69 research outputs found
Competition between shear banding and wall slip in wormlike micelles
The interplay between shear band (SB) formation and boundary conditions (BC)
is investigated in wormlike micellar systems (CPyCl--NaSal) using ultrasonic
velocimetry coupled to standard rheology in Couette geometry. Time-resolved
velocity profiles are recorded during transient strain-controlled experiments
in smooth and sand-blasted geometries. For stick BC standard SB is observed,
although depending on the degree of micellar entanglement temporal fluctuations
are reported in the highly sheared band. For slip BC wall slip occurs only for
shear rates larger than the start of the stress plateau. At low entanglement,
SB formation is shifted by a constant , while for more
entangled systems SB constantly "nucleate and melt." Micellar orientation
gradients at the walls may account for these original features.Comment: 4 pages, 4 figures, to appear in Phys. Rev. Let
Self-diffusion of rod-like viruses in the nematic phase
We measure the self-diffusion of colloidal rod-like virus {\it fd} in an
isotropic and nematic phase. A low volume fraction of viruses are labelled with
a fluorescent dye and dissolved in a background of unlabelled rods. The
trajectories of individual rods are visualized using fluorescence microscopy
from which the diffusion constant is extracted. The diffusion parallel
() and perpendicular () to the nematic director is
measured. The ratio () increases monotonically with
increasing virus concentration. Crossing the isotropic-nematic phase boundary
results in increase of and decrease of when
compared to the diffusion in the isotropic phase ().Comment: 7 pages, 4 figures, to appear in Europhysics Letter
Interplay between a hydrodynamic instability and a phase transition: the Faraday instability in dispersions of rodlike colloids
Strong effects of the Faraday instability on suspensions of rodlike colloidal
particles are reported through measurements of the critical acceleration and of
the surface wave amplitude. We show that the transition to parametrically
excited surface waves displays discontinuous and hysteretic features. This
subcritical behaviour is attributed to the shear-thinning properties of our
colloidal suspensions thanks to a phenomenological model based on rheological
data under large amplitude oscillatory shear. Birefringence measurements
provide direct evidence that Faraday waves induce local nematic ordering of the
rodlike colloids. While local alignment simply follows the surface oscillations
for dilute, isotropic suspensions, permanent nematic patches are generated by
surface waves in samples close to the isotropic-to-nematic transition and above
the transition large domains align in the flow direction. This strong coupling
between the fluid microstructure and a hydrodynamic instability is confirmed by
numerical computations based on the microstructural response of rodlike viruses
in shear flow.Comment: 8 pages, 6 figure
Fast Diffusion of Long Guest Rods in a Lamellar Phase of Short Host Particles
We investigate the dynamic behavior of long guest rod-like particles immersed
in liquid crystalline phases formed by shorter host rods, tracking both guest
and host particles by fluorescence microscopy. Counter-intuitively, we evidence
that long rods diffuse faster than short rods forming the one-dimensional
ordered smectic-A phase. This results from the larger and non-commensurate size
of the guest particles as compared to the wavelength of the energy landscape
set by the lamellar stack of liquid slabs. The long guest particles are also
shown to be still mobile in the crystalline smectic-B phase, as they generate
their own voids in the adjacent layers.Comment: 5 pages, 4 figures, Accepted in Phys. Rev. Let
Glasses of dynamically asymmetric binary colloidal mixtures: Quiescent properties and dynamics under shear
We investigate mixing effects on the glass state of binary colloidal
hard-sphere-like mixtures with large size asymmetry, at a constant volume
fraction phi = 0.61. The structure, dynamics and viscoelastic response as a
function of mixing ratio reflect a transition between caging by one or the
other component. The strongest effect of mixing is observed in systems
dominated by caging of the large component. The possibility to pack a large
number of small spheres in the free volume left by the large ones induces a
pronounced deformation of the cage of the large spheres, which become
increasingly delocalised. This results in faster dynamics and a strong
reduction of the elastic modulus. When the relative volume fraction of small
spheres exceeds that of large spheres, the small particles start to form their
own cages, slowing down the dynamics and increasing the elastic modulus of the
system. The large spheres become the minority and act as an impurity in the
ordering beyond the first neighbour shell, i.e. the cage, and do not directly
affect the particle organisation on the cage level. In such a system, when
shear at constant rate is applied, melting of the glass is observed due to
facilitated out-of-cage diffusion which is associated with structural
anisotropy induced by shear.Comment: 8 pages, 7 figures, Proceedings of the 4th International Symposium on
Slow Dynamics in Complex Systems, Sendai, 2-7 December 201
Self-diffusion of Rod-like Viruses Through Smectic Layer
We report the direct visualization at the scale of single particles of mass
transport between smectic layers, also called permeation, in a suspension of
rod-like viruses. Self-diffusion takes place preferentially in the direction
normal to the smectic layers, and occurs by quasi-quantized steps of one rod
length. The diffusion rate corresponds with the rate calculated from the
diffusion in the nematic state with a lamellar periodic ordering potential that
is obtained experimentally.Comment: latex, 4 pages, 4 figures, accepted in Phys. Rev. Let
When bigger is faster: a self-van Hove analysis of the enhanced self-diffusion of non-commensurate guest particles in smectics
We investigate the anomalous dynamics in smectic phases of short host rods
where, counter-intuitively, long guest rod-shaped particles diffusive faster
than the short host ones, due to their precise size mismatch. In addition to
the previously reported mean-square displacement, we analyze the time evolution
of the Self-van Hove functions G(r,t), as this probability density function
uncovers intrinsic heterogeneous dynamics. Through this analysis, we show that
the dynamics of the host particles parallel to the director becomes
non-gaussian and therefore heterogeneous after the nematic-to-smectic-A phase
transition, even though it exhibits a nearly diffusive behavior according to
its mean-square displacement. In contrast, the non-commensurate guest particles
display Gaussian dynamics of the parallel motion, up to the transition to the
smectic-B phase. Thus, we show that the Self-van Hove function is a very
sensitive probe to account for the instantaneous and heterogeneous dynamics of
our system, and should be more widely considered as a quantitative and
complementary approach of the classical mean-square displacement
characterization in diffusion processes.Comment: 10 pages, 9 figures, the article has been accepted by the Journal of
Chemical Physic
Nematic-Isotropic Spinodal Decomposition Kinetics of Rod-like Viruses
We investigate spinodal decomposition kinetics of an initially nematic
dispersion of rod-like viruses (fd virus). Quench experiments are performed
from a flow-stabilized homogeneous nematic state at high shear rate into the
two-phase isotropic-nematic coexistence region at zero shear rate. We present
experimental evidence that spinodal decomposition is driven by orientational
diffusion, in accordance with a very recent theory.Comment: 17 pages, 6 figures, accepted in Phys. Rev.
Kinetic pathways of the Nematic-Isotropic phase transition as studied by confocal microscopy on rod-like viruses
We investigate the kinetics of phase separation for a mixture of rodlike
viruses (fd) and polymer (dextran), which effectively constitutes a system of
attractive rods. This dispersion is quenched from a flow-induced fully nematic
state into the region where the nematic and the isotropic phase coexist. We
show experimental evidence that the kinetic pathway depends on the overall
concentration. When the quench is made at high concentrations, the system is
meta-stable and we observe typical nucleation-and-growth. For quenches at low
concentration the system is unstable and the system undergoes a spinodal
decomposition. At intermediate concentrations we see the transition between
both demixing processes, where we locate the spinodal point.Comment: 11 pages, 6 figures, accepted in J. Phys.: Condens. Matter as
symposium paper for the 6th Liquid Matter Conference in Utrech
- …