Spontaneous Motion Of Droplet On Shape Gradient Copper Surface

Two methods for eliminating three phrase contact line pinning of droplet and utilizing shape gradient surface to induce droplet directionally spontaneous motion were demonstrated which were pure water droplet moving on slippery shape gradient copper surface and two-component droplet(containing propylene glycol,PG)moving on Ag/Cu shape gradient composite surface,respectively.Firstly,superhydrophobic copper surface was prepared via chemical immersion and then dipped in poly-(dimethylsiloxane)(PDMS)to obtain slippery surface.The combination of superhydrophobicity and PDMS oil layer critically eliminated the droplet contact line pinning and the sliding angle of droplet on surface was only 3°.The unbalanced interfacial tension from the shape gradient offered the actuating force.Shape gradient surfaces with gradient angles varying from 6°to 12°were fabricated and the effects of gradient angle on the velocity and position of droplet motion were discussed.Experiments showed that with a larger gradient angle,droplets moved faster but stopped at a nearer position.The mechanism of droplet motion depending on momentum conservation law on the shape gradient surface was systematically discussed,and the theoretical model analysis was well matched with the experimental results.Moreover,a pattern with multiple gradient branches based on the same mechanism to operate droplet directional motion and aggregation was designed.The facile approach proposed could be a promising option for liquid transportation and aggregation in open microfluidics,lab chip and microchemical reactors application.Secondly,two-component droplet was formed by mixing PG and water.The unbalanced evaporation rate of droplet surface resulted in the surface tension gradient,i.e.Marangoni stress which pulled liquid towards the center along the top of the droplet and therefore eliminated contact line pinning.The effects of PG concentration on droplet motion and shape restoration were discussed.Ag/Cu shape gradient composite surface was fabricated to induce droplet directional motion.Shape gradient composite surfaces with gradient angles varying from 6°to 12°were fabricated and the effects of gradient angle on the velocity and position of droplet motion were discussed.Experiments showed that with a larger gradient angle,droplets moved faster but stopped at a nearer position.Moreover,a pattern with double gradient branches based on the same mechanism for the color reaction of methyl orange droplet and HCl droplet was designed.It would be considerably significant for the design of novel smart materials as behaviors of droplet transportation and aggregation could be controlled effectively which had promising applications for droplet in open microfluidics and microchemical reactors.