Presidential Young Investigator Award: Sub-Triple Liquids and Crystallization in Supercritical Fluids

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Research Summary: Sub-triple liquids (metastable liquids below their triple point): This project includes an experimental study of the phase transition (unique to sub-triple liquids) involving simultaneous boiling and crystallization, completion of a theoretical study of tensile instability (loss of tensile strength), and investigation of the molecular origin of density anomalies (liquids which contract when heated). The connection between tensile instability and density anomalies was derived by the PI. Crystallization in supercritical fluids: This portion of the project involves completion of equipment construction and preliminary experiments in which continuous precipitation occurs into a fluid whose temperature, pressure, and composition can be independently manipulated. Novelty/Innovation: Sub-triple liquids are an unexplored state of matter (the term was coined by the PI). Their behavior includes tensile instabilities (a decrease in tensile strength as the temperature is lowered, an experimentally observed anomaly first explained by the PI) and a phase transition involving rapid crystallization and boiling coupled with liquid disappearance, a phenomenon not to be found elsewhere in nature. Crystallization from supercritical fluids is a novel method for the production of uniform crystals and monodisperse polymer particles. The most innovative concept in this project is the attainment of uniform, controllable conditions within the fluid where nucleation occurs through a manipulation of pressure instead of temperature. Pressure perturbations travel at the speed of sound, thus guaranteeing uniformity throughout the fluid medium and are applied to a highly compressible fluid near its critical point, thus giving rise to large changes in density and hence solvent power. Technical Impact/Practical Significance: Tension generated when a degassed liquid is cooled inside a rigid container is ideally suited for the non-destructive testing of adhesion on flat surfaces. This is a problem of vital importance in microchip manufacture (e.g., photo-resist adhesion testing). The experimentally observed equilibrium of a sub-triple liquid and a crystal suggests the novel idea of creating solids which are uniformly stressed. Since it is otherwise impossible to subject a solid to isotropic tension, this process (tension crystallization) could lead to the production of materials with unique properties.