Numerical analysis of deformation and breakup of a compound droplet in microchannels

Abstract

Compound droplets with different sizes are increasingly used in industrial production and academic research. This study aims to improve understanding of the dynamical behaviors of the compound droplet moving in microchannels. The compound droplet consisting of one inner core is initially concentric and placed at the entrance of a circular channel with a cone at the downstream region. Following this primary channel is a secondary channel that is circular and straight. Manipulation of the droplet is assisted by a flow introduced via the gap between two channels. The numerical results show that when passing through these confined channels, the droplet experiences a finite deformation or breakup mode. In each mode, the deformation or breakup of the compound droplet is also different when changing the values of the parameters including the capillary number, the ratio of the droplet radii, the ratio of the interfacial tension, the size of the secondary channel, the gap size and the cone angle. The finitely deformed droplet can be either nearly ellipsoidal with a deformation index D of around unity (i.e., first deformation mode) or highly elongated with a high D (i.e., second deformation mode). Breakup of the compound droplet happens to the outer droplet, and the breakup point is located either in front of the inner droplet (i.e., first breakup mode) or behind it (i.e., second breakup mode). The simple droplets yielded in the first breakup mode has a higher total volume than those in the second breakup mode. It is found that the droplet experiences the first deformation mode to the first breakup mode, then the second deformation mode and finally the second breakup mode when Ca increases. The regime diagrams of these modes, based on these parameters, are also presented.