Directed Motion Of Propylene Glycol-Water Mixed Droplets On Composite Gradient Surface

Droplet movement is widely used in the micro-surface equipment,and is also a research hotspot in the fields as rapid detection and analysis,fog collection,condensation and heat transfer.In order to realize the long-distance spontaneous movement of droplets on the solid surface,this work takes the mixed droplets of propylene glycol(PG)and water as the research object to explore the directed movement on the two kinds of composited surfaces,i.e.,wettability gradient connected with shape gradient,and wettability gradient overlaid on shape gradient surface.The two-component droplets have the unique Marangoni effect,which formed the interface tension gradient from the edge to center of the droplet,providing the possibility for droplets to move without contact line pinning resistance.This work studied the mobility of droplet moving on a wedge-shaped surface(CA=18.6°)with different wedge angles.The moving distance decreases with increase of the angle,while the average velocity increases with the angle.The moving performance of droplets was also studied on the wettability gradient(64.6°-18.6°)surfaces with different lengths.When the length is reduced to 15 mm(i.e.wettability gradient large enough),the droplets can move automatically.In order to further increase the moving distance,composite surfaces with a rectangular wettability gradient connected with a wedge-shaped gradient were further designed.In the composite surface,the influence of wettability gradient length and wedge angle on the droplet movement was studied.As a result,there is the best wettability gradient length and wedge angle to make the droplet move the farthest.The mixed droplets were found capable of moving without back-end pinning,which is expected to have potential applications in the field of rapidly exposing heat exchange surfaces to achieve heat transfer enhancement and self-driving surface micromixers.Different from the wettability gradient connected with shape gradient composite surface,the wettability gradients(64.6°-18.6°)with different lengths were constructed at the tip of the wedge-shaped surface(i.e.,wettability gradient overlaid on shape gradient surface)to study the wettability gradient enhanced movement of droplets on the wedge-shaped surface.The movement of droplets on the wedge-shaped surface can be clearly divided into two stages: the first stage is the front end spreading and the back end is pinned to the origin for a short time,and the second stage is the back end moving forward with the front end.When the wettability gradient length decreases from 25 mm to 15 mm,the moving distance and speed increase with the length of wettability gradient.But continue to decrease the gradient length,the moving distance of the droplet decreases while the speed still slightly increases.This phenomenon is well explained by a mathematical model established in this work to calculate the force of the wettability gradient on the droplet.Three simple T-shaped self-driven surface micro-mixers composed of a mixing zone with uniform wettability and a transportation zone with different gradients were finally designed to test the effect of wettability gradient on the drainage ability of droplets.In the constructed composite transport zone of wettability gradient overlaid on wedge surface,the mixed droplet was able to keep moving and thoroughly secede from the mixing zone,indicating the wettability gradient surface may effectively enhance the droplet drainage ability.This work is expected to have potential applications in the fields of open system microfluidic flow,virus analysis/detection,and lab-on-a-chip.