Chapter 1 Introduction
3.3 Results and Discussion
3.3.6 Water immersion and droplet impact behavior on CNT arrays with different wettabilityarrays with different wettability
As mentioned earlier, CNT arrays can be turned superhydrophobic by exposing them to vacuum annealing treatments. Not only do these CNT arrays exhibit a very high static contact angle, their capability of repelling water is unparalleled. Such capabil- ity can be seen clearly when these CNT arrays are submerged in water. In a shallow pool of water, the water-free surface is deformed near the surface such that walls of water are formed around it, leaving the surface dry. This phenomenon indicates that water is being repelled strongly by the bulk surface of superhydrophobic CNT arrays (Figure 3.22). In a deep pool of water, it exhibits a silvery mirror-like surface, indi- cating the presence of liquid-vapor-solid interface between water and the bulk surface of superhydrophobic CNT arrays (Figure 3.23). Interestingly, this liquid-vapor-solid interface only exists between water and the bulk surface of superhydrophobic CNT ar- rays, and does not exist between water and the bulk surface of hydrophobic as-grown CNT arrays. Instead of silvery mirror-like color, the as-grown CNT arrays, which
Figure 3.21. Low magnification SEM images of CNT arrays after vacuum annealing treatment (a) and oxygen plasma treatment (b). Both CNT arrays exhibit similar microscale surface roughness and morphology, with no apparent effect of oxidation and reduction.
Figure 3.22. (a) Top view, tilted view, and side view of an as-grown CNT array (denoted by §) and a superhydrophobic CNT array (denoted by‡). (b) Schematic of (a) that highlights the deformation of water free surface (indicated by an arrow) near the surface of superhydrophobic carbon nanotube array.
Figure 3.23. (a) Tilted view of an as-grown CNT array (denoted by §) and a su- perhydrophobic CNT array (denoted by ‡). (b) Schematic of (a) that highlights the presence of a film of trapped air at the interface between water and the surface of superhydrophobic carbon nanotube array.
hydrophobicity is slightly less than that of superhydrophobic CNT arrays, show their original black color.
According to Luo et al. (2010) and McHale et al. (2010), a bright region on the surface of CNT arrays corresponds to a strong light reflection, due to the presence of a thin air cushion trapped between water and the surface. Judging from the size of the reflective area, it can be confirmed that the entire surface of the arrays is covered by a continuous layer of air. This trapped air cushion keeps the entire surface completely dry and prevents a direct contact between water and the surface to be made anywhere on the surface. Indeed, this air film is the signature interface of an ideal Cassie state superhydrophobic surface.
It is widely known that superhydrophobicity of a surface can be explained by Cassie state, Wenzel state, or a transition between these two states (Lafuma and Quere, 2003; Wang and Jiang, 2007). For both states, the static contact angle for water is very high, typically higher than 150◦ (Wang and Jiang, 2007; Bhushan et al., 2009). In the case of a Cassie state, there exists a liquid-vapor-solid interface between
Figure 3.24. (a) Time-lapse images of the water droplet impacting the surface of as grown CNT arrays. Scale bar indicates 5 mm.
water and the surface, such that only a tiny fraction of water is in a direct contact with the surface asperities. Because of such interface, the adhesion between water and the surface is negligible. Hence, both contact angle hysteresis and roll-off angles for water are very low, typically lower than 10◦. In the case of a Wenzel state, the interface between water and the surface is dominated by a liquid-solid interface, such that water is always in intimate contact with the surface asperities. This results in a high adhesion between water and the surface such that both contact angle hysteresis and roll-off angles for water are high.
To demonstrate that the dry oxidized and vacuum annealed treated CNT arrays are, indeed, hydrophilic and hydrophobic respectively, the dynamic effect of a free- falling water droplet on the surface of these CNT arrays was qualitatively compared.
As a free-falling water droplet hits the surface of the hydrophobic as-grown arrays, it deforms heavily and then bounces off the surface (Figure 3.24). However, because the as-grown arrays are just hydrophobic, there is always a small portion of the droplet pinned to the surface, preventing the droplet from bouncing off completely from the surface. This pinning phenomenon was more pronounced when the water droplet hit
Figure 3.25. (a) Time-lapse images of the water droplet impacting the surface of hydrophilic CNT arrays. Scale bar indicates 5 mm.
the surface of the more hydrophilic CNT arrays. For example, on the CNT arrays with an average static contact angle of 30◦, the droplet is always pinned to the surface and comes to rest shortly after the impact (figure 3.25). A totally different behavior is observed on superhydrophobic CNT arrays, where the free-falling water droplet bounces completely off the surface (Figure 3.26). In contrast to another published study (Jung and Bhushan, 2008), the droplet pinning does not occur on the super- hydrophobic CNT arrays, even at a considerably high impact velocity of 2.22ms−1. Detailed investigation of water droplet impact behavior on superhydrophobic CNT arrays can be read in Chapter 5.