More Like this

1. Interfacial effects determine nonequilibrium phase behaviors in chemically driven fluids

   https://doi.org/10.1073/pnas.2501145122  

   Cho, Yongick; Jacobs, William M (July 2025, Proceedings of the National Academy of Sciences)

   Coupling between chemical fuel consumption and phase separation can lead to condensation at a nonequilibrium steady state, resulting in phase behaviors that are not described by equilibrium thermodynamics. Theoretical models of such “chemically driven fluids” typically invoke near-equilibrium approximations at small length scales. However, because dissipation occurs due to both molecular-scale chemical reactions and mesoscale diffusive transport, it has remained unclear which properties of phase-separated reaction–diffusion systems can be assumed to be at an effective equilibrium. Here, we use microscopic simulations to show that mesoscopic fluxes are dependent on nonequilibrium fluctuations at phase-separated interfaces. We further develop a first-principles theory to predict nonequilibrium coexistence curves, localization of mesoscopic fluxes near phase-separated interfaces, and droplet size-scaling relations in good agreement with simulations. Our findings highlight the central role of interfacial properties in governing nonequilibrium condensation and have broad implications for droplet nucleation, coarsening, and size control in chemically driven fluids. 

   more » « less
2. Surface tension measurements reveal charge-driven surfactant depletion in microdroplets approaching the Rayleigh limit

   https://doi.org/10.1039/D6SC03152H  

   Ahmed, Firoz; Jacobs, Michael I (July 2026, Chemical Science)

   The net charge on droplets is predicted to play a crucial role in accelerating chemical reactions in microdroplets by either altering reaction energetics or changing surface compositions. However, there are few experimental studies that have explored how the net charge of microdroplets alters their physicochemical properties. Here, we present a new technique to investigate how net charges on levitated microdroplets affect their surface tensions and the partitioning of surfactants to the air–water interface. The technique is validated by measuring how the resonant surface oscillation frequency (ωobs) of microdroplets changes with increasing net charge. For simple one- and two-component microdroplets, ωobs predictably decreases with charge. This enables the determination of surface tension of microdroplets (6–20 µm radius) with net charges approaching the Rayleigh limit (QR), which represents the maximum net charge a droplet can stably carry before coulombic fission. To demonstrate how this new technique can be used to measure charge-driven changes in surface composition, we measured the dependence of surface tension on charge for microdroplets containing cetrimonium bromide (CTAB). For ∼8 µm-radius microdroplets containing 0.5 mM CTAB, the surface tension was observed to increase from ∼50 to ∼65 mN m−1 as net positive charge on droplets increased from low charge to QR. A similarly-sized change in surface tension was not observed in negatively charged droplets nor droplets with CTAB concentrations larger than the critical micelle concentration. These initial results demonstrate that the net charge on droplets can alter the equilibrium partitioning of charged surfactants to the air–water interface. Ultimately, the new approach to measure the surface tensions of highly-charged microdroplets enables investigations into how charge alters interfacial concentrations of molecules to promote microdroplet chemistry. 

   more » « less
3. Nonequilibrium statistical thermodynamics of multicomponent interfaces

   https://doi.org/10.1073/pnas.2121405119  

   Rauscher, Phillip M.; Öttinger, Hans Christian; de Pablo, Juan J. (June 2022, Proceedings of the National Academy of Sciences)

   Nonequilibrium interfacial thermodynamics has important implications for crucial biological, physical, and industrial-scale transport processes. Here, we discuss a theory of local equilibrium for multiphase multicomponent interfaces that builds upon the “sharp” interface concept first introduced by Gibbs, allowing for a description of nonequilibrium interfacial processes such as those arising in evaporation, condensation, adsorption, etc. By requiring that the thermodynamics be insensitive to the precise location of the dividing surface, one can identify conditions for local equilibrium and develop methods for measuring the values of intensive variables at the interface. We then use extensive, high-precision nonequilibrium molecular dynamics (NEMD) simulations to verify the theory and establish the validity of the local equilibrium hypothesis. In particular, we demonstrate that equilibrium equations of state are also valid out of equilibrium, and can be used to determine interfacial temperature and chemical potential(s) that are consistent with nonequilibrium generalizations of the Clapeyron and Gibbs adsorption equations. We also show, for example, that, far from equilibrium, temperature or chemical potential differences need not be uniform across an interface and may instead exhibit pronounced discontinuities. However, even in these circumstances, we demonstrate that the local equilibrium hypothesis and its implications remain valid. These results provide a thermodynamic foundation and computational tools for studying or revisiting a wide variety of interfacial transport phenomena. 

   more » « less
4. Wormlike emulsion droplets

   https://doi.org/10.1103/3g85-ncsj  

   Abacousnac, Jatin; Chen, Wenjun; Brujic, Jasna; Grier, David G (July 2025, Physical Review Research)

   Forming an interface between immiscible fluids incurs a free-energy cost that usually favors minimizing the interfacial area. An emulsion droplet of fixed volume therefore tends to form a sphere, and pairs of droplets tend to coalesce. Surfactant molecules adsorbed to the droplets' surfaces stabilize emulsions by providing a kinetic barrier to coalescence. Here, we show that the pressure exerted by bound surfactant molecules also competes with the droplet's intrinsic surface tension and can reverse the sign of the overall surface free energy. The onset of negative surface tension favors maximizing surface area and therefore favors elongation into a wormlike morphology. Analyzing this system in the Gibbs grand canonical ensemble reveals a phase transition between spherical and wormlike emulsions that is governed by the chemical potential of surfactant molecules in solution. Predictions based on this model agree with the observed behavior of an experimental model system composed of lipid-stabilized silicone oil droplets in an aqueous surfactant solution. 

   more » « less
5. Interfacial Assembly of Bacterial Microcompartment Shell Proteins in Aqueous Multiphase Systems

   https://doi.org/10.1002/smll.202308390  

   Abeysinghe, A_A_Dharani T; Young, Eric J; Rowland, Andrew T; Dunshee, Lucas C; Urandur, Sandeep; Sullivan, Millicent O; Kerfeld, Cheryl A; Keating, Christine D (April 2024, Small)

   Abstract Compartments are a fundamental feature of life, based variously on lipid membranes, protein shells, or biopolymer phase separation. Here, this combines self‐assembling bacterial microcompartment (BMC) shell proteins and liquid‐liquid phase separation (LLPS) to develop new forms of compartmentalization. It is found that BMC shell proteins assemble at the liquid‐liquid interfaces between either 1) the dextran‐rich droplets and PEG‐rich continuous phase of a poly(ethyleneglycol)(PEG)/dextran aqueous two‐phase system, or 2) the polypeptide‐rich coacervate droplets and continuous dilute phase of a polylysine/polyaspartate complex coacervate system. Interfacial protein assemblies in the coacervate system are sensitive to the ratio of cationic to anionic polypeptides, consistent with electrostatically‐driven assembly. In both systems, interfacial protein assembly competes with aggregation, with protein concentration and polycation availability impacting coating. These two LLPS systems are then combined to form a three‐phase system wherein coacervate droplets are contained within dextran‐rich phase droplets. Interfacial localization of BMC hexameric shell proteins is tunable in a three‐phase system by changing the polyelectrolyte charge ratio. The tens‐of‐micron scale BMC shell protein‐coated droplets introduced here can accommodate bioactive cargo such as enzymes or RNA and represent a new synthetic cell strategy for organizing biomimetic functionality. 

   more » « less