Freezing Water with Sonoluminescing Bubbles

当温度降到摄氏零度时，水冻结成固体冰是每天都会发生的事情。但水可以表现出各种冻结的形式，有时是结晶固体，有时是无序的凝胶，这取决于它被冻结的程度(和速度)。这些独特的形式有时具有奇特的特性，密度很高，或者非常粘稠。创造这些不寻常形式的水来研究它们的非凡特性，通常需要在极低温度下的高压室中进行限制，而与室壁的接触会影响水冻结的方式。该项目旨在创造一种不需要使用压力室的新形式的冰。聚焦的激光将产生单独的纳米大小的气泡，声波将这些气泡增长到毫米大小。然后，这些气泡将被允许在加压水中破裂，这一过程通常被称为“声空化”。当气泡破裂时，随着气泡移动的水会迅速加压，因为它正在收缩到很小的体积。通过控制声学，破裂气泡外的水压在几纳秒内就可以达到10,000个大气压，导致水冻结成一个单独的冰球，不与任何容器表面接触。通过使用成像超声和激光散射等诊断学，研究人员将探索这些非凡的冰球的机械性能和分子结构。通过了解水在超高压和纳秒时间尺度下是如何冻结的，研究人员可以通过各种方式使用这些知识。首先，由于冰球与普通水具有如此不同的性质，因此有可能创造一种具有新颖的光学和声学性质的混合体“冰-水”，这些性质在几毫秒内改变回水性质。其次，目前有许多生物医学和工业过程使用声空化，从超声波清洗机到对比超声成像，再到食品的乳化和提取。通过系统地研究这些剧烈的气泡破裂事件，这项研究将揭示工业和医学中常见的类似但能量较低的过程。有大量数据和理论一致认为，在低温或更温和的温度下，准静态近平衡压缩到Gpa压力的结果是结晶冰(例如，Ice VII)。关于快速压缩的研究较少，虽然其中一些研究表明存在无定形的高密度和/或高粘度相，但最近一项关于ns冲击波压缩的研究[Gleason等人，将水压缩成冰的冻结动力学。莱特牧师。119,025701(2017年)]似乎明确地显示了结晶冰七的形成。相比之下，首席研究员实验室最近的实验[苏科维奇等人]在高环境压力下水中大的单个气泡坍塌的结果。太棒了。Rev.E.95(2017年)43101]表示在ns时间尺度上，围绕着坍塌的声致发光气泡的水“冻结”。其结果是向球形冰球的明显相变，表现出超弹性或超粘性。然而，正如Sukovich等人所说，数据是不完整的。实验只有高速成像作为诊断，没有进行参数调查。这个项目将通过研究高压气泡破裂导致的冰球的形成、持续时间和材料特性来填补这一知识空白。材料特性将通过使用高频时间反转辅助的声散射(探测弹性模数)和同时的飞秒激光散射(探测分子键)来研究。这些实验结果将得到精确到纳秒时间尺度和高应变率的分子动力学模拟的补充，这些都是分子动力学模拟所固有的。这些模拟将有助于确定所观察到的相变的性质。作为这项工作的一部分，一名研究生和几名本科生将接受研究方法方面的培训。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

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.