Directional Water Transport in Flexible Nanochannels

Directional liquid transport in nanochannels composed of two-dimensional (2D) materials shows great potential in applications such as liquid diodes and biochemical nanosensors. However, experimental research on water transport at the nanoscale is challenging, and most previous theoretical studies have focused on rigid nanochannels while overlooking the coupling effect of channel flexibility on liquid transport. Considering the deformation of the 2D material, molecular dynamics simulations and theoretical analysis are employed to assess the transport of water droplets in a flexible 2D material nanochannel. The applicability of the Laplace equation at the nanoscale is verified first. And the deformation of the nanochannel confined with water droplets is analyzed using membrane theory, considering a significant interfacial effect at the nanoscale. Regardless of the wettability, thickness, and droplet size of the flexible graphene nanochannel, the droplets consistently move spontaneously toward the center of the channel, except in the case of a critical wettability state, where the water contact angle (WCA) is close to 90°. The passive transport mechanism is further explained from the perspective of the driving force on the droplet and the evolution of the system potential energy, both deduced from the asymmetric deformation of the channel. Relevant results reveal that for both hydrophilic and hydrophobic channels, as the droplet moves toward the channel center, the driving force and system potential energy decrease, reaching zero and a minimum at the center, respectively. However, in the cases of a critical wettability state, the driving force and potential energy remain zero and constant, respectively.