Fluid-Flow Enhanced Electrokinetic Sample Pre-Concentration in a Voltage Gated Nanochannel

Recent years have witnessed much interest in electrically-driven, or electrokinetic, ion-selective transport through nano-fluidic channels for e.g. sample pre-concentration, sensing, ionic current rectification, energy conversion, and single-molecule studies. A common feature among this diverse set of applications is that ion-selectivity occurs as a result of overlapping nano-meter sized electrical double layers surrounding charged nanochannel walls. In this talk, we present a theoretical analysis of ion-selective transport and sample pre-concentration in a nanochannel containing a tunable "gate" electrode. An imposed axial electric field drives ionic transport along the nanochannel; ion-selectivity of the channel is controlled by tuning the (zeta) potential of the gate. Importantly, ions are enriched (pre-concentrated) and depleted at opposite sides of the gate, and we consider the possibility of enhancing the degree of enrichment using pressure-driven fluid flow along the nanochannel. Indeed, a simple scaling analysis suggests that flow can lead to an exponential increase in enrichment. However, a detailed asymptotic and numerical treatment reveals that this enhancement can be lost at large flow rates, for which we provide a physically-based explanation.