Highly Floatable Superhydrophobic Metallic Assembly for Aquatic Applications

Water-repellent superhydrophobic (SH) surfaces promise a wide range of applications, from increased buoyancy to drag reduction, but their practical use is limited. This comes from the fact that an SH surface will start to lose its efficiency once it is forced into water or damaged by mechanical abrasion. Here, we circumvent these two most challenging obstacles and demonstrate a highly floatable multifaced SH metallic assembly inspired by the diving bell spiders and fire ant assemblies. We study and optimize, both theoretically and experimentally, the floating properties of the design. The assembly shows an unprecedented floating ability; it can float back to the surface even after being forced to submerge under water for months. More strikingly, the assembly maintains its floating ability even after severe damage and piercing in stark contrast to conventional watercrafts and aquatic devices. The potential use of the SH floating metallic assembly ranges from floating devices and electronic equipment protection to highly floatable ships and vessels.

. (a and b) Plastic posts with various heights used in our experiments to control the gap distance for various SH Al assemblies. (c and d) Side by side comparison of Al assemblies that are made of normal Al plates and SH Al plates in air (c) and underwater (d).
S-3    S-7 Figure S8. The photos of an assembly with the initial gap distance, H 1 = 2.75 mm (a). The trapped air takes the form of a hyperboloid. The gap distance is then reduced to H 2 = 2.338 mm underwater (b), where the compressed air takes the form of a cylinder of the same diameter as the SH Al plates.
S-8 Figure S9. (a and b) A SH assembly of 0.411 g loaded by an additional weight of 1.021 g floats back to water surface even the assembly with the load is first forced into water. The radius of the assembly is 11.1 mm and the gap distance of 2.75 mm. The added load is a thicker regular Al plate with the radius 11.1 mm and thickness 1.0 mm (c and d). When the centers of the gravity of the load and the SH assembly is not perfectly aligned, the load can capsize and drop off from the assembly. To avoid this, we attached the load to the bottom. Figure S10. The loading capacity scales with R 2 of the SH assembly with a given H 1 (here H 1 = 2.75 mm). Figure S11. Schematics shows that a SH assembly with R = 13.82 m can handle a load of ~ 1000 kg, while the required area treated to be SH is only about 0.5 m 2 (2×pi×(13820 2 -13817 2 ) = 520681 mm 2 = 0.521 m 2 ), as shown by the black outermost frames. Figure S12. Schematics shows that the air volume is replaced by water after drilling holes inside the SH assembly, which highlights the perpetually floatable nature of the assembly even after severe piercing and damage.
S-10   Figure 3, for small H the trapped air between the two plates takes the form of nearly an ideal cylinder that has similar radius (R) as the SH Al disks, as shown in Figure S15. Therefore, the following equation is used to calculate the H min : (1 + 2 + 2 ) 3/2

S-13
where s 0 = s(∞) is the dimple height from the water surface at infinity, s x (s y ) is the first order derivative of s(x, y) with respect to x (y), s xx (s xy , s yy ) is the second order derivative of s(x, y) with respect to x (x and y, y). For a disk, there exists a rotational symmetry and thus, we can write s(x, y) = s(r) in the polar coordinate. Therefore, the Young-Laplace equation can be simplified as By setting ξ = g/σ and s′ (r) = tan φ with φ ∈ (0, ] ( is the CA), the above equation can be converted to the parameter space as For a given s 0 , the above differential equations can be solved numerically. The real solution is obtained when the given s 0 satisfies s 0 = s(φ = 0). When the top Al plate has the same radius as the bottom plate, the maximum gap distance that enables air trapping is = on the surface with different from and satisfying . We calculated the on the diameters and on the CAs of the superhydrophobic Al surfaces (Table S1, Figure 3J and 3K in the manuscript). Schematic images of the formed dimple shape based on Young-Laplace equation and the calculation of H max for our system in polar coordinates is shown in Figure S16.

Movie S1
A single SH Al disk sinks after a force is exerted downward exceeding the loading capacity, but the SH assembly will float back to the surface even forced into water.

Movie S2
A SH assembly floats back to the water surface once the load is released, but the normal Al disks remain sunk.

Movie S3
A comparison of a regular Al assembly (right) and a SH assembly (left). No air is trapped between the two normal Al disks, while an air bubble is clearly seen trapped between the two SH Al disks.

Movie S4
Water bouncing dynamics from a SH Al surface. A water droplet is released from a height of 80 mm (tip to surface) using a micro-syringe fitted tip and the droplet diameter is about 2.8 mm.

Movie S5
An optimized SH assembly floats on the water surface with a maximum loading of 1.021 g, ~ 2.5 times of the weight of the assembly (0.411 g).

Movie S6
A square-shaped SH assembly with only the outermost rim turned to SH shows the same floating behavior as a fully treated assembly; i.e., it floats back to the water surface immediately after a heavy load is released.

Movie S7
A severely damaged SH assembly also floats back to water surface after forced into water. The assembly is pierced six smaller holes each with a diameter of 3 mm and one large hole with a diameter of 6 mm. The holes account for ~20% of the surface area of the assembly.

Movie S8
The severely damaged SH assembly can float back to water surface very fast (~2.5 second) after forced into water as deep as 44 cm (17 inch).