PHYSICAL REVIEW E 96, 013113 (2017) Magnetic-field-driven alteration in capillary filling dynamics in a narrow fluidic channel Srinivas R. Gorthi, Pranab Kumar Mondal, and Gautam Biswas * Department of Mechanical Engineering, Indian Institute of Technology, Guwahati 781039, India (Received 26 March 2017; revised manuscript received 18 June 2017; published 21 July 2017) We investigated pressure-driven transport of an immiscible binary system, constituted by two electrically conducting liquids, in a narrow fluidic channel under the influence of an externally applied magnetic field. The surface wettability was taken into account in the analysis considering that the walls of the channel are chemically treated to obtain various predefined contact angles as required for the study. Alterations in the capillary filling and wetting dynamics in the channel stemming from a complex interplay among different forces acting over the interface were investigated. It was shown that an alteration in the strength of the magnetic field leads to an alteration in the dynamics of the interface, which in turn, alters the filling and wetting dynamics nontrivially upon interaction with the surface tension force due to the wetted walls of the channel. It is emphasized that a contrast in properties of constituents of the binary system gives rise to an alteration in the forces being applied across the interface, leading to an intricate control over the filling and wetting dynamics for a given flow configuration and an applied field strength. We believe that the results obtained from this analysis may aid the design of microfluidic devices used for multiphase transport. DOI: 10.1103/PhysRevE.96.013113 I. INTRODUCTION The displacement of one fluid by another is ubiquitous in different natural processes such as rainfall on window panes, movement of water droplets over lotus leaves, and tear films on the cornea, to name a few. The movement of immiscible binary fluids also finds many applications in technologically relevant areas. A few important applications include movement of biofluids in lab-on-a-chip devices, on-chip bioanalysis, and filling in physiological systems during artificial grafting [1–4]. In view of the numerous applications of immiscible binary fluids in miniaturized systems and devices along with a need for the systematic investigation of different aspects of the underlying interfacial flow characteristics, researchers have recently concentrated on the microscale transport of immiscible binary systems [5–9]. The control over filling of an immiscible system in narrow fluidic pathways, relevant in many areas from biomedical and biochemical processes to cooling in microelectromechanical systems (MEMS), is one of the challenging tasks in the domain of microscale transport [10–12]. When a two-fluid system moves in a narrow fluidic channel, the interfacial transport plays a major role and affects the filling rate in the channel entailing the dynamics of the contact line formed at the fluid–fluid–solid interface changed. It should be mentioned that although alterations in the dynamics of contact line motion, triggered by several factors such as imposition of surface structuring over solid substrates or modulation of the wetting characteristics of the surface, give rise to an overall change in the interfacial dynamics, the ultimate consequence of which is largely reflected in the filling rate and its control in the fluidic system [10–14]. It is important to mention that active control of the filling rate in the context of microscale transport is far from trivial, thus necessitating alternative means of implementation to achieve control in microflows. Paying careful attention to this aspect, researchers have explored different avenues such as * gtm@iitg.ernet.in surface modifications, viz., artificial texturing of the surface, decoration of the surface with patterned wettability gradients, and alterations to the flow actuation mechanism with the aim of realizing better controllability of microscale and nanoscale transport [10,11,13,15–17]. In all cases mentioned above, the motion of the contact line formed between a pair of immiscible fluids is altered as a result of the extensive interplay among the dominating forcing factors, leading to alterations in the filling and wetting dynamics. When employing electroosmotic effects or thermocapillary actuation in realizing microscale multiphase transport, greater maneuverability of the filling rate can be achieved, as reported in the literature [6,18,19]. In contrast, application of a magnetic field in microscale transport has attracted attention as it offers precise controllability of the flow rate in the context of single-phase transport [20–22]. An applied magnetic field gives rise to a body force acting on the fluid mass, which also depends on the electrical conductivity of the fluid, upon interacting with the flow velocity has a forcing effect on the fluid mass, thus interfering with the fluid motion in the process [20,22–24]. Such a paradigm, however, could be extended to alter the interfacial dynamics of transport of an immiscible binary system over a wetted surface, solely by altering the forces acting across the contact line formed at the fluid–fluid–solid interface. It may be mentioned in this context that the underlying physical considerations are nontrivial, because of the intricate nonlinear interactions among different forces stemming from the viscous drag, surface tension effect as modulated by the wettability of the surface, and the Lorentz force due to the applied magnetic field. Although studies are in progress with the aim of achieving finer control in microscale and nanoscale transport, e.g., surface modifications by altering the physicochemical properties of the surface [10,11,25,26], by surface structuring [7,10,27,28], from the presence of nanobubbles over a hydrophobic surface [17,29], and by altering the flow actuation parameters [6,19,30]. There are better prospects in applying magnetic field-induced forcing to provide effective control of the capillary filling rate, not reported so far. The earlier work [30] is augmented in the current investigation with a view to understand the dynamics 2470-0045/2017/96(1)/013113(14) 013113-1 ©2017 American Physical Society