Numerical Simulation Of The Asymmetric Breakup Of Droplet In Microchannels

Microfluidics is the science and technology of systems that process ormanipulate small (10-9to10-18litres) amounts of fluids, using channels withdimensions of tens to hundreds of micrometres. As a technology, microfludics seemsalmost too good to be true. It offers so many advantage and so few disadvantage.Various micro-devices and its applications have been designed in diverse areas, suchas chemical synthesis, biology test etc.Droplet microfludics is one of the rapidly growing field in recent years. Precisecontrol of droplet volume is an attractive advantage compared with conventionaltechnology. In this thesis, the relation between droplet volumes and other parameters,such as capillary number, the asymmetry of flow field etc, will be studiedsystematically.In part2, a two-dimensional spectral boundary element method has beenemployed and modified to help us understanding the physics of breakup of a dropletin an cross-slot microchannel. The process of breakup has been described in detail andthe results show an good accuracy. In this part, asymmetric breakup or unbreakup of adroplet in cross-slot have been compared methodically. The volume ratio of twodaughter droplets has been calculated and explained in detail.In part3, a general understanding of asymmetric breakups of a droplet due to theasymmetry of flow fields has been developed. By employing a volume of fluid(VOF)-based method, the asymmetric breakup of a droplet suspended in anaxisymmetric extensional flow generated in a cross-like device is investigatednumerically. A general parameter As which is in the range of0to1and characterisesthe asymmetry of flow field is defined. A regime map is depicted to show the fourtypical regimes (no breakup, one-side breakup, retraction breakup and direct breakup)which have been observed in the breakup process. Three major factors which arecritical to the process of asymmetric breakups (the capillary number Ca determiningthe cutting speed, the asymmetry As of flow fields which causes translation of thedroplet perpendicular to the cutting direction, and the initial volume ratio roof theright to left part of the droplet) have been analysis, a simple theoretical model basedon power laws is proposed to predict the volume ratio roof two unequal daughter droplets. Differentiating from the symmetric breakup, the effect of the asymmetry hasbeen studied carefully and systematically, and it could be generated to describediverse asymmetric situations qualitatively.