Nano-Scale Wettability of Free-Standing Capped Carbon Nanotube Arrays

Countless organisms in nature have adapted high-aspect-ratio micro-/nano- fibrillar arrays on their functional surfaces for achieving special and often optimized functionalities using earthly abundant materials. Some prominent examples include lotus leaves, cactus spines, gecko toe pads, eyes of bees, spider silks and butterfly scales. Given a particular local environment, these ‘natural designs’ are intrinsically multifunctional, responsive to external stimuli, biocompatible, biodegradable, energy efficient and effective. One common challenge these natural fibrillar systems have to deal with is the presence of environmental water in all its three forms (i.e., vapor, liquid or solid) surrounding their habitats. It is critical for the animals, insects, and plants to interact with the ubiquitous environmental water in a way such that particular functions can be achieved or sustained. 

At the core of nanoscience and nanotechnology, rationally mimicking nature offers a promising route to create multifunctional superstructures that capture organisms and biological materials’ intriguing responsive and self-adjusting properties. Carbon nanomaterials (e.g., carbon nanotubes) have been widely investigated as advanced structural, energy, and biomedical materials. Due to their high specific surface area, excellent electrical, thermal and mechanical properties, carbon nanomaterials are the ideal building blocks for achieving multi-functionality in the practice of biomimicry. Previous work has demonstrated that hierarchical vertically aligned multi-walled carbon nanotube (VA-MCNT) arrays can achieve ten folds of adhesive force comparing to the fibrillar structures of the gecko toe pads. However, little is known with regard to their wettability at the ultimate atomistic level, and how this may influence the adhesive performance and/or self-cleaning capabilities, despite water condensation/bridging has proven to common phenomenon at this length scale. 

In present study, molecular dynamics (MD) simulations were performed using Large-Scale Atomic/Molecular Massively Parallel Simulator (LAMMPS). A typical system consists of arrays of simplified vertically aligned single walled carbon nanotubes (VA-SWCNTs) with water boxes initially positioned in close proximity to the capped nanotube tips. Periodical boundary conditions are implemented in x, y and z directions with proper spacing. TIP4P water model is adopted. Water-water cohesive forces are descried by overlaying columbic and Lennard-Jones (LJ) potentials. Carbon atoms in CNTs are modeled using AIREBO (Adaptive Intermolecular Reactive Empirical Bond Order) potential. The water-CNTs adhesion is described by Lennard-Jones potentials. The effects of tube spacing, tube aspect ratio, chirality (armchair vs. zigzag) on the shape and position of the water nano-droplet and hydrogen bonding (or H-Bonds) were systematically investigated. It is indicated by the simulations that commonly believed hydrophobic defect free CNTs (i.e., carbon sp² hybridization without any dangling bonds) become super-hydrophilic at this length/temporal scale. The critical factors that influence the water shape, position, and the H-Bonds are: 1) tube aspect ratio and 2) tube spacing. Chirality has little effect on the water interfacial behaviors. Future work will focus on the effect of defects on the wettability of VA-SWCNTs, as well as the effect of the water condensation/bridging on the adhesive behaviors and self-cleaning properties of carbon-based bio-inspired fibrillar dry adhesives.