Engineers Design Micropatterned Surface To Control Droplet Flow

With the right pattern of microsized structures, a surface can compel water droplets to move on their own in a desired direction. Engineers want to create such surfaces to make condensers for applications such as collecting water vapor from the air to monitor pollutants or condensing a person’s breath to detect biomarkers of disease. Now, a team of researchers has designed a water vapor condenser that is relatively easy to build and could lead to long-lasting portable analyzers (Langmuir 2014, DOI: 10.1021/la5004462).

When engineers design micropatterned surfaces for condensers, they have to find ways to overcome forces, such as friction, that tend to make water droplets stay in place. Previously reported surfaces have required complex production processes such as growing nanotubes oriented in specific directions or creating alternating hydrophobic and hydrophilic regions. Also, many of those surfaces didn’t last long because they got fouled by contaminants in the water.

Cristina E. Davis, a mechanical engineer at the University of California, Davis, and colleagues overcame these limitations by designing a condenser with a pattern of microscale grooves and ridges arranged in concentric circles on a 20-mm-diameter silicon wafer. The grooves get narrower as they near the center, starting at a width of about 110 µm and ending at about 4 µm. The team left an 8-mm-diameter unpatterned collection region in the center.

To make these patterns in the silicon, the researchers used photolithography and deep reactive-ion etching. They then coated the wafer with a fluorinated polybutadiene film to make the surface hydrophobic. Because of the silicon’s micropatterns, the coating was distributed unevenly, making the condenser more hydrophobic at the outer edge and less so closer to the center. This created a so-called wettability gradient.

Davis says this gradient combines with capillary action from the decreasing size of the grooves to urge the water droplets inward. Small droplets condense on the wafer, and as more accumulate, they merge to form large drops. Once the drops are large enough, they cover an area with a significant wettability gradient. The edge of the droplets nearest the condenser’s center winds up with more potential energy than the edge farthest away. This produces a force that moves the droplets toward the center, while surface tension holds the drop together. Gravity doesn’t appear to play a role in the motion. The water still marches to the center even when the device is flipped upside down, Davis says.

One advantage of this design is that it doesn’t require complicated production, she says. The photolithography methods are the same as those used to make computer chips, making manufacturing straightforward. And though the team didn’t test the condenser’s lifetime, the researchers used the same one for several months without a drop in performance, Davis says.

Manoj K. Chaudhury, a chemical engineer at Lehigh University, thinks the researchers have found a nice method of using micropatterning to create a wettability gradient on a surface. “This method stands a very good chance in practical applications because of its durability,” he says.