Drag Force for Various Loading

We further explored the relation between the amount of loading on sample and drag force.

The pulling force is also 0.15 g in this case but loading on the sample was added up from 0.73 g to 0.97 g. Results are listed in Table 4-6.

Although in all conditions the sample was still floating above the water surface in a non-wetted state, the time it took to travel the same distance still increased with the amount of loading, Figure 4-24. This result indicates that, for a small object moving on water surface, the

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drag force is still crucial. Consequently, for creatures like water strider it is desirable to have body weight as low as possible to make fast movement. Nevertheless, the drag force can also be useful. As can be observed from the striders in a pond or from previous research [10], the fast propelling of body is usually achieved by rear legs, which are deeper into the water, while the front legs displace relatively less volume of liquid.

Table 4-6: The average time for a hydrophobic i-ZnO having varied loading on it.

Force (g) Mean Time (s) STD

0.733 1.646 0.037

0.791 1.726 0.079

0.849 1.738 0.043

0.907 1.860 0.062

0.72 0.74 0.76 0.78 0.80 0.82 0.84 0.86 0.88 0.90 0.92 1.6

1.8 2.0

Mean Time (s)

Loading (g)

Figure 4-24: The average time increased for sample with increasing loading on it.

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Chapter 5 Conclusion and Future Work

Superhydrophobic inverse opal was successfully prepared by electrophoresis, electrodeposition, and hydrophobic treatment. The fabricated structure exhibited static contact larger than 155 degrees. This structure was utilized to investigate the surface tension-enhanced buoyancy. The theoretical maximum buoyant force was calculated via Laplace equation for comparison with experimental values. It was found that the total buoyant force was boosted simply with a superhydrophobic structured coating on top. This phenomenon was from several aspects including (i) stabilizing larger angle and thus higher loading, (ii) hindering further movement or wetting on the top surface, (iii) impeding the advancement of contact line at the edge. Moreover, the effects of interspacing and drag force were also studied. Adequate interspacing was deemed important for maximizing displaced volume. We also pointed out superhydrophobic coating at the bottom of a bulk object can reduce fluidic drag to a small amount and the drag increased with increasing loading on it. These findings are believed to be helpful for future design of water-walking robots, marine surveillance and any other devices floating on water.

In the end, we point out some of the work that could be done in the future. First, more elaborate measurements of force can be achieved by using high-sensitivity balance system, Data-physics DCAT 11 or Hystron TS 75 TriboScope for example. Second, even more superhydrophobic surfaces (>160°) should be put to test to verify if it can induce a larger buoyancy. Third, objects of different materials, density, and sizes could be fabricated to enhance buoyancy to weight ratio. Fourth, facile approach to fabricate superhydrophobic surfaces would facilitate more practical applications.

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