Active particles-boundary interactions

The pressure of an ensemble of active particles is a fundamental but much debated property. Whether an equation of state relating pressure to number density exists for the different families of active particles, how this pressure depends on particle-particle interactions and on particle interactions with surrounding walls, or what is the role of geometry and wall curvature are typical questions. Establishing the dependence of the mechanical pressure of active systems on system parameters permits the exploration of different perspectives in active matter, notably self-assembly, phase separation, and encapsulation. 

When encapsulated inside a droplet, the Janus particles are randomly distributed. However, under light illumination, they move but preferably towards the boundary of the droplet (see the Figure). Consequently, the drop deformation becomes less. This is in principle because of increasing the effective surface tension by the activity. 

In this work, we have measured the mechanical resistance engendered by an assembly of active spherical and noninteracting particles to the deformation of a drop in which they are encapsulated. From a systematic study over an extended range of its dependence on active particle number density and particle velocity, as well as fluid and particle properties, it turns out that the measured mechanical resistance or pressure can be described by an expression for the pressure of an assembly of active particles proposed some years ago.