Drop Generation in Cross-Flow of Liquid Rotating in Rigid Body Motion

Various methods have been developed to generate monodisperse drops of a dispersed phase (DP) liquid in an immiscible continuous phase (CP) liquid, which include the membrane emulsification method and the microfluidic drop generation. This study proposes an easy-to-adopt drop generation method using cross-flow: a DP liquid is injected through a stationary vertical syringe needle into a CP liquid rotating in rigid body motion. The developed method was tested and characterized using de-ionized water as the DP liquid and mineral oil as the CP liquid. Drops were generated mainly either in the dripping mode or the jetting mode, and the former resulted in higher monodispersity. Smaller drops were generated when a thinner syringe needle was used, the average flow speed of the DP liquid through the needle was decreased, or the linear flow speed of the CP liquid at the needle location was increased. Especially, the power–law relationship was observed between the drop diameter and the flow speeds, and the dripping-to-jetting transition (DJT) was observed when the Weber number of the DP liquid was about 5.