title: Pinch-off of microfluidic droplets with oscillatory velocity of inner phase flow.
abstract: When one liquid is introduced into another immiscible one, it ultimately fragments due to hydrodynamic instability. In contrast to neck pinch-off without external actuation, the viscous two-fluid system subjected to an oscillatory flow demonstrates higher efficiency in breaking fluid threads. However, the underlying dynamics of this process is less well understood. Here we show that the neck-thinning rate is accelerated by the amplitude of oscillation. By simply evaluating the momentum transfer from external actuation, we derive a dimensionless pre-factor to quantify the accelerated pinch-off. Our data ascribes the acceleration to the non-negligible inner fluid inertia, which neutralizes the inner phase viscous stress that retards the pinch-off. Moreover, we characterize an equivalent neck-thinning behavior between an actuated system and its unactuated counterpart with decreased viscosity ratio. Finally, we demonstrate that oscillation is capable of modulating satellite droplet formation by shifting the pinch-off location. Our study would be useful for manipulating fluids at microscale by external forcing.
authors: Department of Mechanical Engineering, the University of Hong Kong, Hong Kong.|HKU-Zhejiang Institute of Research and Innovation (HKU-ZIRI), 311300, Hangzhou, Zhejiang, China.
authors: Pingan
authors: P
authors: Zhu
authors: Department of Mechanical Engineering, the University of Hong Kong, Hong Kong.|HKU-Zhejiang Institute of Research and Innovation (HKU-ZIRI), 311300, Hangzhou, Zhejiang, China.
authors: Xin
authors: X
authors: Tang
authors: Department of Mechanical Engineering, the University of Hong Kong, Hong Kong.|HKU-Zhejiang Institute of Research and Innovation (HKU-ZIRI), 311300, Hangzhou, Zhejiang, China.
authors: Ye
authors: Y
authors: Tian
authors: Department of Mechanical Engineering, the University of Hong Kong, Hong Kong.|HKU-Zhejiang Institute of Research and Innovation (HKU-ZIRI), 311300, Hangzhou, Zhejiang, China.
authors: Liqiu
authors: L
authors: Wang