Granular mixing · DEM simulation

Too Many Small Grains Can Stop Segregation

Main claim

In a vertically vibrated binary-sized granular mixture, an excess of small particles produces an effective repulsion between large particles and suppresses their segregation. Adding more small grains can keep the large grains apart instead of making the mixture separate.

Simulation of a vibrated binary-sized granular mixture with large red and small brown particles — A vibrated mixture of large and small particles in a confined geometry.

Problem

Granular mixtures tend to separate when their particles differ in size, which is troublesome in manufacturing and transport. Adding small grains might seem likely to worsen size segregation, yet the structure of a dense, vibrated mixture can change non-monotonically with composition. We asked whether composition alone could instead stabilize a mixed state.

Approach

We performed three-dimensional discrete element method simulations of large and small frictional spheres confined between parallel walls and driven by vertical vibration. The packing fractions of both species were varied systematically. Particle configurations were characterized using radial distribution functions, structure factors, and hexagonal order, and the trend was tested against changes in system size, size ratio, vibration, and size dispersity.

What we found

When small particles are scarce, the large particles segregate. Increasing the small-particle fraction eventually removes that segregation and produces an effective repulsion between large particles. For a size ratio of three, the large particles can also develop hexagonal order at a lower large-particle fraction than in a single-component system. The suppression remained across several tested conditions.

Why it matters

The finding offers a counterintuitive route to mixing: adding enough of the smaller component can keep larger grains apart rather than promote separation. Beyond its interest as nonequilibrium structure formation, the mechanism may help guide composition choices in chemical, food, and pharmaceutical processes where uniformity matters.

Keywords

granular segregation, granular mixing, DEM simulation, effective interaction, vertical vibration.

Paper

Fumiaki Nakai, Kiwamu Yoshii, “Reducing segregation in vibrated binary-sized granular mixtures by excessive small particle introduction,” Granular Matter 27, 7 (2025).

Published article Open-access preprint