# Interactions between Phase-Separated Liquids and Membrane Surfaces
## 7 Active Processes and Nonequilibrium States in Cells Modify Biomolecular Condensate Properties  
When biomolecular condensates are in the environment of the cell, they are subject to various means of regulation to fine-tune and control their function. Multi-droplet systems, especially in the absence of emulsifiers, are not usually in an equilibrium state. Instead, they undergo coarsening processes such as Ostwald ripening in which smaller droplets in a two-phase system diffusively lose material to larger droplets until a single large droplet remains (the true equilibrium state) [95; 96]. Cells, however, maintain multi-droplet systems of condensates, leaving open the question of what mechanisms enable the maintenance in those systems of out-of-equilibrium states. One mechanism likely to be relevant in biological systems is found in active matter: Liquid-liquid phase-separated systems coupled with nonequilibrium chemical reactions (so-called \"active emulsions\") can counteract Ostwald ripening and remain a monodisperse system of droplets [97]. The dynamics of the evolution of biomolecular condensate droplet systems can differ depending on a variety of different kinds of \"active,\" nonequilibrium chemical reactions as well as factors such as spatiotemporal differences in stimuli affecting protein affinities and undriven but reactive systems. Reviews by [PERSON], [PERSON], and [PERSON] [98] and [PERSON] and [PERSON] [99] explore these modulations of phase separation kinetics in detail. We will turn our focus to how, specifically, membrane-associated biomolecular condensates are controlled by active processes and other related regulatory phenomena.  
Artificial Regulatory Mechanisms Reveal Biomolecular Condensate Responses to Nonequilibrium Processes  
A simple nonequilibrium system is that of a complex coacervate droplet with a constant applied electric field. When an electric field is applied to a coacervate, a number of morphological irregularities--most notably, vacuolization (the uptake of the dilute aqueous phase)--occur [100]. Liposomes at the surface or interior of droplets under such conditions can have various effects on this behavior. Vacuolization may be suppressed, even to the point that droplet fragmentation becomes the dominant phenomenon as was observed for neutral vesicles permeating a charge-balanced coacervate, or it may occur and lead to the movement of surface vesicles to align with the electric field as was observed for a coating of negatively-charged vesicles on a positively-charged coacervate surface [91].  
Another artificial regulatory mechanism is the fine-tuning of biomolecular condensate properties through introduction of non-native stimulus-sensitive protein domains. Membrane-bound optoDroplet proteins, which contain a light-sensitive domain that promotes phase separation when illuminated, have recently been used to demonstrate that phase separation can provide cellular \"memory\" of stimuli. Phase separation occurred within a locally illuminated region, but when the whole cell was subsequently illuminated, most of the droplets remained in the originally stimulated area due to diffusive principles similar to those governing Ostwald ripening [101]. A membrane-associated biomolecularcondensate of endocytic proteins Eps15 and Fcho1 has recently been identified. Fine-tuning the interaction strength between these proteins through an artificial light-sensitive domain allowed researchers to generate endocytic structures that were \"abortive\" (highly transient), \"productive,\" or \"stalled\" (long-lived but unproductive), in order from weakest to strongest interactions [102].