— The image on the left shows the glass ball’s “plop” as it enters the water. The ball on the right is coated with a hydrophobic layer that causes it to repel water, creating a bigger splash (Image: Lydéric Bocquet)
When a spherical object such as a ball is dropped onto water, it sometimes pierces the surface with a gentle plop, and sometimes slams through creating a big splash.
Lydéric Bocquet at the Claude-Bernard University in Lyon, France, and colleagues found these varied impacts a puzzling phenomenon. Why would two spheres of the same size, shape and material create such different effects?
Now Bocquet believes his team has found the answer: the molecular treatment of the surface of the spheres whether it attracts or repels water is paramount, he says.
The team tried balls of various materials, including aluminium, steel and glass, before choosing the sphere with the most hydrophilic (water attracting) surface. This turned out to be a glass ball, cleaned thoroughly with hydrogen peroxide, sulphuric acid and alcohol. Watch the ball plop gently though the water (requires Quicktime, 1.5 MB).
Making a splash
The researchers then coated that same ball with a nano-layer of a hydrophobic (water repelling) material, called silane. The layer was just one molecule thick. This time, watch the glass ball create a huge splash as it strikes the water (2.4 MB).
The reason for the difference is that during impact, the hydrophobic molecules create an air cavity where the water molecules retreat, which leads to the splash, Bocquet explains. In contrast, liquid reaches up the sides of the hydrophilic glass surface, so no air can enter, and no splash occurs.
The finding could prove useful in reducing splashing that occurs during high-speed water impacts, for example during air-to-water torpedo entries, he says. Here, a nano-layer could prevent air bubbles forming at the tip of the torpedo.
Journal reference: Nature Physics (DOI: 10.1038/nphys545)