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A feature of carbon nanotubes that offers intriguing possibilities for future research is the inner hollow cavity. Only a few nanometers in diameter and a few micrometers in length, such a cavity should allow for interesting nanoscale experiments if it could be filled with other materials in a systematic way through capillarity. For instance, the cavity could act as a nanometer-size test tube with motions restricted to a low dimensional environment. It could also be used as a nanometer-size mold to make quantum wires. [Zeolites have been extensively studied for such purposes (1-3).] However, little is known about the capillarity and wetting of nan tubes, which is essential for practical use of this inner cavity.
Theoretical calculations first predicted that open nanotubes should act as "nanostraws" and draw in molecules from vapor or fluid phases (4). It was then demonstrated that when reacting with lead and air closed nanotubes could be filled by a lead compound (5). However, in subsequent experiments we found that if nanotubes were first opened at their tips and exposed to molten lead or other metals in an inert atmosphere, these were not drawn in by capillarity (6). We suggested at the time that debris at the entrance of the open nanotubes or pressure differences might account for the differences in the results (6). However, there may be other reasons (7).
The key to this problem is the surface energies of the interaction between the liquid and the solid surface of the nanotube. This is a problem of wetting, for wetting and capillarity are intimately related (8). Wetting is necessary for observing capillarity action, as can be understood from the Laplace equation
(1) (Equation omitted)
where r is the radius of curvature, delta-P the pressure difference across the liquid-vapor interface, gamma the surface tension, and (character omitted) the liquid-solid contact angle. The size of the contact angle (character omitted), shown in Fig. 1, is critical because delta-P will be negative if (character omitted) is larger than 90deg. (Fig 1 omitted) In such a case, the contact angle is said to be nonwetting and pressure must be applied to cause the liquid to enter the tube. So, if one wanted to fill nanotubes with a given substance by capillarity, the substance must...