Normal stresses in shear thickening granular suspensions.

Area of Science:

• Rheology of complex fluids
• Granular material science
• Fluid dynamics

Background:

• Granular suspensions exhibit normal stresses when sheared, but their behavior is debated.
• Understanding these stresses is crucial for applications involving dense particle flows.

Purpose of the Study:

• To investigate the transition between positive and negative normal stresses in shear thickening granular suspensions.
• To elucidate the roles of frictional and hydrodynamic interactions in determining normal stress components.
• To characterize the system's relaxation time and fluid flow dynamics.

Main Methods:

• Performed oscillatory and rotational rheology measurements.
• Systematically varied particle diameters and parallel-plate gap sizes.
• Utilized a two-fluid model to separate phase contributions.

Main Results:

• Observed a transition from positive to negative normal stress with changing parameters.
• Frictional interactions correlate with positive normal stresses.
• Hydrodynamic interactions become dominant with larger particles or smaller gaps, leading to negative normal stresses.
• Determined a relaxation time dependent on pore and gap sizes.

Conclusions:

• Normal stress behavior in granular suspensions is tunable via particle size and confinement.
• Both frictional and hydrodynamic forces play critical, competing roles.
• The findings provide insights into granular flow physics and material design.