Control Of The Interface Between Immiscible Liquid-liquid Two-phase Microflows And In-situ Metal Interconnection In Microchannels

Microfluidics is an interdisciplinary subject related to fluid mechanics,chemical engineering,bio-engineering,micro-nano technology and other fields.Improved manufacturing and integration technology lead to multi-functional and intelligent microfluidic devices.Electrical interconnection with good processing compatibility is one of the key technology to realize such devices.In this paper,in-situ metal interconnection synthesized by chemical reactions taking place at the interface between immiscible two-phase parallel flows with low Reynolds number was studied.A pressure-driven system was designed and built.High-pressure gas or water head was used as the pressure source.The pressure-driven system had the advantages of high pressure resolution and large pressure regulation range.This made it possible to control the interface between two-phase microflows precisely.It was found that the interface morphology could be affected by the wall properties.When constant water head was applied as the pressure source,the parallel flow was maintained to be stable for more than half an hour.Flow pattern diagrams for oleic acid-water microflows in channels of 240 ?m in width and 10 ?m in depth were mapped according to the dricing pressures.This made it possible for us to understand the balance of pressure in such flow systems better.Special attention was paid on transitions between different flow patterns.The influences of the liquid-liquid interaction and the solid-liquid interaction on flow pattern transitions were studied by changing the concentration of surfactants,temperature,and the wall conditions.A rather complicated transition behavior indicated that the liquid-solid interaction played an important role in modulating the flow pattern at small Reynolds numbers.By applying a certain level of vacuum at the outlet of the channels,the liquid-solid interaction became weaker and the flow resistances for both liquids were remarkably lowered.Stable parallel flow at Re <10^-2 had been obtained without any surface modifications,additional channel structures,or surfactants.A straight and stable interface was successfully established and maintained stable for hours.In-situ metal synthesis was made by the reaction between the octanol solution of cupric oleate and the aqueous solution of ascorbic acid.An easy-disassembly and leak-tight bonding method based on selective oxygen plasma treatment of PDMS was proposed,in order to characterize the interface reaction products in the microchannels.Chips made by such method could not only be used with high-pressure fluid but also be disassembled easily without destroying the internal products in the microchannels afterwards.Partially continuous submicron-micron Cu interconnect wires had been obtained.The submicron-micron Cu wires were mainly composed of cup-shaped particles arranged one-by-one.The interface reactions were controlled by both emulsification and diffusion.The concentration of reactants near the parallel interface changed due to the interface reactions and the interface tension changed simultaneously.These changes made the interface unstable.The liquid-liquid interface appeared to be emulsified due to its instability.Nanoparticles were formed with the emulsion droplets acted as soft templates and trapped on the surfaces of the droplets.The reaction proceeded continuously with the reactants diffused towards the surface of the emulsion droplets.The cup-shaped particles were obtained by self-assembly and growth of the nanoparticles.The factors of interface reaction were analyzed,and it pointed out that interface tension and interface stability was the key to improve the continuity of submicron-micron Cu wires.