Research On Mechanical Behavior And Damage Mechanism Of Nanobubbles And Biological Membranes Under Shock Loading

Blast waves can cause trauma to the brain,auditory system,lungs and other parts of the human body.In order to understand the nature and mechanism of the trauma caused by the blast waves,this paper used molecular dynamics（MD）simulation and related theoretical methods to explore the dynamic mechanical behavior and damage mechanism of different nanosystems under shock loading.The nanosystems were respectively nanobubble in water,biological membrane in water,and a nanosystem composed of water,nanobubble and biological membrane.Biological membrane was a phospholipid bilayer membrane composed of dipalmitoylphosphatidylcholine（DPPC）molecules.Revealing the dynamic damage mechanism of shock waves to human tissues at the molecular level can provide an important theoretical reference for the protection of blast injury and the treatment of personnel.In addition,these studies have great development potential and application value in the field of biomedicine.Firstly,the shock compression simulations of pure water were carried out using MD method.The results showed that the generation and propagation of shock waves could be simulated in pure water.At the same time,the Hugoniot relationship curve between the particle velocity of pure water and the shock velocity was obtained.The Hugoniot relationship curve of pure water was in good agreement with the experimental data,which verified that the force field and water model used were feasible.Secondly,the collapse mechanism of carbon dioxide and oxygen nanobubbles induced by shock waves was systematically analyzed.The influence of factors such as the gas density in the nanobubbles,the diameter of the nanobubbles and the shock wave impulse on the density and pressure of the system,and the particle velocity of the nanojet before and after the complete collapse of nanobubbles was revealed.The morphological distribution of nanobubbles containing carbon dioxide and oxygen before and after the complete collapse of nanobubbles by shock waves was observed.The results showed,before the complete collapse of nanobubbles,the maximum velocity of the nanojet formed by the collapse of nanobubbles containing fewer gas molecules was larger than that of vacuum and nanobubbles containing more gas molecules.After the complete collapse of nanobubbles,the gas molecules caused the velocity of the nanojet to decay,and finally the maximum velocity of the nanojet containing gas molecules was less than that of the vacuum nanojet.Under the action of large impulse shock wave,the collapse time of the nanobubbles was short,and the density and peak pressure of the system were larger,and the maximum velocity of the nanojet was greater and the impact force was stronger after the nanobubbles were completely collapsed.The larger diameter nanobubbles took a long time to collapse,and after the nanobubbles collapsed,the density and peak pressure of the system were smaller,and the shock wave propagated more slowly,but the maximum velocity of the nanojet was greater and the impact force was stronger.The greater the maximum velocity of the nanojet,the longer the distance the gas molecules of the gas-containing nanobubbles were dispersed in the impact direction,and the deeper the depression depth.Then,the impact response of the DPPC membrane in water under different shock wave intensities was discussed.As the impulse of the shock wave increased,the membrane became increasingly disordered and the folds became more severe with more water molecules in the hydrophobic area.The displacement,velocity,order parameters and thickness changes of the membrane were analyzed.It was found that the peak velocity and maximum strain of the membrane have a linear relationship with the shock wave impulse,and the fitting relations were obtained,respectively:v_max=0.93117+6.7×10^-3I andε_max=0.27186+4.51765×10^-4I(v_maxis the peak velocity,ε_maxis the maximum strain,I is the shock wave impulse).The critical impulse interval in which the membrane damage recovery could be observed during the impact process was obtained:It was observed that the membrane damage was recoverable during the impact when the impulse was less than127 m Pa·s,but no membrane damage recovery was observed when the impulse was greater than 153 m Pa·s.The membrane has the mechanical properties of recovery after impact.A recovery simulation was performed on the damaged membrane.The results showed that all systems recovered after a few nanoseconds,the order parameter increased to a constant value,and the membrane thickness was reduced to a constant value.Finally,a nanosystem of water,nanobubble and DPPC membrane was established.The molecular dynamics method was used to simulate the damage to the membrane by shock-induced nanobubble collapse.The influencing factors considered were the impact conditions imposed by the outside,the density of the gas in the nanobubbles and the diameter of the nanobubbles.The dynamic damage process of the membrane was observed,and the membrane damage under various impact conditions was quantitatively evaluated according to the area of the nano-holes on the membrane and the depth of the depressions.The critical impact conditions for the formation of holes in the membrane were analyzed.It was found that the gas molecules weakened the impact damage of the jet to the membrane.And the larger the number of gas molecules,the smaller the area of pores produced by the membrane and the shallower the depth of the depression,indicating that the impact damage was weakened more obviously.In addition,the recovery mechanism of membrane pores containing gas molecules was revealed.Before the recovery,gas molecules gathered at the membrane pore.When the recovery ended,the gas molecules were scattered everywhere,and the membrane pore was closed.