Experimental And Theoretical Investigation On Compound Flow Focusing

Compound flow focusing is one kind of capillary flows, which can smoothly stretch an interface into a thin liquid jet by means of physical shear stress, and the jet will finally break up into monodisperse droplets due to the flow insta-bility. This steady, easy-to-handle and controllable technique does not require rigorous conditions in producing droplets and particles at micro/sub-micro scale with important output in fields of scientific researches and practical applications. In this dissertation we mainly focus on the electro-flow focusing (EFF) and co-flow focusing (CFF). The main contents are described as follows:1. The experimental device based on a well-designed coaxial needle that is convenient for observation and process control has been developed. The effects of configuration parameters and control parameters on the coaxial menisci are investigated using an improved experimental setup, and the values of the liquid jet diameter d at the focusing orifice exit and the disturbance wavelength λ on the jet surface are measured as the gas pressure drop ΔPg and the liquid flow rate Q vary. Finally, the encapsulation pattern of the inner core in the outer shell is investigated.2. Temporal instability of an electrified liquid jet of radius R in the core of a high-speed gas stream is studied to better understand the EFF technique. T-wo types of physical models with and without viscosities of fluids are considered, respectively. The inviscid one utilizes uniform basic flow and axial electric field, while the viscous one considers both the axial and radial electric fields and employs appropriate velocity profiles based on the pipe flow for inner liquid and the error function for surrounding gas stream. The results demonstrate that the axisym-metric instability and the helical instability are two most unstable modes in the EFF problem. The effects of surface tension, viscosities, velocities and densities on the jet instability are studied. Then the significances of both the free charge initially imposed on the interface and the axial electric field in the EFF process are highlighted. It is shown that as the free charge density Q0 increases, both the axisymmetric and the helical instabilities can be definitely promoted and the transition between them arises for sufficiently large Q0 as the axial electric field intensity increases. Finally, the wavelengths of the most unstable disturbances obtained from the axisymmetric temporal instability analysis are compared with experimental measurements and a good agreement is achieved.3. A theoretical model in terms of linear stability analysis of CFF is estab-lished. The coaxial liquid jet consisting of a cylindrical core of radius R1 surround-ed by an annular liquid of outer radius R2, is moving in a surrounding focusing fluid. Considering the viscosities of all three fluids, non-uniform velocity profiles including a pipe flow for the core and shell liquids and an error function for the focusing fluid are employed. Using the classical temporal instability analysis, four different unstable modes are obtained. The effects of the major parameters on the instability of the coaxial jet are studied, and the coupling effects of the inner and outer interfaces are analyzed. The results indicate that the stretching mode is the most unstable mode in most cases; With the increase of the interfacial ten-sion and the decrease of the viscosities, the coaxial liquid jet will become more unstable; The influence of the basic flow on the temporal instability presents a good symmetry; The transition between the axisymmetric and the asymmetric breakup arises when the focusing fluid has low viscosity and high velocity; The outer interface can promote the development of the perturbations on the inner interface, and can enhance the breakup of the inner interface.4. The co-flow focusing is adopted to generate stimuli-responsive microbub-bles (SRMs) that comprise 2H,3H-Decafluoropentane (HFC) suspension of silver nanoparticles (SNPs) in a lipid shell, and a scaling law is derived to predict the size of produced SRMs. Then, an optical droplet vaporization (ODV) process with respect to the SRMs is carried out on a well-designed experimental platform, ver-ifying the technical feasibility of the ODV process that will be available for drug delivery applications. Finally, a theoretical model based on the ODV process is built up, and the influences of main parameters are analyzed, and the theoretical predictions are in good agreement with the experimental results.