In 1752, a French mathematician called Jean le Rond d’Alembert proposed a paradox. He said that an ideal object with a free-slip surface would have a drag of zero when moving through a liquid. Proving it has been a bit more difficult, but a new study is close to an answer.
Led by the King Abdullah University of Science and Technology in Saudi Arabia, scientists have found a way to create a bubble around an object that dramatically reduces the drag as it falls through water. The findings are published in Science Advances.
The study involved dropping two 2-centimeter-wide balls into water. One was heated to a very high temperature, about 400°C (750°F), while the water was heated to 95°C (200°F). The second ball was coated in a superhydrophobic coating, one that repels water, and dropped into water at room temperature.
In both experiments, the researchers found that the drag on the balls could be reduced by up to 90 percent using these methods. Although it’s not completely drag free, it’s a major step in that direction.
“In the study of fluid dynamics, we’ve always used a hypothetical sphere because we haven’t been able to create an object with a free-slip surface – until now,” said Dr Evert Klaseboer of the Institute of High Performance Computing, one of the study’s co-authors, in a statement.
“It could become a textbook example of some fundamental hydrodynamic theories.”
In the first experiment using temperature, the ball immediately boiled the water as it entered the liquid. This created a stable layer of water vapor around the ball, encasing it in gas and reducing the drag.
The second experiment is perhaps more appealing, as it worked at room temperature. Using a spray-on water repellent commonly used for side mirrors on cars, it was able to maintain a similar layer of gas between the ball and the water, again dramatically reducing the drag.
Similar technologies have been employed before in things like supercavitating torpedoes, which use a bubble of steam to reduce skin friction drag and allow the torpedo to reach speeds of more than 400 kilometers per hour (250 mile per hour).
However, these can only reduce drag by 10 to 20 percent. A 90 percent reduction, as achieved in this study, is a significant improvement. Time will tell what practical applications this has, but we’re sure d’Alembert would be impressed.