New Platform for Gravitational Microfluidic Using Ferrofluids

International audienceAmong the large variety of microfluidic platforms, surface devices are a world apart. Electrowetting systems are used to control the displacement of droplets among predetermined pathways. More confidential, superhydrophobic surfaces are more and more described as new elements to guide spherical droplet reactors. As such, they can exhibit confinement properties analogous to channel-based microfluidics. In this article, we describe a new strategy to use superhydrophobic surfaces as a permanently tilted microfluidic platform, on which droplets containing iron oxide nanoparticles are guided with permanent magnets. These droplets are fed with water through a capillary tube until their weight exceeds the magnetic field force. Thus, the volume at which the droplet rolls off the surface is only governed by the initial quantity of magnetic nanoparticles and the tilting angle of the surface. This phenomenon provides a strategy for droplet dilution in a simple and reproducible manner, which is not that easy in microchannels, and a key advantage of open systems. As a proof of concept, we used this platform to prepare magnetic filaments by a salting-out process already described in large batches. By reducing salt concentration on the platform, we are able to control the electrostatic attractive interactions between iron oxide nanoparticles coated with poly(acrylic acid) and a positively charged polyelectrolyte [poly(diallyldimethylammonium chloride)]. The formation of nanostructured filaments was conducted in 2 min while more than 30 min was required for dialysis. Our results also illustrate the power of microfluidic reaction processes because such magnetic filaments could not be obtained through direct batch dilution because of mixing issues. Such microfluidic platforms could be useful for the efficient and simple dilution of systems where reactivity is controlled by concentration