Friction law and hysteresis in granular materials.

Area of Science:

• Physics
• Geophysics
• Materials Science

Background:

• Macroscopic friction in granular materials often exhibits velocity weakening as flow rate increases.
• This phenomenon can lead to catastrophic intermittent events such as earthquakes and landslides.

Purpose of the Study:

• To theoretically and numerically investigate velocity weakening in simple granular materials.
• To understand the role of acoustic noise and particle softness in friction instability.

Main Methods:

• Theoretical modeling of granular material friction.
• Numerical simulations to study the phenomenon.
• Analysis of acoustic noise and contact dynamics.

Main Results:

• Velocity weakening occurs even with identical static and dynamic microscopic friction coefficients.
• Softer particles reduce or eliminate velocity weakening.
• Acoustic noise and 'soft spots' (regions of rolling contacts) are identified as key drivers of instability.
• A microscopic theory predicts scaling behaviors for acoustic noise, sliding contacts, and velocity.

Conclusions:

• The study provides a microscopic theory for nonmonotonic friction laws in granular materials.
• The findings explain previously unexplained observations and offer experimentally testable predictions.
• The phenomenon persists even for infinitely hard particles, contrary to intuition.