**3. Carbon nanotubes (CNTs)**

Carbon nanotubes, seamless rolled-up graphite sheets, are tubular networks of sp2 -hybridized carbon atoms; these 2D structures are considered to be one-dimensional due to their high aspect ratio (1–2 nm dia., length in mm) [22]. Produced using techniques such as chemical vapor deposition, graphite evaporation, etc., these can either be single- or multi-walled depending on the production route [23]. Due to several unusual characteristics including nanoscale diameters, non-polar surfaces, CNTs are being investigated for nanoscale water desalination and selective transport in nanochannels. The wetting behavior of CNTs, nanotube forests and arrays, effects of nanoscale confinement, and tunability are areas of intense research [24, 25].

 The effect of various conditions on the wetting behavior of CNTs with water has been investigated by several researchers. Key factors were found to be the structure of CNTs, tube's size, the strength of van der Waals interaction between water and carbon, spontaneous imbibition of water in CNTs, surface energies, and temperature [26, 27]. MD simulations on SWCNTs in the temperature range 270–370 K showed an increase in capillary filling and water uptake with increasing temperature. This was attributed to increased wettability and reduced viscosity of water with temperature along with small increases in water imbibition [28].

 The wetting behavior inside small capillaries or nanochannels can be very complex depending on intermolecular interactions between the solid and the liquid, van der Waals attraction, surface tension, line tensions along the radius of the nanotubes, tube diameter, etc. [29]. The adsorption of liquid atoms as mono/ multilayers on capillary/CNT walls can distort the meniscus profile during the filling of nanochannels and may affect the wetting behavior due to disjoining pressure induced by molecular interactions. Surface roughness can also play an important role during nanoscale confinement, especially when the characteristic variations in surface roughness become comparable to the channel diameters. Surface chemistry is another key factor affecting wettability and flow through nanochannels [30].

A number of investigations have been reported on "nanopumping" to pump liquids through nanotubes based on electrical or mechanical properties of CNTs including studies on the flow of viscous fluids, activation of fluid flow, friction with nanowalls, etc. [31]. The wetting behavior of CNTs has been investigated with a number of polymers, e.g., polyvinylidene fluoride, maleic acid anhydride,

 polycaprolactone, polyurethane acrylate, etc. Chemical surface treatments were used to modify the wettability of nanochannels [32]. The wetting of nanotube forests and arrays has been investigated with several liquids including water, glycerol, ethylene glycol, propylene carbonate, olive oil, dimethyl sulfoxide, and nitromethane with contact angles ranging between 60 and 157° [33]. CNT forests have been used to grow hematite nanochain arrays, as filters to capture micro/nanocontaminants in water, and as gas diffusion arrays with contact angles up to 150° [34].
