# Contractile and chiral activities co-determine the helicity of swimming droplet trajectories

## TL;DR

This study uses computer simulations to explore how combined contractile and chiral activities influence the complex swimming behaviors and trajectories of active fluid droplets, revealing diverse motility modes and their biological relevance.

## Contribution

It introduces a minimal model of chiral active fluids that uncovers new complex motility behaviors, including oscillatory and helical swimming, driven by combined active stresses.

## Key findings

- Diverse motility modes including oscillatory and helical swimming.
- Chirality of swimming can oppose microscopic torque dipole chirality.
- Resemblance of droplet motility to behaviors of some protozoa.

## Abstract

Active fluids are a class of non-equilibrium systems where energy is injected into the system continuously by the constituent particles themselves. Many examples, such as bacterial suspensions and actomyosin networks, are intrinsically chiral at a local scale, so that their activity involves torque dipoles alongside the force dipoles usually considered. Although many aspects of active fluids have been studied, the effects of chirality on them are much less known. Here we study by computer simulation the dynamics of an unstructured droplet of chiral active fluid in three dimensions. Our model only considers the simplest possible combination of chiral and achiral active stresses, yet this leads to an unprecedented range of complex motilities, including oscillatory swimming, helical swimming, and run-and-tumble motion. Strikingly, while the chirality of helical swimming is the same as the microscopic chirality of torque dipoles in one regime, the two are opposite in another. Some of the features of these motility modes resemble those of some single-celled protozoa, suggesting that underlying mechanisms may be shared by some biological systems and synthetic active droplets.