arXiv subjects

Umesh Dhumal

Publications and source records attributed to Umesh Dhumal.

4 recordsLinked to original sources

arXiv2026

Multiple Emulsions (W/O/W) for Confined Precipitation of Drug Nanoparticles

Multiple emulsions offer a compelling route to confine nucleation and growth during drug precipitation, yet their practical use is frequently limited by kinetic fragility and sensitivity to formulation and processing conditions. Here, we develop an ultrasound-assisted, two-stage emulsification strategy to generate water-in-oil-in-water (W/O/W) multiple emulsions with sufficient stability to function as templates for forming drug-rich submicron particulates. We first establish an operating window using simple W/O emulsions, showing that increased Tween~80 concentration and intensified sonication (higher amplitude and larger probe) yield smaller droplets and reduced coarsening tendencies. Using this window, W/O/W emulsions are formulated and systematically screened via surfactant pairing across ionic, non-ionic, and polymeric stabilizers. Ionic--non-ionic combinations provide the most favorable droplet-size control, with CTAB--Tween~80 emerging as a practically robust formulation. Cyclohexane was selected as a reproducible platform oil for downstream precipitation using the lead CTAB--Tween~80 formulation. Finally, curcumin-loaded W/O/W constructs generate curcumin-rich submicron particulates, supporting multiple emulsions as experimentally accessible microstructured environments for particle engineering of poorly soluble drugs.

cond-mat.soft↗

arXiv2025

Cross-Interaction Softness as a Route to Microphase Separation in Binary Colloidal Systems

Understanding how interparticle interactions govern phase behavior is central to controlling self-organization in multicomponent soft-matter systems. In particular, the role of cross-interactions between unlike components remains insufficiently understood. Here, we systematically investigate how cross-interaction character controls phase behavior in binary mixtures of hard and soft particles using coarse-grained modeling, Reference Interaction Site Model (RISM) theory, and molecular dynamics simulations. Four representative systems are examined that differ only in whether interactions between unlike particles are bounded or hard-sphere. We show that penetrable (bounded) cross-interactions are both necessary and sufficient to induce microphase separation, even in the absence of attractive forces. Such systems exhibit dispersed states, macrophase separation, and microphase-separated morphologies characterized by finite-wavelength compositional ordering. In contrast, purely hard-sphere cross interactions suppress microphase separation entirely, despite strong local clustering. Comparison between theory and simulations reveals qualitative agreement in phase topology, while simulations additionally capture hierarchical and multiscale ordering near crossover regimes. These findings establish cross-interaction softness as a fundamental design principle for controlling phase behavior in multicomponent colloidal and soft-matter systems.

cond-mat.soft↗

arXiv2025

The elusive fluid-and-crystal coexistence state in simulations of monodisperse, hard-sphere colloids

Monodisperse, purely repulsive, hard spheres (MPRHS) are an important model system for mechanistically exploring phase behavior in atomic systems and colloids. Since the 1940s, phase transitions in these systems have been obtained via simulation, theory, and experiments. But there is a gap in this literature: despite decades of reports of phase transition from one pure state to another, no computational studies report spontaneous phase separation into coexisting domains of liquid and crystal regions. This gap owes its origin to the underlying mechanism of entropically-driven phase separation in MPRHS - the competition between short-range entropy and long-range entropy. Frenkel proposed that spontaneous phase separation in simulations of up to 1,000,000 particles would require more than 317,000,000 years to sample enough microstates to converge to phase separated macrostate. Some brute-force simulations do show brief spontaneous coexistence but a metastable crystal or fluid subsequently overtakes the system. To bypass these difficulties, many studies use seeding, gravity, or direct construction of liquid and solid phases to study interfacial energy and nucleation rates of MPRHS systems. WCA potentials have also been used to bypass metastability, where softness provides free volume, lowers osmotic pressure and the energy barrier. It is argued that the transition path taken in bypassing metastability is mechanistically the same as without triggers. But as acknowledged by Alder & Wainwright, explicit observation of spontaneous coexistence is central to computational prediction of first-order transition. Such observation would provide satisfying demonstration of Frenkel's entropy exchange mechanism. We explore literature revealing these interesting behaviors and conclude that computational demonstration of Frenkel's mechanism for MPRHS awaits large systems with careful hardness perturbations.

cond-mat.soft↗

arXiv2025

Frenkel's entropy-exchange mechanism in monodisperse, nearly hard-sphere colloids: minimal perturbations to access fluid-crystal coexistence

Entropically driven fluid-solid transitions in monodisperse, purely repulsive hard spheres (MPRHS) are well established in theory, simulation, and experiment for atomic and colloidal systems. For MPRHS, however, coexistence is usually located via bulk free-energy calculations; the underlying microscopic balance between configurational and vibrational entropy is left implicit. Frenkel clarified this mechanism explicitly as an exchange of long-range configurational entropy for short-range vibrational entropy, but in the pristine MPRHS limit the nucleation barrier near coexistence is so high that phase separation is predicted only on astronomical time scales. Consistent with this, even unbiased simulations do not show spontaneous, equilibrium fluid-crystal coexistence; transient mixtures are mostly overtaken by a single phase; observed coexistence is still algorithmically-driven. Nearly hard-sphere colloid experiments do observe fluid-crystal coexistence, but always in the presence of unavoidable triggers such as gravity, walls, and polydispersity. We treat the hard-sphere phase diagram as settled and ask how the entropic exchange mechanism can be revealed in nearly hard-sphere colloidal simulations. We probe the mechanism on finite time scales by introducing minimal perturbations that trigger phase separation: small reductions in hardness that increase locally accessible free volume (and thus gently increase vibrational entropy), and 2-4% distributed crystal seeds. These perturbations produce coexisting fluid and crystal domains with crystal fraction, phase envelope and osmotic pressure that, with systematically increasing particle hardness, approach the hard-sphere limit. These results demonstrate that slight enhancements to vibrational entropy provide a dynamically accessible route to realizing the long-range/short-range entropy exchange required for phase separation.

cond-mat.soft↗