Using biology as an inspiration, we seek to understand physical principle that allow building smart materials, which exhibit designed responses upon changes in external stimuli. On the one hand, we explorer passive materials, where responses originate from phase transitions while the system stays close to equilibrium. On the other hand, we explorer active systems, which are driven away from equilibrium.
AN elastic mesh (red) constrains droplets (green), which then arrange in a regular pattern.
Elastic matrices control liquid droplets
Biomolecular condensates exhibit an incredibly complicated environment in biological cells. For instance, droplets in the cytosol are restricted by long, stable polymers that form the cytoskeleton. Similarly, droplets in the nucleus can only form by deforming the dense chromatin. To understand how the elastic properties of cellular components affect biomolecular condensates we study an idealized situation. In particular, we collaborate with Eric Dufresne's lab at ETH Zürich to analyze oil droplets induced in elastic PDMS gels. These systems display surprisingly mono-disperse emulsions, which we could link to a non-equilibrium process where droplets cavitate by breaking the surrounding mesh. Experiments with a stiffness gradient reveal that droplets dissolve in stiff regions, forming a dissolution front. Meanwhile, the dissolving droplet material diffuses through the dilute phase and accumulates in soft regions. To understand these dynamics, we developed a theoretical model that describes how the elastic environment squeezes droplets. This model explains the observed experimental data quantitatively. A coarse-grained version of the model reveals that the dissolution front moves diffusively and it generally allows to predict how stiffness profiles affect droplet arrangements. Taken together, our work identifies how elastic gradients control droplets, which will help to disentangle all the processes affecting biomolecular condensates in cells.
Qiang et al., Nonlocal Elasticity Yields Equilibrium Patterns in Phase Separating Systems, PRX 2024
Vidal-Henriquez et al., Cavitation controls droplet sizes in elastic media, PNAS 2021
Vidal-Henriquez et al., Theory of droplet ripening in stiffness gradients, Soft Matter 2020
Rosowski et al., Elastic stresses reverse Ostwald ripening, Soft Matter 2020
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.