Head-on collision of two equal-size nanoscale argon droplets with relative speed less than 10 m/s is investigated using a parallel cellular molecular dynamics code (PCMD). Previous studies showed that bouncing only occurred within a narrow range of head-on collision conditions, which was mainly attributed to the existence of background gas between the droplets. However, through simulations by thoroughly varying the head-on collision conditions, we have found that bouncing can easily occur as long as the relative speed is less than a critical value, which the magnitude strongly depends on the background gas pressure. This critical value of relative speed generally decreases with increasing background gas pressure. We attribute the bouncing between nanoscale droplets to the vaporizing atoms emitting from the head-on surfaces of the two droplets, which becomes the dominated factor under vacuum condition.
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