# Pairing, waltzing and scattering of chemotactic active colloids

## TL;DR

This paper investigates the complex dynamical interactions of chemotactic active colloids, revealing bound states, scattering behavior, and bifurcations driven by their chemical interactions and propulsion speeds.

## Contribution

It introduces a detailed analysis of two-body interactions of chemotactic colloids, highlighting novel bound states, scattering phenomena, and the role of non-reciprocal dynamical forces.

## Key findings

- Chemotactic colloids can form active dimers with translational motion.
- Anti-chemotactic colloids always scatter, preventing bound states.
- Transitions between bound states depend on propulsion speed and initial conditions.

## Abstract

An interacting pair of chemotactic (anti-chemotactic) active colloids, that can rotate their axes of self-propulsion to align {parallel (anti-parallel)} to a chemical gradient, shows dynamical behaviour that varies from bound states to scattering. The underlying two-body interactions are purely dynamical, non-central, non-reciprocal, and controlled by changing the catalytic activity and phoretic mobility. Mutually chemotactic colloids trap each other in a final state of fixed separation; the resulting `active dimer' translates. A second type of bound state is observed where the polar axes undergo periodic cycles leading to phase-synchronised circular motion around a common point. These bound states are formed depending on initial conditions and can unbind on increasing the speed of self propulsion. Mutually anti-chemotactic swimmers always scatter apart. We also classify the fixed points underlying the bound states, and the bifurcations leading to transitions from one type of bound state to another, for the case of a single swimmer in the presence of a localised source of solute.