Numerical Simulation Study On The Effect Of Microchannels Geometries On Asymmetric Breakup Of A Droplet

Microfluidics is new multiple discipline which is the science and technology of systems that process or manipulate complex fluids,and draw many people's attention.Droplet-microfluidics,one of branches of it,including the formation of droplets and control of droplets(transfer,sorting,mixing,merging and breaking,etc),has been applied to many fields such as biology,chemistry,pharmacology and so on.The breakup of droplets plays an important role in the emulsion industries,chemical processes involving immiscible phases.The methods to achieve the breakup of droplets in microchannels are active splitting and passive splitting mechanisms.In our paper,the passive method is mainly applied due to its easy to control.Passive asymmetric breakups of a droplet could be done in many microchannels of various geometries.In part 2,in order to study the effects of different geometries on the asymmetric breakup of a droplet,four types of asymmetric microchannels with the topological equivalence of geometry are designed,which are T-90,Y-120,Y-150 and I-180 microchannels.A three-dimensional(3D)volume of fluid(VOF)multiphase model is employed to investigate the asymmetric rheological behaviors of a droplet numerically.Three regimes of rheological behaviors as a function of the capillary numbers Ca and the asymmetries As have been observed.A power law model based on three major factors(Ca,As and the initial volume ratio r0)is employed to describe the volume ratio of two daughter droplets.The analysis of pressure fields shows that the pressure gradient inside the droplet is one of the major factors causing the droplet translation during its asymmetric breakup.Besides the above similarities among various microchannels,the asymmetric breakups in them also have some slight differences.It is disclosed that I-180 microchannel has the smallest critical capillary number,the shortest splitting time,and is hardest to generate satellite droplets.In part 3,three dimensional numerical simulation of the asymmetric breakup of a droplet in microchannel with an obstacle is carried out using the VOF method,and four regimes of direct breakup,retraction breakup,retraction non-breakup and nonbreakup were observed,respectively.The asymmetric breakup of a droplet with an initial volume is affected by the outer flow and the obstacle.The critical capillary numbers decreases with the increase of the length of an obstacle or the decrease of the height of an obstacle.The critical capillary numbers increases along with the increase of the initial volume of the droplet when L/Lob is bigger than 0.68,and decrease along with the increase of the initial volume of the droplet when L/Lob is smaller than 0.68.Those results will help the designing of microchannels in the future.