Streaming Currents in a Single Nanofluidic Channel

We report measurements of the streaming current, an electrical current generated by a pressure-driven liquid flow, in individual rectangular silica nanochannels down to 70 nm in height. The streaming current is observed to be proportional to the pressure gradient and increases with the channel height. As a function of salt concentration, it is approximately constant below $\sim 10\mathrm{m}M$, whereas it strongly decreases at higher salt. Changing the sign of the surface charge is found to reverse the streaming current. The data are best modeled using a nonlinear Poisson-Boltzmann theory that includes the salt-dependent hydration state of the silica surface.

Figure 1

(a) Nanochannels are fabricated by bonding two pieces of fused silica. The top piece contains an etched nanochannel and two 1 mm diameter holes for fluid connections. (b) Side view of the nanochannel. (c) Streaming current as a function of pressure for a 140 nm high channel at low- and high-salt concentrations. Lines are linear fits. (d) Schematic illustration of the origin of the streaming current.

Figure 2

Streaming conductance as a function of KCl concentration for a 140 nm high channel. Lines show model curves of constant surface charge, constant zeta potential, and the chemical equilibrium model, treated here in this Letter.

Figure 3

Streaming conductance as a function of KCl concentration for five different channel heights. Data points are averages of two data sets. The dashed lines represent the chemical equilibrium model for a single set of parameters, whereas the solid lines represent the same model when the parameters $C$ and $p\mathrm{K}$ are different for each curve.

Figure 4

Effect of the sign of the surface charge on the streaming current. The streaming current is shown as a function of pressure for a 279 nm high nanochannel at low- and high-salt concentration. Data are shown for a negatively charged silica surface, and for a polylysine-coated surface that is positively charged. Lines indicate linear fits.