Directed Motion Of Aqueous Droplet With Propylene Glycol On Interfacial Tension Gradient Copper Surface

The main findings of this work are summarized as follows,First,a homogeneous hydrophilic film with contact angle of 25°?pure water?was prepared on the copper sheet by an alkali-assisted oxidation method.Scanning electron microscopy and X-ray diffraction analysis showed that the thin film was composed of Cu?OH?₂with a needle-like morphology and many voids among the microstructure.On the surface,the aqueous droplet with an appropriate amount of PG presents the performance of shape restoration due to the Marangoni while the pure water and pure Propylene glycol?PG?droplet just pin the contact line.The droplet with 30%PG showed the optimal shape restoration and the shape restoration is suppressed in the presence of fog.Inspired by the shape restoration,wedge-shaped patterned surfaces and wettability gradient surfaces of Cu?OH?₂/Cu were prepared,on which the droplet with the proper amount of PG can move without back end pining while pure water and pure PG droplet just spread.The droplet velocity decreases with increasing PG content and at the same PG content?30%?,the droplet velocity decreases with increasing fog flow velocitySecondly,a homogeneous hydrophilic film was prepared on the copper sheet by replacement technique,and the contact angle of pure water was 25°.According to scanning electron microscopy and X-ray diffraction analysis,the film was composed of densely packed spherical Ag particles with few voids on the surface.On the Ag/Cu surface,the droplet with PG restores shape faster than on the Cu?OH?₂/Cu surface,and the liquid residue was also less.In the directional motion experiment,it was found that the morphology of Cu?OH?₂ is more conducive to the spreading of front end while the morphology of Ag is in favor of shrinking of back end.Therefor the segmented Ag@Cu?OH?₂/Cu wedge-patterned surfaces with Ag film on the narrow end of track and Cu?OH?₂ on the wide end is fabricated to enhance the moving of droplet with 30%PG.In addition,the wetting gradient and wedge patterning were also combined to prepare a composite gradient surface of Ag/Cu,which increased the speed and displacement of the droplet directional motion.Finally,based on the directed motion of droplet on surface with composite gradient,the micro-reactor is fabricated to achieve the aggregation of two droplets.