Active Brownian Particles are a useful model system for understanding emergent properties in the dynamics living organism due to the simplicity of the model. In its simplest form, it consists of identical disc shaped particles to move along a surface with a prescribed velocity. Due to thermal fluctuations (noise), the particles deviate from their trajectories and gradually change direction. Additionally, the particles are unable to pass through or over one another. Given just these simple ingredients the particles will form clusters resulting in regions of high and low concentration, an example of phase separation driven by an active process (i.e. one that is driven by energy converted into motion by the particles). The dynamics of this phase separated state are still quite rich though, as clusters can move, rotate and occasionally disperse. You should be able to see all of these phenomena using the simple interactive simulation below. For further background reading, try the following papers: M.E.Cates and J.Tailleur EPL (2013), Stenhammar et al. PRL (2013), and Stenhammar et al. Soft Matter (2014).
To run the simulation, enter a value for the particle packing fraction and box size first. Then click start to run the code. Particle velocity and angular diffusion constant can be changed dynamically with the sliders. If the code runs slowly, try reducing the box size or packing fraction. If it starts to stutter after starting a new run, try refreshing the page.