[Paper Review] Dynamically Correlated Region in Sheared Colloidal Glasses Revealed by Neutron Scattering

Zhe Wang, Takuya Iwashita+7 more|arXiv (Cornell University)|Sep 21, 2017

Material Dynamics and Properties1 references3 citations

TL;DR

This study identifies a dynamically correlated region (DCR) in sheared charged colloidal glasses using in-situ small-angle neutron scattering (SANS) and rheometry. The DCR, sustained by extended-range electrostatic interactions, acts as a local elastic barrier resisting configurational rearrangement, with its size and stress response quantitatively matching macroscopic shear stress at low to moderate shear rates, revealing a key microscopic origin of shear thinning and high viscosity in strongly interacting colloids.

ABSTRACT

The microscopic deformation mechanism of charged colloidal glasses with extended-range interactions under shear is investigated by in-situ small-angle neutron scattering, and a dynamically correlated region (DCR) is identified. This short-lived region provides the resistance to the configurational rearrangement imposed by the external deformation, as evidenced by the evolution of the size of DCR in the shear thinning regime and the quantitative agreement between the local stress sustained by DCR and the macroscopic stress from rheological measurements at low and mediate shear rates. This finding suggests that DCR is an important quantity for microscopically addressing the flow and deformation behavior of strongly interacting colloids.

Motivation & Objective

• To investigate the microscopic deformation mechanism of charged colloidal glasses under shear, focusing on the role of extended-range inter-particle interactions.
• To identify and characterize a dynamically correlated region (DCR) responsible for resistance to configurational rearrangement during shear flow.
• To establish a quantitative link between the local stress in the DCR and the macroscopic stress measured by rheometry.
• To clarify how inter-particle potential influences nonlinear rheological behavior, particularly shear thinning and viscosity enhancement.
• To explore the evolution of deformation heterogeneity through the size and dynamics of the DCR across varying shear rates.

Proposed method

• Conducted in-situ small-angle neutron scattering (SANS) on sheared colloidal suspensions to probe real-space structural and dynamic heterogeneities.
• Performed steady shear rheometry on two systems: charged colloids with extended-range electrostatic repulsion and hard-sphere suspensions with only short-range repulsion.
• Compared SANS spectra in flow-velocity gradient and flow-vorticity planes to detect anisotropic scattering patterns indicative of localized deformation.
• Defined the DCR as a spatial region of coherent, elastic deformation spanning a few particle diameters, where particles maintain neighbor correlations under shear.
• Calculated the microscopically determined stress in the DCR as $\sigma_{DCR} = G'\gamma_M$, where $G'$ is the local elastic modulus and $\gamma_M$ is the maximum shear strain in the DCR.
• Correlated the DCR size with shear rate and Péclet number to assess the evolution of deformation heterogeneity and its relation to shear thinning.

Experimental results

Research questions

• RQ1 How does the extended-range electrostatic potential influence the microscopic deformation mechanism in sheared colloidal glasses?
• RQ2 What is the spatial extent and mechanical role of the dynamically correlated region (DCR) in resisting configurational rearrangement under shear?
• RQ3 To what extent does the stress sustained by the DCR quantitatively match the macroscopic shear stress measured by rheometry?
• RQ4 How does the size of the DCR evolve with increasing shear rate, and what does this imply for deformation heterogeneity and shear thinning?
• RQ5 At what shear rate does the DCR cease to dominate the mechanical response, and what mechanism takes over?

Key findings

• A dynamically correlated region (DCR) spanning a few particle diameters was identified in sheared charged colloidal glasses, sustained by extended-range electrostatic repulsion.
• The size of the DCR decreases with increasing shear rate, indicating diminishing deformation heterogeneity, consistent with shear thinning behavior.
• The microscopically determined stress in the DCR ($\sigma_{DCR}$) shows quantitative agreement with the macroscopic shear stress ($\sigma_{CC}$) from rheometry at low to moderate shear rates ($Pe \leq 1$, $\dot{\gamma} \leq 10\,\text{s}^{-1}$).
• The DCR acts as a local elastic barrier that resists topological rearrangement, explaining the significantly higher viscosity in charged colloids compared to hard spheres at low and moderate shear rates.
• At high shear rates ($Pe \gg 1$), $\sigma_{DCR}$ deviates from $\sigma_{CC}$, indicating that hydrodynamic effects dominate and the DCR loses its controlling role.
• The breakdown of local deformation coherency in hard-sphere suspensions—evidenced by mismatch between $g_2^{-2}(r)$ and $-r dg(r)/dr$—confirms the absence of a DCR, highlighting the unique role of long-range interactions.