Apparent wall slip in non-Brownian hard-sphere suspensions

Area of Science:

• Fluid dynamics
• Materials science
• Colloid science

Background:

• Apparent wall slip is a phenomenon where particle concentration decreases near a wall in suspensions.
• Understanding wall slip is crucial for predicting the flow behavior of concentrated suspensions.
• Existing models often simplify or overlook the complex interplay of forces causing slip.

Purpose of the Study:

• To analyze apparent wall slip in hard-sphere suspensions below the jamming limit.
• To extend existing constitutive equations by incorporating gravitation and wall-particle repulsion potentials.
• To investigate the shear-dependent nature of wall slip and its effect on flow curves.

Main Methods:

• Utilizing a continuum level diffusion model.
• Extending a prior constitutive equation with additional potentials for gravity and wall repulsion.
• Analyzing shear-rate-dependent particle migration fluxes.

Main Results:

• Both gravitational and wall-repulsion effects are inherently shear-independent.
• Shear-rate-dependent particle migration fluxes lead to a non-linearly shear-dependent net slip effect.
• Larger slip is observed at lower shear rates.

Conclusions:

• The interplay of migration fluxes results in a non-linear shear dependence of wall slip.
• The model predicts a mild shear-thickening regime at the transition from small to intermediate shear rates.
• This work provides a more nuanced understanding of wall slip in hard-sphere suspensions.