CiteULike: dchen's Prasad

Granular flow in rotating cylinders with noncircular cross sections

DVN Prasad — 2008-06-08T23:50:12-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 77, No. 4. (2008)

An experimental and theoretical study is carried out of the flow of granular material in cylinders with different cross-sectional shapes rotated about their axes. The flow of particles in such geometries is confined to a shallow layer at the free surface. The length and thickness of the layer shrink and expand periodically with rotation of the cylinder, resulting in chaotic advection and improved mixing of passive tracers. Experimental results obtained by flow visualization are reported for quasi-two-dimensional mixers half filled with glass beads. A depth-averaged flow model to predict the time-varying layer thickness profile is presented, along with a perturbation solution in terms of a small parameter k, which is the ratio of the maximum layer thickness to the half length of the layer (L), at the cross-section orientation when the length is minimum. To the lowest order [O(k0)], the model predicts that the layer profiles scaled with L() at different mixer orientation angles () are identical and the same as that for a circle. The measured layer thickness profiles averaged over different orientations of noncircular mixers match reasonably well with the theory, but the standard deviations are larger for the noncircular cylinders compared to the circle. The O(k) perturbation solution and the full theory both predict that the scaled layer thickness varies periodically; the deviations are proportional to the rate of change of the length with orientation. The perturbation solution gives results close to those from the numerical solution except at cylinder orientations when the length of the flowing layer changes sharply. The measured variation of the scaled midlayer thickness with orientation for all geometries is well predicted by the theory.

Shake-gels: shear-induced gelation of laponite-PEO mixtures

J Zebrowski — 2008-04-24T22:24:38-00:00

Colloids and Surfaces A: Physicochemical and Engineering Aspects, Vol. 213, No. 2-3. (12 February 2003), pp. 189-197.

Suspensions of clay particles (laponite), mixed with poly(ethylene oxide) (PEO) undergo a dramatic shear thickening when subjected to vigorous shaking, which transforms them from a low viscosity fluid into a [`]shake-gel', a solid with elasticity sufficient enough to support its own weight. The shake-gel is reversible, relaxing back to a fluid with a relaxation time that is strongly dependent on PEO concentration. Shake-gels are observed for PEO concentrations slightly below the threshold for complete saturation of the laponite particles by the polymer. Light scattering measurements confirm that the PEO is adsorbed on the surface of the laponite particles, and suggests that shear induces a bridging between the colloidal particles, resulting in a gel network which spans the system. Desorption of the polymer reduces the bridging and thus relaxes the network.

Universal features of the ﬂuid to solid transition for attractive colloidal particles

Prasad — 2008-04-24T18:37:13-00:00

Faraday Discuss., 2003

Attractive colloidal particles can exhibit a fluid to solid phase transition if the magnitude of the attractive interaction is sufficiently large, if the volume fraction is sufficiently high, and if the applied stress is sufficiently small. The nature of this fluid to solid transition is similar for many different colloid systems, and for many different forms of interaction. The jamming phase transition captures the common features of these fluid to solid translations, by unifying the behavior as a function of the particle volume fraction, the energy of interparticle attractions, and the applied stress. This paper describes the applicability of the jamming state diagram, and highlights those regions where the fluid to solid transition is still poorly understood. It also presents new data for gelation of colloidal particles with an attractive depletion interaction, providing more insight into the origin of the fluid to solid transition.

Limits to Gelation in Colloidal Aggregation

S Manley — 2008-04-23T23:55:21-00:00

Physical Review Letters, Vol. 93, No. 10. (2004), 108302.

Time-Dependent Strength of Colloidal Gels

S Manley — 2008-04-23T21:08:32-00:00

Physical Review Letters, Vol. 95, No. 4. (2005)

Colloidal silica gels are shown to stiffen with time, as demonstrated by both dynamic light scattering and bulk rheological measurements. Their elastic moduli increase as a power law with time, independent of particle volume fraction; however, static light scattering indicates that there are no large-scale structural changes. We propose that increases in local elasticity arising from bonding between neighboring colloidal particles can account for the strengthening of the network, while preserving network structure.

Microscopic Structure and Elasticity of Weakly Aggregated Colloidal Gels

AD Dinsmore — 2007-09-19T16:41:52-00:00

Physical Review Letters, Vol. 96, No. 18. (2006)

We directly probe the microscopic structure, connectivity, and elasticity of colloidal gels using confocal microscopy. We show that the gel is a random network of one-dimensional chains of particles. By measuring thermal fluctuations, we determine the effective spring constant between pairs of particles as a function of separation; this is in agreement with the theory for fractal chains. Long-range attractions between particles lead to freely rotating bonds, and the gel is stabilized by multiple connections among the chains. By contrast, short-range attractions lead to bonds that resist bending, with dramatically suppressed formation of loops of particles.