The Two-Plate Predecessor: Guo Group's 2019 Floatable Superhydrophobic Aluminium Assembly (Zhan et al.)

Press coverage of the later floating metal tubes says that Chunlei Guo's laboratory "first demonstrated superhydrophobic floating devices in 2019" with two treated disks. This is the paper behind that line: "Highly Floatable Superhydrophobic Metallic Assembly for Aquatic Applications" by Zhibing Zhan, Mohamed ElKabbash, JinLuo Cheng, Jihua Zhang, Subhash Singh and Chunlei Guo, received 28 August 2019, accepted 6 November 2019 and issued 26 December 2019 under an open-access CC-BY licence. The paper gives methods, a buoyancy model and measured numbers, which the press coverage of the tubes does not.

What problem the paper says it addresses

The authors argue that water-repellent surfaces have two weaknesses that limit practical use. Fully submerged, such a surface eventually wets and loses the air layer that makes it repellent. And the fine texture that produces the effect wears away under abrasion. A single treated metal disk does float, the paper notes, but if it is pushed down past its loading capacity it sinks and may not float again even when brought back up.

Their answer takes its inspiration from the diving bell spider (Argyroneta aquatica), which carries air underwater on its body and stores it in a dome-shaped web among aquatic plants, and from fire ants, which link into rafts that trap air among their bodies. The idea taken from both is that several water-repellent faces arranged around a space can hold a large volume of air.

The design

The assembly is two parallel aluminium plates joined by a short plastic post (cut from tubing and sanded to height, glued with epoxy). Only the two facing inner surfaces are made superhydrophobic; the outer faces are left as plain aluminium. According to the authors, this protects the fragile texture: the plain outer faces take whatever contact there is, while the treated faces stay in the interior, away from abrasion. When the assembly is pushed under, a bubble stays trapped between the plates and lifts it back to the surface once released. An identical pair of untreated disks traps no air and stays on the bottom.

The post is a ring cut from plastic tube with an outside diameter of about 7.0 mm and a wall 0.9 mm thick; the circular plates in the experiments had radii from 3.2 to 11.1 mm. The paper describes the treated faces as enclosed but does not say whether the rim between the plates is sealed.

How the surfaces were made

The plates (0.2 mm and 1.0 mm aluminium, 99.9% purity) were scanned with an amplified Ti:sapphire laser producing 65 fs pulses at 800 nm and up to 1 kHz, at about 0.8 mJ per pulse (0.4 and 0.2 mJ were also used to tune the result), a stage speed of 1 mm/s and 100 μm between scan lines. The laser leaves microgrooves covered in nanostructures and turns the metal pitch black. Soaking in a 0.01 M stearic acid solution in aqueous ethanol then lowers the surface energy. The paper reports a water contact angle of about 54° for bare aluminium and about 168° after both treatments, above the conventional 150° threshold for superhydrophobicity. This is the two-scale texturing described in Hierarchical Micro/Nanoscale Surface Texturing.

What was measured

Gap limits. The spacing between the plates, H, decides whether the device works. The authors calculate a minimum gap of about 0.68 mm for 0.2 mm plates, the point where trapped air just balances the weight, against an observed 0.761 mm; they attribute the difference to the post and glue. At the other end, too wide a gap cannot hold air at all. In one series, air was trapped at 1, 2 and 3 mm but not at 4 mm. Modelling the water surface with the Young–Laplace equation, they find that this maximum gap rises with plate radius before levelling off, and is larger for more water-repellent surfaces (samples at roughly 153°, 160° and 168°).

Load. Some air escapes as the assembly goes under, and more is lost at wider gaps, so a wider gap does not simply mean more lift. For plates of radius 11.1 mm the best buoyancy came at a gap of about 2.75 mm. An assembly built to those dimensions returned to the surface carrying 2.5 times its own weight. The measured maximum load, 1.021 g, sits close to the calculated buoyancy of 0.01043 N (equivalent to 1.064 g). A square version treated only along a frame about 3 mm wide also trapped air and refloated.

Duration. A loaded assembly left in water for 2 months stayed afloat throughout, with no deterioration observed.

Damage. Drilling six 3 mm holes and one 6 mm hole, which the paper says removed about 20% of the structure, did not stop the assembly floating. The damaged device rose back from a 44 cm deep container in about 2.5 s. The authors' explanation is that water simply replaces air in the pierced volume while the remaining treated faces keep holding their share, so water does not flood the whole device as it would through a breached hull.

What the evidence does and does not show

The measurements are small-scale bench tests on centimetre-sized parts carrying about a gram. The paper's figure of a 1-ton load for an assembly of radius 13.82 m with about 0.5 m² of treated area is a scaling calculation, not a test. The applications listed, from personal flotation devices and protection for electronic equipment to boats, ships and large vessels, are the authors' projections.

Several limits are stated in the paper itself. The gap experiments were done about 1 cm under water in a beaker about 5 cm deep, and the authors explicitly set aside the effect of deep-water pressure as a question for later work. Air loss during submersion made trapped volume hard to predict from the gap alone. For the load test, an extra 1.0 mm aluminium plate was fixed to the bottom of the assembly to keep it from overturning as it rose. The 2-month trial reports no test conditions beyond the loaded device staying in water, and no independent replication is reported.

Connection to the tube work

This paper fills in what the press item in Superhydrophobic Metallic Tubes: Chunlei Guo's Floating Aluminum Tubes — A source Dossier summarises in one sentence. The later tube design is presented as fixing the disks' weakness at extreme angles. The 2019 paper does not test tilting. For the mechanism in general see Air-Trapping Buoyancy (Plastron-Based Flotation) and Superhydrophobicity; for the lab see Chunlei Guo and University of Rochester.

References

• Zhan Z., ElKabbash M., Cheng J., Zhang J., Singh S., Guo C., "Highly Floatable Superhydrophobic Metallic Assembly for Aquatic Applications," DOI 10.1021/acsami.9b15540; open-access text at https://pmc.ncbi.nlm.nih.gov/articles/PMC6938389/.
• Funding acknowledged in the paper: Bill & Melinda Gates Foundation (OPP1157723), US Army Research Office (W911NF-15-1-0319), National Science Foundation (1701163 and 1722169). The authors declare no competing financial interest.