Wetting of solid surfaces is a complex and subtle phenomenon which has been studied carefully over the past 200 years. A good understanding of wetting can explain many key physical interactions at interfaces, notable examples being in lubrication, composite materials, and capillarity. Wetting phenomena continue to intrigue the scientific community due to the complexity of this seemingly simple process. In recent years, specific nano-scale aspects of wetting have been revealed, highlighting the importance of a molecular-level understanding of wetting. The study of nanotube wetting encompasses the old/new, as well as nanoscale aspect of these endeavors. Proven importance of nanotubes as fillers in ultra-strength nanocomposites, where the interfacial interactions in the nanocomposite are controlled by wetting, lends a technological push to the field. Inorganic nanotubes (INT) formed from tungsten and molybdenum disulfides disperse very well in a variety of polymers, enabling preparation of nanocomposites with enhanced mechanical properties, thermal stability and improved rheological behavior. Nonetheless, the nature of the interaction between a nanotube and polymer liquid has received little attention and is poorly understood.

Here we present a combined experimental and theoretical study on the microscopic interaction of WS₂ nanotubes (INT-WS₂) with water. The unique experimental approach is based on manipulation and pull-out of individual nanotubes from water films while monitoring the forces generated with a cantilever in an atomic force microscope (AFM). This method draws on concepts of the classic Wilhemy Balance Technique, while exploiting the exquisite force control of the AFM. The AFM experiments were contrasted with parallel experiments in an environmental scanning electron microscope (ESEM). Detailed theoretical calculations based on density functional theory (DFT) predicted well the interaction energy for large, closed cap nanotubes, but vastly underestimated the interaction energy with small, open-ended nanotubes. For those small diameter tubes, force-field molecular dynamics (MD) simulations together with a thermodynamic analysis qualitatively explain the observed behavior, strongly implicating a dominant capillary effect. Visualization of the pullout in the ESEM together with AFM force traces allow precise modelling of the meniscus formation during pullout, reflecting the energetics of the interface at, and inside the nanotube wall.

Acknowledgment: Supported by the Israel National Nano-Initiative, the Israel Science Foundation, H. Perlman Foundation. and Act 211 Government of the Russian Federation, contract № 02.A03.21.0006.