Mechanism And Experimental Study On Coalescence And Release Of Double-emulsion Droplets By AC Electrokinetics

Microfluidics is the science and technology of systems with integrated channels on the microscale,through which small volume of fluids and active particles suspending inside can flow in designed structures that are controlled and manipulated by external physical field systematically.With the rapid advance of MEMS technology,people now are able to fabricate microfluidic chips that are highly integrated,cross-scale and controllable,therefore it is widely used in biomedical,new material and frontal engineering fields.One important subcategory of microfluidics is droplet microfluidics,which generates and manipulates discrete droplets through immiscible multiphase flows inside microchannels;it is used in biochemical analyses and micro-nano material synthesis,such as single cell assay,biomolecules analyses and synthesis of nanoparticles.Microdroplets are highly monodispersed and controllable,therefore they provide great accommodations for microreactions.During the process of microreactions,many manipulations are required for droplets,including droplet transport,sorting,coalescence,split and release.Droplet controlled coalescence and release are critical steps for the performance of microreactions.However,there is a lack of methods to realize effective coalescence and rapid release of double-emulsion droplets.Considering these requirements,this thesis studies the following topics:Considering the electrokinetic behavior of single-emulsion droplets,we analyzed the mechanical behavior of double-emulsion droplets in AC electric field.Based on the Maxwell-Wagner interfacial polarization theory,we built the 2D physical model for droplet interface deformation and EHD flow in AC electric field.We studied droplet deformation and interfacial flow by numerical simulation to get the regular law behind this phenomenon.The dependence of droplet shell thickness,core and medium conductivities on interfacial relaxation is studied.The theoretic study lays the foundation for experimental investigation of droplet coalescence and release.To improve the biocompatibility and avoid cross contamination during microreactions,a novel method of controlling core coalescence and therefore conducting microreactions by AC electric fields inside double-emulsion droplets is proposed.We design a three-layer capillary microfluidic device for generating single or multiple-core droplets,and fabricate a PDMS(polydimethylsiloxane)microfluidic device for conducting droplet coalescence experiments.After that,two different cores are encapsulated in double-emulsion droplets and droplet coalescence experiments are conducted by AC electric fields,followed by quantitative analysis of the effect of solution conductivity,electric field strength and frequency on coalescence performance.This experiment lays the foundation for further sequential coalescence of double-emulsion droplets.In practical applications,sequential reaction of different reagents is always required other than single-step reactions,which demanding the ordered coalescence of multiple droplets.To prove this possibility,we further conduct experiments for sequential coalescence of triple-core double-emulsion droplets.Double-emulsion droplets with three different cores are prepared by the capillary device,and the droplets are triggered to experience sequential core coalescence.Discrepancy in core volume or conductivity are used to control the order of the coalescence.It is found that cores with larger volume or conductivity dominates the dipole moment of the overall droplet,therefore the droplets' rotation tends to make them parallel to the direction of electric field,and they fuse first.This sequential coalescence method is further applied to two-step enzyme reactions for detection of glucose concentration.In microfluidic or biological systems,it is always required to transport and release different actives to local spot.Double-emulsion droplets featuring exquisite and controllable structures provide the possibility for this requirement.Therefore,we experimentally investigate the controlled release of single,double and tri ple-core droplets.A prerequisite for droplet release is an ultrathin droplet shell.We use a special capillary device to prepare droplets with shells thinner than 200 nm.The controlled release of the three types of droplets are realized.In addition,the shell thickness of single-core droplet can be measure by comparing the diameter of the shell-formed sphere and the original droplet.Finally,this method is used to encapsulate and release nanoparticles and yeast cells,further proving its value for potential practical applications.