This is another epic video by the famous The Action Lab YouTube channel. This time he covers a recent finding on significantly lowering the drag or friction force a projectile experiences when moving in a fluid, in this case water. This is mainly done in two ways, both giving similar results and both involve generating an air filled cavity to surround the object.
The 1st method uses the Leidenfrost effect and is the phenomenon that occurs when very hot objects make contact with water; the water boils rapidly forming a gas layer around the object and giving it a super-low friction surface. In this case, a metal ball was super heated and dropped in water. The Leidenfrost effect results in a large tear-drop torpedo air-filled cavity forming around the ball.
The 2nd method involves making the ball super hydrophobic, to repel the water. One way of doing this is by placing a super hydrophobic substance, such as lycopodium powder, on the water surface. When the ball drops through the water / powder surface, there is an air cavity formed as the powder seeks to keep the water away from the air cavity. The benefit of this method is that it can be done at room temperatures.
The air pocket dramatically lowers the drag friction, even up to 10 times of that for a similar torpedo solid shape. This is because the water drags along the air instead of the ball; much like a ball bearing lowers friction by minimizing contact with an object. This has some obvious possible applications, such as super fast and efficient submarines and other water craft. Pretty epic stuff!
The Action Lab video:
Paper and More Resources
The original paper and other resources can be viewed in the following links.
Science.org paper: https://www.science.org/doi/10.1126/sciadv.1701558
PDF: https://www.science.org/doi/epdf/10.1126/sciadv.1701558
Science.org article: https://www.science.org/content/article/watch-these-hot-balls-cut-through-water-knife-through-butter
Science.org video:
