PHYSICAL REVIEW APPLIED 10, 034057 (2018) Steady and Oscillatory Lorentz-Force-Induced Transport and Digitization of Two-Phase Microflows Joydip Chaudhuri, 1 Tapas Kumar Mandal, 1,2 and Dipankar Bandyopadhyay 1,2, * 1 Department of Chemical Engineering, Indian Institute of Technology Guwahati, Assam 781039, India 2 Centre for Nanotechnology, Indian Institute of Technology Guwahati, Assam 781039, India (Received 7 March 2018; revised manuscript received 29 May 2018; published 26 September 2018) We explore pumping and digitization of two-phase flow patterns inside a microchannel with the help of a unidirectional and oscillating Lorentz force. For this purpose, an electric field has been coupled with an oscillating (or unidirectional) magnetic field to generate a sinusoidal (or unidirectional) Lorentz force in the channel filled with a pair of Newtonian, immiscible, and electrically conducting fluids. Applica- tion of the steady or oscillating Lorentz force is found to enhance the throughput of a pressure-driven flow in conjunction with the mixing of the phases by creating discrete and miniaturized flow structures. Numerical simulations show that the application of Lorentz force in an oil-water stratified flow leads to the digitization of the flow patterns together with enhanced transport due to the magnetohydrodynamic pumping of the fluids. The size and frequency of the flow patterns and the throughput of the flow can be noninvasively altered by tuning the intensity of the electric or magnetic field, frequency of the mag- netic field, and fluid properties. An oscillatory Lorentz force with periodic change in direction can lead to time-periodic forward and backward motions of the fluids to prompt a unique reciprocating motion of the flow features while they translate along the channel. The oscillation frequency of some of the flow fea- tures is found to follow a linear correlation with the frequency of the magnetic field suitable for pumping applications. The proposed pumping and digitization strategies can be of significance in the design and development of next-generation microscale reactors, mixers, pumps, and microelectromechanical systems (MEMS) devices. DOI: 10.1103/PhysRevApplied.10.034057 I. INTRODUCTION Digitization of flow patterns into morphologies with rel- atively higher surface area to volume ratio inside microflu- idic devices has shown significant potential in improving the proficiency of a number of cutting-edge applications. These include microreactors [1–3], bio-analysis tools [4,5], microelectromechanical systems (MEMS) [6,7], therapeu- tic or diagnostic devices [8,9], emulsifiers [10,11], and energy harvesters [12]. The studies related to the pre- cise control on the generation, actuation, movement, and throughput of diverse flow patterns at different time and length scales are also of fundamental importance because a better understanding of the complex physics associ- ated with them can help in expanding their domains of applicability [13–19]. In particular, it is now well under- stood that the mass, momentum, and heat transfers inside highly confined microsystems are often limited by diffu- sive length and time scales, which restrict their efficiency and applicability [20–23]. The studies on the creation and applications of microscale gas bubbles or liquid droplets * dipban@iitg.ac.in involving gas-liquid [24–26] or liquid-liquid [27,28] flows have revealed that the use of the regular pressure-driven flows in the commercial processes are largely limited by their weaker transport properties and smaller through- put. Thus, much research activity has been observed in the recent past, which involves the enhancement of the transport features through the digitization of flow patterns inside the microfluidic devices [20–32]. For example, the externally applied fields have been employed to disrupt the regular pressure-driven flow pat- terns to improve the transport properties [33–45]. The non- invasive integration of the applied dc and ac electric fields are found to effectively alter the balance of the capillary, inertial, gravitational, and viscous forces in the pressure- driven microflows to improve the mass, momentum, and heat transfer as well as throughput. The additional electro- hydrodynamic (EHD) stresses at the surface or interface due to the accumulation of either bound or free charges can lead to facile deformation, disruption, or actuation of the flow patterns [42–45]. Apart from the electric fields, the transportation of fluids through the lab-on-a-chip devices is achieved by the use of magnetohydrodynamics (MHD) flows [46–49]. In particular, the application of Lorentz 2331-7019/18/10(3)/034057(13) 034057-1 © 2018 American Physical Society