“This comprehensive theory helps to verify that what we were seeing in our experiments was real. It's also now something that can be used in nanoscale engineering.”Haneesh Kesari
At very small scales, adhesive forces are dominant. In a finding that could be useful in nanoscale engineering, new research shows how minute amounts of surface roughness can influence stickiness. (Image courtesy of Kesari Lab/Brown University)Scientists from Brown University in the US have made a breakthrough that helps our understanding of how surfaces adhere at the nanoscale, a discovery that could bring innovation to engineering at the nanoscale and be useful in the manufacture of new micro- and nanoscale devices.
As reported in Scientific Reports [Deng, W., Kesari, H., Sci Rep. (2019) DOI: 10.1038/s41598-018-38212-z], researchers Weilin Deng and Haneesh Kesari demonstrated how tiny differences in the roughness of a surface can result in surprising changes in how two surfaces stick to each other, improving our knowledge of the adhesive van der Waals forces that are key at this level. They developed a theoretical framework on how particular levels of roughness can cause surfaces to exert different amounts of force on each other that is dependent on whether they're being pulled apart or pushed together.
As Kesari points out, “At the sub-micron scales, the adhesive forces become dominant, while the force due to gravity is essentially meaningless by comparison”. To engineer at such scales they needed a better theory of how adhesive forces deform and shape material surfaces, and combine with surface roughness to affect how surfaces stick to, and slip over, each another. He had been using atomic force microscopy to examine the physical features of a surface at the micro-scale, with a cantilever and a small needle suspended from one end that is pulled across a surface, with how much it moves being measurable.
This set-up was modified by replacing the needle with a small glass bead that came into contact with a substrate made of a polymer called PDMS. The cantilever measured the forces that the two surfaces exerted on each other – as the bead and the PDMS came close together or just touching, there was an attractive force between the two. As they came fully into contact and the cantilever continued to push down, the force reversed, with the two solids attempting to push each other away. On the cantilever being again raised and the solids moved apart, the attractive force returned until the gap was large enough for the force to disappear entirely.
The amount of attractive force between the bead and the substrate was dependent on whether the cantilever was on its way up or on its way down, which was surprising as you have the same separation distance but the forces are different when you're loading or unloading. The effect was found to be due to surface roughness, and a mathematical model was devised on how this might affect adhesion, predicting the amount of work needed to separate two surfaces increases steadily as roughness increases to a certain point. After that, the toughness drops off quickly. Such an understanding of adhesion is could help in the design of devices with micro- and nanoscale moving parts, as well as in the nanoscale patterning of surfaces.