Motion of viscous drops on superhydrophobic surfaces due to magnetic gradients

Microliter droplets that contain paramagnetic particles can be moved on superhydrophobic surfaces using magnetic fields. Paramagnetic microparticles form chains at concentrations ranging from 0.1 to 10 wt% and are used to move, coalesce, and split drops of water as well as drops of biological fluids. Video analysis of dextran solution drops to investigate the effect of viscosity on drop movement on LPDE surfaces suggests that paramagnetic particle chain orientation compensates for viscosity increases from 1.2 mPa s (water) to 125 mPa s (20%, w/v – Dextran 428) in order to maintain drop movement. Interestingly, such changes in chain orientation are not present for drops moving on silicon nanowire (Si NW) superhydrophobic surfaces even at higher viscosities 470 mPa s (30% w/v – Dextran 428). On Si NW surfaces, drops with high viscosity can be moved even with particle concentrations as low as 0.5%. Higher particle concentrations (2%) are needed to displace drops on LPDE surfaces. This new approach to so-called “discrete” microliter-scale fluidics has the advantages of faster and more flexible control over drop movement, manipulation, and detection of solution components.