Magnetofluidic Control Of The Formation And Breakup Of Ferrofluid Droplets In Microchannels

In recent decades,with the rapid development of microchemical technology,microfluidics combining with magnetic particles as the micro-magnetofluidics has attracted increasing attention in the industrial application and academic research.Since the ferrofluid was discovered in the early 1960 s,it has been used extensively in various research fields such as biology,medical treatment and chemical analysis.The formation,breakup and other hydrodynamics of ferrofluid droplets in the microchannel were investigated experimentally by using a high speed digital camera.The main contents in this thesis are listed as follows:The droplet formation mechanism of squeezing-to-dripping in the microfluidic T-junction was investigated experimentally.The experiment was conducted in the square microchannel with the width of 400 ?m.Three different flow regimes including the squeezing,transition and dripping regimes were observed in the microfluidic T-junction and the transition mechanisms between them were studied.The area of squeezing regime decreases with increasing the magnetic flux density,which results in more ferrofluid droplets moving from the squeezing regime into the transition regime.Meanwhile,the magnetic force has little influence on the dripping regime.The transition lines for the transition and the dripping regime almost unchanged over the magnetic field.The Influences of the flow rate ratio,magnetic flux density and relative position between the magnet and microchannel on the droplet size were also determined,with and without the applied magnetic field.Moreover,the temporal evolutions of the thread tip length and the minimum width of thinning neck under four different magnetic flux densities were investigated.The experimental results showed that both the two temporal evolutions could be divided into two stages: expansion stage and pinch-off stage.The breakup of ferrofluid droplets in a microfluidic Y-junction was investigated,and the magnetofluidic control for droplet breakup was achieved.The asymmetric breakup of the ferrofluid droplet or non-breakup with filtering the mother droplet into a desired branch to separate it from the satellite droplet was implemented by an external magnetic field.The breakup processes of ferrofluid droplets with and without the magnetic field were studied systematically.The influences of both the flow rate ratio between the continuous phase and dispersed phase and the magnetic flux density on the sizes of daughter droplets were determined.It was found that the attractive magnetic force shifted the mass center of mother droplet in the upstream main channel,which accordingly facilitated the asymmetric breakup of the droplet at the downstream Y-junction.A power function correlation for precisely predicting the sizes of daughter droplets was proposed by introducing the magnetic Bond number:V_u/V_l=1+0.05?Q_d/Q_c?^-0.95?Ca?^0.06?Bo_m?^0.38.The flow patterns of ferrofluid droplets in the microfluidic Y-junction were investigated experimentally.Four flow patterns were observed including breakup with permanent obstruction?BPO?,breakup with temporary obstruction?BTO?,breakup with permanent tunnel?BPT?and non-breakup?NB?.The instantaneous evolution of minimum width of the neck could be considered as a function of the time in the above flow patterns with four different magnetic flux densities.It was found that the critical values of minimum width of the neck were significantly related to the channel structure and size,but independent of the fluid properties and the magnetic field.In addition,the controllable magnetic force could promote the pattern transition between the breakup and non-breakup of ferrofluid droplets.