Hydrodynamics control shear-induced pattern formation in attractive suspensions

Abstract

Dilute suspensions of repulsive particles exhibit a Newtonian response to flow that can be accurately predicted by the particle volume fraction and the viscosity of the suspending fluid. However, such a description fails when the particles are weakly attractive. In a simple shear flow, suspensions of attractive particles exhibit complex, anisotropic microstructures and flow instabilities that are poorly understood and plague industrial processes. One such phenomenon, the formation of log-rolling flocs, which is ubiquitously observed in suspensions of attractive particles that are sheared while confined between parallel plates, is an exemplar of this phenomenology. Combining experiments and discrete element simulations, we demonstrate that this shear-induced structuring is driven by hydrodynamic coupling between the flocs and the confining boundaries. Clusters of particles trigger the formation of viscous eddies that are spaced periodically and whose centers act as stable regions where particles aggregate to form flocs spanning the vorticity direction. Simulation results for the wavelength of the periodic pattern of stripes formed by the logs and for the log diameter are in quantitative agreement with experimental observations on both colloidal and noncolloidal suspensions. Numerical and experimental results are successfully combined by means of rescaling in terms of a Mason number that describes the strength of the shear flow relative to the rupture force between contacting particles in the flocs. The introduction of this dimensionless group leads to a universal stability diagram for the log-rolling structures and allows for application of shear-induced structuring as a tool for assembling and patterning suspensions of attractive particles.

On the shear thinning of non-Brownian suspensions: Friction or adhesion?

“…Particle suspensions are sensitive to shearing conditions and therefore, the samples were first pre-sheared at a constant shear rate of 200 s -1 for 300 s to ensure the same shear history, followed by a resting period of 300 s to allow sample equilibration (Khandavalli and Rothstein, 2015;Ma et al, 2017;Varga et al, 2019). Steady state flow sweeps were conducted at increasing shear rates () from 10 -3 s -1 to 10 3 s -1 .…”

“…Visualisation of dense particle suspension microstructure is challenging and thus optical imaging and actual quantification of the shear-induced characteristics of such systems has been fairly limited; to date very few relevant studies exist mainly focusing on smooth or spherical particles and biological samples, such as blood (Gunes et al, 2008;Kaliviotis et al, 2016;Lin et al, 2018;Ma et al, 2008;Massaro et al, 2020;Tanner, 2015;Varga et al, 2019).…”

Highly Filled Glycerol/Graphite Suspensions as Fluidic Soft Sensors and Their Responsive Mechanism to Shear

“…The resistance between two opposite sides of a cubic entity can be determined assuming N × N × N microstructures connected in series and parallel (Figure S13, Supporting Information). The resistance is deduced as follows (details in the Supporting Information) where z (represents the conductivity of Ldn) and k RTD are constants, while r CDT and k ir (represents the conductivity variation due to non‐uniform aggregation [ 24 ] ) may vary with time during shear. To be simple, here k ir is regarded as a constant, 1.…”

Effect of Particle Specific Surface Area on the Rheology of Non-Brownian Silica Suspensions

“…First, a pre-shearing step was conducted at a shear rate of 200 s −1 for 5 min to eliminate any prior shearing history in the material, followed by a 5 min resting period to allow restoration of properties [ 24 , 25 , 26 ]. Nondestructive oscillatory frequency sweeps were performed in the linear viscoelastic region ( ) and at angular frequencies varying from 1 to 600 rad/sec.…”