(565f) Discussion on Nontraditional Methods of Addressing of Liquid Droplets By DC Electric Field

Addressing of discrete volumes of fluids in microfluidic structures to a particular detection/reaction spot is an important task in many applications. If properly imposed, DC electric field can induce electrophoretic transport to efficiently control a droplet trajectory. It is well known that water droplets dispersed in a dielectric fluid can be charged due to a faradaic reaction at a microelectrode surface. Unfortunately, there are many other two- or multiphase liquid systems, e.g. oil droplets dispersed in water phase or two immiscible water phases (based e.g. on polyethyleneglycol), in which charging cannot be carried out by this simple method. Here we will discuss three alternative techniques that could be employed in liquid addressing:

i) We have observed that electrophoretic addressing of droplets is possible in complex systems that consist of two immiscible water phases. Surface electric charge is spontaneously formed at the interface. We will show experimental arrangement, in which oriented motion of droplets was observed. Dependences of migration velocity on applied electric field will be presented.

ii) Another mechanism is related to water in oil systems. Instead of the faradaic current injection from an external electric circuit, passive charging can be carried out to get a charged droplet. The passive charging is based on the use of an electrode from a material that spontaneously dissolves in a water solution. We will critically evaluate this mechanism in the view of our experimental results.

iii) Oil in water droplets can be manipulated by DC electric field when an ionic surfactant that adsorbs to the water-oil interface is used. Electric field polarity and properly selected ionic surfactant can lead to oriented transport of oil droplets. Preliminary results that we have achieved will also be presented.