Study On Micro-scale Interface Phenomena In Two-phase Flow

The multiphase flow with various interfacial phenomena and complex fluid dynamics behaviors widely exists in numerous industrial processes. In recent years, the combination of the multiphase flow and microfluidics has been becoming a cutting-edge technology to gain new insight into various applications. The two-phase flow in microchannel is of prime importance in this technology. Thus this thesis systematically investigates the involved droplet(bubble) formation, secondary breakup and coalescence processes in two-phase microfluidic flow, by using a high speed digital camera. The content of the thesis is arranged as follows:The first part of this thesis focuses on the active control of the ferrofluid droplet generation in a microfluidic flow-focusing device. The expanding and breakup dynamics of the thread of a controllable dispersed phase under different flow rates in a microfluidic flow-focusing device was studied. The influences of the flow rates, magnetic flux density and magnetic field direction on the formation and breakup processes were extensively investigated. The whole formation processes of ferrofluid droplets under no magnetic field(NM), radial magnetic field(RM) and axial magnetic field(AM) were investigated and compared. It was found that the volume of ferrofluid droplets can be actively controlled by the magnetic field. Both the radial magnetic field and axial magnetic field remarkably affect the expanding and breakup process of the thread, respectively. The variation of the minimum width of the ferrofluid thread with the remaining time could be scaled with a power law.Subsequently, the breakup dynamics of ferrofluid droplet in a symmetric microfluidic T-junction under magnetic field and the feedback effect on the breakup of bubbles at a microfluidic T-junction were investigated experimentally. In the first part, the breakup dynamics of the ferrofluid droplet under magnetic fields was studied. The whole breakup processes of the ferrofluid droplets under uniform and non-uniform magnetic fields were exhaustively considered. The influences of both the flow rates and magnetic flux density on the breakup were studied. Results showed that the type of breakup process could be changed and the breakup frequency of ferrofluid droplets could be actively modulated by applying a uniform magnetic field. While under non-uniform magnetic field, the asymmetric breakup of ferrofluid droplets could be realized. The non-uniform magnetic field can exert attraction on ferrofluid droplets in a dissymmetrical way to avoid breakup. In addition, the asymmetrical breakup processes of bubbles without external field at a microfluidic T-junction divergence were also studied. The asymmetrical breakup stems from the feedback effect of bubble behaviors at the T-junction convergence in a loop with the symmetrical branches. The feedback effects of asymmetrical collision and staggered flow of bubble pairs on bubble behavior at the T-junction divergence were mainly investigated. The result showed that the feedback effect is negligible at relatively low flow rates when no collision of bubble pairs occurs. And the bubble pair asymmetrical collision at T-junction convergence or an amplified effect of structured blemish of microchannel at relatively high flow rates is primarily responsible for the asymmetric breakup of bubbles at the T-junction divergence.Next, the emphasis is on the generation of satellite droplet(bubble). Firstly, the formations of satellite droplet during neck filament thinning in a microfluidic flow-focusing and a T-junction were investigated. In this part, the breakup dynamics of the neck during droplet formation process in a flow-focusing junction and droplet breakup process in a T-junction were primarily concerned. The results demonstrated that these exists an inevitable slow thinning process in last stage due to a new balance between viscous force and capillary force and the neck filament maintains a roughly constant volume in last stage. The final breakup point no longer locates at the center of the neck, in turn, the breakup occurs at the two ends of the neck caused by uneven surface tension generating a satellite droplet. The size of the satellite droplet is proportional function of the capillary number of the continuous phase. Secondly, the satellite droplets(bubbles) stemming from the shear-induced tail breakup of droplets(bubbles) flowing in a straight microfluidic channel were studied. The study is mainly focused on the influences of both the discrete droplets(bubbles) size and two-phase average flow velocity on the formation of tip stream in the rear of a droplet(bubble). It was found that the droplet deformation increased with the droplet length or the capillary number. There exists a critical droplet(bubble) length dependent on capillary number, beyond which the tip stream ejecting tiny daughter droplets(bubbles) will take place. The generated tiny droplets are usually three orders of magnitude smaller than primary droplet.The final part of this thesis aims at the coalescence of droplets(bubbles). At first, bubble coalescence at a microfluidic T-junction convergence was presented. The responses of bubble collisions at the T-junction convergence have been investigated within a wide range of dimensionless bubble size and capillary number Ca. Colliding coalescence, squeezing coalescence and non-coalescence were observed at the junction. The results showed that for whatever colliding coalescence or squeezing coalescence, the coalescence efficiency decreased with the increase of the two phase superficial velocity in moderate liquid viscosities, and the transition from colliding to squeezing coalescence due to the increase of the two phase superficial velocity enhanced the coalescence of bubbles. The decrease of the bubble size and the augment of the liquid viscosity were not conducive to the coalescence. Then the coalescences of ferrofluid droplets under various magnetic fields were investigated. A continuous dripping ferrofluid from a capillary was studied in this work under various magnetic fields applied. The emphasis was given to the consecutive coalescence and breakup between two coaxial ferrofluid droplets with conic edges attracting one another under applied magnetic field. By means of a high-speed digital camera, it could be observed that a small amount of ferrofluid would pierces the tip surfaces of the approaching droplet cones and then forms a liquid bridge or neck with a visible gap in the order of tens of micrometers between the leading edges. The inertia of the ferrofluid originating from the magnetic attraction fields becomes the driving force at the initial stage of coalescence instead of capillary force. After the coalescence, a ferrofluid column was formed; A critical magnetic flux density above which the column will collapse was found and different pinch-off patterns were observed by varying the intensity of magnetic flux density.