CiteULike: dchen's fluctuation

The interface in demixed colloid–polymer systems: wetting, waves and droplets

Dirk Aarts — 2008-06-20T23:21:17-00:00

Soft Matter, 2007, 3, 19 - 23, DOI: 10.1039/b608479f

Phase transitions in colloid–polymer mixtures have attracted a large amount of attention over the last 20 years (W. C. K. Poon, J. Phys.: Condens. Matter, 2002, 14, R859; R. Tuinier, J. Rieger and C. G. de Kruif, Adv. Colloid Interface Sci., 2003, 103, 1). By comparison, the interfacial tension between the coexisting phases has received little attention. Here, we show that the ultralow interfacial tension in fluid–fluid demixed colloid–polymer systems, which is roughly one million times smaller than in ordinary liquids, manifests itself in a wide variety of interface characteristics and processes. Discussed are the interfacial wetting behaviour close to a hard wall, the thermal capillary waves at the free interface and the process of droplet coalescence and breakup. These subjects can be studied in a single experiment by combining modern soft matter chemistry and laser scanning confocal microscopy. This combination allows a further exploration of a broad range of interface issues.

Evidence for three-dimensional unstable flows in shear-banding wormlike micelles

Lydiane Bécu — 2008-06-11T00:06:30-00:00

We report on an experimental study of the shear-banding phenomenon in the concentrated wormlike micellar system CTAB at 20 wt. % in D2O. Time-resolved velocity profiles are recorded using ultrasonic velocimetry simultaneously to global rheological data. Our results confirm the studies performed previously by Fischer and Callaghan [Phys. Rev. E 64, 011501 (2001)]. Time averaged velocity profiles display an unsheared “nematic gel.” In the range of applied shear rate, the flow field exhibits very fast temporal fluctuations. Suspicions for the presence of three-dimensional flow are evidenced and possible causes for a three-dimensional instability are discussed together with the coupling of wall slip to bulk dynamic.

Interplay between hydrodynamic and Brownian fluctuations in sedimenting colloidal suspensions

JT Padding — 2008-01-23T07:40:30-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 77, No. 1. (2008)

We apply a hybrid molecular dynamics and mesoscopic simulation technique to study the steady-state sedimentation of hard sphere particles for Peclet number (Pe) ranging from 0.08 to 12. Hydrodynamic backflow causes a reduction of the average sedimentation velocity relative to the Stokes velocity. We find that this effect is independent of Pe number. Velocity fluctuations show the expected effects of thermal fluctuations at short correlation times. At longer times, nonequilibrium hydrodynamic fluctuations are visible, and their character appears to be independent of the thermal fluctuations. The hydrodynamic fluctuations dominate the diffusive behavior even for modest Pe number, while conversely the short-time fluctuations are dominated by thermal effects for surprisingly large Pe numbers. Inspired by recent experiments, we also study finite sedimentation in a horizontal planar slit. In our simulations distinct lateral patterns emerge, in agreement with observations in the experiments.

Simulation of density segregation in vibrated beds

C Zeilstra — 2008-06-09T19:22:35-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 77, No. 3. (2008)

We have investigated by numerical simulation the density segregation of fine equal-sized bronze and glass particles subject to vertical vibrations. The model was found to be capable of predicting the two main segregation forms (“bronze on top” and “sandwich”) in roughly the same regions of the phase diagram as was found experimentally by Burtally et al. We investigated the effects of pressure air forcing, friction and restitution of kinetic energy in collisions, and box size on the segregation behavior. We find that next to the interstitial air friction also has a large influence on the formation of the sandwich structure.

Capillarylike Fluctuations at the Interface of Falling Granular Jets

Yacine Amarouchene — 2008-06-05T20:44:07-00:00

Physical Review Letters, Vol. 100, No. 21. (2008)

We study the interface fluctuations of a granular jet falling under gravity and show that for small scales they are the analog of the thermally induced capillary waves. Experimental results from radial height and velocity fluctuations, static correlation functions and capillary ripple velocities allow us to estimate a granular surface tension. The ultralow interfacial tensions measured (of the order of 100 µN/m) can be rationalized using a simple model.

Velocity fluctuations in a steadily sheared model foam

Ian Ono — 2008-04-29T00:58:30-00:00

Physical Review E, Vol. 67, No. 6. (18 June 2003), 061503.

Nonlinear stress and fluctuation dynamics of sheared disordered wet foam

Ethan Pratt — 2008-04-29T00:41:34-00:00

Physical Review E, Vol. 67, No. 5. (May 2003), 051402.

Activated hopping and dynamical fluctuation effects in hard sphere suspensions and fluids

Erica Saltzman — 2007-10-29T19:49:58-00:00

The Journal of Chemical Physics, Vol. 125, No. 4. (2006)

Single particle Brownian dynamics simulation methods are employed to establish the full trajectory level predictions of our nonlinear stochastic Langevin equation theory of activated hopping dynamics in glassy hard sphere suspensions and fluids. The consequences of thermal noise driven mobility fluctuations associated with the barrier hopping process are determined for various ensemble-averaged properties and their distributions. The predicted mean square displacements show classic signatures of transient trapping and anomalous diffusion on intermediate time and length scales. A crossover to a stronger volume fraction dependence of the apparent nondiffusive exponent occurs when the entropic barrier is of order the thermal energy. The volume fraction dependences of various mean relaxation times and rates can be fitted by empirical critical power laws with parameters consistent with ideal mode-coupling theory. However, the results of our divergence-free theory are largely a consequence of activated dynamics. The experimentally measurable alpha relaxation time is found to be very similar to the theoretically defined mean reaction time for escape from the barrier-dominated regime. Various measures of decoupling have been studied. For fluid states with small or nonexistent barriers, relaxation times obey a simple log-normal distribution, while for high volume fractions the relaxation time distributions become Poissonian. The product of the self-diffusion constant and mean alpha relaxation time increases roughly as a logarithmic function of the alpha relaxation time. The cage scale incoherent dynamic structure factor exhibits nonexponential decay with a modest degree of stretching. A nearly universal collapse of the different volume fraction results occurs if time is scaled by the mean alpha relaxation time. Hence, time-volume fraction superposition holds quite well, despite the presence of stretching and volume fraction dependent decoupling associated with the stochastic barrier hopping process. The relevance of other origins of dynamic heterogeneity (e.g., mesoscopic domains), and comparison of our results with experiments, simulations, and alternative theories, is discussed. ©2006 American Institute of Physics

Jamming and Fluctuations in Granular Drag

I Albert — 2007-09-27T03:46:04-00:00

Physical Review Letters, Vol. 84, No. 22. (29 May 2000), 5122.

We investigate the dynamic evolution of jamming in granular media through fluctuations in the granular drag force. The successive collapse and formation of jammed states give a stick-slip nature to the fluctuations which is independent of the contact surface between the grains and the dragged object; thus implying that the stress-induced collapse is nucleated in the bulk of the granular sample. We also find that while the fluctuations are periodic at small depths; they become “stepped” at large depths; a transition which we interpret as a consequence of the long-range nature of the force chains.

Thermal fluctuations of the shapes of droplets in dense and compressed emulsions

Hu Gang — 2008-04-27T00:26:02-00:00

Physical Review E, Vol. 52, No. 6. (1 December 1995), 6289.

Stress Fluctuations in a 2D Granular Couette Experiment: A Continuous Transition

Daniel Howell — 2008-04-27T00:21:13-00:00

Physical Review Letters, Vol. 82, No. 26. (28 June 1999), 5241.

Density fluctuations in vibrated granular materials

Edmund Nowak — 2007-06-03T19:15:17-00:00

Physical Review E, Vol. 57, No. 2. (February 1998), 1971.

We report systematic measurements of the density of a vibrated granular material as a function of time. Monodisperse spherical beads were confined to a cylindrical container and shaken vertically. Under vibrations; the density of the pile slowly reaches a final steady-state value about which the density fluctuates. We have investigated the frequency dependence and amplitude of these fluctuations as a function of vibration intensity Γ. The spectrum of density fluctuations around the steady state value provides a probe of the internal relaxation dynamics of the system and a link to recent thermodynamic theories for the settling of granular material. In particular; we propose a method to evaluate the compactivity of a powder; first put forth by Edwards and co-workers; that is the analog to temperature for a quasistatic powder. We also propose a stochastic model based on free volume considerations that captures the essential mechanism underlying the slow relaxation. We compare our experimental results with simulations of a one-dimensional model for random adsorption and desorption.

Elastic energy, fluctuations and temperature for granular materials

L Kondic — 2008-04-26T22:48:36-00:00

EPL (Europhysics Letters), Vol. 67, No. 2. (2004), pp. 205-211.

We probe, using a model system, elastic and kinetic energies for sheared granular materials. For large enough P/Ey (pressure/Young's modulus) and P/rv2 (P/kinetic energy density) elastic dominates kinetic energy, and energy fluctuations become primarily elastic in nature. This regime has likely been reached in recent experiments. We consider a generalization of the granular temperature, Tg, with both kinetic and elastic terms and that changes smoothly from one regime to the other. This Tg is roughly consistent with a temperature adapted from equilibrium statistical mechanics.

Nonequilibrium dynamics and fluctuation-dissipation relation in a sheared fluid

Ludovic Berthier — 2008-04-25T19:42:56-00:00

The Journal of Chemical Physics, Vol. 116, No. 14. (2002), pp. 6228-6242.

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Stress Fluctuations for Continuously Sheared Granular Materials

Brian Miller — 2008-03-24T22:01:06-00:00

Physical Review Letters, Vol. 77, No. 15. (7 October 1996), 3110.

Experiments on continuously sheared granular materials (glass spheres) with diameters 1.0≤ d ≤5 mm show large fluctuations in the normal stress; σ( t ). Experiments are carried out in an annular Couette geometry for rotation rates 0.003 < θ̇ < 1 rad/s. Power spectra from σ( t ); P (ω); show rate invariance in the fluctuations: θ̇ P (ω) is a function only of ω / θ̇; independent of θ̇. θ̇ P (ω) depends relatively weakly on d . The distributions of stresses ρ σ are similar to recent predictions for static arrays; but the width of ρ σ varies only weakly with d ; suggesting stronger spatial correlation effects than expected from theory.

Critical Fluctuations of Tense Fluid Membrane Tubules

Jean Fournier — 2008-03-19T17:24:33-00:00

Physical Review Letters, Vol. 98, No. 1. (2007)

We show that, contrary to planar membranes under tension, tense membrane tubules exhibit important critical fluctuations originating from unidimensional Goldstone modes. The latter yield unexpected behavior, such as correlations extending over the whole tube length and the increase of the fluctuating area over the projected area with increasing tension.

A simple physical model of liquid - glass transition: intrinsic fluctuating interactions and random fields hidden in glass-forming liquids

Hajime Tanaka — 2008-03-04T20:48:17-00:00

Journal of Physics: Condensed Matter, Vol. 10, No. 14. (1998), pp. L207-L214.

We propose that glass-forming liquids are intrinsically under the influences of both fluctuating interactions and random fields well-known in the field of spin systems. This is due to the frustration between the isotropic and anisotropic parts of effective intermolecular interactions. Our model indicates the existence of two key temperatures relevant to the glass transition, the density ordering point and the Vogel - Fulcher temperature . Between and , a system has features similar to the `Griffiths phase', while below it has those peculiar to the `spin-glass phase'. This picture naturally and universally explains vitrification behaviour from its strong to its fragile limit.

Spectral diffusion in liquids with fluctuating solvent responses: Dynamical heterogeneity and rate exchange

Ranko Richert — 2008-03-01T23:13:46-00:00

The Journal of Chemical Physics, Vol. 115, No. 3. (2001), pp. 1429-1434.

Dynamical fluctuation effects in glassy colloidal suspensions

Kenneth Schweizer — 2008-03-01T22:36:00-00:00

Current Opinion in Colloid & Interface Science, Vol. 12, No. 6. (December 2007), pp. 297-306.

Fundamental understanding of heterogeneous dynamics in concentrated glassy hard sphere fluids and colloidal suspensions, even at the single particle level, requires major theoretical advances. Recent simulations and confocal microscopy experiments suggest strong nongaussian dynamical fluctuation effects and activated transport emerge well before an apparent kinetic glass transition is reached. New theoretical approaches that can predict the observable signatures of intermittent large amplitude motions and the associated fluctuation phenomena are discussed. Comparisons are made with experiments, computer simulations, and prior theory for average dynamical properties.

Heterogeneous dynamics in liquids: fluctuations in space and time

Ranko Richert — 2007-12-26T12:51:28-00:00

Journal of Physics: Condensed Matter, Vol. 14, No. 23. (2002), pp. R703-R738.

The disordered nature of glass-forming melts gives rise to non-Arrhenius and non-exponential behaviour of their dynamics. With respect to the microscopic details involved in the structural relaxation, these materials have remained an unsolved puzzle for over a century. The observation of spatial heterogeneity regarding the dynamics provides an important step towards understanding the relation between the macroscopic properties of soft condensed matter and the molecular mechanisms involved. On the other hand, dynamic heterogeneity is the source of several new questions: What is the length scale and persistence time associated with such clusters of relaxation time? What is the signature of heterogeneity at high temperatures and in the glassy state? How do these features depend on the particular material and on the correlation function used for probing these heterogeneities? This work attempts to review the various approaches to heterogeneous dynamics and the generally accepted results, as well as some controversial issues. Undoubtedly, heterogeneity has provoked a number of novel experimental techniques targeted at studying glass-forming liquids at the molecular level. It will be emphasized that the picture of heterogeneity is a requirement for rationalizing an increasing number of experimental observations rather than just an alternative model for the dynamics of molecules.