Microscopic structure and dynamics of shear-thinning suspensions of polydisperse, repulsive vesicles

TL;DR

This study explores the microscopic structure and flow behavior of polydisperse vesicle suspensions, revealing a shear-thinning transition linked to structural changes and crowding effects, with implications for dense charged suspensions.

Contribution

It uncovers a shear-thinning transition in polydisperse vesicle suspensions linked to structural and dynamic changes, highlighting entropic effects from polydispersity and crowding.

Findings

Shear-thinning occurs at low concentrations.

Structural peak in static structure factor appears at transition.

Dynamics slow down with two-step decay in correlation functions.

Abstract

We investigate the rheology, microscopic structure, and dynamics of an industrially relevant dispersion made of cationic surfactant vesicles, from dilute to concentrated conditions. We find that these suspensions exhibit a shear-thinning behavior at relatively low concentrations. At the microscale, this corresponds to a well-defined transition in both the structure, marked by the appearance of a peak in the static structure factor, and the dynamics, which slow down and develop a two-step decay in the correlation functions. This low-concentration transition is particularly surprising in light of experiments showing that for surfactant vesicles of similar composition the interactions should be purely repulsive. This leads us to propose that the observed structural and dynamic transition could arise, as an entropic effect, from the large sample polydispersity coupled to crowding. The shear-thinning behavior is thus interpreted as the nonlinear response of this transient structure to the imposed flow. Our work suggests that similar effects might be a generic feature of dense, highly polydisperse charged suspensions.