We report on the results of extensive molecular dynamics simulation of water imbibition in carbon nanotubes (CNTs), connected together by converging or diverging nanojunctions in various configurations. The goal of the study is to understand the effect of the nanojunctions on the interface motion, as well as the differences between what we study and water imbibition in microchannels. While the dynamics of water uptake in the entrance CNT is the same as that of imbibition in straight CNTs, with the main source of energy dissipation being the friction at the entrance, water uptake in the exit CNT is more complex due to significant energy loss in the nanojunctions. We derive an approximate but accurate expression for the pressure drop in the nanojunction. A remarkable difference between dynamic wetting of nano- and microjunctions is that, whereas water absorption time in the latter depends only on the ratios of the radii and of the lengths of the channels, the same is not true about the former, which is shown to be strongly dependent upon the size of each segment of the nanojunction. Interface pinning-depinning also occurs at the convex edges.

The phenomenon of water filling of heterogeneous nanocapillaries composed of hydrophilic and hydrophobic zones is studied by molecular dynamics simulation. The heterogeneous nanocapillaries have been constructed by reducing the strength of Lennard-Jones (LJ) interaction between oxygen and wall atoms in certain domains of otherwise pristine carbon nanotubes. The modified or reduced LJ interaction is tuned to a value that corresponds to the hydrophobic characteristics. Nanochannels of different radii and lengths in which up to 50% of atoms had hydrophobic interactions have been investigated. Our simulations show that carbon nanotubes (CNTs) with a small hydrophobic zone within will eventually be invaded by water; however, the existence of a small hydrophobic region in the tube entrance might interrupt the flow of water molecules for an unknown time interval. This retardation time is found to be a random variable, sometimes magnitudes larger than the filling time of a pristine CNT. When the nanotube consists of alternate hydrophilic and hydrophobic blocks of equal size, water occupancy progresses between filled and partially filled states or shows filling/emptying transitions, depending on the size of the nanotube and the extent of the hydrophobic zones. The uncertainty in the retardation time still remains when the wall atoms next to the entrance have the hydrophobic characteristic.

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