The simple process of a liquid wetting a solid surface is controlled by a plethora of factors-surface texture, liquid droplet size and shape, energetics of both liquid and solid surfaces, as well as their interface. Studying these events at the nanoscale provides insights into the molecular basis of wetting. Nanotube wetting studies are particularly challenging due to their unique shape and small size. Nonetheless, the success of nanotubes, particularly inorganic ones, as fillers in composite materials makes it essential to understand how common liquids wet them. Here, we present a comprehensive wetting study of individual tungsten disulfide nanotubes by water. We reveal the nature of interaction at the inert outer wall and show that remarkably high wetting forces are attained on small, open-ended nanotubes due to capillary aspiration into the hollow core. This study provides a theoretical and experimental paradigm for this intricate problem.
MD simulations; capillary; in situ microscopy; inorganic nanotubes; wetting.
Conflict of interest statement
The authors declare no conflict of interest.
TEM micrograph of the two kinds of WS
2 nanotubes used in this series of experiments. ( A) A 30-nm-diameter INT with open cap and no oxide filling (type I); ( B) A 100-nm INT, with large number of outer walls and a closed cap (type II). ( C) SEM image of condensed water droplets on an assortment of cooled (type II) WS 2 nanotubes. In the Inset, zoom-in on two single water droplets and their manually measured contact angles.
A) Initially, the INT-tip touches the surface of the water film. ( B) Just before the snapping out of the nanotube-tip from the water “cone” (denoted as the maximum force point, l). ( C) Image of the water film and the nanotube right after the separation; the position of the edge of the tube represents the overall distance traveled by the cantilever during pullout ( X).
Setup of the ESEM pullout experiments.
A represents a nanomanipulator with an AFM probe attached at its end. B is an illustration of a cooled stainless-steel stub with water film formed on its left side. The stub was kept at 4 °C and water vapor pressure of 6.5–7 torr. ( C) Expanded view of the AFM nanotube tip just before touching the water film. In the lower part, SEM micrographs taken in FIB. ( D) WS 2 nanotubes are protruding out of the blade edge, and an AFM tip is shown approaching it. ( E) A nanotube was attached using a platinum source, and the AFM tip is ready for use.
An illustration of the pullout experiment with calculation of the pullout force. (
A) The nanotube is positioned at its starting point where the end of the nanotube is in contact with the water film surface (dotted line). ( B) The maximum water protrusion is presented with height l. ( C) The nanotube was pulled out of the water film, and the total distance traveled by the nanomanipulator, from the starting point, is measured and denoted as X. The red dashed line represents the initial point of the water film.
An ESEM micrograph during a pullout test. In this micrograph, the nanotube was pushed to penetrate the water film surface. In this early stage, the water film was not yet pierced and the nanotube bent. One can also notice the concave shape of the water surface, implying a strong resistance of the water film to the nanotube penetration.
The cross-sections of the water meniscus calculated using Eqs.
S2 and S3 for different stages of the pullout corresponding to catenoid (meniscus) heights I = 0.1, 0.2, 0.3, 0.4, and 0.5 μm (from the Bottom to the Top). The Inset shows an ESEM image of a nanotube with diameter of 25 nm pulled out from the water surface with I indicated.
A) Normalized pullout forces of INT measured in ESEM (blue rhombi) and AFM (red squares). ( B) Pullout work vs. the nanotube (outer) diameters. Blue rhombi are the results calculated by the DFT calculations. Red squares are results calculated from the AFM pullout experiments (see text).
A and B are SEM micrographs of large-diameter INT-WS 2 collapsed at their ends. The collapsed elliptical structure exists only at the INT end. The rest of the nanotube has a cylindrical shape. TEM micrographs of WS 2 nanotubes. C is a top view of a WS 2 nanotube, one can notice the brighter end of the nanotube indicating a less dense area. ( D) A closer examination of the INT reveals that the end of the nanotube has a hollow core and most likely a collapsed edge. Away from the nanotube end, remains of tungsten oxide can be seen in the core of the nanotube.
Several examples of optimized models for different WS
2|H 2O interfaces with zigzag configuration of the edges were considered in the DFT calculations. ( 1) W-terminated edges of 2H–WS 2 are saturated with water molecules (blue colored circle); ( 2) one of the W-terminated edges of 2H–WS 2 is saturated with hydroxyl groups (purple colored circle), whereas positive counterions occur in the bulk of the H 2O slab (Zundel H 5O 2 + cation is represented by cyan full circle). Water molecule (blue circle) is attached to the other W-terminated plan; the (0001) WS 2 surface is covered with monomolecular water layer with dense ( 3) and open-work packing ( 4). Red full circles represent oxygen; blue, Mo; yellow, S; and H atoms in white.
The estimated binding energy
E between water surface and the tip of a multiwalled WS b 2 nanotube dependence on the parameters of nanotube—diameter and number of walls k.
Side views on the imbibition of H
2O drop during 1 ns into the fragments of double-walled (24,0)@(36,0) 2H–WS 2 nanotubes with frontal tip consisting of S- and W-terminated inner and outer walls ( 1), or W- and S-terminated inner and outer walls ( 2), with frontal tip consisting of S- and W-terminated inner and outer walls of WS 1.9 stoichiometry ( 3). Pullout using open end of a (28,28) nanotube from a CCl 4 film deposited on graphite ( 4). For clarity, only the CCl 4 molecules are depicted, not the nanotube walls. Corresponding radial distributions of O and H atoms within the nanotube cavities are depicted on the right panels.
Side and head-on views of the model tips used in MD simulations of the pullout of small WS
2 nanotubes from the vicinity of thin water film (mode 2): double-walled (24,0)@(36,0) 2H–WS 2 nanotube with capped rear end ( A), (24,0)@(36,0) 2H–WS 2 nanotube with capped frontal end ( B), and double-walled (32,0) 2H–WS 2 nanostripe ( C).
Evolution of the potential energy with the retracted distance of a WS
2 tip during MD simulations of pullout tests from the surface of water film. WS 2 tips of different morphology were taken into account: open-ended and capped (24,0)@(36,0) nanotubes, double-walled (32,0) nanostripe. Lines indicate the break-off point of the nanotube–water contact distance.
Side-view screenshots of the MD simulations of pullout tests from the surface of water film using WS
2 nanotips of different morphology: open-ended (24,0)@(36,0) nanotube ( A), capped (24,0)@(36,0) nanotube ( B), and double-walled (32,0) nanostripe ( C). The MD time and the distance of the tip withdrawn from the film are shown below.
Plot of the capillary energy Δ
G estimated for water imbibition into WS 2 nanotubes with length h = 250 nm, vs. inner diameter (determined by the outer diameter of nanotube D and the number of layers comprising the wall, k. The wetting angle between H 2O and the inner core of the WS 2 nanotube is assumed to be 89.80°. Experimental values of the pullout work W obtained after AFM measurements are plotted for comparison as the black dots with the black curve serving only as a guide for the eye.
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