Passive electrophoresis in microchannels using liquid junction potentials.

Abstract

The formation of the liquid junction potential (LJP) is a well-studied phenomenon that occurs in the presence of ionic concentration gradients. Although the LJP has been well characterized, its impact has generally been overlooked in microfluidic applications. The characteristics of flow in microfluidic channels cause this phenomenon to be particularly important, both as a source of deviation from anticipated results and as a tool capable of being harnessed to perform useful tasks. It is demonstrated that LJPs formed in microchannels can induce appreciable electrophoretic transport of charged species without the use of electrodes or an external power supply. This process is demonstrated in an H-filter (an H-shaped microfluidic channel used to bring two fluids into contact allowing extraction of diffusing species from one stream to another) by generating junction potentials between two flowing streams containing different concentrations of strong electrolytes and observing the mass transport of the charged dye fluorescein between those streams. It is shown that the LJP can be controlled to either accelerate or decelerate mass transport across a fluid interface in the absence of an interposed membrane. A preliminary mathematical description of the phenomena is offered to support the hypothesis that the observed mass transport is a result of the LJP. Possible practical microfluidic applications of electrophoretic transport without electrodes are discussed.