Loading Tests

4.2.3 Loading Tests

Experiments were carried out for measuring the maximum total buoyant forces, angles, and other conditions. A PMMA container (30 cm × 2.5 cm × 5 cm) was filled with 200 mL DI water. The analyzed samples were carefully placed on the water surface. A camera (Nikon D90) with 28-105 macro lens was set up by the container to observe the changing angle of water. One piece of weight was added each time. Each one has weight 0.05803 g with a standard deviation of 0.00067 g. An illustration of the set-up is shown below.

Figure 4-12: The loading measurement.

4.2.3.1 Total Buoyant Force vs. Angle ϕ

We carried out experiments investigating the changing angles ϕ vs. the applied force. The ϕ is the angle measured with respect to vertical line. Samples included ITO with or without hydrophobic treatment (HP), and inverse ZnO opal with or without hydrophobic treatment. All the results are plotted in Figure 4-14. For structured surface and ITO side, “up” indicates such surfaces are upward, contacting the air, and “down” indicates they are downward, contacting the water.

We combined all the seven conditions for comparison in Figure 4-15. The trends appear similar at small ϕ, indicating that the meniscus profile is independent of samples. Moreover, contrary to what we initially anticipated that the superhydrophobic inverse structures under the

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sample should be able to support much greater additional loading. The reason was later found out that when several structures are too close to each other, the extra volume provided by surface tension would cancel each other. The details are examined in the next section by conducting calculation and experiments on a larger scale. Besides, compared to pure hydrophobic ITO, the superhydrophobic inverse structure in water still gives 0.1 to 0.15 g increase. It is from a layer of gas blanket, i.e. the air trapped below the sample, which, though not much, offers extra displaced volume. It can be observed from the silver-like appearance when the sample is immersed in water (Figure 4-16). This corresponds with the findings by Qinmin Pan et al [44] that a film of air would surround structured superhydrophobic surfaces and thus provide extra buoyancy.

Furthermore, we also examined the relationship between the static angle and maximum loading capacity in Figure 4-17. The result shows that the total buoyant force is proportional to the static contact angle, especially for superhydrophobic inverse structure. Though having contact smaller than ITO, non-hydrophobic inverse structure exhibits better loading capacity.

This phenomenon is due to the improvement of contact line pinning by the porous structure, suggesting possible existence of metastable Cassie state [5]. Overall the loading behavior followed the prediction in Section 4.2.2 that total buoyant force should be in positive relation with the angle ϕ. The experimental shapes of menisci at various angles were compared with theoretical shapes in Figure 4-18, in which experimental menisci (right) show well agreement with theoretical menisci (left). Nevertheless, the errors in measuring the angle ϕ and the force came from a few aspects. First, we could not control the center of weight precisely at the center, so all sides might not pierce the water at the same time, consequently reducing the maximum loading. Second, the weight was placed with tweezers as gently as possible.

However, we could not avoid the suddenly pushing down and decelerating of the weight.

Third, the ITOs were not perfectly smooth at the edge, bringing about some undesirable

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effects. Fourth, the angles were manually measured and inevitably resulted in values smaller than actual values. These errors possibly lead to lower loading capacity, and they may be countered by more careful micro-conduct of experiments.

Figure 4-13: Optical images for (a) 0.661, (b) 1.179 , (c) 1.696, and (d) 2.271 g. The angle changes with applied weight.

Table 4-2: Static contact angles, max angle ϕ, and maximum force for various surfaces on top.

ITO-noHP-glass ITO-noHP-ito ITO-HP i-ZnO i-ZnO-HP Static Contact

Angle (deg)

49.05 87.51 99.27 78.64 155.45

Max Angle ϕ (deg)

150.7 177.9 160.7 175.0 195.8

Max Force (g) 1.84±0.06 1.87±0.05 2.01±0.03 1.99±0.09 2.21±0.06

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Figure 4-14: Angle ϕ vs. total buoyant force for (a) ITO-noHP-up, (b) ITO-noHP-down, (c) ITO-HP, (d) i-ZnO-noHP-down, (e) i-ZnO-noHP-up, (f) i-ZnO-HP-down, and (g) i-ZnO-HP-up.

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0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4

100 120 140 160 180 200

ITO-noHP-down ITO-noHP-up ITO-HP-up

i-ZnO-noHP-down i-ZnO-noHP-up i-ZnO-HP-down i-ZnO-HP-up

Angle (deg)

Total Force (g)

Figure 4-15: Combination of plots for total loading vs. the angles ϕ.

Figure 4-16: Silver-like appearance on the superhydrophobic inverse opal structures.

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40 60 80 100 120 140 160

1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4 2.5

Total Force (g)

Contact Angle (deg)

Figure 4-17: The total buoyant force vs. their static contact angles.

Figure 4-18: Meniscus shapes at various angles. (a) 111° (b) 122° (c) 145° (d) 170° (e) 185°

and (f) 202°

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4.2.3.2 Force vs. Structured Surfaces

Further discussion of two cases, ITO-HP and i-ZnO-HP-up are provided here.

We found that for the case of superhydrophobic ZnO inverse opal on top, angle ϕ can exceed 180 degrees as in Figure 4-13 (d) and Figure 4-14 (g). The assumption made by Y.S.

Song et al [45] and A. R. Pennar [46] were not valid. They assumed the angle contacting the object cannot exceed 180° because the surface tension force would then reach a maximum.

Actually, one should also take into account the pressure force term, which is not necessarily maximized when the surface tension force reaches maximum.

The corollary of such structured superhydrophobic surface on top of the sample is that it can induce a larger floatability by several means: (i) Stabilize larger angle and thus sustain higher loading, (ii) hinder further movement or wetting on the top surface, (iii) impede the advancement of contact line at the edge, (iv) reveal reversibility once the loads are taken away (Figure 4-19). The reason for that is the increase of total energy when water contacts the superhydrophobic structures. The effect of water droplet pinning at the edge has been addressed by others as Gibbs’ inequality from the book “The Collected Works of J. Willard Gibbs” in 1957. As stated by David Quere [47] and J. F. Oliver [48] that the angle ϕ at the edge can vary by:

𝜙 (eqn. 4-10)

where θ₀ is static contact angle, and ω is the angle subtended by the side wall, 90° in our case.

Since all the samples have identical geometry, the maximum angle at the edge should be the same for all cases. However, this is not true as this angle varied from one case to another, especially for superhydrophobic ZnO inverse opals. This is ascribed to the existence of a structured superhydrophobic surface, which is necessary for enhancing this effect. The effect is also useful for application in solving the liquid outflow phenomena [49].

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Figure 4-19: Demonstration of reversibility, (a) loaded, (b) unloading, and (c) unloaded.

4.2.3.3 Force vs. Surface Tension

We added another parameter, surface tension, into the maximum force to verify the applicability of the calculation. By adding a minute amount of surfactant, Triton X-100, into pure water, the surface tension can be easily tuned with little variation of other properties, such as density and viscosity. The solution was prepared by dissolving certain amount of triton in one liter of DI water and heating it at 50°C for one hour. Hydrophobic ITO samples were used in these tests. The results are summarized in Table 4-3 and Figure 4-20.

The experimental results also coincide with the predicted trend of maximum loading and surface tension. As the concentration of triton increases, both the surface tension and contact angle decrease. As a result, the maximum loading force increases correspondingly.

Nevertheless, the mismatch between theoretical and experimental values is because the contact line is more prone to movement in the presence of surfactant.

Table 4-3: The characteristics and results for various surfactant concentrations.

Triton Conc. per liter 0 ul 4 ul 20 ul 100 ul 500 ul

Static Contact Angle (deg) 109.01 105.05 95.18 77.14 56.27

Surface Tension (mN/m) 72 50.7 43.8 30.8 30.6

Max Force (g) 2.01±0.03 1.78±0.08 1.48±0.11 1.14±0.10 1.05±0.06

Theoretical Max Force (g) 2.64 2.61 2.50 2.24 1.83

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Figure 4-20: Maximum buoyant force vs. (a) triton concentration, (b) surface tension, and (c) static contact angle. (d) All these trends follow the theoretical calculation in Section 5-2.

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