EXPERIMENTAL STUDY OF CONCENTRATION POLARIZATION AT A MICROCHANNEL-NANOCHANNEL INTERFACE

摘要

Recent advances in fabrication methods allow us to study and leverage the unique flow regimes offered by nano-scale fluidic channels, [1-3] and recent work suggests that the physics of microchannel/nanochannel interfaces present opportunities for novel methods of sample preconcentration and analysis. [4-6] In nanochannels, channel height is of the same order of the electric double layer (EDL) thickness, leading to a decreased electrical resistance relative to the fluidic resistance of the channel. More importantly, analyte molecules undergoing electrophoresis spend a significant amount of time within EDLs. This has a profound effect on the interfaces between micro- and nanochannels. In particular, for negatively charged walls and a nanochannel in series with two microchannels, the concentration of ions (of both signs) increases on the cathodic side of the nanochannel and decreases on the anodic side. This phenomenon is called concentration polarization (CP) or the exclusion enrichment effect. [4, 5] There is a dearth of basic studies of these phenomena and the coupling of electroosmotic flow with concentration polarization. We present experimental validation of a computational model which predicts the development of concentration polarization. Furthermore, we will show preliminary results demonstrating focusing and separation of analyte anions in the cathodic side microchannel. This focusing is due to a balance of advection and electrophoretic migration. Anionic analytes focus and separate according to electrophoretic mobility.