Experimental Investigation Of Capillary Flow Focusing Upon External Actuation

Microscale droplets(i.e.,microdroplets)are of great value in scientific research and engineering applications.In recent years,a variety of microfluidic methods at mi-croscales have been developed for producing microdroplets with various structures in one step.Among them,microfluidic chip and glass microcapillary are two most com-mon techniques to prepare uniform microdroplets.However,both these techniques suffer the limitation of low throughput.Capillary flow focusing(i.e.,flow focusing)is another microfluidic technique that has the potential to overcome the throughput lim-itation.Flow focusing represents the phenomenon where a high-speed micro/nanoscale flow forms a steady cone as it is injected through a capillary needle and focused by the other fluid.A liquid jet is emitted from the tip of the cone,passes through the orifice and breakup into microdroplets due to the instability.Although flow focusing is able to reach high productivity,its application in microdroplet fabrication is hindered by its low monodispersity in the jetting mode.We propose to overcome this limitation by introducing active droplet generation in flow focusing so that the jet breakup process can be precisely controlled by the external actuation for high throughput production of uniform droplets.In this thesis,the experimental and numerical simulation methods are combined to study the breakup process of the liquid jet in active flow focusing(AFF).The results and conclusions are briefly given as follows:1.Experimental research is carried out in single-axial AFF.The jet breakup pro-cess and microdroplet generation have been quantitatively studied in the most common cone-jet mode by applying external disturbances.As the focused liquid is actuated by mechanical vibration with a sinusoidal signal,the droplet size in the synchronized mode can be precisely determined by the focused liquid flow rate and the vibration frequency,independent of other parameters such as ge-ometries of the device and physical properties of the liquids.Moreover,the jet breakup length can be directly controlled by vibration amplitudes modulated by the meniscus formed in the AFF process.Furthermore,the formation of micro-droplets upon external excitation does not be affected by the physical properties of the fluid(e.g.viscosity,density,interfacial tension,etc.)and geometric struc-ture parameters(e.g.,capillary size and orifice diameter,etc.).In addition,the synchronization regime exists near the natural breakup frequency of the jet,and it can be adjusted by changing the driving phase flow.2.Both the experimental and numerical studies have been carried out for the coax-ial active flow focusing process.The coaxial jet breakup process involves the dynamic phenomena more complicated than those of single-axial flow focusing.As the ratio of the inner and the outer flow rates is relatively low,the interface is weakly coupled,usually accompanied by the formation of uncontrollable multi-core emulsions.As this ratio is high,the interface between the inner and the outer flows is strongly coupled,resulting in single-core droplets.The influence of the excitation application and the instability of the coaxial jet are investigated when the outer phase is excited.The effects of experimental parameters such as exter-nal frequency,flow rate and excitation amplitude on the preparation of double emulsion droplets by coaxial AFF are discussed in detail.The study shows that the excitation on the outer phase of the focused liquid can disturb the system,and then dominate the coaxial jet breakup process,which is verified in numeri-cal simulations.The size of the double emulsions can be effectively controlled by changing the excitation frequency.In addition,the length of the outer jet can be controlled by adjusting the amplitude of the disturbance and the relative shell thickness of the single-core double-emulsion droplets can be controlled in a wide range.Thus,the morphology and the size of the double-emulsion droplets can be precisely controlled.Finally,simple demonstration experiments(i.e.,fabri-cation of CO2 capture capsules with ETPTA and hydrogel microspheres under oil-free condition)are conducted by changing the external phase material and the type of driving fluid,which proves its versatility.Coaxial AFF enables single-channel preparation of single-core double-emulsion droplets in the jetting regime may gain broad applications.3.The multi-axial active flow focusing technique has been developed for high-throughput production of dual-core double emulsions in a single step.These emulsions have two distinct inner cores with their sizes independently controlled by individual inner flow rates with accuracy.The influences of external frequency and the ratio of the inner and the outer flow rates on the non-axisymmetric com-pound jet are studied in detail.Meanwhile,the mix-core double emulsions can be prepared by adjusting the needle configuration(i.e.,the relative height of the inner and outer needles)and the concentration can be controlled by varying the two inner flow rates.