Density Functional Theory of Liquids Applications to Nanofluidic Devices

1. Density Functional Theory of LiquidsApplications to Nanofluidic Devices Nanofluidic devices hold the promise to analyze, separate, and detect specific molecules with great sensitivity, as well as to produce and store energy. The simplest fluidic devices have 2 charged walls separated by an electrolyte. A current is measured when the ions are driven by voltage or by pressure. • microfluidic devices • large with >~1000 nm (1 m) distance between the walls • current is dominated by bulk region (i.e., ions in the wide space between the walls) • nanofluidic devices • small with10-1000 nm distance between the walls • current is dominated by the structure of the liquid/wall interface (i.e., “edge effects” dominate) • the key to making nanofluidic devices viable is understanding the surface/liquid interface W. Reisner et al. PNAS(2010) J. Fu et al. Nat. Nanotech. (2007)

2. Density Functional Theory of LiquidsApplications to Nanofluidic Devices Density functional theory (DFT) of liquids is a powerful computational technique to model the surface/liquid interface that defines nanofluidic device properties. • DFT computes results in seconds on a single CPU • generalizes classic Poisson-Boltzmann theory by giving ions size • includes ion-ion and ion-interface correlations that are generally neglected • these correlations can dominate the current properties of nanofluidic devices (next slide) • allows fast exploration of material parameters to predict novel properties of • nanofluidic devices (next slide) • ions near dielectric or metal interfaces (e.g., those found in fuel cells or batteries) (below) The ARO grant funded the first theory for ions of different sizes at low-dielectric or metallic interfaces. lines = new DFT; symbols = simulations divalent ions near variouslow-dielectric interfaces mixture of CaCl2 and KClnear a metal interface

3. Density Functional Theory of LiquidsApplications to Nanofluidic Devices When applied to nanofluidicdevices, DFT predicts novel device properties: • currents change sign with CaCl2 concentration • DFT is one of the few theories to correctly model charge inversion, a counter-intuitive phenomenon produced by ion correlations • DFT reproduces experiments of CaCl2 charge inversion where the current changes sign from positive (Ca2+ dominated) to negative (Cl-dominated) (solid line=theory; symbols=experiments) • qualitatively different behavior for pressure- and voltage-driven CaCl2 current • voltage-driven flow (right) current density is positive, but can change sign for pressure-driven flow (left) • net pressure-driven current is negative as shown in top panel (●) • applications may include molecular separation and energy storage ● DFT aids design of unprecedented materials by predicting its properties.