Magnetically actuated transport of ferrofluid droplets over micro-coil array on a digital microfluidic platform

Magnetic manipulation of liquid droplets in microfluidic environment offers a promising tool for sample handling in lab-on-chip devices. Biofunctionalized ferrofluid droplets can effectively carry a measured volume of analytes or reagents on a flat microfluidic platform, executing key tasks of a micrototal analysis system (μ-TAS). However, achieving precise control using on-chip miniaturized magnetic coils is challenging and requires delicate combination of operating parameters, e.g., magnetizing current and timing of switching, fluid viscosity, droplet size, etc. Here we present a numerical analysis of magnetic manipulation of an immiscible, microliter-scale ferrofluid droplet over a thin aqueous film on a solid substrate using embedded micro-electromagnet coils. The numerical model is first validated against the experimentally observed droplet trajectory in a simple, single-coil configuration. Subsequently, two-dimensional manipulation of the ferrofluid droplets on the liquid film is predicted numerically when the magnetic field is produced by a sequentially switched array of square spiral microelectromagnets. By adjusting the operating parameters, we show that the droplet can be moved in predefined meandering path over an active substrate area. The transport is broadly classified into viscosity- and inertia-influenced regimes. Transport time of the droplet for the viscous regime is expressed in terms of a generalized group-variable involving the operating parameters. The study is important for selecting the design bases for a magnetically manipulated sample handling system for digital microfluidic platforms.