Discrete magnetic microfluidics

We present a method to move and control drops of water on superhydrophobic surfaces using magnetic fields. Small water drops (volume of $5–35\mu \mathrm{l}$ ) that contain fractions of paramagnetic particles as low as 0.1% in weight can be moved at relatively high speed $(7\mathrm{cm}∕\mathrm{s})$ by displacing a permanent magnet placed below the surface. Coalescence of two drops has been demonstrated by moving a drop that contains paramagnetic particles towards an aqueous drop that was previously pinned to a surface defect. This approach to microfluidics has the advantages of faster and more flexible control over drop movement.