Manipulating shear-induced non-equilibrium transitions in colloidal films by feedback control

Abstract

Using Brownian Dynamics (BD) simulations we investigate non-equilibrium transitions of sheared colloidal films under controlled shear stress σxz. In our approach the shear rate [small gamma, Greek, dot above] is a dynamical variable, which relaxes on a time scale τc such that the instantaneous, configuration-dependent stress σxz(t) approaches a pre-imposed value. Investigating the dynamics under this "feedback-control" scheme we find unique behavior in regions where the flow curve σxz([small gamma, Greek, dot above]) of the uncontrolled system is monotonic. However, in non-monotonic regions our method allows to select between dynamical states characterized by different in-plane structure and viscosities. Indeed, the final state strongly depends on τc relative to an intrinsic relaxation time of the uncontrolled system. The critical values of τc are estimated on the basis of a simple model.

Depinning and heterogeneous dynamics of colloidal crystal layers under shear flow

“…For the one-component bilayer-and trilayer system, the number density ρd 3 = 0.85 and the slit-pore width L z = 2.2d, 3.2d are chosen following previous studies [16,18]. The particle interaction parameters are set according to experimental setups for particles with diameter d ≈ 26 nm and valency Z = 35 [26,40], yielding κd ≈ 3.2.…”

“…Following previous studies, the total particle interaction potential is truncated at a cutoff radius r c ≈ 3d and shifted accordingly [16,18]. To mimic the slit-pore geometry, the colloids are confined by two plane-parallel soft walls extended infinitely in x-and y-direction and separated in z-direction by a distance L z (see Fig.…”

“…Exposing such highly correlated systems to shear flow (along * s.gerloff@tu-berlin.de † klapp@physik.tu-berlin.de a direction within the plane of the walls) leads to a breakdown of crystalline in-plane ordering after overcoming a "critical" shear rate, and a subsequent recrystallization at higher shear rates, as both computer simulations [15,16] and experiments [17] reveal. In two earlier publications [16,18] we have analyzed this behavior in detail, for the exemplary case of a colloidal bilayer (of monodisperse particles) under constant shear rate [16] or constant stress [18] (both of these external control strategies can be experimentally realized). One main conclusion was that the breakdown of crystalline order is related to "depinning" transitions in terms of the layer velocity from a locked into a running (sliding) state [16].…”

Shear-induced laning transition in a confined colloidal film

“…10(b)]. The shear stress σ xz reveals a strongly nonmonotonic behavior with several multivalued regimes where the shear rate corresponding to a certain stress value is not uniquely defined (as already reported earlier by us [28]). In particular, the ranges ofγ where the stress decreases imply that the corresponding (laned or shearmolten) state is mechanically unstable and would not be observable in a simulation (or experiment) at constant stress.…”

“…Following previous studies [27,28] we consider a model colloidal suspension where two spherical particles (diameter d) with distance r ij interact via a combination of a repulsive Yukawa potential, u Yuk (r ij ) = W exp [−κr ij ] /r ij and a repulsive soft-sphere potential u SS (r ij ) = 4 (d/r ij ) 12 . The interaction parameters are set to W/(k B T d) ≈ 123 (where k B T is the thermal energy) and the inverse Debye screening length is set to κ d ≈ 3.2 (for details, see Ref.…”