Publication: Kinetics of Phase Transitions in Multicomponent Fluid Mixtures Using Computer Simulations: Role of Surface Potential and Mixture Composition
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2025-06-17
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Indian Institute of Technology, Jodhpur
Abstract
Phase transitions are transient processes in which a thermodynamically unstable or metastable state evolves to a stable state driven by minimizing the appropriate free energy. The instability or metastability occurs due to sudden changes in external parameters such as temperature, pressure, etc. Subsequently, the system undergoing phase transition finds itself in a far-from-equilibrium unstable state characterized by complex spatiotemporal pattern formation, where the patterns account for domains enriched in the preferred states. The phase transition dynamics are subject to the nonuniformities present in the form of defects, such as domain interfaces costing free energy, and the system proceeds by annealing these defects. Such an annealing process adopts nonequilibrium dynamics in the growth and coarsening of the domains, commonly referred to as domain growth, coarsening kinetics, and phase-ordering kinetics. The objectives of my thesis comprise systems that exhibit domain growth and coarsening kinetics in phaseseparating binary mixtures (AB). In our studies, the quenched mixture is unstable to infinitesimal long-wavelength fluctuations and spontaneously decomposes into A-rich and B-rich domains demarcated by interfaces. This process is a widely used paradigm for the coarsening kinetics and is known as spinodal decomposition (SD). Phase separation is recurring in most far-from-equilibrium systems. One of the primary focuses of my work is to analyze the commonalities and distinctions in these systems. The problems we address in our thesis emerge when we add surfaces to the phaseseparating mixtures. The phase-separating mixture in contact with a surface is commonplace in metallurgy, chemical, and biological systems. For immiscible mixtures, if the surfaces preferentially attract one of the species, the surface becomes the origin of composition waves, known as surface-directed spinodal decomposition (SDSD) waves. These SDSD waves are composed of alternating segments of the wetting (rich in the preferred component) and depletion layers (lacking preferred particles). Using numerical simulations, we specifically study the wetting kinetics in the systems undergoing SDSD with functional surface properties. Our focus remains on elucidating the roles played by composition ratio (A : B), hydrodynamics in the system, and the chemical and physical heterogeneity of the surface in contact. To begin with, we add a flat wall represented by an integrated wall potential V (z) with an attractive part ⇠ z−n, where z and n denote its depth below the horizontal fluid layer and its interaction range, respectively. We find that for critical compositions (A : B = 50 : 50), the wetting-layer thickness R1(t) at very early times exhibits a potentialdependent growth regime of R1(t) ⇠ t↵ with ↵ = 1/(n + 2) being the growth exponent. The potential-dependent growth next crosses over to a universal fast-mode regime with ↵ = 3/2. In contrast, much slower logarithmic behavior in R1(t) is seen initially for a short-ranged surface potential (n ! 1). Remarkably, a similar rapid growth with ↵ = 3/2 is seen in this case too. We invoked Siggia’s arguments to explain the fast-mode kinetics, where novel long-range correlations develop in the early phase separation stage, resulting in coating mechanism. Secondly, we investigate the role of the composition ratio in the binary mixture for a similar attractive long-range potential V (z). We show that R1(t) in early times slows down if the minority component (minority wetting) wets the surface. However, the potential-dependent growth of R1(t) ⇠ t1/(n+2) is obtained when the majority component (majority wetting) wets the surface. The contrasting results corroborate a local barrier of thickness greater than the thermal correlation length above the wetting layer for minority wetting. In this case, the probability of particles crossing the barrier to feed the wetting layer is lower. We also report the recovery of the potentialdependent growth for the minority wetting by weakening the local barrier. Furthermore, we replace the flat walls with amorphous ones to investigate the latetime growth regimes of R1(t). By choosing the amorphous walls, we could avoid surfaceinduced crystallization in the mixture and examine the wetting kinetics in SDSD for times much later than previously reported by molecular simulation. Using morphological characterization, we identify relevant length scales of phase ordering in the direction parallel [L||] and perpendicular [L?] to the wetting wall. Our results on the length scales show widely accepted growth exponents of ↵ = 1/3 (Lifshitz-Slyzov (LS) law) and 1 (Siggia’s mechanism), which are also universal. However, we could not observe the fast-mode as the coating mechanism was absent due to the roughness of the amorphous wall. Furthermore, we highlight the possible orientational e↵ects of the multilayeredstructures on domain coarsening near the surface. In the end, we replaced chemically homogeneous surfaces with chemically patterned substrates. By setting the pattern’s periodicity commensurate with the mixture’s inherent length scale, we notice the transposition of surface patterns to the fluid mixture in contact. The emerging patterns are denoted as a transient surface-registry regime exhibiting a dynamical crossover from surface-driven to other universal coarsening regimes. Moreover, we provide details of the scaling behavior for the registry formation and melting times as a function of the pattern size Mx. We further assess lateral domain morphologies and examine the crossover using correlation functions, structure factors, and domain length scales. The kinetics of SDSD are of great technological and scientific importance, o↵ering a pathway to curate and design functional materials by enabling precise control over their domain morphologies. Given the above, acquiring a detailed understanding of the process is beneficial.
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Zaidi, Sayed Shuja Hasan (2019).Kinetics of Phase Transitions in Multicomponent Fluid Mixtures Using Computer Simulations: Role of Surface Potential and Mixture Composition (Doctor's thesis).Indian Institute of Technology, Jodhpur