Floating beads of water act as tiny test tubes

Droplets of water dancing over a hot skillet may be the chemical factories of the future.

On a hot surface, water droplets hover on a cushion of their own evaporation. The drops become superheated and negatively charged, which creates ideal conditions for making nanoproducts, researchers report October 29 in Nature Communications. The chemists exploited these drops to make products such as porous metal, heat-resistant foam and metal coatings.

Because the reaction requires only water, instead of hazardous or waste-producing solvents, the study authors say the technique offers a cheap, environmentally friendly way to manufacture nanoscale materials.

“The experiment itself is very simple,” says Mady Elbahri, a nanochemist at the University of Kiel in Germany. “All you need is a hot plate.”

Elbahri got the idea to use the droplets in 2006 while making pancakes at home. After washing his hands, he accidentally shook some water onto a hot plate and noticed the levitating water droplets that resulted.

The phenomenon is known as the Leidenfrost effect, named for the chemist who first described it in 1756. Intrigued, Elbahri and colleagues used a heat-sensing camera to find that on a surface heated to 250° Celsius, the water droplets’ bottoms become overheated and undergo evaporation while the tops stay cooler.

The fast bottom-side evaporation causes the water to ionize, with negatively charged hydroxyl ions staying in the water while positively charged ions predominate in the vapor. The negative ions create a basic pH in the droplet, which Elbahri knew would be ideal for nanochemistry.

To test droplets as tiny reactors, Elbahri and colleagues combined water and an acidic solution of salt containing gold atoms. When they dripped the liquid onto a hot surface, the basic environment in the drop converted the salt to gold nanoparticles that formed metal clusters that colored the solution red.

 

Using other salts, the team found that the hot drops could also form other materials: nanoscale coatings around tiny objects; porous metals that, because of their absorption of light, could find use in solar energy; and a hybrid metal-polymer foam that could be useful on spacecraft for its light weight and heat resistance.

Elbahri admits there are still challenges ahead, such as scaling up the reaction to make larger quantities of nanoproducts.

Bryce Tappan, a nanochemist at Los Alamos National Laboratory in New Mexico, says the method could allow nonspecialists to make a wide variety of nanomaterials. In all, Tappan calls the technique a “triumph of clever over complicated.”

Editor’s Note: This story was updated on November 5, 2013, to clarify the identities of the products the researchers made in droplets and their possible applications.

 

WOBBLING WATER  On a hot metal plate, a ball of water containing gold salt (yellow droplet) floats and sways on its own positively charged vapor. The droplet gradually becomes negatively charged and the salt converts to gold nanoparticles that clump into a red-hued metal (red droplet). Credit: Elbahri et al