Elastic flow instability in nanotube suspensions

We report an elastic instability associated with flow-induced clustering in semidilute non-Brownian colloidal nanotubes. Rheo-optical measurements are compared with simulations of mechanical flocculation in sheared fiber suspensions, and the evolving structure is characterized as a function of confinement and shear stress. The transient rheology is correlated with the evolution of highly elastic vorticity-aligned aggregates, with the underlying instability being somewhat ubiquitous in complex fluids.     

Nematic-wetted colloids in the isotropic phase: pairwise interaction, biaxiality, and defects

We calculate the interaction between two spherical colloidal particles embedded in the isotropic phase of a nematogenic liquid. The surface of the particles induces wetting nematic coronas that mediate an elastic interaction. In the weak wetting regime, we obtain exact results for the interaction energy and the texture, showing that defects and biaxiality arise, although they are not topologically required. We evidence rich behaviors, including the possibility of reversible colloidal aggregation and dispersion. Complex anisotropic self-assembled phases might be formed in dense suspensions.     

Modelling the evaporation of thin films of colloidal suspensions using dynamical density functional theory

Recent experiments have shown that various structures may be formed during the evaporative dewetting of thin films of colloidal suspensions. Nanoparticle deposits of strongly branched 'flower-like', labyrinthine and network structures are observed. They are caused by the different transport processes and the rich phase behaviour of the system. We develop a model for the system, based on a dynamical density functional theory, which reproduces these structures. The model is employed to determine the influences of the solvent evaporation and of the diffusion of the colloidal particles and of the liquid over the surface. Finally, we investigate the conditions needed for 'liquid-particle' phase separation to occur and discuss its effect on the self-organized nanostructures.     

Dynamic domain formation in membranes: Thickness-modulation-induced phase separation

A simple model investigates the amplification of fluctuations on membranes constituted of two lipids having different lengths. Van der Waals and electrostatic interactions across the lipid bilayer result in a destabilization favoring thickness variations of the membrane. Close to spontaneous demixing of the two components, the additional gain in free energy due to thickness undulations shifts the stability boundary which promotes phase separation into domains. Interestingly, this effect can be induced by an applied electric field or membrane potential. In biological systems, the dynamic model presented here indicates that electric fields might be important for controlling phase separation and the formation of domains called rafts.Received: 26 March 2004, Published online: 15 July 2004PACS: 87.16.Dg Membranes, bilayers, and vesicles - 68.35.Rh Phase transitions and critical phenomena - 82.70.Uv Surfactants, micellar solutions, vesicles, lamellae, amphiphilic systems, (hydrophilic and hydrophobic interactions)     

Transport coefficients and mechanical response in hard-disk colloidal suspensions

We investigate the transport properties and mechanical response of glassy hard disks using nonlinear Langevin equation theory.We derive expressions for the elastic shear modulus and viscosity in two dimensions on the basis of thermalactivated barrier-hopping dynamics and mechanically accelerated motion.Dense hard disks exhibit phenomena such as softening elasticity,shear-thinning of viscosity,and yielding upon deformation,which are qualitatively similar to dense hard-sphere colloidal suspensions in three dimensions.These phenomena can be ascribed to stress-induced "landscape tilting".Quantitative comparisons of these phenomena between hard disks and hard spheres are presented.Interestingly,we find that the density dependence of yield stress in hard disks is much more significant than in hard spheres.Our work provides a foundation for further generalizing the nonlinear Langevin equation theory to address slow dynamics and rheological behavior in binary or polydisperse mixtures of hard or soft disks.     

Lattice model for colloidal gels and glasses

We study a lattice model of attractive colloids. It is exactly solvable on sparse random graphs. As the pressure and temperature are varied, it reproduces many characteristic phenomena of liquids, glasses, and colloidal systems such as ideal gel formation, liquid-glass phase coexistence, jamming, or the re-entrance of the glass transition.     

Static and time-dependent properties of polymer gels around the volume phase transition

Recent progress in the study of the volume phase transition of polymer gels is reviewed. The phenomenological theories of swelling equilibrium and phase transition of gels are summarized, and some basic experimental results on poly-N-isopropylacrylamide (NIPA) gels are compared with the prediction from these theories. Special attention is paid to the elastic properties of the gel network near the volume phase transition. The effect of external stresses on the swelling and the phase transition is analyzed. Some anomalous and unique characteristics revealed in NIPA gels such as shape- and size-dependent swelling and phase transition properties, curious phase coexistence, and domain structure are presented. Experimental results on some time-dependent phenomena such as phase separation, spinodal decomposition, and pattern formation are also presented and discussed. Some problems inherent to gels from biological bodies are briefly discussed.     

Inter-lamellar interactions modulated by addition of guest components

We have investigated the effects of a guest component (polymer or spherical colloidal particle) confined between flexible lamellar slits on the inter-lamellar interaction by means of a small-angle X-ray scattering technique and a neutron spin echo technique. The dominant interaction between flexible lamellar membranes without guest components is the Helfrich mechanism. The addition of a neutral polymer into the lamellar phase induces an attractive inter-lamellar interaction and finally destabilizes the lamellar phase. On the other hand, spherical colloidal particles confined between flexible lamellar membranes reduce the undulational fluctuations of lamellae and bring a repulsive inter-lamellar interaction. The behavior of the layer compression modulus of the lamellar membrane containing colloidal particles is well described by the entropical repulsive inter-lamellar interaction driven by steric hindrance.Received: 26 March 2004, Published online: 4 May 2004PACS: 82.70.Uv Surfactants, micellar solutions, vesicles, lamellae, amphiphilic systems, (hydrophilic and hydrophobic interactions) - 61.25.Hq Macromolecular and polymer solutions; polymer melts; swelling - 83.80.Hj Suspensions, dispersions, pastes, slurries, colloids - 89.75.Fb Structures and organization in complex systems     

Equilibrium cluster phases and low-density arrested disordered states: the role of short-range attraction and long-range repulsion

We study a model in which particles interact with short-ranged attractive and long-ranged repulsive interactions, in an attempt to model the equilibrium cluster phase recently discovered in sterically stabilized colloidal systems in the presence of depletion interactions. At low packing fractions, particles form stable equilibrium clusters which act as building blocks of a cluster fluid. We study the possibility that cluster fluids generate a low-density disordered arrested phase, a gel, via a glass transition driven by the repulsive interaction. In this model the gel formation is formally described with the same physics of the glass formation.     

Phase separation in mixtures of colloids and long ideal polymer coils

Colloidal suspensions with free polymer coils which are larger than the colloidal particles are considered. The polymer-colloid interaction is modeled by an extension of the Asakura-Oosawa model. Phase separation occurs into dilute and dense fluid phases of colloidal particles when polymer is added. The critical density of this transition tends to zero as the size of the polymer coils diverges.     

The width of Liesegang bands: A study using moving boundary model and simulation

The pattern formation in reaction–diffusion systems was studied by invoking the provisions contained in the moving boundary model. The model claims that the phase separation mechanism is responsible for separating the colloidal phase of precipitants into band and non-band regions. The relation between the band separation and its width are invariably related to the concentration of the reacting components. It was observed that this model provides critical condition for the band formation in semi-idealized diffusion systems. An algorithm for generating the band structure was designed, and the simulated pattern shows a close resemblance with the experimentally observed ones.     

Instabilities in wormlike micelle systems

Shear-banding is ubiquitous in complex fluids. It is related to the organization of the flow into macroscopic bands bearing different viscosities and local shear rates and stacked along the velocity gradient direction. This flow-induced transition towards a heterogeneous flow state has been reported in a variety of systems, including wormlike micellar solutions, telechelic polymers, emulsions, clay suspensions, colloidal gels, star polymers, granular materials, or foams. In the past twenty years, shear-banding flows have been probed by various techniques, such as rheometry, velocimetry and flow birefringence. In wormlike micelle solutions, many of the data collected exhibit unexplained spatio-temporal fluctuations. Different candidates have been identified, the main ones being wall slip, interfacial instability between bands or bulk instability of one of the bands. In this review, we present experimental evidence for a purely elastic instability of the high shear rate band as the main origin for fluctuating shear-banding flows.      

Shear-induced migration in colloidal suspensions

Using Brownian dynamics simulations, we perform a systematic investigation of the shear-induced migration of colloidal particles subject to Poiseuille flow in both cylindrical and planar geometry. We find that adding an attractive component to the interparticle interaction enhances the migration effect, consistent with recent simulation studies of platelet suspensions. Monodisperse, bidisperse and polydisperse systems are studied over a range of shear-rates, considering both steady-states and the transient dynamics arising from the onset of flow. For bidisperse and polydisperse systems, size segregation is observed.     

Correlations in the elastic response of dense random packings

Results are presented for the autocorrelation function of the vortexlike nonaffine piece of the linear elastic displacement field in dense random bidisperse packings of harmonically repulsive disks in 2D. The autocorrelation function is shown to scale precisely with the length of the simulation cell in systems ranging from 20 to 100 particles across. It is shown that, to first order, the displacement fields can be thought to arise from the action of uncorrelated local random forcing of a homogeneous elastic sheet, and a theory is presented which gives excellent quantitative agreement with the form of the correlation functions. These results suggest measurements to be made in many types of densely packed, random materials where the elastic displacement fields are accessible experimentally such as granular materials, dense emulsions, colloidal suspensions, etc.     

Heterogeneous diffusion in a reversible gel

We introduce a microscopically realistic model of a physical gel and use computer simulations to study its static and dynamic properties at thermal equilibrium. The phase diagram comprises a sol phase, a coexistence region ending at a critical point, a gelation line determined by geometric percolation, and an equilibrium gel phase unrelated to phase separation. The global structure of the gel is homogeneous, but the stress is only supported by a fractal network. The gel dynamics is highly heterogeneous and we propose a theoretical model to quantitatively describe dynamic heterogeneity in gels. We elucidate several differences between the dynamics of gels and that of glass formers.     

Novel experimentally observed phenomena in soft matter

Soft materials such as colloidal suspensions, polymer solutions and liquid crystals are constituted by mesoscopic entities held together by weak forces. Their mechanical moduli are several orders of magnitude lower than those of atomic solids. The application of small to moderate stresses to these materials results in the disruption of their microstructures. The resulting flow is non-Newtonian and is characterized by features such as shear rate-dependent viscosities and non-zero normal stresses. This article begins with an introduction to some unusual flow properties displayed by soft matter. Experiments that report a spectrum of novel phenomena exhibited by these materials, such as turbulent drag reduction, elastic turbulence, the formation of shear bands and the existence of rheological chaos, flow-induced birefringence and the unusual rheology of soft glassy materials, are reviewed. The focus then shifts to observations of the liquid-like response of granular media that have been subjected to external forces. The article concludes with examples of the patterns that emerge when certain soft materials are vibrated, or when they are displaced with Newtonian fluids of lower viscosities.     

Aging, yielding, and shear banding in soft colloidal glasses

Soft colloidal interactions in colloidal glasses are modeled using suspensions of multiarm star polymers. Using a preshearing protocol that ensures a reproducible initial state ("rejuvenation" of the system), we report here the evolution of the flow curve from monotonically increasing to one dominated by a stress plateau, demonstrating a corresponding shear-banded state. Phenomenological understanding is provided through a scalar model that describes the free-energy landscape.