Leaping puddles create new rules for water physics

(Press-News.org) Water droplets have a unique ability: They can leap from a surface on their own. 

This can happen for a variety of reasons, such as when a surface repels water or when heat is involved, such as a water or oil droplet skittering across a hot pan.

It also happens at a very small scale. Up to this point, researchers have observed droplets up to 3 millimeters in diameter exhibiting this behavior. When droplets are larger than that, gravity prevents it from jumping.

A new study published in Nature identifies a previously unreported way to get a puddle of water up to a centimeter wide to jump into the air, something that could support applications from surface cleaning to 3D printing. 

Members of Associate Professor Jiangtao Cheng’s lab conducted the study, including first author Wenge Huang and co-authors Mohammad Shamsodini Lori and Yuanhao Cheng. Cheng’s team collaborated with researchers at the Hong Kong University of Science and Technology (Guangzhou) and the Wuhan University of Technology.

The team discovered that a puddle of water up to one centimeter wide can jump into the air if a bubble trapped inside it bursts. This works especially well on surfaces that repel water.

Bigger jumpers The observations that drove this study came from nature. On a dewy spring morning, small droplets of water form on plant leaves. As the plants release oxygen through their leaves, air bubbles pass into those droplets and become suspended in the liquid. The bubbles rise and burst, causing the droplet to whip downward off the leaf.

Huang, a Ph.D. student, was born in the countryside of South China, where lotus pools are common. From his youth, he watched dew form on lotus leaves and noticed air bubbles trapped inside the droplets that sometimes burst. He began studying the physics behind the phenomenon more formally after publishing bubble-driven droplet actuation research in Nature Physics in 2024.

Members of Cheng’s team wondered if the bursting bubbles could move larger volumes of water, potentially breaking the 3-millimeter limit. They found that the bursting bubbles not only broke that barrier but also affected how high droplets could jump. The larger the bubble, the higher the jump. A large bubble inside a small droplet propelled it even higher.

The reason for this, according to the findings, is how the bursting energy is used. When the bubble bursts, 90 percent of the energy is directed at the base of the droplet. Most of that energy drives propulsion, allowing larger droplets to overcome gravity.

Bubbles put to work The use of jumping bubbles has potential for many applications.

Self-propelled jumping droplets have widespread applications in surface cleaning, condensation heat transfer, hydrogen production. This phenomenon can also shed light on anti-icing and anti-frosting surface designs.

Because the droplets are producing motion without the need for any fuel, that energy can be harnessed in small-scale energy harvesters. Larger droplets could raise the energy output of these devices, or produce larger harvesting devices, as the study develops.

Water also has sticky properties and can pick up particles on the surface it touches. Cheng’s own team has already studied this application for COVID-19 sensing, and more water picking up more particles could lead to an expansion of this field.

Perhaps one of the widest horizons for this science is in the realm of 3D printing. In this area, bursting bubbles can precisely deliver printed material to a micro- or nano-sized area, creating a high level of 3D printing precision. Cheng’s team included some examples of this application in the study.

“We have achieved, for the first time, the passive jumping of water puddle in the unprecedented centimeter scale on lotus-leaf-like surfaces, which has never been accomplished and reported in previous works,” Cheng said. “Through studying the synergistic interplay between bubble bursting, fluidic jetting and droplet jumping, this work reveals a previously unexplored mechanism of water wave impact in fluid-structure interactions and offers a promising strategy for droplet actuations and the directional printing of particles in additive manufacturing.”

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