Capacitive effects and transport in nanofluidic diodes

Nanofluidics has been an emergent topic during the last decade (Bocquet et al., 2010). Because of the huge increase of surface/volume ratio, the surface electrokinetic effects could have a strong impact in the recovery of osmotic energy (van der Heyden et al., 2006).

In addition, experimental works have shown that it is possible to build nanofluidic diodes using nanopores whose surface charge is not uniform (Karnik et al., 2007). A current rectification, typical of semi-conductor diodes, is observed when a potential difference is applied to nanochannels. An intrinsic aymmetry of the electrochemical potential causes this effect (Vlassiouk et al., 2007).

Moreover, the dimensions of nanopores compared to the electrical double layer can lead to concentration-polarization layers with ions accumulation inside the pore or at its entrance (Dlugolecki et al., 2010), phenomena which could be promoted by surface charge discontinuity.

A nanofluidic diode is made of a thin layer (20 nm) of alumina deposited on a 50 nm thick SiN membrane. A hole of about 50 nm in diameter is then drilled allowing the passage of electrolytes and fluid. We characterize experimentally the device submitted to gradients (electric potential, pressure, salinity). Both transient and stationary effects are studied.