Manipulation of magnetic ionic liquid droplets and their application in energy harvesting

Abstract

Controlling discrete fluid droplets using magnetic fields on a hydrophobic surface offers new opportunities in microfluidics. Manipulation of two pure magnetic ionic liquids (MILs) [Emim][FeCl4] and [Bmim][FeCl4] was investigated on a hydrophobic surface under the influence of an external magnetic field. The effects of increasing and decreasing magnetic field strength on the shape of the MIL droplets were first studied at three different temperatures. Both MILs showed good robustness to temperature variations. Moreover, the hysteresis phenomena of height and contact line diameter were observed for both MIL droplets with different volumes. Next, the movement of the MIL droplets under the actuation of a horizontal moving magnet was investigated. It was found that the maximum attainable speed increased linearly with the droplet volume, and it was up to 22 mm s−1 for a 10 µl [Emim][FeCl4] droplet when the surface magnetic field strength was 400 mT. Finally, a electrostatic energy harvester using conductive MIL droplets rolling across a charged electret film was studied. An average output power of 78 nW was obtained with a 20 µl droplet. The manipulation of individual real pure MIL droplets without adding or dispersing magnetic particles was persistent and neither evaporation nor phase separation took place.