冲击波诱导水中纳米气泡塌陷的分子动力学分析

Molecular dynamics analysis of shock wave-induced nanobubble collapse in water

人体中含有的纳米气泡受冲击波诱导塌陷后产生的强冲击高速纳米射流会对人体组织产生创伤.本文运用分子动力学方法,分析了冲击波引起的水中纳米气泡的塌陷行为,纳米气泡分为三种:真空、含二氧化碳和氧气纳米气泡.同时探讨了不同气体分子数、纳米气泡的直径和冲击波的冲量等因素对水中纳米气泡塌陷行为的影响.研究发现在真空纳米气泡中加入气体分子后并没有影响冲击波的传播,但在纳米气泡完全塌陷前,与真空和含1368个二氧化碳分子(或含1409个氧气分子)的纳米气泡相比,含718个二氧化碳分子(或含733个氧气分子)的纳米气泡塌陷形成的纳米射流的最大速度较大.在气泡完全塌陷后气体分子致使纳米射流的速度衰减,最终含气体分子的纳米射流的最大速度小于真空的.此外,还发现在大冲量时,纳米气泡的塌陷时间短,同一时刻冲击波经过时的密度、压力更大,气泡塌陷后纳米射流的最大速度较大,冲击力比小冲量增强很多.较大直径的纳米气泡塌陷时间长,同一时刻冲击波经过时的密度、压力较小,冲击波传播较慢,但纳米射流的最大速度较大,纳米射流冲击力更强.纳米射流的最大速度越大,含气纳米气泡的气体分子在冲击方向分散的距离更远,凹陷深度更深.

The nanobubbles contained in the human body are induced to collapse by the shock wave,and thus produce a strong impact and high-speed nanojet,resulting in trauma to human tissues.The collapse of nanobubbles in water caused by shock waves is investigated by molecular dynamics.Nanobubbles are divided into three types:vacuum nanobubble,carbon dioxide nanobubble,and oxygen nanobubble.The influence of factors such as the number of gas molecules,the diameter of the nanobubbles,and the impulse of the shock wave on the bubble collapse are considered separately.The results show that the addition of gas molecules to vacuum nanobubbles does not affect the propagation of shock waves.However,before the nanobubbles are completely collapsed,the maximum velocity of the nanojet formed by the collapse of nanobubbles containing 718 carbon dioxide molecules(or 733 oxygen molecules)is larger than that of vacuum and nanobubbles containing 1368 carbon dioxide molecules(or 1409 oxygen molecules).After the nanobubbles are completely collapsed,the gas molecules cause the velocity of the nanojet to decay,and finally the maximum velocity of the nanojet containing gas molecules is less than that of the vacuum nanojet.In addition,it is also found that the collapse time of nanobubbles is short at high impulse,and the density and pressure when the shock wave passes at the same time are both greater.After the bubble collapses,the maximum velocity of the nanojet is larger,and the impact force is much stronger than that at a small impulse.Larger diameter nanobubble has a longer collapse time,and the density and pressure when the shock wave passes at the same time are both smaller,and the shock wave propagation is slower,but the maximum speed of the nanojet is larger.The impact is stronger.The greater the maximum velocity of the nanojet,the greater the distance that is dispersed by the gas molecules of the gas-containing nanobubbles in the impact direction will be and the deeper the depression.