Shockwave-induced structural changes of lipid flat disk and transition to vesicle

The transition from lipid flat disks to vesicles under shock waves is essential for producing nanosized vesicles during sonication. We perform non-equilibrium molecular dynamics simulations to examine how shock waves interact with a lipid flat disk. The lipid disk consists of coarse-grained saturated phospholipid models and is approximately 30 nm in diameter in the gel phase. Shock waves are simulated using a piston-driven method, with piston speeds limited to 1.0 km s^-1 or less. When a planar shock wave strikes, the disk's structural changes depend on the impact angle and the shock intensity. A disk with its rotation axis parallel to the shock direction decreases in thickness while maintaining its circular shape. In contrast, a disk with its rotation axis perpendicular to the shock wave direction undergoes radial compression in the shock propagation direction, causing a temporary increase in ellipticity. Behind the shock front, lipid molecules become disordered, as indicated by a reduction in the average P2 order parameter of lipid chains and the gel fraction in the disk. This suggests that shock waves can trigger the phase transition of lipid disks from the gel to the liquid phase. The shock's intensity and the resulting structural changes influence subsequent vesicle formation. During recovery, vesicles often form from the disk after exposure to higher-intensity shock waves or after a temporary anisotropic disk induced by a side impact. This highlights the importance of impact angle. These structural changes in lipid flat disks caused by shock waves may help in understanding and controlling vesicle sizes through sonication.