Dynamics of acoustically actuated droplet breakup and manipulation in a flow-focusing microfluidic device

Droplets of sizes in the range of micro- to nano-scales found many applications in biomedical industry, food processing industry, chemical and mechanical engineering. Control on the size of droplets produced in microfluidic devices can be achieved through passive (only controlled by the flow fields of the two immiscible fluids) and active methods (external actuation provided). The present work focuses on the investigation of a novel method with the incorporation of a piezoelectric actuator to control the droplet formation process in a flow-focusing device. Acoustic actuation causes a boundary-induced microstreaming flow in the two immiscible fluids at the cross-junction of the device. The scale of the microstreaming flow is within the Stokes layer and is periodic in nature. Phase-resolved micro-particle-image-velocity (μPIV) is applied for extracting the global, phase-averaged and periodic components from the microstreaming flow. Vibrating fluid-fluid interfaces are observed and acoustic actuation causes a reduction in the droplet size. Characterization of droplet formation under acoustic actuation is performed. The effects of the control parameters, flow rate ratio, voltage and frequency of acoustic actuation, continuous phases with different viscosities on droplet formation are characterized. Droplet size decreases with increase in the voltage and frequency of acoustic actuation. Continuous phases with higher viscosities cause a restrained vibration motion at the fluid-fluid interface with a reduced extent of vibration. Since droplet formation mainly depends on the extent of the vibration motion, acoustic actuation is more effective in affecting the breakup of droplets in a continuous phase with a lower viscosity. The evolution of the interfacial curvature during the whole droplet formation process in a flow-focusing device is investigated. Two critical curvature values that determine the detachment of liquid thread from walls of orifice and the onset of pinch-off are identified. The rate of evolution mainly depends on the flow rates of the two immiscible fluids and the flow through the gap between the liquid thread and the walls of the orifice. Acoustic actuation reduces the time for triggering the onset of pinch-off as the vibrating interface hinders the process of relaxation of interfacial energy and making pinch-off easier to trigger. Keywords: droplet formation dynamics, immiscible microfluidics, micro-particle-image-velocimetry (μPIV), acoustofluidics, microchannel flow, microelectromechanical systems (MEMS