Mixing and segregation rates in sheared granular materials

Area of Science:

• Granular physics
• Complex systems
• Statistical mechanics

Background:

• Vertical size segregation in granular materials, often termed the Brazil-nut effect, typically accelerates with increasing particle size disparity.
• Previous models predict faster segregation with greater size differences.

Purpose of the Study:

• To experimentally investigate the effect of particle size ratio on the mixing and segregation rates in sheared bidisperse granular materials.
• To explore the influence of confining pressure on these segregation dynamics.

Main Methods:

• Experiments were conducted on dense bidisperse granular materials under shear.
• The particle size ratio was systematically varied.
• Confining pressure was manipulated to observe its effect on mixing and segregation.

Main Results:

• Mixing rates at low confining pressure align with percolation theory predictions.
• An anomalous nonmonotonic segregation rate was observed, peaking at intermediate particle size ratios.
• Increased confining pressure significantly reduced mixing and segregation rates for particles with large size differences.

Conclusions:

• The observed anomalous segregation behavior suggests that simple size disparity is insufficient to explain segregation dynamics.
• A species-dependent distribution of forces within the granular system is proposed as the cause of the anomalous segregation.
• Force distribution within granular materials is a critical factor influencing segregation, especially under varying confining pressures.