**1. Introduction**

 Surface wetting, capillarity, adhesion, and surface tension-related processes across solid-liquid interfaces have been the focus of extensive theoretical and experimental research in fields such as natural sciences, agriculture, geophysics, technology, water management, biological, and environmental sciences. Intermolecular interactions play a key role in the ability of a liquid to maintain contact with a solid surface, and a balance between cohesive and adhesive forces determines the overall wetting behavior of the system. Some of the wettability applications include superhydrophobic surfaces, dynamics of oil spills, ground water flows, disease transmission, chemical leaching, nanotechnology, and several other real-life applications [1, 2].

 The wetting behavior of nanomaterials such as carbon nanotubes (CNT), graphene, graphyne, nanoparticles, and nanoengineered surfaces is an area of intense experimental and theoretical research activity [3–5]. Intermolecular interactions are of crucial importance for controlling nanoscale material behavior in various aspects of nanotechnology, nanodevices, and their applications. Other areas of research include the flow of liquids inside nanochannels, tuning of nanotube forests and arrays for modifying wetting characteristics, development of nanogrippers for manipulating carbon nanotubes for electro-mechanical devices, nanoscale surface treatments for producing hydrophilic or superhydrophobic surfaces [6].

Several advanced optical or microscopic experimental techniques are being used for nanoscale wetting investigations. While the macroscopic contact angle at the solid-liquid interface can be measured using conventional optical techniques, advanced microscopic or indirect techniques are required for micro and nanoscale investigations. It is also a common practice to determine an "effective contact angle" at nanoscale distances due to limited/poor contact at the liquid meniscus, confinement issues, and the influence of long-range forces [7].

 A brief overview of measurement techniques is provided next. Surface forces technique determines the influence of separation distance on forces (±10 nN) between two surfaces with the help of capacitive sensors or springs; distances can be controlled down to 0.1 nm with help of piezoelectric positioners. Wilhelmy method measures the force exerted during contact of liquid with a solid specimen for an indirect determination of contact angle [8]. This technique has been used to determine the wetting behavior of nanowires (500 nm dia.) and nanoneedle probes. Widely used sessile drop technique determines contact angle directly from the profile of a liquid drop atop a solid substrate with help of video cameras or telescopic arrangements. This technique can be used to continuously monitor dynamic changes in contact angles as a function of various system variables [9].

Interference microscopy technique computes contact angles (15–30°) from the 3D contours and profiles of liquid droplets by using interference fringe patterns formed at the solid/liquid and liquid/vapor interfacial region [10]. Atomic force microscopy scans the sample surface with a sharp tip to monitor surface topology while maintaining a constant interaction force through feedback loops; a modified nanodispenser tip is used for micro- and nanodroplets [11]. Scanning Polarization Force Microscopy uses electric polarization forces for mapping topography contours for distances in the range 10–20 nm, thereby minimizing the risk of surface contact by probing tip, an aspect especially valuable for soft and liquid specimens [12].

Extensive theoretical research is also being carried out using theoretical modeling, analysis, Monte Carlo, and molecular dynamics computer simulation on a variety of relevant issues. Some of the problems being investigated include the influence of nanoscale confinement on surface tensions, wetting behavior inside small capillaries and nanochannels, effect of line tension, liquid adsorption on capillary walls, and precursor films, the role of nanoscale surface roughness on confinement and wetting among others [13, 14].

In this chapter, a brief overview on the nanoscale wetting behavior of graphene and carbon nanotubes is presented along with factors influencing the wetting phenomena and basic system characteristics of nanomaterials.
