Droplets Motion Dominated By Evaporation Rate Gradient And Related Theoretical Research

The movement of droplet on a solid surface can be triggered by a gradient in the surface energy of the substrate,for example,using chemical or physical methods to make a surface with gradually changed wettability,or by the Marangoni effect arising from heterogeneities in the surface tension of the liquid,which can be achieved by temperature gradient or concentration gradient.Here,we have experimentally shown a droplets motion induced by a new mechanism.It is formed by two closed pure liquid droplets when evaporating on a solid surface.In this system,the substrate is inert and uniform without any heterogeneities in the surface energy,also the droplet must be the pure one-component liquid,so that there is no Marangoni effect.The only difference is the non-uniform vapor density between the inner side of the two droplets and the outer side,giving rise to a gradient in the evaporation rate across the two droplets.The droplets always attract each other,moving from the high evaporation side to the low evaporation side,to reduce the energy dissipation related with the flow.We choose n-hexane as the model droplet in this study.It can form a stable droplet with a contact angle of 6.5°,conforming to the assumption used in the theoretical model with R>>H,and it can move flexible suffering low contact angle hysteresis,so the movement is repeatable.We use a custom-made MatLab code to track the droplets motion so that we can get the radius of the contact line,shortest edge-to-edge distance as well as the length of the long/short axis,to demonstrate the velocity and the deformation of the droplets.The results indicate that,the droplets move slowly at the first and accelerate in the end,which is consistent with the theoretical prediction.By varying the distance between the droplets or the volume of one of the droplets,the velocity can be tuned effectively.Furthermore,we have observed two asymmetric multi-ring deposition pattern from two evaporating droplets of gold nanorods in aqueous solution,making a further exploration in the effect of both non-uniform evaporation and moving contact line on the deposition pattern.