Triggering granular avalanches with ultrasound.

Area of Science:

• Physics
• Geophysics
• Materials Science

Background:

• Granular flows, common in nature, are often triggered by vibrations.
• The precise mechanisms behind granular flow initiation and behavior remain incompletely understood.
• Understanding these triggers is crucial for predicting natural hazards like landslides and earthquakes.

Purpose of the Study:

• To investigate how nanometer-amplitude ultrasound triggers shear instability in granular layers.
• To elucidate the role of acoustic lubrication in granular flow dynamics.
• To connect granular flow behavior to fundamental friction models.

Main Methods:

• Applying high-frequency, low-amplitude ultrasound to granular layers near their static threshold.
• Observing and analyzing the resulting granular flow regimes (inertial flow, creep).
• Investigating the influence of ultrasound removal on flow cessation.

Main Results:

• Ultrasound successfully triggered shear instability and subsequent granular flow.
• Observed flow exhibited either self-accelerated inertial or creeplike behavior.
• Flowing ceased upon removal of the ultrasound.
• The effects were attributed to shear acoustic lubrication reducing interparticle friction.

Conclusions:

• Nanometer-amplitude ultrasound can effectively trigger granular flows through friction reduction.
• The observed phenomena align with bistability in velocity-weakening friction models.
• This research provides insights into seismic wave-induced landslides and earthquakes.