Mechanism And Experimental Research On Controllable Encapsulation And Coalescence Of Inner Droplets In Double-emulsion Drops

As a new fluid control technique,droplet-based microfluidics makes the nanoliter-scale microreactions occur in the droplet level,which has become an effective means for micro-scale biological and chemical reactions and process detections.As a microreactor,droplets can effectively control the diffusion,accelerate the mixing,improve the detection sensitivity.Its application covers single cell analysis,drug screening,personal care and other interdisciplinary areas.Single emulsion droplets,however,are prone to sample leakage under the action of external factors such as wall wetting,interfacial disturbance and so on,resulting in cross-contamination.And it is difficult to control multi-component droplets.The unique core-shell structure of the double-emulsion drops can effectively protect the reaction samples and reaction products in the core droplets,while avoiding the pollution of the external reaction environment.Moreover,the multi-core double-emulsion drop provides an important carrier for the precise handling of multi-component droplets.The droplet-based microfluidic chip is a research platform that aims to provide methods for the application of various fields.However,the micro-scale reactions are highly diversified,and a single droplet-based microfluidic system is difficult to meet the needs of diverse microreactions.Therefore,a variety of droplet forms and manipulation methods are required to achieve the microreactor function under different conditions to meet the diverse microreaction requirements.This thesis is based on the above issues.Based on the basic theory of two-phase flow in the microchannel,the formation of droplets and the core coalescence mechanism of dual-cored water-in-oil-in-water(W/O/W)double-emulsion drops are analyzed.The simulation model of droplet generation based on co-flow method is established,and the key factors influencing the size and frequency of droplet formation are simulated and analyzed,which provides support for the design of droplet-based glass capillary microfluidic devices.A two-dimensional model of dual-cored W/O/W dual-emulsion drop is established for force analysis and simulation.The electric force and hydrodynamic stress of the inner droplets under the electric polarization is analyzed.The relationship between the electric field frequency,the voltage amplitude,the co re conductivity and the electrotriggered core coalescence of dual-cored double-emulsion drops is obtained,which provides the theoretical foundation for the follow-up experiments.From the simple and practical point of view,a glass capillary microfluidic system is designed for “one step” encapsulation of two reagents with varying concentrations in W/O/W double-emulsion drops without stopping the experiment and changing solutions.Using this method,microreactions under different conditions can be achieved with saving a lot of reagents.The relationship between the size and shell thickness of the drops and the flow rate of each phase is deduced according to the mass conservation equation.Monodispersed double-emulsion drops with different sizes and shell thicknesses are generated as experimental verification.After curing the PDMS shell,the double-emulsion drops can be converted to microcapsules with nice robustness.These capsules can maintain their structure for up to 7 months with no rupture observed on their shell,which allow for their encapsulants to be preserved and investigated for a long term.Finally,the feasibility of the double-emulsion drops used for microreactors under different conditions were verified by the enzyme-catalyzed reaction and the neutralization reaction.From the flexible point of view,a microfluidic platform is designed and built for the generation of dual-cored W/O/W double-emulsion drops.By adjusting the salt concentrations between the inner cores and the external medium,the swelling and deformation behavious of double-emulsion drops under different osmotic pressure is studied,including the core coalescence after uniform swelling and core rupture after nonuniform swelling.The feasibility of the double-emulsion drops used for microreactors was verified by the formation of CaCO3 after fusing two inner cores containing CaCl2 and Na2CO3 solutions by osmotic pressure.Especially in the absence of electrical energy under the harsh conditions,nano liter-scale microreactions can be achieved only through the salt concentration difference.From the instant-controlled point of view,the core coalescence of dual-cored W/O/W double-emulsion drops in continuous flow is achieved by the polarization of AC electric field.The relationship between the flow rate of continuous flow,electric field frequency,voltage amplitude,conductivity of inner cores and the core coalescence efficiency of double-emulsion drops with high throughput is studied.By adjusting the influence parameters reasonably,the core coalescence efficiency of dual-cored W/O/W double emulsion droplets in continuous flow can be reached up to 95%.The continuously electrotriggered core coalescence provides technical support for the high-throughput nanoliter-scale microreactions.In addition,to illustrate the feasibility of the continuously electrotriggered core coalescence as instant-controlled nanoliter-scale microreactors,enzyme-catalyzed reactions for glucose detection and fabrication of alginate microgels for immobilization of yeast cells are conducted using this approach.