Shock-induced nanobubble collapse and its applications

The shock-induced collapse of nanobubbles in water is investigated using molecular dynamics simulations based on a reactive force field. Monitoring the collapse of a cavitation nanobubble, we observe a focused nanojet at the onset of bubble shrinkage and a water hammer shock wave upon bubble collapse. The nanojet length scales linearly with the nanobubble radius, as observed in experiments on micron-to-millimeter size bubbles. The shock induces dramatic structural changes, including an ice-VII-like structural motif at a particle velocity of approximately 1 km/s. The incipient ice VII formation and the calculated Hugoniot curve are in good agreement with experimental results. Moreover, a substantial number of positive and negative ions appear when the nanojet hits the distal side of the nanobubble and the water hammer shock forms. Furthermore, two promising applications of shock-induced nanobubble collapse have been explored. Our simulations of poration in lipid bilayers due to shock-induced collapse of nanobubbles reveal penetration of nanojets into lipid bilayers. The nanojet impact generates shear flow of water on bilayer leaflets and pressure gradients across them, which transiently enhance the bilayer permeability by creating nanopores through which water molecules translocate across the bilayer. The effects of nanobubble size and temperature on the porosity of lipid bilayers are examined. Finally, the shock-induced collapse of CO2-filled nanobubbles in water is investigated. The energetic nanojet and high-pressure water hammer shock formed during and after collapse of the nanobubble trigger mechano-chemical H2O-CO2 reactions, some of which lead to splitting of water molecules. The dominant pathways through which splitting of water molecules occur are identified.本研究基于反应力场（reactive force field），采用分子动力学模拟（molecular dynamics simulations）方法，探究了水中冲击波诱导纳米气泡的溃灭过程。在监测空化纳米气泡（cavitation nanobubble）的溃灭过程中，我们观察到气泡收缩初期会形成聚焦型纳米射流（nanojet），而在气泡溃灭时刻则会产生水击冲击波（water hammer shock wave）。纳米射流的长度与纳米气泡半径呈线性比例关系，该现象与微米至毫米级气泡的实验观测结果相符。冲击波可引发显著的结构变化：当粒子速度约为1 km/s时，会形成类冰VII（ice-VII）结构基元（structural motif）。初始冰VII的形成过程与计算得到的雨贡纽曲线（Hugoniot curve）均与实验结果吻合良好。此外，当纳米射流撞击纳米气泡的远端侧壁并形成水击冲击波时，体系中会产生大量正负离子。进一步地，本研究还探索了冲击波诱导纳米气泡溃灭的两项潜在应用方向。针对纳米气泡冲击波诱导溃灭引发脂质双层膜（lipid bilayers）穿孔的模拟研究表明，纳米射流可穿透脂质双层膜。纳米射流的撞击会在双层膜小叶（bilayer leaflets）表面诱导水体剪切流，并在膜两侧形成压力梯度；通过生成纳米孔道实现水分子跨膜转运，进而暂时性提升脂质双层膜的通透性。本研究还考察了纳米气泡尺寸与温度对脂质双层膜孔隙率（porosity）的影响。最后，本研究探究了水中充二氧化碳纳米气泡（CO₂-filled nanobubbles）的冲击波诱导溃灭过程。纳米气泡溃灭前后形成的高能纳米射流与高压水击冲击波，会触发H₂O-CO₂机械化学反应，其中部分反应会导致水分子发生裂解。本研究明确了水分子裂解的主要反应路径。