A Study On Ion Transport Characteristics In Nanopores And Nanochannels

Nanofluidic devices are based on fluid flows in structures with critical dimension ??100 nanometers.They have potential applications in biological sensing,sample preparation,and water processing.Because of the multi-physics coupling between fluids/electric fields/ions in the transport at nano-scale,complicated non-linear phenomena are usually present in such devices.The non-linear characteristics are usually intrinsic to the transport,and the underlying mechanisms are still unclear.This work investigates the ion transport characteristics in nanopores and nanochannels that have weakly overlapping electrical double layers.Numerical simulations and 1-D mathematical model are developed to reveal the non-linear I-V characteristics from perspectives of electro-osmotic flow(EOF)and screening ionic charges.On the experimental side,nanopores devices are fabricated and their electrical properties are measured.The main contributions are as follows:(1)The simulation model for basic nano-channel structures is established.Transport processes are simulated by Poisson-Nernst-Planck(PNP)equations and Poisson-Nernst-Planck-Stokes(PNP-S)equations.(2)The suppression of ion conductance by EOF at moderate biases are revealed.The impact of fluid on the overall transport processes is examined by COMSOL Multiphysics and PROPHET,and then analyzed by 1-D mathematical model.It is shown that EOF drives ion depletion zone into the channels and suppresses the ion conductance.Furthermore,a limiting-conductance behavior is identified,which is intrinsically different from the classical limiting-current behavior.(3)We verify that screening ionic charges can be locally inverted under significant ionic transport at high biases.The relationship between inversion and vortex is revealed by COMSOL Multiphysics and mathematical analysis.It is shown the charge inversion process results in a body-force torque which may cause the vortex generation,which provides an explanation for nonlinear characteristics in nanofluidic devices.(4)Basic nanopore devices are fabricated and measured electrically.FIB and TEM enable us to drill nanopores with good size control.The size could be further reduced by ALD.Electrical measurement system is established and basic measurement is conducted.