Introduction to Active Particle Systems

Overview

Questions

  • How can we model particles with self-induced active forces?

  • What types of information can we learn from non-equilibrium systems?

Objectives

  • Introduce the Active Brownian Particle (ABP) model.

  • Define Motility-Induced Phase Separation (MIPS).

  • Discuss the mechanics of rigid bodies responding to active forces using torque as an example.

Introduction to Active Particles

Active particles are particles that can consume stored energy or energy from the environment and convert it into directed motion. There are many different types of active particles with different mechanisms for this realized self-propulsion. A simple example is an active Janus particle which is a particle with 2 or more regions of distinct physical properties. Activity can be induced in different Janus particles in a variety of ways; one commonly studied active Janus particle is a polystyrene bead coated with platinum on one side, which will be propelled forward if submerged in a solution containing hydrogen peroxide by a catalytic chemical reaction occuring on the platinum coating. This localized reaction propels the particle forward. In this example, thermal and chemical energy is being consumed to generate the kinetic motion of the (now active) Janus particle.

In the previous HOOMD-blue tutorials, most of the systems have been models typically used to investigate equilibrium properties where the steady state is independent of kinetic parameters. When studying active matter, however, we are no longer considering equilibrium processes, and we shift our attention to non-equilibrium dynamics. Regardless, we can set up the initial state of the simulation in a very similar manner to previous methods we have used in earlier hoomd MD simulation tutorials.

Active Particles and their Phenomena:

There are a few categories of active particles. Here we introduce the commonly discussed Active Brownian Particle (ABP). Cates and Tailleur have a helpful figure distinguishing this active particle type from others their 2015 Annual Review Condensed Matter Physics Paper. The left image shows an example cartoon trajectory of an alternate type of active particle called a Run-and-Tumble particle (Fig. 1a), and the right shows an example cartoon trajectory for an ABP (Fig. 1b). This tutorial focuses on modeling dry active matter with ABPs.