Active droplets are a class of active matter, where the material forming the droplets is turned over by chemical reactions. One can show that these chemical reactions must be driven by an external energy input, implying that these droplets exist in an non-equilibrium environment. We develop theoretical models to understand the behavior of such active droplets. Such active droplets can for instance be found in biological condensates.
Nucleation of active droplets
Active droplets are subject to thermal fluctuations, which affect their shape and how the nucleate. While the fluctuations of the reactions are usually negligible, the turnover generally hinders droplet formation and suppresses nucleation. Consequently, the simplest driven chemical reactions that we study oppose the formation of droplets, leading to suppressed Ostwald ripening and droplet initiation. However, there are also more complicated active droplets, where a catalytically active core allows controlled nucleation and localized chemical reactions actually facilitate droplet formation.
Active droplets center internal particles
Active droplets exhibit diffusive fluxes, which can be used to center solid particles inside the droplets. Such control over the droplet morphology explains the structure of centrosomes, which are biomolecular condensatein cells. More generally, driving droplets with chemical reactions might allow biological cells to control droplet formation to structure their interior.
Spontaneously dividing active droplets
Active droplets are described as a combination of phase separation and driven chemical reactions that affect the droplet material. Generically, this leads to compositional fluxes between the droplet phase and its surrounding. In the typical case of externally maintained droplets, where droplet material is produced in the surrounding of the droplet, the spherical droplet shape can become unstable and droplets may divide spontaneously. This process happens until the droplet density high enough such that the active emulsion is stable; see video on the left.
Stable emulsions
Controlling the formation and stabilizing droplets is important in many fields ranging from the food industry to cosmetics and medicine. Furthermore, there is more and more evidence that droplets also play an important role to organize the interior of biological cells. Indeed, we propose that centrosomes are liquid droplets. One problem with liquid droplets is that they try to combine to form on large droplet, which is energetically more favorable.
The video on the left shows that chemical reactions influencing the physical properties of the droplet material can prevent this droplet coarsening. We study generic physical models of droplet formation under the influence of chemical reactions to identify the necessary conditions where multiple droplets are stable. This improves our understanding of droplet formation inside cells and might also benefit technical applications.