Peter and Hummer illustrate using molecular dynamics simulations that nonpolar pores less than 0.5 nm in diameter can successfully block ions, but increasing the pore diameter to 1 nm allows the passage of ions. They observe a drastic increase in the diffusion coefficient of ions once the tube diameter is above this critical value, and this critical diameter can also be linked to changes in the water structure within the NT. This critical diameter has also been noted in a study by Beu, which included a polarizable model for water and solute ions. Beu found that polarizability has a negligible influence on the ionic currents through CNTs 6 nm in length with diameters ranging from 1 to 1.6 nm. Ions are thought to move through the tube almost frictionlessly, but as ions enter the NT they encounter an energy barrier that increases as the pore radius decreases. As a result, ion currents show a strong dependence on pore radius. In recent experiments, Yu et al report on the gated ionic diffusion through densely packed CNT membranes with both 3-nm NT and 3-nm interstitial pore diameters. At a temperature of 298 K, ions become trapped in the membrane by discontinuous water clusters. However, ion diffusion rates were shown to increase upon increasing the temperature or exposing the membrane to ultrasound. Although the membrane is able to gate the flow of ions, it is not selective to a particular ion type.