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.
