Large amplitude oscillatory microrheology

Abstract

SynopsisWe study the motion of a colloidal particle as it is driven by an oscillating external force of arbitrary amplitude and frequency through a colloidal dispersion. Large amplitude oscillatory flows (LAOFs) are examined predominantly from a phenomenological perspective in which experimental measurements inform constitutive models. Here, we investigate a LAOF from a microstructural perspective by connecting motion of the probe particle to the material response while making no assumptions a priori about how stress relaxes in the material. The suspension exerts nonconservative, hydrodynamic forces on the probe, while distortions in the particle configuration exert conservative forces: Brownian and interparticle forces, for example. The relative importance of each of these contributions to particle motion evolves with the degree of displacement from equilibrium. When the force on the probe is weak, the linear microviscoelasticity of the suspension is probed [see, e.g., Khair and Brady, J. Rheol. 49, 1449-1481]. When oscillation rate is slow, the steady microrheology is probed [see, e.g., Squires and Brady, Phys. Fluids 17, 073101 (2005); Khair and Brady, J. Fluid Mech. 557, 73-117 (2006)]. This article develops a micromechanical model that recovers these limiting cases and then uses the same model to reveal the microrheology of colloidal dispersions deformed by a probe driven with arbitrary force amplitude and frequency. A chief result of this work is the discovery of a regime in which the resistance to motion of the probe particle is on average weaker than the resistance the probe experiences when deformed by high frequency oscillation. This hypoviscous effect arises when the reciprocating motion of the probe particle opens a a)

Thermally reversible nanoparticle gels with tuneable porosity showing structural colour

“…However, the additional attraction provided by the DNA hybridization leads to a slightly narrower S(q) at higher volume fractions for the DNA-NPs. This is in agreement with the findings that strongly attracting colloids simply show more rapid kinetic arrest when quenched into the spinodal decomposition region 3, 14 than weaker short-ranged attractive interactions of less than a few k B T. 47 It is also interesting to note that the cord analysis data (Table 1) suggest that the PEG-NP gels are more compact than the DNA-NP gels. In all three volume-fractions studied the average pore sizes are systematically larger and the gel arms smaller in the PEG-NP gels.…”

Active microrheology of colloidal suspensions: Simulation and microstructural theory

“…They observed that as the external force increases the distribution of colloidal particles around the probe becomes more anisotropic, with large accumulation of particles in front and a depleted zone or wake behind the probe. This work was extended to consider the effect of pair hydrodynamic interactions in dilute suspension [Khair & Brady, 2006], the role of probe particle's shape [Khair & Brady, 2008], relation between stress and force-induced diffusivity of the probe [Zia & Brady, 2012], and the case of having an oscillating external force/velocity [Swan et al, 2014]. The predictions of the dilute theories are in qualitative agreement with available experimental work [Sriram et al, 2010] and Brownian Dynamics simulations [Carpen & Brady, 2005].…”

“…We show that the source of the dispersion in CF is the hydrodynamic interactions of the probe with its neighboring bath particles. This is the first study that takes into account the effect of hydrodynamic dispersion on the structure and viscosity of colloidal suspensions in active microrheology; the previous theories ignore this effect by either assuming the suspension is dilute and the interactions are limited to pair particles [Squires & Brady, 2005;Khair & Brady, 2006;Swan & Zia, 2013;Swan et al, 2014], or the HI is entirely ignored [Gazuz et al, 2009;Voigtmann & Fuchs, 2013].…”

The impact of hydrodynamics on viscosity evolution in colloidal dispersions: Transient, nonlinear microrheology

“…More fundamentally, it provides a compelling connection between fluctuation and dissipation away from equilibrium, extending Einstein's equilibrium fluctuation‐dissipation theory of Brownian motion to strong departures from equilibrium (H. C. W. Chu and R. N. Zia, in preparation). Experimental studies, theoretical models, and dynamic simulations of active microrheology have recovered steady‐state non‐Newtonian behaviors measured via traditional shear rheology, including flow thinning and thickening, flow‐induced diffusion, normal stress differences, and linear viscoelasticity . Comparison between rheological measurements obtained via viscometric flows, for example, simple shear, and those obtained via microrheology, reveal many qualitative similarities as well as important differences.…”