Boiling: Where are the bubbles?

It’s common kitchen wisdom that when water reaches the boiling point, it bubbles. But a group of scientists has recently found a way to bypass this process by manipulating texture and chemistry. Their findings, published in the journal Nature, could one day enhance kitchens, boost drag-reduction in ships, or improve the flow in microfluidic devices.

Neelesh Patankar, a professor of mechanical engineering at Northwestern University, collaborated on the research with Ivan Vakarelski of King Abdullah University of Science and Technology in Saudi Arabia and Derek Chan of University of Melbourne in Australia.

This movie shows the cooling of 20 mm hydrophilic (left) and superhydrophobic (right) steel spheres in 100 C water. The spheres’ initial temperature is about 380 C. The bubbling phase of boiling is completely eliminated for steel spheres with superhydrophobic coating.
Credit: Ivan U. Vakarelski of King Abdullah University of Science and Technology, Saudi Arabia

To boil all this down, look no further than your kitchen:

A cook trying to find out if a pan is hot will put a drop of water on the surface. If it’s hot enough (much hotter than the boiling point of water), the drop “kind of skitters around,” Patankar says. “That is because the pan is so hot that the liquid evaporates and forms a vapor film on which the drop is levitated.” This is called the Leidenfrost effect, named after Johann Leidenfrost, the 18th-century German doctor who observed it.

But when the temperature drops to 100 degrees Celsius, water’s boiling point, the insulating vapor film collapses and bubbles rise out.

Patankar and his colleagues found a way to eliminate this explosive bubbling phase (called nucleate boiling) by changing the texture of a surface and making it extremely water repellant. They took two-centimeter steel spheres that were sprayed with a superhydrophobic coating and heated them to 400 degrees Celsius. Then they dropped them into room-temperature water.

The special coating created tiny “peaks and valleys,” about 1/100th the width of a strand of hair, on the balls’ surface. As the surface cools down to the boiling point, the liquid will “not want” to touch it, Patankar explains. “The liquid will land on the surface like a person lying on a bed of nails.” The liquid and solid had little contact, the vapor film remained intact, and no bubbling occurred.

In comparison, the scientists heated smooth, hydrophilic (water-loving) spheres to 750 C. and put them in water. At the boiling point, the vapor film collapsed and the water began to furiously bubble.

Patankar envisions many uses for the “water-hating” or “water-loving” surface textures they’ve studied, from better heat-transfer equipment in the kitchen to techniques that may one day reduce biofouling in the oceans. “We’re showing the concept that you can control phase by using chemistry and texture, and it can really impact many areas.”

But Patankar is most excited about the potential for dew harvesting, using an artificial turf with a water-loving coating that “would really suck out moisture from the air.” In his native India, where cricket is a popular sport, “nobody likes to pitch in the evenings, because … the dew starts forming,” he says. Even though there is little rainfall during much of the year, so much dew forms on the grass that the ball has to be dried with a towel “so it doesn’t slip out of the pitcher’s hand.

“If you can engineer it,” he says, “a village of 300 can be supplied with water from dewfall.”