Engineers at Northwestern University have identified the ideal surface ‘roughness’ required for keeping surfaces dry underwater.
The team analyzed various types of surfaces that remain dry, and determined the reason that this is possible. The texture of a surface must have an ideal “roughness” for it to stay dry when it is submerged in water for a long period of time. The team observed that the valleys in the surface roughness had to be on the nanoscale, and have a width of less than a micron. However, these nanoscale valleys can have a significant impact on the macroscopic scale.
Produced by Paul Jones
A better understanding of the manner in which the surfaces are able to effectively deflect water could enable the deflecting property to be utilized on a large scale in a wide range of industries, including pipe coatings and antifouling surfaces for shipping.
“The trick is to use rough surfaces of the right chemistry and size to promote vapor formation, which we can use to our advantage,” said Neelesh A. Patankar, a theoretical mechanical engineer who led the research.
“When the valleys are less than one micron wide, pockets of water vapor or gas accumulate in them by underwater evaporation or effervescence, just like a drop of water evaporates without having to boil it. These gas pockets deflect water, keeping the surface dry,” he said.
The research team has demonstrated and explained the nanoscale mechanics that are involved in helping a surface stay dry underwater. They conducted studies using various types of materials, some which had the required surface roughness, and some which did not. The experiment was conducted for a period of four months, during which the samples that had the nanoscale roughness stayed dry. The team placed other samples in harsh environments, where they also remained dry. In these harsh environments, the ambient liquid was devoid of dissolved gas.
“It was amazing and what we were hoping for,” said Patankar, a professor of mechanical engineering in the McCormick School of Engineering and Applied Science. “My lab likes to defy normal experience. In this work, we looked for properties that manipulate the water phase changes we know.”
Some aquatic insects, including water striders and water bugs, use the same surface roughness strategy. These insects have small hairs on the body surfaces. The spacing between these hairs is less than a micron, which enable retention of gas between the hairs.
“These gas-retaining insects have surface properties consistent with our predictions, allowing them to stay dry for a long time,” said Paul R. Jones, the study’s first author. He is a Ph.D. student in Patankar’s research group.
The nanoscopic structure of surfaces was the focus of the study. This structure at the nanoscale is similar to a carpet’s texture that is made up of valley-shaped pores that are between spike-like elevations. When the surfaces are submerged in water, water tries to stick to the top of the tiny spike-like elevations, and air and water vapor accumulate in the valley-shaped pores in between. The trapped water vapor and the air in the cavities combine to form a gaseous layer that prevents moisture from seeping into the surface.
“When we looked at the rough surfaces under the microscope, we could see clearly the vacant gaps — where the protective water vapor is,” Patankar said.
Until now, researchers had not been able to prevent water vapor from condensing within the pore, which causes the surface to get wet. However, the molecular key to preventing this process has been discovered by the team. They showed that when the valleys had a width of less than a micron, they had the ability to contain trapped vapor and air in their gaseous states. This increases the strength of the seal, and prevents wetness.
The study paper, entitled “Sustaining Dry Surfaces Under Water,” has been published in the journal 'Scientific Reports'.
Researchers from ETH Zürich, Switzerland, Massachusetts Institute of Technology, University of Illinois at Chicago, University of Denmark, and the Arizona State University also took part in this study.
The research was supported by the Initiative for Sustainability and Energy at Northwestern (ISEN).