The diffusion of an ammonia molecule （NH3） in water was investigated by molecular dynamic simulations. It is found that the diffusion shows negative correlation with its dipole orientation.
Based on a simple model, we theoretically show that asymmetric transportation is possible in nanoscale systems experiencing thermal noise without the presence of extemal fluctuations. The key to this theoretical advance is that the correlation lengths of the thermal fluctuations become significantly long for nanoscale systems. This differs from macroscopic systems in which the thermal noises are usually treated as white noise. Our observation does not violate the second law of thermodynamics, since at the nanoscale, extra energy is required to keep the asymmetric structure against thermal fluctuations.
Using molecular dynamics simulations,we show that an asymmetrically shaped nanoparticle in dilute solution possesses a spontaneously curved trajectory within a finite time interval,instead of the generally expected random walk.This unexpected dynamic behavior has a similarity to that of active matters,such as swimming bacteria,cells,or even fish,but is of a different physical origin.The key to the curved trajectory lies in the non-zero resultant force originated from the imbalance of the collision forces acted by surrounding solvent molecules on the asymmetrically shaped nanoparticle during its orientation regulation.Theoretical formulae based on microscopic observations have been derived to describe this non-zero force and the resulting motion of the asymmetrically shaped nanoparticle.
Nan ShengYuSong TuPan GuoRongZheng WanZuoWei WangHaiPing Fang
Using molecular dynamics simulations, we show that free diffusion of a nanoscale particle (molecule) with asymmetric structure critically depends on the orientation in a finite timescale of picoseconds to nanoseconds. In a timescale of ~100 ps, there are ~10% more possibilities for the particle moving along the initial orientation than moving opposite to the orientation; and the diffusion distances of the particle reach ~1 nm. We find that the key to this observation is the orientation-dependence of the damping force to the moving of the nanoscale particle and a finite time is required to regulate the particle orientation. This finding extends the work of Einstein to nano-world beyond random Brownian motion, thus will have a critical role in the understanding of the nanoscale world.
Anesthetics are extremely important in modem surgery to greatly reduce the patient,s pain. The understanding of anesthesia at molecular level is the preliminary step for the application of anesthetics in clinic safely and effectively. Inert gases, with low chemical activity, have been found to cause anesthesia for centuries, but the mechanism is unclear yet. In this review, we first summarize the progress of theories about general anesthesia, especially for inert gas narcosis, and then propose a new hypothesis that the aggregated rather than the dispersed inert gas molecules are the key to trigger the narcosis to explain the steep dose-response relationship of anesthesia.
