Reversing the Brazil-nut effect: competition between percolation and condensation

Area of Science:

• Physics
• Materials Science
• Fluid Dynamics

Background:

• Granular mixtures exhibit complex behaviors when subjected to external forces.
• The Brazil-nut effect describes the phenomenon where larger particles rise in a shaken mixture.
• Theoretical models predict a reverse Brazil-nut effect, where larger particles sink.

Purpose of the Study:

• To experimentally investigate the transition between the Brazil-nut effect and its reverse form in binary granular mixtures.
• To explore the influence of particle size, material, and excitation on segregation patterns.
• To demonstrate the controllability of particle demixing.

Main Methods:

• Utilizing vertically shaken binary mixtures of spherical beads with varying diameters and materials.
• Systematically varying external excitation parameters to observe segregation dynamics.
• Analyzing the direction of particle demixing (upward or downward movement of larger particles).

Main Results:

• Observed both the classic Brazil-nut effect and the predicted reverse Brazil-nut effect.
• Demonstrated that the direction of demixing is highly sensitive to the external excitation.
• Showcased the ability to switch between the two effects for a given mass density ratio by adjusting excitation.

Conclusions:

• The segregation behavior in binary granular mixtures is controllable through external excitation.
• Experimental evidence supports the existence and tunability of the reverse Brazil-nut effect.
• Findings have implications for understanding and manipulating granular flows and separations.