Study On The Droplet Formation And Breakup Dynamics In Microchannels

Microchannels have been used in many fields such as chemical analysis,chemical synthesis and biomedicine for its advantages of high flux,microscale and easy control.The researches on the generations and manipulations of microdroplets are also favored by scholars.In this paper,the formation and breakup of microdroplets in different microchannels are studied by experiments and numerical simulations,and the hydrodynamic behaviors of multiphase flow in the microchannel are discussed.The principle of the droplet generation driven by temperature difference is solubility varities induced from temperature.For the n-hexane-methanol system,the two groups were dissolved into homogeneous liquid at high temperature.At low temperature,the hydrocarbon phase and the alcohol phase were formed by stratification,and the droplets were formed under the restriction of the microchannel space.The flow patterns such as slug flow,dripper flow and homogeneous flow are observed in experiments.The droplet size decreases with the increase of the homogeneous phase flow rate and increases with the increase of the initial supersaturation of n-hexane.Numerical simulations of droplet generation in cross flow microchannels are presented.The effect of asymmetry on the droplet generation caused by the change of the unilateral flow velocity and the change in the geometry of the inlet side of the continuous phase channel is studied.The droplet generation process is divided into three flow regimes:slug flow,dripper flow and jet flow.The factors affecting droplet size include dispersed phase velocity,continuous phase velocity and microchannel geometries.The droplet size is positively correlated with the velocity of dispersed phase,and negatively correlated with the velocity of continuous phase.The movement tracks and droplet shapes has changed due to the asymmetry caused by varies of the inlet parameters of single continuous phase.Rheological behaviors of a droplet moving in the micro-channel with a linear obstacle off the channel axis are investigated numerically by employing Volume of Fluid methods.Five different hydrodynamic patterns of the rheological behaviors of the droplet have been observed.The effects of the length ratio L of the mother droplet to the linear obstacle on the critical capillary number of the droplet breakup have been particularly studied.When L is less than a minimum value L_min,the larger the droplet is,the smaller the critical capillary number Ca_crr is;when L is greater than L_min,the larger the droplet is,the greater Ca_cr is.The curve of Ca_crr as a function of L has a minimum point at L_min,which means that the droplet with L=L_min could be broken up most easily.This is caused by the competition of the enhancing?positive?factors and the hindering?negative?factors on the droplet breakup,which has been analyzed carefully here.When other parameters are fixed,it is very interesting to find out that the change of the obstacle length does not change the value of L_min,which is fixed to 0.66 for the conditions discussed.