. This whole
sequence is very robust to concentration changes in the explored range ( w/w). We further demonstrate that the maximum stress
and the corresponding strain both depend on the
applied shear rate and on the waiting time between preshear
and shear start-up: they remain roughly constant as long as is
smaller than some critical shear rate and they
increase as weak power laws of for
[...].Comment: 18 pages, 14 figures, accepted for publication in Soft Matte
Collective effects at frictional interfaces
We discuss the role of the long-range elastic interaction between the
contacts inside an inhomogeneous frictional interface. The interaction produces
a characteristic elastic correlation length (where
is the distance between the contacts, is the elastic constant of a
contact, and is the Young modulus of the sliding body), below which the
slider may be considered as a rigid body. The strong inter-contact interaction
leads to a narrowing of the effective threshold distribution for contact
breaking and enhances the chances for an elastic instability to appear. Above
the correlation length, , the interaction leads to screening of
local perturbations in the interface, or to appearance of collective modes ---
frictional cracks propagating as solitary waves
Critical review on biofilm methods
Biofilms are widespread in nature and constitute an important strategy implemented by microorganisms to survive in sometimes harsh environmental conditions. They can be beneficial or have a negative impact particularly when formed in industrial settings or on medical devices. As such, research into the formation and elimination of biofilms is important for many disciplines. Several new methodologies have been recently developed for, or adapted to, biofilm studies that have contributed to deeper knowledge on biofilm physiology, structure and composition. In this review, traditional and cutting-edge methods to study biofilm biomass, viability, structure, composition and physiology are addressed. Moreover, as there is a lack of consensus among the diversity of techniques used to grow and study biofilms. This review intends to remedy this, by giving a critical perspective, highlighting the advantages and limitations of several methods. Accordingly, this review aims at helping scientists in finding the most appropriate and up-to-date methods to study their biofilms.The authors would like to acknowledge the support from the EU COST Action BacFoodNet FA1202
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