Research On Lossless Manipulation Of Droplet With Controllable Volume

Lossless manipulation of microdroplets is one of the most important applications of the extreme wettability surface.Lossless manipulation of microdroplets with different volume has a great practical application value in the area of local chemical reactions,biochemical separation,and lab-on-chip devices.Superhydrophobic and underwater superoleophobic surfaces could be used for the lossless contact or impact to water droplets in the air and non-polar organic liquids underwater respectively.Appropriate method of droplet transportation is able to realize the lossless manipulation based on the above surface.In this work,underwater superoleophobic surface with long-term stability of wettability was prepared on Al surface using electrochemically etched and boiling-water immersed(EEBWI)special machining technique,then turned into a superhydrophobic surface after modified by stearic acid(STA).An equipment for lossless manipulation of water droplets in the air and non-polar organic liquids underwater was designed and constructed based on the obtained extreme wettability surfaces,and studies were conducted for analyzing the process of lossless manipulation.The main content is as follows,(1)Method combined of electrochemical etching and boiling-water immersion techniques was proposed to prepare underwater superoleophobic and superhydrophobic surfaces on Al substrate in view of the high cost,the non-environment friendly characteristic,the low efficiency,and the poor time effectiveness of the existing preparation methods.The wettability of obtained samples was researched;the micro-morphology,crystal structures,and chemical composition of the prepared superhydrophobic and underwater superoleophobic surfaces on Al substrate were analyzed to explore the formation mechanism of special morphology on the surface;the change law of wettability with the exposure time in the air was tested.Results show that,the micrometer-scale rectangular-shaped plateaus and step-like structures and nanometer-scale needle-like structures are formed on the Al surface after electrochemical etching and boiling-water immersion,which has a superhydrophilicunderwater superoleophobic(SUS)property with a underwater dichloromethane contact angle of 164±3° and sliding angle of 3.6±0.7°;while after modified by STA,it transforms into a superhydrophobic surface with a water contact angle of 165±2° and sliding angle of 2±1°;the long-time stability of the EEBWI SUS Al surface is more than one year.(2)The above preparation method was adopted to fabricate a superhydrophobic hole on Al rod for putting up a lossless manipulation device based on the transferring method of negative pressure suction.Water droplets in the air could be transferred using the developed device for overcoming the disadvantages of the existing methods of droplet transportation.The feasibility of manipulating water droplets was studied;the change law of the operable maximum and minimum volume with the diameter of hole was analyzed,and the relevant operated experiments were also conducted;the reversible and repetitive property of the method of droplet transportation was observed using specific experiments.Results show that,the device can manipulate and transfer water droplets with a large range of controllable volume in the air flexibly;the operable limiting maximum volume of water droplets even reaches to 140 μL with the corresponding limiting maximum size of the hole of 6.4 mm;theoretical results agreed well with the experimental ones;and the transferring method is excellently reversible and repeatable.(3)Underwater superoleophobic hole on Al bar was fabricated using the above EEBWI technique.A lossless manipulation device with flexible operations,volume controllable ability and high efficiency was designed to transfer the underwater non-polar organic liquid adopting a micro-pump to supply negative pressure for capturing droplets.The feasibility of manipulating underwater non-polar organic droplets was studied;the change law of the operable maximum and minimum volume of organic liquids with the diameter of hole was analyzed by conducting the relevant underwater operated experiments for manipulating dichloromethane;microdroplet-based micromixing was carried out by the developed device.Results show that,the device can be used to transfer underwater non-polar organic liquids in a large range of controllable volume;the operable limiting maximum volume of dichloromethane reaches to 187 μL with the corresponding limiting maximum size of the hole of 7.1 mm;the experimental results can verify the theoretical analysis well;the current device is capable of micromixing two microdroplets,which demonstrated the application to droplet-based reactors,the transfer of valuable reagents,and accurate control of miniature chemical reactions.