Collaborative Research: Fluid Polyamorphism: Theory, Experiment and Simulation

Overview: Integrated theoretical, experimental, and computational studies of fluid polyamorphism are proposed. Fluid polyamorphism, the existence of two distinct amorphous structures in a single-component condensed fluid, is a surprisingly ubiquitous, yet poorly understood, phenomenon. It is either found or predicted for a broad group of very different materials, including helium, sulfur, carbon, phosphorous, silicon, cerium, tin tetraiodide, tellurium, and hydrogen. This phenomenon is also hypothesized for deeply supercooled water, presumably located a few degrees below the empirical limit of homogeneous ice formation. It is proposed to develop and verify a generic phenomenological approach to describe polyamorphism in a single-component fluid, either in the presence (supercooled water, silicon, silica, etc.) or absence (helium, sulfur, phosphorous) of fluid-fluid phase separation. To specify the phenomenology, a fluid with thermodynamic equilibrium between two distinct interconvertible molecular or supramolecular structures will be considered. A global equation of state, which generically describes both vapor-liquid and liquid-liquid equilibria in single-component fluids, including metastable states under negative pressures will be formulated. The phenomenology will be verified by simulations of molecular models of various systems exhibiting fluid polyamorphism and by experimental studies. In particular, novel computational studies of chirality-driven liquid-liquid phase separation and equilibrium polymerization with and without liquid-liquid separation are proposed. A primary question to be answered is the distinction and interplay between events that are thermodynamically and kinetically controlled. The interplay between crystallization, fluid phase separation, and fluid structural relaxation, which underlies the routes to phase formation and interconversion of the alternative states will be studied computationally. The relation between the two-state phenomenology and two-scale soft-repulsion potentials that generate a liquid-liquid transition in a single-component system will be investigated and the microscopic nature of the corresponding order parameter will be identified. The developed phenomenology will be extended to binary solutions. A solute may suppress unwanted crystallization, thus revealing the liquid polyamorphism emanating from the hypothesized liquid-liquid transition in the case of pure water, where it is experimentally difficult to access. High-resolution calorimetric measurements and simulations of binary-solutions that exhibit a liquid- liquid critical line emanating from the liquid-liquid critical point of the pure solvent will be performed to verify this approach. Intellectual Merit: The proposed research addresses, from complementary theoretical, experimental and computational perspectives, one of the most interesting open questions in the physics and chemistry of condensed matter, namely the possibility of the existence of two amorphous phases in a single-component fluid. From experimental studies of novel ideal solutions of ionic liquids in water to molecular simulations of chirality-driven liquid-liquid transitions, and from phenomenological approaches based on two-state ideas to computational explorations of highly stretched water, the proposed work will expose to rigorous scrutiny an unusually broad range of phenomena (cavitation, metastable phase coexistence, chiral symmetry breaking, polymerization transitions), under the unifying conceptual approach. The concept of two competing interconvertible amorphous structures marks a paradigm shift that significantly broadens fluid polyamorphism from its original narrow scope to a cross-disciplinary field that addresses a wide class of systems and phenomena with interconversion of alternative molecular or supramolecular states, such as reversible conformations of macromolecules (such folding/unfolding of macromolecules or self-assembly in lyotropic liquid crystals) or interconversion of sterioisomers. Broader Impacts: Understanding the nature of fluid polyamorphism is fundamental to the advancement of knowledge in key areas of natural sciences, from physical chemistry and physics of soft matter to materials science, petrochemistry, and biology. This cross-disciplinary collaborative research has unusually broad implications, ranging from atmospheric science (supercooled water being central to cloud microphysics) and cryobiology (tissue preservation, life under extreme conditions), to the design of high-density active-matter systems (where intriguing analogies to glassy and supercooled states have been recently suggested) and to the development of international standards for thermophysical properties of water. The proposed studies of negative pressures are important in applications including capillary flow of water in trees, medical ultrasound, droplet ejection in inkjet printers, and cavitation damage in ship propeller blades.