Manipulation of liquid droplets using amphiphilic, magnetic one-dimensional photonic crystal chaperones

The controlled manipulation of small volumes of liquids is a challenging problem in microfluidics1-4, and it is a key requirement for many high-throughput analyses and microassays5,6. One-dimensional photonic crystals made from porous silicon have been constructed with amphiphilic properties7. When prepared in the form of micrometre-sized particles and placed in a two-phase liquid such as dichloromethane/water, these materials will accumulate and spontaneously align at the interface. Here we show that superparamagnetic nanoparticles of Fe3O4 can be incorporated into the porous nanostructure, allowing the materials to chaperone microlitre-scale liquid droplets when an external magnetic field is applied. The optical reflectivity spectrum of the photonic crystal displays a peak that serves to identify the droplet. Two simple microfluidics applications are demonstrated: filling and draining a chaperoned droplet, and combining two different droplets to perform a chemical reaction. The method provides a general means for manipulating and monitoring small volumes of liquids without the use of pumps, valves or a microfluidic container.