37 research outputs found
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
Fluctations and noise in time-resolved light scattering experiments : measuring temporally heterogeneous dynamics
We use Time Resolved Correlation (TRC), a recently introduced light
scattering method, to study the dynamics of a variety of jammed, or glassy,
soft materials. The output of a TRC experiment is cI(t,tau), the time series of
the degree of correlation between the speckle patterns generated by the light
scattered at time t and t+tau. We characterize the fluctuations of cI by
calculating their Probability Density Function, their variance as a function of
the lag tau, and their time autocorrelation function. The comparison between
these quantities for a Brownian sample and for jammed materials indicate
unambiguously that the slow dynamics measured in soft glasses is temporally
heterogeneous. The analogies with recent experimental, numerical and
theoretical work on temporal heterogeneity in the glassy dynamics are briefly
discussed.Comment: date de la fin de redaction : 06/09/200
Signature of elasticity in the Faraday instability
We investigate the onset of the Faraday instability in a vertically vibrated
wormlike micelle solution. In this strongly viscoelastic fluid, the critical
acceleration and wavenumber are shown to present oscillations as a function of
driving frequency and fluid height. This effect, unseen neither in simple
fluids nor in previous experiments on polymeric fluids, is interpreted in terms
of standing elastic waves between the disturbed surface and the container
bottom. It is shown that the model of S. Kumar [Phys. Rev. E, {\bf 65}, 026305
(2002)] for a viscoelastic fluid accounts qualitatively for our experimental
observations. Explanations for quantitative discrepancies are proposed, such as
the influence of the nonlinear rheological behaviour of this complex fluid.Comment: 4 pages, 4 figure
Unexpected drop of dynamical heterogeneities in colloidal suspensions approaching the jamming transition
As the glass (in molecular fluids\cite{Donth}) or the jamming (in colloids
and grains\cite{LiuNature1998}) transitions are approached, the dynamics slow
down dramatically with no marked structural changes. Dynamical heterogeneity
(DH) plays a crucial role: structural relaxation occurs through correlated
rearrangements of particle ``blobs'' of size
\cite{WeeksScience2000,DauchotPRL2005,Glotzer,Ediger}. On approaching
these transitions, grows in glass-formers\cite{Glotzer,Ediger},
colloids\cite{WeeksScience2000,BerthierScience2005}, and driven granular
materials\cite{KeysNaturePhys2007} alike, strengthening the analogies between
the glass and the jamming transitions. However, little is known yet on the
behavior of DH very close to dynamical arrest. Here, we measure in colloids the
maximum of a ``dynamical susceptibility'', , whose growth is usually
associated to that of \cite{LacevicPRE}. initially increases with
volume fraction , as in\cite{KeysNaturePhys2007}, but strikingly drops
dramatically very close to jamming. We show that this unexpected behavior
results from the competition between the growth of and the reduced
particle displacements associated with rearrangements in very dense
suspensions, unveiling a richer-than-expected scenario.Comment: 1st version originally submitted to Nature Physics. See the Nature
Physics website fro the final, published versio
Partial jamming and non-locality in dense granular flows
Dense granular flows can exhibit non-local flow behaviours that cannot be predicted by local constitutive laws alone. Such behaviour is accompanied by the existence of diverging cooperativity length. Here we show that this length can be attributed to the development of transient clusters of jammed particles within the flow. By performing DEM simulation of dense granular flows, we directly measure the size of such clusters which scales with the inertial number with a power law. We then derive a general non-local relation based on kinematic compatibility for the existence of clusters in an arbitrary non-homogenous flow. The kinematic nature of this derivation suggests that non-locality should be expected in any material regardless of their local constitutive law, as long as transient clusters exist within the flow
Instabilité de Faraday dans les fluides complexes
Since Faraday's founding work in 1831, it is known that the surface of a vertically vibrated fluid layers undergoes the Faraday instability when the forcing acceleration is greater than some critical value. Surface waves appear at the surface and form geometrical patterns characterised by a critical wavelength. We focus on the behaviour in complex fluids to such a hydrodynamic instability. Whe show that a strong coupling between the instability and the fluid microstructure may occur depending on the complex fluid. In semi-dilute wormlike micelles solutions, this coupling gives rise to standing elastic waves. In dilute wormlike micelles solutions, a shear-thickening phenomenom may be induced by the surfaces waves. And in suspensions of rod-like colloids localised alignment takes place.Il est connu depuis 1831 que les fluides vibrés verticalement sont sujets à l'instabilité de Faraday. Au-dessus d'une certaine accélération dite accélération critique, des ondes stationnaires apparaissent à la surface du fluide et forment un motif géométrique caractérisé par une longueur d'onde critique. Dans ce travail nous montrons que la microstructure d'un fluide complexe peut se coupler fortement à l'instabilité et induire différents types de comportements selon le fluide étudié. Dans des solutions semi diluées de micelles géantes, la forte viscosité du fluide engendre des ondes stationnaires dans la hauteur du fluide. Dans des solutions diluées de micelles géantes, le cisaillement engendré par les ondes de surface conduit à un chargement de la microstructure se traduisant par un phénomène de rhéo-épaississement. Enfin, dans des suspensions de bâtonnets rigides, le couplage entre instabilité et microstructure induit un alignement localisé des bâtonnets
Instabilité de Faraday dans les fluides complexes
Il est connu depuis 1831 que les fluides vibrés verticalement sont sujets à l'instabilité de Faraday. Au-dessus d'une certaine accélération dite accélération critique, des ondes stationnaires apparaissent à la surface du fluide et forment un motif géométrique caractérisé par une longueur d'onde critique. Dans ce travail nous montrons que la microstructure d'un fluide complexe peut se coupler fortement à l'instabilité et induire différents types de comportements selon le fluide étudié. Dans des solutions semi diluées de micelles géantes, la forte viscosité du fluide engendre des ondes stationnaires dans la hauteur du fluide. Dans des solutions diluées de micelles géantes, le cisaillement engendré par les ondes de surface conduit à un chargement de la microstructure se traduisant par un phénomène de rhéo-épaississement. Enfin, dans des suspensions de bâtonnets rigides, le couplage entre instabilité et microstructure induit un alignement localisé des bâtonnets.Since Faraday's founding work in 1831, it is known that the surface of a vertically vibrated fluid layers undergoes the Faraday instability when the forcing acceleration is greater than some critical value. Surface waves appear at the surface and form geometrical patterns characterised by a critical wavelength. We focus on the behaviour in complex fluids to such a hydrodynamic instability. Whe show that a strong coupling between the instability and the fluid microstructure may occur depending on the complex fluid. In semi-dilute wormlike micelles solutions, this coupling gives rise to standing elastic waves. In dilute wormlike micelles solutions, a shear-thickening phenomenom may be induced by the surfaces waves. And in suspensions of rod-like colloids localised alignment takes place
Fractional Hoppinglike Motion in Columnar Mesophases of Semiflexible Rodlike Particles
We report on single-particle dynamics of strongly interacting filamentous fd virus particles in the liquid-crystalline columnar state in aqueous solution. From fluorescence microscopy, we find that rare, discreteevents take place, in which individual particles engage in sudden, jumplike motion along the main rodaxis. The jump length distribution is bimodal and centered at half- and full-particle lengths. Our Browniandynamics simulations of hard semiflexible particles mimic our experiments and indicate that full-lengthjumps must be due to collective dynamics in which particles move in stringlike fashion in and betweenneighboring columns, while half jumps arise as a result of particles moving into defects. We find that thefinite domain structure of the columnar phase strongly influences the observed dynamics