Sol-Gel transition of concentrated colloidal suspensions

Diffusing-wave spectroscopy (DWS) was used to follow the sol-gel transition of concentrated colloidal suspensions. We present a new technique based on a sandwich of two scattering cells aimed to overcome the problem of nonergodicity in DWS of solidlike systems. Using this technique we obtain quantitative information about the microscopic dynamics all the way from an aggregating suspension to the final gel, thereby covering the whole sol-gel transition. At the gel point a dramatic change of the particle dynamics from diffusion to a subdiffusive arrested motion is observed. A critical-power-law behavior is found for the time evolution of the maximum mean square displacement delta(2) probed by a single particle in the gel.     

Effects of the rate of evaporation and film thickness on nonuniform drying of film-forming concentrated colloidal suspensions

In this paper, we report on nonuniform distribution of film-forming waterborne colloidal suspensions above the critical concentration _c of the colloidal glass transition during drying. We found that colloidal suspension films dry nonuniformly when the initial rate of evaporation E and/or the initial thickness l₀ are high. We found that a Peclet number Pe, defined as Pe = El₀/D, where D is the diffusion coefficient of the colloids in the diluted suspensions, does not predict uniformity of drying of the concentrated suspensions, contrary to the reported work on drying of diluted suspensions. Since the colloidal particles are crowded and their diffusive motion is restricted in concentrated suspensions, we assumed that above _c water is transported to the drying surface by hydrodynamic flow along the osmotic pressure gradient. The permeability of water through channels between deforming particles is estimated by adapting the theory of foam drainage. We defined a new Peclet number Pe by substituting the transport coefficient of flow (defined as the permeability divided by the viscosity, multiplied by the osmotic pressure gradient) for the diffusion coefficient. This extended Peclet number predicted the nonuniform drying with a criterion of Pe > 1. These results indicate that the mechanism of water transport to the drying surface in concentrated suspensions is water permeation by osmotic pressure, which is faster than mutual diffusion between water and particles --that has been observed in diluted suspensions and discussed by Routh and Russel. The theory fits well the experimental drying curves for various thicknesses and rates of evaporation. The particle distribution in the drying films is also estimated and it is indicated that the latex distribution is nonuniform when Pe > 1.     

Dynamics of polymers in flowing colloidal suspensions

Using hydrodynamic simulations we examine the behavior of single polymers in a confined colloidal suspension under flow. We study the conformations of both, collapsed and noncollapsed polymers. Our results show that the presence of the colloids has a pronounced effect on the unfolding and refolding cycles of collapsed polymers, but does not have a large effect for noncollapsed polymers. Further inspection of the conformations reveals that the strong flow around the colloids and the direct physical compressions exerted on a globular polymer diffusing in between colloidal shear bands largely facilitate the initiation and unraveling of the globular chains. These results are important for rheological studies of (bio)polymer-(bio)colloid mixtures.     

Onset of buckling in drying droplets of colloidal suspensions

Minute concentrations of suspended particles can dramatically alter the behavior of a drying droplet. After a period of isotropic shrinkage, similar to droplets of a pure liquid, these droplets suddenly buckle like an elastic shell. While linear elasticity is able to describe the morphology of the buckled droplets, it fails to predict the onset of buckling. Instead, we find that buckling is coincident with a stress-induced fluid to solid transition in a shell of particles at a droplet's surface, occurring when attractive capillary forces overcome stabilizing electrostatic forces between particles.     

Dynamics and sedimentation velocity in charge-stabilized colloidal suspensions

Summary Charge-stabilized suspensions are characterized by the strong electrostatic interactions between the particles so that rather dilute systems may exhibit strong correlation resulting in a well-developed short-range order. This microstructure, quantitatively described by the pair distribution functiong(r), is rather different from that of (uncharged) hard spheres. It is shown how this difference affects the ?hydrodynamic function?H(k), which appears in the expression for the first cumulant Γ(k)=k ² D _eff(k)=k ² H(k)/S(k) of the dynamic autocorrelation function. Without hydrodynamic interaction,H(k)=D ⁰, which is the free-diffusion coefficient. Using pairwise additive hydrodynamic interaction and the lowest-order many-body theory of hydrodynamic interaction, it is found thatH(k) can deviate considerably fromD ⁰ even for systems of volume fractions ϕ as low as 10⁻³. These effects are more pronounced for collective diffusion than for self-diffusion. SinceH(k=0) is closely related to the sedimentation velocity, we have studied this quantity as a function of volume fraction. It is found that (H(0)/D ⁰) −1 scales asφ ^1/3 at low ϕ in salt-free suspensions. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

Dynamics in dense suspensions of charge-stabilized colloidal particles

The dynamic behavior of charge-stabilized colloidal particles in suspension was studied by photon correlation spectroscopy with coherent X-rays (XPCS). The short-time diffusion coefficient, D(Q) , was measured for volume concentrations φ ⩽ 0.18 and compared to the free particle diffusion constant D₀ and the static structure factor S(Q) . The data show that indirect, hydrodynamic interactions are relevant for the system and hydrodynamic functions were derived. The results are in striking contrast to the predictions of the PA (pairwise-additive approximation) model, but show features typical for a hard-sphere system. The observed mobility is however considerably smaller than the one of a respective hard-sphere system. The hydrodynamic functions can be modelled quantitatively if one allows for an increased effective viscosity relative to the hard-sphere case.     

Nonequilibrium Brownian dynamics simulations of shear thinning in concentrated colloidal suspensions

The effect of interparticle forces on shear thinning in concentrated aqueous and nonaqueous colloidal suspensions was studied using nonequilibrium Brownian dynamics. Hydrodynamic interactions among particles were neglected. Systems of 108 particles were studied at volume fractions of 0.2 and 0.4. For the nonaqueous systems, shear thinning could be correlated with the gradual breakup of small flocs present because of the weak, attractive secondary minimum in the interparticle potential. At the highest shear rate for=0.4, the particles were organized into a hexagonally packed array of strings. For the strongly repulsive aqueous systems, the viscosity appeared to be a discontinuous function of the shear rate. For=0.4, this discontinuity coincided with a transition from a disordered state to a lamellar structure for the suspension.     

Shear zones and wall slip in the capillary flow of concentrated colloidal suspensions

We image the flow of a nearly random close packed, hard-sphere colloidal suspension (a "paste") in a square capillary using confocal microscopy. The flow consists of a "plug" in the center while shear occurs localized adjacent to the channel walls, reminiscent of yield-stress fluid behavior. However, the observed scaling of the velocity profiles with the flow rate strongly contrasts yield-stress fluid predictions. Instead, the velocity profiles can be captured by a theory of stress fluctuations originally developed for chute flow of dry granular media. We verified this both for smooth and rough walls.     

Energy landscape picture of overaging and rejuvenation in a sheared glass

Molecular simulations and an energy landscape analysis are used to investigate the effects of shear on aging in a glass. Shear beyond the yield point is shown to change the state of a glass such that it resembles (but is not identical to) a different stage in the aging process. A cycle of large strain rejuvenates the glass by relocating the system to shallower energy minima, while a cycle of small strain overages the glass by relocating the system to deeper energy minima. The balance between overaging and rejuvenation is controlled by how well the glass was initially annealed.     

Rejuvenation and overaging in a colloidal glass under shear

We report the modifications of the microscopic dynamics of a colloidal glass submitted to shear. We use multispeckle diffusing wave spectroscopy to monitor the evolution of the spontaneous slow relaxation processes after the samples have been submitted to various straining. We show that high shear rejuvenates the system and accelerates its dynamics, whereas moderate shear over-ages the system. We analyze these phenomena within the frame of the Bouchaud's trap model.     

Microstructure identification during crystallization of charged colloidal suspensions

We use a polarized light microscope in its orthoscopic and conoscopic arrangements and laser light diffraction to study the effect of particle volume fraction and cell thickness on the microstructure of crystallizing suspensions of negatively charged polystyrene microspheres. Deionized suspensions of these particles nucleate at random sites in the bulk of the suspension to give a variety of structures, orientations and sizes. Orthoscopic observation of the Bragg diffraction colors between crossed polars and conoscopic inspection of the interference figures reveal structural details. We find that the crystallites grow by parallel stacking of the (111) layers to single and twin fcc structures. At moderate volume fractions, Φ ≈ 0.09, the structures are essentially “frozen” in space by their neighbors. At lower concentrations, Φ ≈ 0.05, the crystallites are larger with smoother boundaries and exhibit a range of colors. In thick cells, L ≥ 200 μm, and Φ ≤ 0.05, the colored crystallites become dark with time as they align with the (111) planes parallel to the cell walls. In thin, 50μm cells and Φ ≤ 0.05, this alignment is enhanced. We demonstrate that striated crystallites with lamellae of alternating colors and varying width are polysynthetic fcc twins with (111) twin plane. The number density of twin crystals and the frequency of striations decrease with decreasing volume fraction.     

Modeling of concentrated suspensions

The constitutive equation of a concentrated suspension of spherical particles in a Newtonian medium is derived. To this end the method of local volume averaging is employed. To calculate the contribution of the particles to the stress tensor it is assumed that the stress generated in the interstitial holes between the particles is negligible compared to the stress generated in !he narrow gaps separating the particles. The use of the resulting expression is demonstrated with two examples on a cubical arrangement of particles: pure shear and simple shear. Furthermore, the validity of the lubrication approximation employed in this work is checked against the results derived by Nunan and Keller for periodic suspensions.     

Pattern formation during the drying of a colloidal suspension

Receding contact lines of colloidal suspensions are studied in the presence of drying, inside Hele-Shaw cells. At high velocity the contact line movement is continuous and the particle deposition is uniform. At small velocity, a periodic pinning-unpinning of the contact line is observed leading to a patterning of the substrate. We focused on the correlation between the deposition pattern and the pinning force that grows during the pinning. Our results strongly indicate that this pinning force is proportional to the macroscopic slope of the deposit and accounted by a simple capillary balance.     

Towardsab Initio simulations of concentrated suspensions

Determination of many-body interactions between particles of arbitrary shape in a viscous fluid is a key problem in the simulation of concentrated suspensions. Three-dimensional flows involving such complex fluid-solid boundaries are beyond the scope of spatial methods, even on supercomputers. Boundary integral methods convert the three-dimensional PDE to a two-dimensional integral equation. Unfortunately, conventional boundary methods yield Fredholm integral equations of the first kind, and dense linear systems which are too large for accurate solution. We have pursued a different boundary integral formulation, which yields Fredholm integral equations of the second kind; these arc amenable to iterative solution. The velocity representation involves a compact operator, so a discrete spectrum results. Wielandt deflations give dramatic reductions in the spectral radius and accurate solutions are obtained after only a few iterations (typically less than 10). An analytic construction of the spectrum for sphere sphere interactions confirms these numerical results. The mathematics is similar to that encountered in the mixing ofd-atomic orbitals to form bonding/antibonding molecular orbitals in transition metals. The memory-saving version of our code can be implemented directly on a dedicated MicroVAX to solve problems involving clusters of less than a dozen particles. For a fixed number of processors, the algorithm grows essentially asN ², whereN is the system size, so computational times are readily estimated on more powerful super-minicomputers and supercomputers using standard dot-product benchmarks. The algorithm is especially ideal for gigaflop and teraflop parallel array processors under construction in a number of computer companies; an analysis of the spectrum reveals that asynchronous iterative methods will converge, leading the way to a rigorous formulation of screening concepts for suspended particles of arbitrary shape.     

Shear-driven solidification of dilute colloidal suspensions

We show that shear-induced solidification of dilute charge-stabilized colloids is due to the interplay between shear-induced formation and breakage of large non-Brownian clusters. While their size is limited by breakage, their number density increases with shearing time. Upon flow cessation, the dense packing of clusters interconnects into a rigid state by means of grainy bonds, each involving a large number of primary colloidal bonds. The emerging picture of shear-driven solidification in dilute colloidal suspensions combines the gelation of Brownian systems with the jamming of athermal systems.     

Shear-induced configurations of confined colloidal suspensions

We show that geometric confinement dramatically affects the shear-induced configurations of dense monodisperse colloidal suspensions; a new structure emerges, where layers of particles buckle to stack in a more efficient packing. The volume fraction in the shear zone is controlled by a balance between the viscous stresses and the osmotic pressure of a contacting reservoir of unsheared particles. We present a model that accounts for our observations and helps elucidate the complex interplay between particle packing and shear stress for confined suspensions.     

Laponite: aging and shear rejuvenation of a colloidal glass

We study the nonlinear rheological behavior and the microscopic particle dynamics for a colloidal glass, to see whether recently developed models for driven glassy systems can be applied to predict the rheology. Qualitatively, all the findings predicted by the models can be retrieved in our system. Notably, the viscosity decreases strongly with the shear rate. Since it is difficult to predict non-Newtonian viscosities of colloidal systems due to long-ranged hydrodynamic interactions, this shows the promise of this approach for predicting flow behavior. In addition, the measurements allow us to relate the microscopic diffusion dynamics to the macroscopic viscosity of the system.