Why tiny water droplets zoom weirdly on some surfaces

Condensation is part of the cooling process in industrial and high-powered computer. New research investigates why droplets there act strangely.

When a thin layer of oil covers a special type of surface, condensed water droplets seemed to be randomly flying across the surface at high velocities, merging with larger droplets, in patterns not caused by gravity.

“They’re so far apart, in terms of their own, relative dimensions”—the droplets have a diameter smaller than 50 micrometers—”and yet they’re getting pulled, and moving at really high velocities,” says Patricia Weisensee, assistant professor of mechanical engineering & materials science in the McKelvey School of Engineering at Washington University in St. Louis.

“They’re all moving toward the bigger droplets at speeds of up to 1 mm per second.”

Their results appear in the journal Soft Matter.

Filmwise or dropwise condensation

In the most common type of condensation in industry, water vapor condenses to form a thick layer of liquid on a surface. This method is known as “filmwise” condensation. But another method has been shown to be more efficient at promoting condensation and the transfer of heat that comes along with it: dropwise condensation.

It has been used on traditionally hydrophobic surfaces—those that repel water such as the Teflon coating on a non-stick pan. However, these traditional non-wetting surfaces degrade rapidly when exposed to hot vapor. Instead, a few years ago, researchers discovered that infusing a rough or porous hydrophobic surface with a lubricant, such as oil, leads to faster condensation. Importantly, these lubricant-infused surfaces (LIS) led to the formation of highly mobile and smaller water droplets, which are responsible for most of the heat transfer when it comes to condensation and evaporation.

During the process, however, the movement of water droplets on the surface seemed erratic—and fast. “They move at a really high velocity for their size”—about 100 microns —”just by sitting there,” Weisensee says.

“The question is, ‘Why are they moving?’ ”

Capillary forces

Using high-speed microscopy and interferometry to watch the process play out, Weisensee and her team were able to discern what was happening and the relationships among droplet size, speed, and oil viscosity.

They created water vapor and watched as small droplets formed on the surface. “The first process is that small droplets coalesce and form bigger droplets,” Weisensee says. Capillary forces cause the oil to grow up and over the droplets, forming a meniscus—a curved layer of oil surrounding the droplet.

The oil is continuously moving around, trying to strike a balance as it covers different-sized droplets in different places on the surface—if a large droplet forms here, the meniscus stretches over it, causing the oil layer to contract somewhere else. Any smaller droplets in the area of contraction are swiftly pulled to the larger droplets, leading to oil-rich and oil-poor regions.

During the process, larger droplets are essentially clearing the space, which in turn makes room for the formation of more small droplets.

Since most of the heat transfer (about 85 percent) occurs via these small droplets, using LIS for dropwise condensation should be a more efficient way to disperse heat and get water from vapor. And since the droplets are very small, less than 100 microns in diameter, condensation can occur in a smaller area.

Making space

There’s another benefit, too. During “traditional” condensation, gravity is the force that clears water from the surface, making room for new droplets to form. The surface is placed vertically, and the water simply runs off. Since capillary forces are doing the work in dropwise condensation on liquid-infused surfaces, however, the orientation of the surface is of no consequence.

“It could potentially be used on personal devices,” where orientation is constantly changing, she says, “or in space.” And because the entire process is more efficient than traditional condensation, Weisensee says, “This might be a nice way of clearing up space without having to rely on gravity.”

Going forward, Weisensee’s team will measure heat transfer to determine if the smaller droplets during dropwise condensation on LIS are, in fact, more efficient. They also plan to investigate different surfaces in order to maximize droplet movement.

Source: Washington University in St. Louis

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