Manipulations of microfluidic droplets using electrorheological carrier fluid.

Electrorheological (ER) fluids are a type of ''smart'' colloid capable of reversible viscosity variations, or even solidification, in response to an applied electric field. The response time can be as short as a few milliseconds. By using the ER fluid as the carrier fluid in microfluidic chips, we report the generation and manipulation of microdroplets and bubbles via integrated, digitally controlled micro-electrodes equipped with a feedback system. By utilizing the strong electric response of the ER fluid, the flow rate can be easily controlled digitally, thereby making tunable the size of the droplets generated and their separations. In particular, ordering change in a chain of droplets is demonstrated. The maneuverability presented in this paper may have potential applications in a variety of lab chips for chemical reactions, bioassays, as well as microfluidic logic computation.