Active microrheology of colloidal suspensions of hard cuboids

Abstract

By performing dynamic Monte Carlo simulations, we investigate the microrheology of isotropic suspensions of hard-core colloidal cuboids. In particular, we infer the local viscoelastic behaviour of these fluids by studying the dynamics of a probe spherical particle that is incorporated in the host phase and is dragged by an external force. This technique, known as active microrheology, allows one to characterise the microscopic response of soft materials upon application of a constant force, whose intensity spans here three orders of magnitude. By tuning the geometry of cuboids from oblate to prolate as well as the system density, we observe different responses that are quantified by measuring the effective friction perceived by the probe particle. The resulting friction coefficient exhibits a linear regime at forces that are much weaker and larger than the thermal forces, whereas a non-linear, force-thinning regime is observed at intermediate force intensities.

Insight into the Viscoelasticity of Self-Assembling Smectic Liquid Crystals of Colloidal Rods from Active Microrheology Simulations

“…The force pulling the tracer is sufficiently intense to cause a microstructural deformation of the particle distribution around the probe, developing force thinning , where the effective friction decreases with F ∥ . This force-thinning regime has been observed in experiments 57 , 58 and simulations 12 , 18 , 21 , 23 , 25 , 26 and predicted by theory, 17 − 19 , 22 , 59 and it resembles shear thinning in viscoelastic fluids, where the shear viscosity decreases as the shear rate increases. Interestingly, while γ eff,∥ 2σ > γ eff,∥ 3σ at small forces, we notice that γ eff,∥ 2σ < γ eff,∥ 3σ for F ∥ ≥ 2 k B Tσ –1 .…”

“…Finally, at large forces advection of bath particles dominates its Brownian response, and a second plateau γ eff,∥ is observed. Each of these behaviors has been reported in theoretical, − ,, simulation, ,,,,, and experimental , studies on colloidal suspensions.…”

“…Finally, at large forces advection of bath particles dominates its Brownian response, and a second plateau γ eff,∥ is observed. Each of these behaviors has been reported in theoretical, [17][18][19]22,59 simulation, 12,18,21,23,25,26 and experimental 57,58 studies on colloidal suspensions. When the external force is oriented diagonal (F 45°) or perpendicular (F ⊥ ) to n, a qualitatively similar behavior of the effective friction to the parallel case is observed: a Newtonian plateau at small and large forces and a nonlinear regime at intermediate forces.…”

“…Particularly, in constant-force active MR , the tracer is pulled through the host phase by a constant force, F , and develops a long-time steady velocity ⟨ v t ⟩ which is related to the effective friction coefficient of the bath, γ eff , through the Stokes’ drag law F = γ eff ⟨ v t ⟩. Within this context, theoretical − , and simulation ,− works have reported the occurrence of two plateaus for the effective friction at low and high forces and a force-thinning regime representing the nonlinear response of the bath where the friction decreases with the external force. Interestingly enough, despite the significant number of works on the diffusion of guest particles in colloidal dispersions, − only a limited number of them − focus on extracting the viscoelastic properties of LC phases from the dynamic response of the tracer particles.…”

“…To this end, we employ the dynamic Monte Carlo (DMC) technique which has been developed to study the Brownian motion of colloidal particles under the most general conditions. − More recently, the DMC has been extended to investigate the passive and fixed-force active , MR of colloidal suspensions. Formally, the DMC is a Metropolis-based technique that, in the limit of small displacements and rotations, mimics the Brownian motion of particles and reproduces time-dependent events.…”