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 10 mM, 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.

We report on the efficiency of electrical power generation in individual rectangular nanochannels by means of streaming currents, the pressure-driven transport of counterions in the electrical double layer. Our experimental study as a function of channel height and salt concentration reveals that the highest efficiency occurs when double layers overlap, which corresponds to nanoscale fluidic channels filled with aqueous solutions of low ionic strength. The highest efficiency of approximately 3% was found for a 75 nm high channel, the smallest channel measured. The data are well described by Poisson-Boltzmann theory with an additional electrical conductance of the Stern layer.

We theoretically evaluate the prospect of using electrokinetic phenomena to convert hydrostatic energy to electrical power. An expression is derived for the energy conversion efficiency of a two-terminal fluidic device in terms of its linear electrokinetic response properties. For a slitlike nanochannel of constant surface charge density, we predict that the maximum energy conversion efficiency occurs at low salt concentrations. An analytic expression for the regime of strong double-layer overlap reveals that the efficiency depends only on the ratio of the channel height to the Gouy-Chapman length, and the product of the viscosity and the counterion mobility. We estimate that an electrokinetic energy conversion device could achieve a maximum efficiency of 12% for simple monovalent ions in aqueous solution.

We report charge inversion, the sign reversal of the effective surface charge in the presence of multivalent counterions, for the biologically relevant regimes of divalent ions and mixtures of monovalent and multivalent ions. Using streaming currents, the pressure-driven transport of countercharges in the diffuse layer, we find that charge inversion occurs in rectangular silica nanochannels at high concentrations of divalent ions. Strong monovalent screening is found to cancel charge inversion, restoring the original surface charge polarity. An analytical model based on ion correlations successfully describes our observations. DOI: 10.1103/PhysRevLett.96.224502 PACS numbers: 47.57.jd, 66.90.+r, 68.08.ÿp Screening by counterions is of fundamental importance in mediating electrostatic interactions in liquids. For multivalent counterions (Z ions, where Z is the ion valency including the sign), a counterintuitive phenomenon is observed: Screening not only reduces the effective surface charge, but it can also actually cause it to flip sign. This socalled charge inversion (CI) has been proposed to be biologically relevant in, e.g., DNA condensation, viral packaging, and drug delivery [1]. CI is not explained by conventional mean-field theories of screening. Recently, an analytical model was proposed that assumes that Z ions form a two-dimensional strongly correlated liquid (SCL) at charged surfaces [2]. This effect is particularly strong for high Z, and was confirmed experimentally for Z 3 and 4 [3]. Experimental evidence has remained inconclusive for the cases Z 2 and mixtures of Z ions with monovalent ions [4], both of which are biologically relevant given that K , Na , and Mg 2 are the most abundant cations in the cell. The main difficulty is that existing experimental probes become unreliable at high concentrations (*10 mM): Electrophoretic mobility measurements suffer from increasingly low signal to noise at higher salt, whereas surface force measurements are complicated by short-range forces.In this Letter, we investigate CI in individual silica nanochannels at high ionic strength by employing streaming currents as a new method. A streaming current is an ionic current that results from the pressure-driven transport of counterions in the diffuse part of the double layer [5], as illustrated in Fig. 1(b). The Stern layer, where the SCL is formed, is generally accepted to be immobile [6]. Consequently, streaming currents provide a direct measurement of the effective surface charge at the diffuse layer boundary. The well-defined rectangular channel geometry allows for straightforward interpretation. Contrary to other methods, streaming currents remain a reliable probe of the surface charge at high salt, up to 1 M in our experiments. We report unambiguous CI by divalent ions at concentrations above 400 mM. Additionally, we resolve the effect of screening by monovalent salt. We find that monovalent ions reduce CI by high-Z ions, and even cancel CI entirely at sufficiently high monovalent ion concentrations. We succ...

The pressure-driven transport of individual DNA molecules in 175-nm to 3.8-m high silica channels was studied by fluorescence microscopy. Two distinct transport regimes were observed. The pressure-driven mobility of DNA increased with molecular length in channels higher than a few times the molecular radius of gyration, whereas DNA mobility was practically independent of molecular length in thin channels. In addition, both the Taylor dispersion and the self-diffusion of DNA molecules decreased significantly in confined channels in accordance with scaling relationships. These transport properties, which reflect the statistical nature of DNA polymer coils, may be of interest in the development of ''lab-on-a-chip'' technologies. nanofluidics T ransport of DNA and proteins within microf luidic and nanof luidic channels is of central importance to ''lab-ona-chip'' bioanalysis technology. As the size of f luidic devices shrinks, a new regime is encountered where critical device dimensions approach the molecular scale. The properties of polymers like DNA often depart significantly from bulk behavior in such systems because statistical properties or finite molecular size effects can dominate there. DNA confinement effects have been exploited in novel diagnostic applications such as artificial gels (1), entropic trap arrays (2), and solidstate nanopores (3, 4). These advances underline the importance of exploring the fundamental behavior of f lexible polymers in f luid f lows and channels (5-10) that underlie current and future f luidic technologies.Most transport in microfluidic and nanofluidic separation applications is currently driven by electrokinetic mechanisms that result in a uniform velocity profile and low dispersion (11,12). An applied pressure gradient, in contrast, generates a parabolic fluid velocity profile that is maximal in the channel center and zero at the walls. Many important aspects of pressuredriven flows as a transport mechanism remain unexplored despite their ease of implementation and their ubiquity in conventional chemical analysis techniques such as high-pressure liquid chromatography. Our understanding of an object's fundamental transport properties in parabolic flows, mobility and dispersion, is at present based mainly on models for rigid particles (13, 14) that explain several important effects such as the following: (i) hydrodynamic chromatography, the tendency of large particles to move faster than small particles because large particles are more strongly confined to the center of a channel, where the flow speeds are highest, and (ii) Taylor dispersion (15), the mechanism by which analyte molecules are hydrodynamically dispersed as they explore different velocity streamlines by diffusion, an effect that has discouraged the use of pressure-driven flows in microfluidic separation technology. The applicability of rigid-particle models as useful approximations to the transport of flexible polymers is dubious in the regime where the channel size is comparable with the characteristic molecular ...