# Solve a physics debate among friends?September 5, 2013 5:08 AM   Subscribe

My friends and I are having a nearly friendly debate about the effects of superhydrophobic coatings on various items, and we've decided to look here for more input.

I'm not affiliated with them in any way, haven't bought the product, but here is a video of one of these products. It was viral several months ago. Just for fun, one of us is a social worker, one is an electrical engineer, and one is a chemical engineer.

Basically, our debate asks these questions:

1. What would happen to an extremely buoyant item with this coating, assuming that friction wouldn't just peel it off under load. Like, say a boat bottom, or a propeller. Improve speed and performance, decrease it, or no change?

2. What would happen to a perfectly neutrally buoyant item? Would it float HIGHER , would it sink, or would it remain neutral?

3. What would happen it applied to a swimmer? Would he have an easier or harder time in the water?
posted by TomMelee to science & nature (12 answers total) 2 users marked this as a favorite

Regarding point 2: Things float or sink because of their mass as compared to the mass of water they displace. So, in a spherical-cow sort of sense, adding the coating would cause the object to have a higher volume, and assuming that the coating's density is less than the density of the object, it would be slightly more buoyant, but not because of the hydrophobic nature of the coating.

In practice, the difference would be very, very very tiny.
posted by odinsdream at 5:20 AM on September 5

2. Higher because it would bring down a layer of air with it thereby increasing the volume slightly without appreciably increasing the mass.
posted by koolkat at 5:20 AM on September 5

leaving aside propulsion, it will help anything trying to move through the water - see super cavitation.
posted by russm at 5:24 AM on September 5

I think it would be great for a swimmer so long as they weren't entirely covered. (probably wouldn't want to cover: hands, underside of lower arms, and below the knees.
posted by ropeladder at 5:36 AM on September 5

1 & 3 are the same issue and it is being researched for use on ships to improve efficiency.

For 2 I'd imagine negligible change, as it is gravity that is involved, so except for minor changes in volume and density due to the coating, I wouldn't expect to see any real difference.
posted by DamPots at 7:17 AM on September 5

Most others have addressed the issue of buyoancy correctly: the only change is that of the added mass and volume of the coating (there would not be a layer of air "brought down" as one poster mentioned).

For objects moving through water, you would eliminate the boundary layer (water partly "sticking" to the moving object) being dragged and resulting in turbulence (eddies) which is a form of friction. Thus, it should help the object move more easily in water.
posted by aroberge at 7:34 AM on September 5 [1 favorite]

Depending on the surface treated there would indeed be a layer of air brought down using the surface tension of water. Small nooks and crannies would trap air because they wouldn't be able to be wetted. It is why the magic sand (essentially sand treated with a similar coating) appears to have a silvery coating when placed in water also the same as what happens to a finger treated in a similar manner. The water is unable to wet the surface and therefore some air (a very small amount, but because air is so not dense I think it would make a difference) will be trapped between the surface and the water. What else is going to be there a vacuum?
posted by koolkat at 7:41 AM on September 5 [1 favorite]

Others have talked about propulsion and drag. This is about how wetability/hydrophobicity affects flotation.

The size and mass of the item matters here. Anti-wetting coatings usually work by taking advantage of the high surface tension of water. Water has one of the toughest liquid-air barriers in nature. Water-striders, for example, take advantage of this to walk on water. They can't walk on a lower surface tension liquid like an oil.

Flotation behaviours of small items often depend on how it interacts with the water surface tension, and the objects density. Critically, flotation can depend on the surface of the item remaining unwetted, provided that the force of gravity on the object is similar (or less than) the water surface forces. Pollen has a hydrophobic coating, for example, a layer of microscopic spikes spaced closely enough to stand on water, like the water strider. Pollen floats if those spiky bits can keep the water surface away from the bulk of the core. Thoroughly mixed, and with the addition of soap to lower the water surface tension, the hydrophobic surface of the pollen will wet and the pollen will disperse into the water column as nearly neutrally buoyant particles. Break the surface tension and the question of flotation becomes just about relative buoyancy.

For heavier items, larger items, roughly more than a gram (~1/30th of an oz) for most organic material, the effect of surface tension is less important. Size and mass are important. The relative density becomes the sole factor which matters for flotation.

Submerged in water, density, size and shape are the main influence on buoyancy, rather than surface tension. The speed of rising or settling depends mostly on shape, less on hydrophobicity or surface effects. "Shape drag" means particles quickly find terminal sinking or rising speeds, just as objects falling in air reach a terminal velocity---the fluid dynamics are very similar.
posted by bonehead at 7:42 AM on September 5 [1 favorite]

Koolkat, those are are really nice examples of hydrophobic/surface wettability interactions. I may have to steal some of those.
posted by bonehead at 7:52 AM on September 5

What else is going to be there a vacuum?

It's not like water can't touch a hydrophobic material, only that it doesn't want to. I don't think you can argue, in general, that there will always be a layer of air around a submerged hydrophobic object. It really depends on the object in question and what external forces are involved. For instance, the bottom of a hydrophobic boat would not preserve a layer of air underneath it once it started cruising around in the ocean.
posted by grog at 8:13 AM on September 5

Hydrophobic coatings (mostly) work because the air-water surface has a high radius of curvature. What does that mean?

Imagine a heavy rubber sheet covering two objects, a ball and a star. Both are going to look pretty similar under an inflexible sheet, mostly just bumps roughly the size of the objects. Replace that heavy sheet with a light cotton one, and allow it to drape as it wants. This time, the smooth curves of the ball and the pointiness of the star can be seen under the sheet.

Water surfaces are more like heavy rubber than cotton. That's why pollen grains, water striders and hydrophobic sand do what they do: the angles created by the spikes and rough surfaces are small enough to be inside the maximum radius of curvature of the water surface. Just like the star shape under the rubber sheet, this traps a lot of air at the surface. You can see this in the great images koolkat provides above.

Trapped air makes things more buoyant. This effect is small, small enough to mostly affect really small objects, but is still measurable even for larger ones.
posted by bonehead at 8:25 AM on September 5

For instance, the bottom of a hydrophobic boat would not preserve a layer of air underneath it once it started cruising around in the ocean.

I was thinking more about two identical objects, one treated and the other untreated, being dunked into water. I bet though that there would still be a layer of air underneath the boat after it was done cruising around. The force required to remove the water would be high and if they ever reversed the engines the air would be replaced by the bubbles that are made from engines anyways.

I think I need to find a pool and some of this spray and do some experiments waving my treated hand underwater, except I would need an underwater camera also.
posted by koolkat at 9:40 AM on September 5 [1 favorite]