Tapping of Granular Packs: A Model Based on Local Two-Level Systems

Area of Science:

• Condensed matter physics
• Materials science

Background:

• Two-level systems are crucial for understanding the low-temperature behavior of glasses.
• Previous research has observed unique powder properties under repeated tapping by the Chicago group.

Purpose of the Study:

• To investigate the potential role of two-level systems in explaining the observed properties of powders under repeated tapping.
• To model the effects of tapping on powder states and compactivity.

Main Methods:

• Following S. F. Edwards' theoretical framework.
• Analyzing relevant variables: state volumes (V(alpha), V(beta)), activated volume (B), and compactivity (v).
• Modeling tapping-induced transitions (alpha-->beta) and their impact on compactivity.

Main Results:

• Tapping induces transitions that reduce compactivity.
• At low tapping strengths, systems freeze before equilibrium, resulting in logarithmic density growth.
• At higher tapping strengths, equilibrium may be approached, competing with freezing due to increased free volume.

Conclusions:

• Two-level systems offer a potential explanation for powder dynamics under tapping.
• The model predicts different behaviors based on tapping strength and the distribution of volume differences (Delta).
• The study highlights the interplay between transitions, compactivity, and system freezing in granular materials.