Droplet Formation Behavior Of Two-phase Co-flow In A Microchannel With Step-structure Downstream

As an important branch of microfluidics focusing on droplet preparation and manipulation,droplet microfluidics has presented many outstanding technical characteristics,such as high-throughput,low consumption,accelerating mixing and reaction speed,easy to realize encapsulation and separation without causing cross-contamination,therefore providing powerful tools for high-throughput bioanalysis,food emulsification,etc..It has attracted wide interest in basic research and application,including biomedical fields such as single cell analysis,macromolecular sequencing,drug discovery and medical diagnosis,and micro-nano structure materials synthesis,food engineering and fine chemistry,and so on.The current research trend of droplet microfluidics pursues not only higher throughput,but also higher requirements on droplet size and monodispersity,and lower cost of droplet generation system.As the‘step-emulsification mechanism’could prepare droplets with high monodispersity,and with somewhat easy to large-scale droplet preparation through parallel integration,the integration of such a step-emulsification mechanism in conventional microfluidic structures is considered as one of the promising technical ideas to meet the above-mentioned requirements.However,most currently‘stepped’microfluidic droplet-generation devices are fabricated with high-cost technical facilities,focusing on the preparation of fine droplets with diameters below tens of microns.In this paper,we try to couple the step-emulsification geometrical structure with the wide microchannel fabricated with conventioanl CNC（computer numerical control）micro-engraving machine,and discuss the droplet generation behavior when two-phase fluids flows through the coupled microchannel and downstream step geometry（the used device is special recorded as‘stepped co-flowing microfluidic device’）.The main results are as follows:（1）Using PMMA（polymethylmethacrylate）as the substrate,under the use of CNC micro-engraving,three‘stepped co-flowing microfluidic devices’with auxiliary-flow or not were constructed,which couples a two-phase co-flow microchannel with a downstream step towarding reservior,with the feature sizes of the microchannel 200μm wide and 70μm deep.During these processes,we exploed two ways to much simpler bonding of PMMA microchannels,double-sided adhesive bonding and ethanol solvent heating.The results showed a simpler and improved bonding method,that is,only heating to 60℃in 2min two pieces of PMMA surface coated with ethanol solvent could arrived at effective bonding with enduring up to at least 0.5MPa.This improved bonding method could make the construction of PMMA microfluidic device based on low-cost CNC technology much simpler.（2）The droplet generation behavior of water-in-oil under different flow conditions was tested for the thus-fabricated three‘stepped co-flowing microfluidic device’.The results show that:（1）In the range of 1/6～1 flow ratio of two-phase fluid（water/oil）,the device without auxiliary-flow directly generates droplets with a diameter of 80～450μm at the intersection,and the frequency of droplet generation increases with the increase of the total flow rate of two phases.In the measured flow range,the maximum frequency reaches 24s^-1 when the total flow rate is 7μl/min,and the variation trend of these parameters is similar to that of classical flow focusing in literature.The monodispersity of droplet size is less than 3%under a few flow conditions,and even more than 10%under some flow conditions.（2）In the above same device,when realizing two-phase fluid co-flow,the width ratio of two-phase fluid streamlines can be controlled by two-phase flow ratio,and the width ratio of two-phase（water/oil）varies from 1 to 5 within the range of 4～17 flow ratio.In many cases,the formation of droplets at the steps can be observed.However,if there is no auxiliary flow,the generated droplets will move slowly in the channel of the reservoir,and the later generated droplets will easily touch the other generated droplets,resulting in serious merger and retention.（3）In order to overcome the problem of the exitting-droplet retention（merging）in the step region,we developed the third device with two auxiliary-flows parallelled with the co-flow microchannel.Under the action of auxiliary liquid flow,two-phase co-flow can successfully generate droplets at the step.The results show that the device could generate droplets with a diameter of 800～1200μm in the range of two-phase（water/oil）flow ratio of 9～20,and the droplet size distribution is high monodispersity（less than 4%）.These results show that the streamline width of dispesed flow can be adjusted reasonably through the restriction of two-phase co-flow in the microchannel,therefore,it could effectively mitigate the precision limitation of conventioanl CNC machining process.Moreover,by adjusting the flow ratio of two phases in the microfluidic channel thus-fabricated,the preparation of fine droplets and large-calibre droplets（800～1200μm）still with highly monodisperse could be realized,which may expand the droplet preparation capacity of microfluidic step-emulsification.