Fabrication Of Patterned Wettable Surface And Research On Droplet Manipulation

Droplet manipulation has broad application prospects in many engineering and biomedical fields such as biochemistry,microfluidic systems,cell culture,energy harvesting and utilization.One type of method for achieving efficient droplet operation is to guide the droplet movement through external magnetic fields,electric fields,etc.Most of these methods are highly de pendent on external energy input,and can only handle droplets with a specific size range,and their applications are limited.When applying a functional microstructured surface with differences in wettability to droplet manipulation,the energy input is l ow,the range of use is relatively wide,the manipulation device cost is low,and the manipulation accuracy is high.The thesis achieves controlled evaporation of droplets,directional bounce and directional transport by accurately preparing patterned surf aces with differences in wettability.The main research content and results of the paper are as follows :(1)A patterned surface with differences in wettability is precisely prepared on an aluminum-based surface.Using an environmentally friendly sodium chl oride solution as an electrolyte,a micro-nano rough structure is processed on an aluminum substrate by electrochemical anodic etching to obtain an aluminum-based superhydrophilic surface,and then use fluorosilane solution for low surface energy modification to prepare aluminum-based superhydrophobic surface.Then,the machining process is used to localize the hydrophilic region on the aluminum-based superhydrophobic surface,so as to accurately obtain a patterned surface with a difference in wettability.The contact angle,micro-morphology,surface chemical composition and roughness of the patterned wettable surface were characterized.(2)Controlled evaporation of droplets on the patterned wettable surface is achieved.The droplets tend to have constant co ntact angle(CCA)evaporation modes on superhydrophobic surfaces,while they tend to have constant contact radius(CCR)evaporation modes on ordinary hydrophilic surfaces.Because the evaporation rate of droplets on a superhydrophobic surface is slower tha n the evaporation rate of droplets on an ordinary hydrophilic surface,when droplets of the same volume are stored on hydrophilic micropits with different diameters under a superhydrophobic background,controlled evaporation of droplets can be achieved.The thesis mainly studies the controlled evaporation of droplets on the pattern of hydrophilic micropits and the controlled evaporation of droplets on the polygonal hydrophilic pattern.Finally,the numerical method was used to study the heat transfer charac teristics and internal fluidity of the fixed droplets on the hydrophilic and superhydrophobic surfaces.The combined effect of Marangoni flow and internal natural convection led to the formation of two circulation units inside the droplets.(3)By controlling the wetting contrast at the junction,directional bounce of droplets under normal temperature and high temperature conditions is achieved.Under normal temperature conditions,When a droplet hits a substrate with a difference in wettability by a certain amount of offset,it bounces toward the hydrophilic or superhydrophilic area.At the same time,the greater the difference in wettability at the junction,the greater the directional bounce distance of the droplet.Under high temperature conditions,the droplets hit the junction with extreme wettability difference.When the temperature is below 200 ℃,the droplet bounces towards the superhydrophilic region;when the temperature is about 200 ℃,the bounce direction keeps basically vertical upward When the tem perature is higher than 200℃,the droplet moves to the superhydrophobic area after the first bounce.With the increase of the movement time,the droplet bounces on the substrate many times,and the distance toward the superhydrophobic ar ea gradually becomes longer.(4)The directional transportation of droplets under normal temperature and the directional transportation of droplets from high temperature to low temperature under the guidance of temperature gradient are realized.The micro-milling process on the aluminum-based superhydrophobic surface obtained a hydrophilic track with stable wettability.The Laplace pressure difference was used to drive the droplet directional transport and wedge pattern in two circular groove reservoirs with different diameter s.The directional transportation of the upper droplets also realizes the droplet splitting,fusion and anti-gravity transportation.Driven by the temperature gradient,the three liquids of deionized water,absolute ethanol and kerosene are transported dir ectionally from the high temperature area to the low temperature area.When the temperature gradient increases,the droplet migration speed also increases.In addition,the temperature gradient can also realize the short-distance anti-gravity transportation of droplets.