[ { "id": "authors:gkdqr-90866", "collection": "authors", "collection_id": "gkdqr-90866", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20170405-095705528", "type": "article", "title": "Unsteady shear flows of colloidal hard-sphere suspensions by dynamic simulation", "author": [ { "family_name": "Marenne", "given_name": "St\u00e9phanie", "clpid": "Marenne-S" }, { "family_name": "Morris", "given_name": "Jeffrey F.", "clpid": "Morris-J-F" }, { "family_name": "Foss", "given_name": "David R.", "clpid": "Foss-D-R" }, { "family_name": "Brady", "given_name": "John F.", "orcid": "0000-0001-5817-9128", "clpid": "Brady-J-F" } ], "abstract": "The rheology during the start-up and cessation of simple shear flow has been investigated for near hard-sphere colloidal suspensions. Simulations augmented by theoretical analysis are used to determine how the non-Newtonian stress development and relaxation depend on the microstructure. Accelerated Stokesian dynamics (ASD) and Brownian dynamics (BD) simulations are used for 0.05\u2009\u2264\u2009Pe\u2009\u2264\u2009500 in concentrated freely flowing suspensions; the P\u00e9clet number defining the ratio of shear to thermal motion is Pe=3\u03c0\u03b7\u03b3 \u0307a^3/kT with \u03b7 the suspending fluid viscosity, \u03b3 \u0307 the shear rate, and kT the thermal energy. Theoretical predictions based on the Smoluchowski equation for dilute suspensions are made, and these are primarily used for comparison with results from BD simulations in which hydrodynamic interactions are neglected. For suspensions with hydrodynamics, simulations by ASD are used to probe start-up and flow cessation over a large range of Pe; these studies focus on solid volume fraction \u03d5=0.4, with more limited examinations at other \u03d5. The use of both BD and ASD simulations allows us to discriminate hydrodynamic interaction effects on the suspension rheology. The Brownian stresses computed by either method exhibit overshoots of their steady state value during the start-up of shear flow. The overshoots occur at strain amplitudes which depend on Pe, and the overshoot is described by a model based on extension of the concept of cage-breaking from glass dynamics. Results from the relaxation of a sheared suspension show that the distortion of the pair distribution function from its equilibrium form has a fast radial relaxation and a slow angular relaxation. The various rheometric functions (relative viscosity; first and second normal stress differences) are found to respond on different timescales, reflecting their different dependences on the flow-induced structure. A re-examination of steady shear flow allows us to find normal stress differences which tend properly toward zero at small Pe, unlike prior work; the discrepancy is found to be due to finite size scaling, as small simulations used in prior work resulted in excessively large normal stress responses at small Pe.", "doi": "10.1122/1.4979005", "issn": "0148-6055", "publisher": "American Institute of Physics", "publication": "Journal of Rheology", "publication_date": "2017-04", "series_number": "3", "volume": "61", "issue": "3", "pages": "477-501" }, { "id": "authors:je87t-cxf30", "collection": "authors", "collection_id": "je87t-cxf30", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:FOSjfm00", "type": "article", "title": "Structure, diffusion and rheology of Brownian suspensions by Stokesian Dynamics simulation", "author": [ { "family_name": "Foss", "given_name": "David R.", "clpid": "Foss-D-R" }, { "family_name": "Brady", "given_name": "John F.", "orcid": "0000-0001-5817-9128", "clpid": "Brady-J-F" } ], "abstract": "The non-equilibrium behaviour of concentrated colloidal dispersions is studied using Stokesian Dynamics, a molecular-dynamics-like simulation technique for analysing suspensions of particles immersed in a Newtonian fluid. The simulations are of a monodisperse suspension of Brownian hard spheres in simple shear flow as a function of the P\u00e9clet number, Pe, which measures the relative importance of hydrodynamic and Brownian forces, over a range of volume fraction 0.316 [less-than-or-eq, slant] [phi] [less-than-or-eq, slant] 0.49. For Pe < 10, Brownian motion dominates the behaviour, the suspension remains well-dispersed, and the viscosity shear thins. The first normal stress difference is positive and the second negative. At higher Pe, hydrodynamics dominate resulting in an increase in the long-time self-diffusivity and the viscosity. The first normal stress difference changes sign when hydrodynamics dominate. Simulation results are shown to agree well with both theory and experiment.", "doi": "10.1017/S0022112099007557", "issn": "0022-1120", "publisher": "Journal of Fluid Mechanics", "publication": "Journal of Fluid Mechanics", "publication_date": "2000-03-25", "volume": "407", "pages": "167-200" }, { "id": "authors:58167-sy674", "collection": "authors", "collection_id": "58167-sy674", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:FOSjfm99", "type": "article", "title": "Self-diffusion in sheared suspensions by dynamic simulation", "author": [ { "family_name": "Foss", "given_name": "David R.", "clpid": "Foss-D-R" }, { "family_name": "Brady", "given_name": "John F.", "orcid": "0000-0001-5817-9128", "clpid": "Brady-J-F" } ], "abstract": "The behaviour of the long-time self-diffusion tensor in concentrated colloidal dispersions is studied using dynamic simulation. The simulations are of a suspension of monodisperse Brownian hard spheres in simple shear flow as a function of the P\u00e9clet number, Pe, which measures the relative importance of shear and Brownian forces, and the volume fraction, [phi]. Here, Pe = &[gamma]dot;a^2/D0, where &[gamma]dot; is the shear rate, a the particle size and D0 = kT/6[pi][eta]a is the Stokes\u2013Einstein diffusivity of an isolated particle of size a with thermal energy kT in a solvent of viscosity [eta]. Two simulations algorithms are used: Stokesian Dynamics for inclusion of the many-body hydrodynamic interactions, and Brownian Dynamics for suspensions without hydrodynamic interactions. A new procedure for obtaining high-quality diffusion data based on averaging the results of many short simulations is presented and utilized. At low shear rates, low Pe, Brownian diffusion due to a random walk process dominates and the characteristic scale for diffusion is the Stokes\u2013Einstein diffusivity, D0. At zero Pe the diffusivity is found to be a decreasing function of [phi]. As Pe is slowly increased, O(Pe) and O(Pe^3/2) corrections to the diffusivity due to the flow are clearly seen in the Brownian Dynamics system in agreement with the theoretical results of Morris & Brady (1996). At large shear rates, large Pe, both systems exhibit diffusivities that grow linearly with the shear rate by the non-Brownian mechanism of shear-induced diffusion. In contrast to the behaviour at low Pe, this shear-induced diffusion mode is an increasing function of [phi]. Long-time rotational self-diffusivities are of interest in the Stokesian Dynamics system and show similar behaviour to their translational analogues. An off-diagonal long-time self-diffusivity, Dxy, is reported for both systems. Results for both the translational and rotational Dxy show a sign change from low Pe to high Pe due to different mechanisms in the two regimes. A physical explanation for the off-diagonal diffusivities is proposed.", "issn": "0022-1120", "publisher": "Journal of Fluid Mechanics", "publication": "Journal of Fluid Mechanics", "publication_date": "1999-12-25", "volume": "401", "pages": "243-274" } ]