# Navigating Gravity: Competing Effects Result in Opposing Taxis for Different Janus Swimmers

## TL;DR

This study explores how gravity and surface interactions affect the movement of tiny particles called Janus colloids, revealing a method to separate them based on their behavior.

## Contribution

The paper introduces a novel method for separating active particles using their distinct gravitactic behavior near surfaces.

## Key findings

- Platinum- and copper-coated Janus colloids show different gravitactic alignment due to hydrodynamic coupling.
- Cu@SiO2 particles exhibit a narrower orientation distribution compared to Pt@SiO2 particles.
- Active and passive particles can form a lateral distribution similar to a Galton board.

## Abstract

Microorganisms often employ tactic behaviors as a survival strategy, enabling them to move in response to different stimuli. One fascinating example is the use of gravity for their orientation, where gravity provides a directional cue for swimming. Separately, when microswimmers move near solid surfaces, hydrodynamic interactions with the wall can alter their trajectories and reorientation dynamics.

This study investigates the combined effects of gravitational torque and wall‐induced hydrodynamics on the orientation and movement of Janus colloids coated with either platinum or copper, swimming toward their inert side (Pt) or the catalytic half (Cu), respectively. Both particle types exhibit alignment with the gravitational direction on inclined substrates due to bottom‐heaviness; however, Cu@SiO2 particles, with lighter metal caps, show a significantly narrower orientation distribution compared to Pt@SiO2 particles. This enhanced alignment is attributed to the distinct hydrodynamic coupling of catalytic‐forward type swimmers near boundaries.

Using these differences, a simple yet effective method for separating active particles based on their gravitactic behavior is presented. Furthermore, we demonstrate that active and passive particles can develop a lateral distribution analogous to that of a Galton board, highlighting potential applications in educational tools and microfluidic‐based computational systems.

The combined effects of gravity and wall‐induced hydrodynamics on the motion of Janus colloids are investigated. Platinum‐ and copper‐coated silica microswimmers show distinct gravitactic alignment near surfaces due to different hydrodynamic couplings. This distinct gravitactic behavior enables separation of initially mixed active particles.