A private research university in Rhode Island is making substantial strides in the world of nanoscale engineering.

Researchers at Brown University have figured out how things stick together at the nano level, and this discovery could help inform micro and nanoscale device manufacturers.

The researchers have published their findings in Scientific Reports.  The lead author of the study, Haneesh Kesari, says there have been theories about how things stick together at the nanoscale level in the last 100 years, but nothing has explained it as well as their research does.

Source: Kesari Lab / Brown University

The engineers found that at the smallest of scales adhesive forces known as van der Waals forces exist. Kesari, who is also an assistant professor in Brown's School of Engineering, explained in a press statement: 

“At the sub-micron scales, the adhesive forces become dominant, while the force due to gravity is essentially meaningless by comparison. That is why small insects like flies and ants can scale walls and ceiling with no problem. So from a practical perspective, if we want to engineer at those scales, we need a more complete theory of how adhesive forces deform and shape material surfaces, and coupled with surface roughness affect how surfaces stick to, and slip over one another.”

The research has been a decade in the making. Kesari has spent this time bringing solids together and pulling them apart while measuring their atomic force with an atomic force microscope (AFM) apparatus. They determined their findings during practical experiments,  as is detailed in the press release:

“An AFM is a bit like a tiny record player. A cantilever with a small needle hanging from one end is dragged across a surface. By measuring how much the cantilever jiggles up and down, researchers can map out the physical features of a surface. For Kesari’s experiments, he modified the setup slightly. He replaced the needle with a tiny glass bead and used the cantilever to simply raise and lower the bead — bringing it into contact with a substrate and then pulling it back off over and over again. The substrate was of PDMS, a squishy polymer material often used in microscale engineered systems. The cantilever measured the forces that the two surfaces exerted on each other.”

In the experiment, when they brought the bead and the substrate in the closest proximity of each other without touching, the researchers found an attractive force between the two. When they eventually touched, the researchers observed that the two materials were trying to push each other away. What was surprising in this experiment, was the cantilever positioning.

Kesari says that he has not seen any literature to corroborate what he found. He showed that the amount of attractive force between the bean and the PDMS substrate was different depending on whether the cantilever was on its way up or on its way down.

Kesari is the first to uncover that it was surface roughness that played a part in the nanoscale makeup of materials that are in nanoscale proximity of each other. Through his experiments, a broader understanding of adhesion in nanoscale is possible. Explaining how this can be applied to the engineering industries at large, the press release the university put out quoted Kesari:

“For instance, he says, a full understanding of adhesion is helpful in designing micro-electro-mechanical systems - devices with micro- and nanoscale moving parts. Without properly accounting for how those tiny parts may stick and unstick, they may easily grind themselves to pieces. Another application could be using nanoscale patterning of surfaces. It might be possible to use nano-patterned surfaces to make solar panels that resist a build-up of dust, which robs them of their efficiency.”


Works Cited

Brownuniversity. “Research Details Sticky Situations at the Nanoscale.” EurekAlert!, www.eurekalert.org/pub_releases/2019-02/bu-rds020719.php.

University, Brown. “Sticky Situations Discovered in Nanoscale Engineering.” Research & Development, 11 Feb. 2019, www.rdmag.com/news/2019/02/sticky-situations-discovered-nanoscale-engineering.

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