Soft repulsive mixtures under gravity: brazil-nut effect, depletion bubbles, boundary layering, nonequilibrium
Tobias Kruppa1, Tim Neuhaus, René Messina
1Institut für Theoretische Physik II: Weiche Materie, Heinrich-Heine-Universität Düsseldorf, Universitätsstraße 1, D-40225, Düsseldorf, Germany.
The Journal of Chemical Physics
|April 10, 2012
Summary
Heavier particles float on lighter ones due to a "depletion bubble" effect, driven by repulsive forces. This phenomenon, observed in simulations, explains particle layering and mixing under gravity.
Area of Science:
- Soft Matter Physics
- Colloidal Science
- Statistical Mechanics
Background:
- Binary particle mixtures with long-ranged repulsive forces exhibit complex behaviors under external fields.
- The Brazil nut effect, where larger particles rise in a mixture, is a known phenomenon but its underlying mechanisms in repulsive systems require further elucidation.
Purpose of the Study:
- To investigate the behavior of a binary particle mixture with repulsive forces under gravitational fields.
- To explain the Brazil nut effect and associated boundary phenomena using a novel
- depletion bubble
- mechanism.
- To study the mixture's response to periodic gravity changes.
Main Methods:
- Computer simulations of a two-dimensional binary mixture of colloidal repulsive dipoles.
- Theoretical analysis using density functional theory.
- Investigation of systems under constant and time-dependent (periodic) gravity.
Main Results:
- A
- depletion bubble
- mechanism was identified, where more repulsive particles create voids in less repulsive ones, leading to buoyancy.
- This mechanism explains the Brazil nut effect, causing heavier particles to rise above lighter ones.
- Effective attraction to container walls causes boundary layering of heavier particles, which persists even under periodic gravity inversion.
Conclusions:
- The
- depletion bubble
- mechanism provides a unified explanation for buoyancy and boundary phenomena in repulsive binary mixtures.
- The observed effects are general and applicable to various repulsive interactions and dimensions, including colloidal suspensions, charged granulates, and dusty plasmas.
- These findings are experimentally verifiable in settling experiments.
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