Viscosity minimum in bimodal concentrated suspensions under shear

Area of Science:

• Rheology and Soft Matter Physics
• Computational Fluid Dynamics
• Materials Science

Background:

• Classical theories of concentrated suspensions, like Farris's (1968), provide a baseline for understanding suspension behavior.
• Bimodal suspensions, with varying proportions of large and small particles, present complex flow dynamics not fully captured by existing models.
• Understanding shear viscosity in concentrated systems is crucial for predicting material behavior in industrial and natural processes.

Purpose of the Study:

• To investigate the shear viscosity of two-dimensional concentrated bimodal suspensions.
• To identify and analyze the conditions leading to a minimum in shear viscosity as a function of particle size distribution.
• To explore the influence of particle interactions, shear rates, and friction parameters on macroscopic viscosity.

Main Methods:

• Development and application of a computational model for concentrated suspensions under shear.
• Utilizing a linear-response scheme to analyze the dependence of viscosity on shear and friction parameters.
• Simulation of shear geometries imposed by external forces and boundaries, including dynamically imposed shear.

Main Results:

• A shear viscosity minimum was observed as a function of the small-to-large-particle ratio in bimodal suspensions.
• The viscosity minimum's dependence on imposed shear and microscopic drop friction parameters was quantified.
• Macroscopic viscosity decreased with increasing microscopic friction parameters, explained via a two-drop model.

Conclusions:

• The study extends classical suspension theory by identifying a viscosity minimum in bimodal systems.
• Microscopic friction plays a significant role in reducing macroscopic viscosity, offering insights into particle-level interactions.
• Simulation results show qualitative agreement with experimental data from concentrated bimodal emulsions.