Ferrofluid droplet manipulation using an adjustable alternating magnetic field

Magnetically actuated droplet manipulation offers a promising tool for biomedical and engineering applications, such as drug delivery, biochemistry, sample handling in lab-on-chip devices and tissue engineering. In this study, characteristics of an adjustable alternating magnetic field generated by a magnetic coil for droplet manipulation was investigated which enables more control on droplet transport, and it can be considered as a suitable alternative for moving magnets or an array of micro-coils. By adjusting the magnetic flux density, the duty cycle and applied magnetic frequency, the manipulation of water-based ferrofluid droplets with a bio-compatible surfactant for different volumes was comprehensively examined. Also, the platform was able to manipulate the ferrofluid droplets completely immersed in oil. This solves the problem with droplet evaporation which has previously been reported for droplet manipulation on the surface. Furthermore, an analytical model is proposed for the movement of the ferrofluid droplet on the hydrophobic surface. The model predictions are in good agreement with experimental results. Also, the effects of magnetic flux density, duty cycle, frequency and the distance between the coils on the mixing process were studied. Results showed that the droplet movement on the hydrophobic surface was fully synchronized with the generated signal while the droplet moved backward in the oil after the magnetic field was turned off. By decreasing magnetic flux density, droplet volume, duty cycle, as well as increasing applied magnetic frequency, the step lengths become more uniform, the resolution of droplet displacement increases, but the average-velocity decreases. (C) 2019 Elsevier B.V. All rights reserved.