Chapter 1 Introduction
3.3 Results and Discussion
3.3.7 Dispersibility and zeta potential analysis
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.
Figure 3.26. (a) Time-lapse images of the water droplet impacting the surface of superhydrophobic CNT arrays. Scale bar indicates 5 mm.
indeed extremely hydrophobic (Figure 3.27.a). Just like any other highly hydrophobic substances, the extremely hydrophobic CNT clump up together to minimize the in- terface area between the CNT and water. Similar to the phenomenon observed when these superhydrophobic CNT arrays are immersed in a deep pool of water, there exist large scale air pockets encapsulating them such that their effective density becomes extremely low. Consequently, they are always found to float on the surface of water, even after being subjected to ultrasonication. Interestingly, by floating, the interface area between the CNT and water is decreased even further.
Similarly, the hydrophobic CNT from the as-grown CNT arrays could not be dis- persed easily in DI water, although a different behavior can be observed. Instead of floating, these hydrophobic CNT are completely submerged in the water column (Figure 3.27.b). The dispersibility of these hydrophobic CNT in DI water was un- questionably higher than that of superhydrophobic CNT. Since these CNT have a higher concentration of oxygenated groups than that of superhydrophobic CNT, they are more polar and are not encapsulated inside large scale air pockets. However, they are still clumping up together to minimize the interface area between the CNT and
Figure 3.27. Dispersion of CNTs with various wetting properties in DI water. The wettability of the CNT increases from left to right. The nanotubes from (a) cannot be dispersed in DI water. The nanotubes from (b) and (c) arrays precipitated in several hours and days, respectively, after the dispersions were performed. The dispersion of nanotubes from (d) arrays was found stable even after more than two months.
Figure 3.28. Plot of zeta potential of CNTs with various wetting properties in DI water as a function of their oxygen/carbon atomic ratio.
water. Consequently, their effective density becomes higher than water and, instead of floating, they are sinking into the bottom of the water column and immediately forming sedimentation.
A better dispersibility is shown by hydrophilic CNT from the CNT arrays, with an average static contact angle of 75◦. Although sedimentation of CNT is still ob- servable, a small fraction of the CNT is already well dispersed. This behavior can be observed from the color of the dispersion, which has turned dark and is no longer clear (Figure 3.27.c). Obviously, the CNT dispersibility can be improved by exposing them to a longer UV/ozone or oxygen plasma treatment. For example, the CNT from the highly hydrophilic CNT arrays with an average static contact angle of 30◦ can be dispersed quite easily in DI water. In agreement with works by Naseh et al. (2010) and Chen et al. (2010), dispersion of hydrophilic CNT from the highly hydrophilic CNT arrays is very stable and does not form sedimentation even after two months of settling time (Figure 3.27.d).
The dispersibility of CNT arrays in DI water can be quantified by their zeta po- tential measurement. Zeta potential is a measure of electric potential that is exhibited
by any particle in suspension and is measured by the difference of potential at the slipping plane and the bulk suspension medium. Particles with higher magnitude of zeta potential, either positive or negative, have the tendency to repel each other such that they do not coagulate or flocculate. On the other hand, the closer the magni- tude of zeta potential to zero, the less dominant the repulsive force is. Hence, they have a higher tendency to coagulate or flocculate. Since the magnitude of the zeta potential gives an indication of the stability of the dispersion, it is expected that the zeta potential of a dispersion of hydrophilic dry oxidized CNT in DI water will be quite high. In contrast, a dispersion of superhydrophobic vacuum annealed CNT in DI water is expected to exhibit a near zero zeta potential.
As expected, the zeta potential of a dispersion of superhydrophobic CNT with an average O/C ratio of 7% in DI water is almost zero. In agreement with the qualitative observation of the dispersion, a near zero zeta potential means that the dispersion is highly unstable and the superhydrophobic CNT coagulate rapidly in DI water. The value of zeta potential of CNT dispersion decreases monotonically as the increase of O/C ratio (Figure 3.28.c). An increase of O/C ratio by 1% results in an decrease of zeta potential by 10mV. For the dispersion of hydrophilic CNT with an average O/C ratio of 11%, its zeta potential is measured to be -36mV, which in theory should exhibit a moderate stability. Indeed, as observed qualitatively, these hydrophilic CNT exhibit a better dispersibility, although sedimentation of CNT is still observable on the bottom of the dispersion. Dispersion of highly oxidized CNT with an average O/C ratio of 15.5% exhibits a zeta potential of -45mV, which in theory indicates a good dispersion stability. Indeed, qualitative observation confirms that the dispersion of highly oxidized CNT is stable even for more than two months of settling time.