Testing the Wyart-Cates model for non-Brownian shear thickening using bidisperse suspensions

Area of Science:

• Rheology
• Soft Matter Physics
• Colloid Science

Background:

• Shear thickening in concentrated dispersions is widely attributed to stress-induced frictional contacts.
• The Wyart-Cates (WC) model simplifies this by using a single parameter, the fraction of frictional contacts (f), to describe contact microphysics.
• The WC model's applicability to complex, polydisperse systems remains largely untested.

Purpose of the Study:

• To evaluate the validity of the Wyart-Cates model for polydisperse suspensions.
• To investigate the limitations of the WC model when dealing with varying particle sizes and contact types.
• To propose a more comprehensive understanding of shear thickening mechanisms in real-world suspensions.

Main Methods:

• Systematic computational simulations using bidisperse mixtures of spheres.
• Analysis of flow curves and contact mechanics under shear stress.
• Development and testing of a polydisperse analogue of the WC model parameter f.
• Corroboration with systematic experimental studies on model shear-thickening suspensions.

Main Results:

• The Wyart-Cates model accurately predicts shear thickening only in the monodisperse limit.
• Significant deviations from the WC model were observed with increasing bidispersity.
• The model's failure stems from its inability to differentiate between large-large, large-small, and small-small frictional contacts.
• A modified parameter accounting for different contact types improved model fitting.

Conclusions:

• The standard Wyart-Cates model is insufficient for describing shear thickening in polydisperse systems.
• The microphysics of shear thickening is more complex than captured by a single frictional contact fraction.
• Further research is needed to refine models for polydisperse suspensions and understand contact microphysics.
• Experimental validation of theoretical models presents challenges due to system complexities.