THEORETICAL STUDIES OF ELECTROKINETIC TRANSPORT AND SEPARATION IN NANOFLUIDIC CHANNELS

Abstract

Nanofluidics primarily encompasses the fundamental principles and applications concerning fluid flow and transport processes in conduits having at least one characteristic dimension less than 100 nm. Over such length scales, extremely large surface area-to-volume ratios of the devices may give rise to intriguing transport phenomena, remarkably distinctive as compared to what is observed in macrofluidic or microfluidic systems. The present study focuses on three important and inter-related facets of transport phenomena in nanofluidic channels. First, a comprehensive mathematical model is developed to describe electroviscous effects in nanochannels, which may originate from the fact that the counterions in the mobile part of the electrical double layer (EDL) are preferentially transported towards the down-stream end of the flow conduit with a pressure-driven liquid motion. This causes an electrical current, known as the streaming current, to flow in the direction of the imposed fluid motion. However, the resultant accumulation of ions in the downstream section of the channel sets up its own induced electrical field, known as the streaming potential. This field, in turn, generates a current to flow back against the direction of the pressure-driven flow, so as to enhance the effective viscosity of flow (an artifact also known as electroviscous effect). A novel mathematical model is first developed in this work to predict the streaming potential and the resulting electroviscous effects in nanochannels, with adequate considerations of ionic advection due to streaming field induced back electroosmosis, in addition to the pressure-driven ionic transport. In deriving the pertinent equations, it is assumed that the transport phenomena may be described by the continuum conservation equations with appropriate modifications to accommodate special features of interfacial phenomena at small scales. This approach may be adequate for liquid flow systems with characteristic length scales not falling below the order of 1 nm.