Micro-magnetofluidics of ferrofluid droplet formation in a T-junction

Abstract

The combination of microfluidics with magnetism has led to the emergence of a new scientific domain known as micro-magnetofluidics. The present study aims to investigate the interfacial dynamics of ferrofluid droplet formation under magnetic field in a microfluidic T-junction. A non-uniform magnetic field was constructed by placing a permanent magnet at one side of the T-junction. Three typical flow regimes including the slug, slug-dripping transition and dripping flow were observed. The external magnetic field promotes the transition from the slug-dripping transition flow to the slug flow, prolongs the generation cycle of ferrofluid droplets and hinders the ferrofluid droplet formation. As the magnetic flux density increases, both the increasing rate of the length for the thread tip and the expanding rate of the dispersed neck decrease, and the peak value of the minimum width of the dispersed neck increases. The ferrofluid droplet size increases with increasing the magnetic flux density while decreases with increasing the two-phase flow rate ratio and the capillary number. A scaling law is proposed to predict the size of ferrofluid droplets, by taking the two-phase flow rate ratio, capillary number and magnetic Bond number into account.