Arghya Majee

The hierarchical organization of multiphase biomolecular condensates into core-shell architectures is a fundamental problem in soft matter and biophysics. While classical explanations rely on hierarchies of interfacial tension (γ) between coexisting liquids, the ultralow tensions of condensates (0.1–1 μN/m) render such hierarchies potentially fragile. We introduce a robust assembly principle based on Polymer-Assisted Condensation (PAC), in which a single polymer species dictates the entire structure. The polymer nucleates a dense core by recruiting a condensation-incompetent protein (P1). A second incompetent protein (P2), which is repelled or otherwise thermodynamically disfavored from entering the polymer-rich core, is nonetheless recruited to the interface by weak attraction to P1, forming a stable shell. This effective repulsion-driven layering operates across a wide parameter space without requiring γ asymmetries and yields a robust structure that is impervious to concentration fluctuations and environmental perturbations. Phase-field modeling and molecular simulations establish this mechanism and capture key features of nucleolar organization. Our work reveals a general physical pathway for encoding spatial order in soft, multicomponent fluids.

Effet thermoélectrique dans des dispersions colloïdales: Cette thèse porte sur le mouvement de particules colloïdales induit par l’effet thermoélectrique (ou effet Seebeck). Dans un électrolyte soumis à un gradient de température, les ions ont tendance à migrer à des vitesse qui différent d'une espèce à l'autre. On observe alors une accumulation de charge aux bords de l’échantillon. Ce déséquilibre induit un champ électrique qui agit sur les colloïdes chargés présents dans la solution. Cette contribution électrophorétique dans le champ de Seebeck s'additionne à la contribution directe de thermodiffusion. Comme résultat principal,nous obtenons la vitesse phorétique en fonction de la fraction volumique des particules et, dans le cas de polyélectrolytes, du poids moléculaire. Dans la seconde partie, nous étudions l’effet thermoélectrique pour une particule chauffée par absorption d’un faisceau laser. Le gradient de température est alors radial et l’effet Seebeck induit une charge nette dans le voisinage de la particule. Enfin, nous discutons les applications possibles de ce phénomène de thermocharge

We put forward a novel mechanism for the positioning of chromatin within the cell nucleus that does not rely on polymer adsorption to the nuclear lamina. Using a field-theoretic model of a chromatin condensate formed through polymer–protein co-condensation, we show that surface enrichment of chromatin-binding proteins can localize the condensate at the nuclear lamina even when chromatin itself is repelled from the surface. The competition between the tendency of the protein component to wet the surface and the conformational restrictions of the polymer near the impenetrable boundary produces a nonmonotonic chromatin density profile with a maximum at a finite distance from the lamina, consistent with live-cell experiments [Sci. Adv. 7 , eabf6251 (2021)]. We further demonstrate that a non-binding protein competing for surface association can regulate the position of the condensate, providing an alternative pathway by which compositional changes of the lamina can induce chromatin repositioning. A phase diagram of the surface-localization transition reveals how surface affinity, protein availability, and polymer–protein interaction strength control the switch between peripheral and central chromatin configurations. This wetting-based mechanism contrasts with direct polymer adsorption and provides a general framework for understanding how multicomponent condensates interact with responsive biological surfaces.

We present a theory for phase-separated liquid coacervates with salt, taking into account spatial heterogeneities and interfacial profiles. We find that charged layers of alternating sign can form around the interface while the bulk phases remain approximately charge neutral. We show that the salt concentration regulates the number of layers and the amplitude of the layer's charge density and electrostatic potential. Such charged layers can either repel or attract single-charged molecules diffusing across the interface. Our theory could be relevant for artificial systems and biomolecular condensates in cells. Our work suggests that interfaces of biomolecular condensates could mediate charge-specific transport similar to membrane-bound compartments. Published by the American Physical Society 2024

We analyze the changes in the vicinal acidity (pH) at a spherical amphiphilic membrane. The membrane is assumed to contain solvent accessible, embedded, dissociable, charge-regulated moieties. Basing our approach on the linear Debye-Hückel approximation, as well as on the nonlinear Poisson-Boltzmann theory, together with the general Frumkin-Fowler-Guggenheim adsorption isotherm model of the charge-regulation process, we analyze and review the dependence of the local pH on the position, as well as bulk electrolyte concentration, bulk pH, and curvature of the amphiphilic single membrane vesicle. With appropriately chosen adsorption parameters of the charge-regulation model, we find a good agreement with the available experimental data.

We analyze the changes in the vicinal acidity (pH) at a spherical amphiphilic membrane. The membrane is assumed to contain solvent accessible, embedded, dissociable, charge regulated moieties. Basing our approach on the linear Debye-Hückel as well as the non-linear Poisson-Boltzmann theory, together with the general Frumkin-Fowler-Guggenheim adsorption isotherm model of the charge regulation process, we analyse and review the dependence of the local pH on the position, as well as bulk electrolyte concentration, bulk pH and curvature of the amphiphilic single membrane vesicle. With appropriately chosen adsorption parameters of the charge regulation model, we find a good agreement with available experimental data.

We formulate a theory of electrostatic interactions in lipid bilayer membranes where both monolayer leaflets contain dissociable moieties that are subject to charge regulation.

Liquid-liquid phase separation has emerged as one of the important paradigms in the chemical physics as well as biophysics of charged macromolecular systems. We elucidate an equilibrium phase separation mechanism based on charge regulation, i.e., protonation-deprotonation equilibria controlled by pH, in an idealized macroion system which can serve as a proxy for simple coacervation. First, a low-density density functional calculation reveals the dominance of two-particle configurations coupled by ion adsorption on neighboring macroions. Then a binary cell model, solved on the Debye-Huckel as well as the full nonlinear Poisson-Boltzmann level, unveils the charge symmetry breaking as inducing the phase separation between low- and high-density phases as a function of pH. These results can be identified as a charge symmetry broken complex coacervation between chemically identical macroions.

In this paper, we theoretically study the electrostatic interaction between a pair of identical colloids with constant surface potentials sitting in close vicinity next to each other at the fluid interface. By employing a simplified yet reasonable model system, the problem is solved within the framework of classical density functional theory and linearized as well as nonlinear Poisson–Boltzmann (PB) theory. Apart from providing a sound theoretical framework generally applicable to any such problem, our novel findings, all of which contradict common beliefs, include the following: first, quantitative and qualitative differences between the interactions obtained within the linear and the nonlinear PB theories; second, the importance of the electrostatic interaction between the omnipresent three-phase contact lines in interfacial systems; and, third, the occurrence of an attractive electrostatic interaction between a pair of identical metal colloids. The unusual attraction we report largely stems from an attractive line interaction, which although scales linearly with the size of the particle can compete with the surface interactions and can be strong enough to alter the nature of the total electrostatic interaction. Our results should find applications in metal or metal-coated particle-stabilized emulsions where densely packed particle arrays are not only frequently observed but also sometimes required.