Controlling colloidal sedimentation using time-dependent shear

Abstract

Abstract. -Employing a recently developed dynamical density functional theory we study the response of a colloidal sediment above a wall to shear, demonstrating the time dependent changes of the density distribution and its center-of-mass after switching shear either on or off and under oscillatory shear. Following the onset of steady shear we identify two dynamical mechanisms, distinguished by their timescales. Shortly after the onset, a transient enhancement of the packing structure at the wall reflects the self-organization into lanes. On a much longer timescale these effects are transmitted to the bulk, leading to migration away from the wall and an increase in the center-of-mass. Under oscillatory shear flow the center-of-mass enters a stationary state, reminiscent of a driven damped oscillator.Colloidal suspensions under shear have been the subject of intense research during the last decades, both theoretically as well as in simulations and experiments [1][2][3][4]. Dense systems have attracted particular attention, as particle interactions can transmit the effects of shear into directions perpendicular to the flow, leading to nontrivial collective dynamics. An important consequence of applying steady shear is that the intrinsic slow dynamics exhibited by quiescent dense systems (and characterized by observables such as density correlators or mean squared displacements [5][6][7][8][9]) can be sped up drastically in all spatial directions, provided the shear rate exceeds the inverse of the structural relaxation time [10]. The dynamical response becomes richer still when considering timedependent shear fields for which transient dynamics can play a dominant role (e.g. oscillatory shear [11]).In real colloidal systems the buoyant mass of the suspended particles is often nonzero, such that gravity drives the formation of a nonuniform density distribution: A colloidal sediment (or creaming profile, in the event that the buoyant mass is negative). The study of colloidal sedimentation has a long history, effectively starting with the pioneering experimental work of Perrin at the start of the 19th century. While early theoretical works focused upon (a) kruegerm@mit.edu (b) joseph.brader@unifr.ch the influence of hydrodynamic interactions on the sedimentation velocity in dilute systems (see e.g. [12][13][14]), the dynamic settling of denser suspensions has only recently been studied in detail in experiments, Brownian dynamics simulations, and theoretically, using dynamic density functional theory (DDFT) [15][16][17]. Incorporating hydrodynamic interactions into theories of dense suspensions is a difficult task, although recently some progress has been made [18,19].Experiments in which shear flow has been applied to particle sediments (where in the early literature, mostly large, nearly non-Brownian particles were considered) report changes in the viscosity, attributed to a shear induced modification of the underlying microstructure [20]. Moreover, an increase in the height of the sediment has been observed with inc...

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