Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userFormation of Laves Phases in Repulsive and Attractive Hard Sphere Suspensions
113
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
Colloidal Laves phases (LPs) of $MgCu_2$ type are promising precursors for diamond structure photonic materials. They have been predicted for hard sphere binary mixtures, but not yet observed. We here report a time resolved static light scattering study on their formation in a binary mixture of buoyant experimental hard sphere approximants (size ratio $Gamma=0.77$, molar fraction of small spheres $x_S = 0.76$) for volume fractions between melting and the glass transition. In line with theoretical expectation, all samples form LPs of $MgZn_2$ structure on the time scale of weeks to months. $MgNi_2$ structures are absent, $MgCu_2$ structures and randomly stacked LPs prevail at elevated volume fraction. The addition of small amounts of non-adsorbing polymer switches the interaction to depletion attractive and results in significantly accelerated crystallization kinetics and improved crystal quality.
rate research
Read More
Discontinuous shear thickening (DST) observed in many dense athermal suspensions has proven difficult to understand and to reproduce by numerical simulation. By introducing a numerical scheme including both relevant hydrodynamic interactions and granularlike contacts, we show that contact friction is essential for having DST. Above a critical volume fraction, we observe the existence of two states: a low viscosity, contactless (hence, frictionless) state, and a high viscosity frictional shear jammed state. These two states are separated by a critical shear stress, associated with a critical shear rate where DST occurs. The shear jammed state is reminiscent of the jamming phase of granular matter. Continuous shear thickening is seen as a lower volume fraction vestige of the jamming transition.
We present a comprehensive study of the slip and flow of concentrated colloidal suspensions using cone-plate rheometry and simultaneous confocal imaging. In the colloidal glass regime, for smooth, non-stick walls, the solid nature of the suspension causes a transition in the rheology from Herschel-Bulkley (HB) bulk flow behavior at large stress to a Bingham-like slip behavior at low stress, which is suppressed for sufficient colloid-wall attraction or colloid-scale wall roughness. Visualization shows how the slip-shear transition depends on gap size and the boundary conditions at both walls and that partial slip persist well above the yield stress. A phenomenological model, incorporating the Bingham slip law and HB bulk flow, fully accounts for the behavior. Microscopically, the Bingham law is related to a thin (sub-colloidal) lubrication layer at the wall, giving rise to a characteristic dependence of slip parameters on particle size and concentration. We relate this to the suspensions osmotic pressure and yield stress and also analyze the influence of van der Waals interaction. For the largest concentrations, we observe non-uniform flow around the yield stress, in line with recent work on bulk shear-banding of concentrated pastes. We also describe residual slip in concentrated liquid suspensions, where the vanishing yield stress causes coexistence of (weak) slip and bulk shear flow for all measured rates.
Diffusion in bidisperse Brownian hard-sphere suspensions is studied by Stokesian Dynamics (SD) computer simulations and a semi-analytical theoretical scheme for colloidal short-time dynamics, based on Beenakker and Mazurs method [Physica 120A, 388 (1983) & 126A, 349 (1984)]. Two species of hard spheres are suspended in an overdamped viscous solvent that mediates the salient hydrodynamic interactions among all particles. In a comprehensive parameter scan that covers various packing fractions and suspension compositions, we employ numerically accurate SD simulations to compute the initial diffusive relaxation of density modulations at the Brownian time scale, quantified by the partial hydrodynamic functions. A revised version of Beenakker and Mazurs $deltagamma$-scheme for monodisperse suspensions is found to exhibit surprisingly good accuracy, when simple rescaling laws are invoked in its application to mixtures. The so-modified $deltagamma$ scheme predicts hydrodynamic functions in very good agreement with our SD simulation results, for all densities from the very dilute limit up to packing fractions as high as $40%$.
Dense suspensions of model hard-sphere-like colloids, with different particle sizes, are examined experimentally in the glass state, under shear and extensional rheology. Under steady shear flow we detect Discontinuous Shear Thickening (DST) above a critical shear rate. Start-up shear experiments show stress overshoots in the vicinity of the onset of DST related with a change in microscopic morphology, as the sample shows dilatancy effects. The analysis of the normal stress together with direct sample observation by high speed camera, indicates the appearance of positive N1 and dilation behavior at the shear thickening onset. Dilatancy effects are detected also under extensional flow. The latter was studied through capillary breakup and filament stretching experimental setups, where liquid-like response is seen for strain rate lower than a critical strain rate and solid like-behavior for higher strain rates. Monitoring the filament thinning processes under different conditions (volume fractions and strain rates) we have created a state diagram where all responses of a hard-sphere suspension (Newtonian, shear thinning, shear thickening, dilatant) are shown. We finally compare the shear thickening response of these hard-sphere-like suspensions and glasses in shear with that in extensional flow.
We consider the dynamics of a suspension of hard sphere-like particles in the proximity of its glass transition, the region where the intermediate scattering functions show significant aging. The time correlation function of the longitudinal particle current shows no dependence on age and reveals behaviour of ideal super-packed fluid and glass. The power laws of the beta process of the idealised mode coupling theory are exposed directly without reliance on fitting parameters. We proffer a mechanism linking the reversible/ageless dynamics, which constitutes the beta-process, and the irreversible aging dynamics. The latter verifies predictions of spin-glass theories.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
