Characterizing sliding and rolling contacts between single particles

Area of Science:

• Rheology
• Colloidal science
• Tribology

Background:

• Particle contacts govern the rheology of dense, sheared suspensions.
• Microscopic understanding of how roughness and adhesion affect force transmission in contacts is limited.
• Previous studies lack direct experimental measurement of friction coefficients at the particle contact level.

Purpose of the Study:

• To experimentally investigate the microscopic mechanisms of force transmission and relative motion at particle contacts.
• To simultaneously measure normal and tangential forces, sliding, and rolling friction between tailored surfaces and microparticles.
• To elucidate the role of surface roughness and adhesion in dictating particle contact dynamics and rheological properties.

Main Methods:

• Development and application of an innovative colloidal-probe atomic force microscopy technique.
• Simultaneous measurement of normal and tangential forces during particle-surface interactions.
• Tracking of relative sliding and rolling motion to determine coefficients of friction.

Main Results:

• Particles spontaneously roll when traction is sufficient, reducing energy dissipation and prolonging contact duration.
• Friction significantly increases for rough and adhesive surfaces when rolling is hindered.
• Smooth particles with polymer brushes exhibit well-lubricated contacts, minimizing friction.
• Surface roughness promotes load-dependent asperity interlocking, inducing off-axis particle rotations and rolling.
• Smooth, adhesive surfaces lead to rolling along the principal axis of motion.

Conclusions:

• Direct measurement of sliding and rolling friction coefficients is achieved.
• Understanding of how surface properties influence particle contact mechanics and rheology is advanced.
• Results provide crucial data for numerical simulations and offer insights into discontinuous shear thickening.
• Potential for tailoring suspension rheology through engineered surface contacts is demonstrated.