Freezing Water with Sonoluminescing Bubbles

Trustees Of Boston University, Boston

Investigators

Abstract

It is an everyday occurrence for water to freeze into solid ice when the temperature drops to 0 degrees Centigrade. But water can exhibit a variety of frozen forms, sometimes crystalline solids, sometimes disordered gels, depending on how (and how fast) it is frozen. These unique forms possess sometimes fantastic properties, being very dense or very viscous. Creating these unusual forms of water to study their remarkable properties usually requires confinement in high-pressure chambers at extremely cold temperatures, and contact with the chamber wall affects how the water freezes. This project seeks to create a novel form of ice without requiring the use of pressure chambers. Focused lasers will create individual nano-sized bubbles, and acoustic waves will grow these bubbles to millimeter sizes. Then, the bubbles will be allowed to collapse in pressurized water, a process often termed "acoustic cavitation". When the bubbles collapse, the water moving with the bubble rapidly pressurizes because it is contracting down to a tiny volume. By controlling the acoustics, water pressures just outside the collapsing bubble can reach 10,000 atmospheres in only a few nanoseconds, causing the water to freeze into a single ball of "ice" with no contact with any container surface. By employing diagnostics such as imaging ultrasound and laser scattering, researchers will probe both the mechanical properties and molecular structure of these remarkable ice balls. By learning about how water freezes at ultra-high pressures and nanosecond time scales, researchers can use the knowledge in a variety of ways. First, since the ice balls have such different properties from regular water, it may be possible to create a hybrid "ice-water" having novel optical and acoustic properties that change back to water properties in a few milliseconds. Second, there are many biomedical and industrial processes that currently employ acoustic cavitation, ranging from ultrasonic cleaners to contrast ultrasonic imaging to emulsification and extraction of food products. By systematically studying these violent bubble collapse events this research will shed light on similar but less energetic processes commonly occurring in industry and medicine. There is ample data and theory in agreement that the outcome for quasi-static near equilibrium compressions to GPa pressures at cryogenic or more modest temperatures is a crystalline ice (Ice VII, for example). There have been fewer studies of rapid compression, and while some of these studies show an amorphous high density and/or high viscosity phase, a very recent study of ns shock wave compression [Gleason et al., Compression freezing kinetics of water to Ice VII. Phys. Rev. Lett. 119, 025701 (2017)] seems to definitively show the formation of the crystalline Ice VII. In contrast, recent experiments from the principal investigator's lab [Sukovich et al., Outcomes of the Collapses of Large Single Bubbles in Water at High Ambient Pressures. Phys. Rev. E. 95 (2017) 43101] indicate the water surrounding a collapsing, sonoluminescing bubble "freezes" on the ns time scale. The result is an apparent phase transition to a spherical ice ball which displays properties of either hyper-elasticity or hyper-viscosity. However, the data are incomplete, as the Sukovich et al. experiment had only high-speed imaging as a diagnostic, and a parametric investigation was not carried out. This project will fill the gap in knowledge by investigating the formation, duration, and material properties of these ice balls resulting from high-pressure bubble collapse. Material properties will be investigated by employing both high-frequency time-reversal-aided acoustic scattering (probing the elastic modulus) and simultaneous femtosecond laser scattering (probing the molecular bonding). These experimental results will be complemented by molecular dynamics simulations at precisely the nanosecond time scales and high strain rates exhibited by the experiments and which are intrinsic to molecular dynamics simulations. These simulations will help determine the nature of the phase transition observed. A graduate student and several undergraduate students will be trained in research methods as a part of this work. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.