Drag force in a cold or hot granular medium.

Area of Science:

• Physics
• Granular Materials Science
• Soft Matter Physics

Background:

• Granular materials exhibit complex behaviors influenced by external stimuli like vibration.
• Understanding the dynamics of objects moving through granular media is crucial for various applications.
• The concept of granular temperature, analogous to thermodynamic temperature, helps characterize granular system energy.

Purpose of the Study:

• To experimentally investigate the resisting force on a large intruder disk in a 2D granular packing.
• To analyze the influence of vibration levels and intruder velocity on the drag force.
• To establish a framework based on granular temperatures to explain the observed force regimes.

Main Methods:

• Experimental measurement of drag force on an intruder disk in a bidimensional granular packing.
• Systematic variation of intruder velocity and packing vibration levels.
• Analysis of force data in relation to granular temperatures (bulk and local).

Main Results:

• Two distinct drag force regimes were observed: velocity-independent in "cold" (low vibration) and velocity-dependent in "hot" (high vibration) granular media.
• The drag force behavior in both regimes is explained by the balance of bulk granular temperature (Tb) and local granular temperature (T0).
• Experimental data collapse when plotted against the temperature ratio T0/Tb, revealing a critical velocity (V0c) for regime transition.

Conclusions:

• The study provides a unified understanding of drag forces in granular media based on granular temperatures.
• A critical velocity V0c marks the transition between "hot" and "cold" regimes.
• An inertial regime, with force scaling as V0^2, is predicted at higher velocities (V0 > V0i).