# Colloidal Brazil nut effect in microswimmer mixtures induced by motility contrast

## TL;DR

This study investigates the segregation behavior of microswimmer mixtures under a light intensity field, revealing a colloidal Brazil nut effect where heavier particles can float above lighter ones due to motility contrast and external torques.

## Contribution

It introduces the concept of a colloidal Brazil nut effect in active particle mixtures, combining experimental, numerical, and theoretical approaches to explain segregation phenomena.

## Key findings

- Segregation occurs near intensity minima with distinct species localization.
- A strong aligning torque maps the system onto an effective equilibrium model.
- Heavier particles can float above lighter ones under certain conditions.

## Abstract

We numerically and experimentally study the segregation dynamics in a binary mixture of microswimmers which move on a two-dimensional substrate in a static periodic triangular-like light intensity field. The motility of the active particles is proportional to the imposed light intensity and they possess a motility contrast, i.e., the prefactor depends on the species. In addition, the active particles also experience a torque aligning their motion towards the direction of the negative intensity gradient. We find a segregation of active particles near the intensity minima where typically one species is localized close to the minimum and the other one is centered around in an outer shell. For a very strong aligning torque, there is an exact mapping onto an equilibrium system in an effective external potential that is minimal at the intensity minima. This external potential is similar to (height-dependent) gravity, such that one can define effective `heaviness' of the self-propelled particles. In analogy to shaken granular matter in gravity, we define a `colloidal Brazil nut effect' if the heavier particles are floating on top of the lighter ones. Using extensive Brownian dynamics simulations, we identify system parameters for the active colloidal Brazil nut effect to occur and explain it based on a generalized Archimedes' principle within the effective equilibrium model: heavy particles are levitated in a dense fluid of lighter particles if their effective mass density is lower than that of the surrounding fluid. We also perform real-space experiments on light-activated self-propelled colloidal mixtures which confirm the theoretical predictions.