Magnetic droplet actuation on natural (Colocasia leaf) and fluorinated silica nanoparticle superhydrophobic surfaces

Abstract

Interest in digital microfluidics (DMF), where discrete droplets are manipulated in a miniaturized device, has grown rapidly due to the versatility that arises from non-linear control of fluids. Adding superparamagnetic particles to the fluid allows the droplet to be manipulated by magnetic fields, such as that from a simple permanent magnet, removing some of the complexity of previous DMF devices and adding enhanced robustness. The magnetic field strength need not be high to control commercially available magnetic silica microparticles, however a hydrophobic surface is required to provide reduced friction. In this study, three types of hydrophobic surfaces were explored for their use in the magnetic actuation of water droplets containing magnetic particles. A conventional fluoropolymer film was found to have excessive frictional resistance to effectively actuate droplets. A natural superhydrophobic surface, the Colocasia leaf, was found to have too much surface microstructure to allow the magnetic particles to move freely over the surface. The commercially available Ultra-Ever Dry®, on the other hand, was suitable for effective magnetic actuation of aqueous and mixed droplets as it provided a robust, virtually frictionless surface. Based on fluorinated silica nanoparticles, this surface allowed a 20μL water droplet to be manually pulled by a magnet at speeds in excess of 550mms−1.