Manufacturing Bits: Feb. 11

How things stick together
Using a metrology technique called atomic force microscopy (AFM), Brown University has gained more insights into the theory of adhesion or how things stick together.

Understanding the theory of adhesion also has some practical applications. It could pave the way towards a new class of MEMS or nanoscale devices. Nanoscale patterning is another potential application.

For years, researchers have been exploring the concept of adhesion, or how two surfaces adhere to each other. One theory to explain adhesion at a smaller scale is van der Waals forces. These forces involve a “distance-dependent interaction between atoms or molecules,” according to Wikipedia.

In simple terms, van der Waals forces explain how geckos walk effortlessly along walls. This is mainly due to electrostatic interaction, according to Wikipedia.

To gain a better understanding of how those forces work, researchers from Brown used a metrology tool called an AFM. An AFM uses a tiny probe or needle to conduct 3D measurements in structures. In operation, the AFM traces the surface and it records the “X” and “Y” coordinates. Then, it goes up and down, which enables the “Z” coordinates.

Instead of a needle, researchers from Brown used a tiny glass bead. The bead was brought in contact with a substrate based on a polymer material called polydimethylsiloxane (PDMS). The AFM measured the forces.

Here’s what surprised researchers: “The amount of attractive force between the bead and PDMS substrate was different depending on whether the cantilever was on its way up or on its way down,” according to researchers from Brown. The research also “show that miniscule differences in the roughness of a surface can cause surprising changes in the way two surfaces adhere to each other. Certain levels of roughness, the studies show, can cause the surfaces to exert different amounts of force on each other depending upon whether they’re being pushed together or pulled apart.”

“At the sub-micron scales, the adhesive forces become dominant, while the force due to gravity is essentially meaningless by comparison,” said Haneesh Kesari, an assistant professor in Brown’s School of Engineering. “That is why small insects like flies and ants can scale walls and ceilings 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.”

Weilin Deng, a Ph.D. student at Brown, added: “People have worked on adhesion for over 100 years, but none of the existing theories captured this. Over the course of this work, we showed with experiments that this really exists and now we have a theoretical framework that captures it.”

Cryo-force spectroscopy
Using a metrology technique called cryo-force spectroscopy, the University of Basel has developed a new way to examine the properties of complex DNA structures.

The technology is based on AFM. In this case, though, the AFM is subjected to cryogenic temperatures.

DNA is a molecule that carries the genetic instructions in living things. A complete set of DNA is called the genome. For some time, the industry has been working on more complex structures called DNA origami. For this, the idea is to fold DNA strands, creating 3D structures. DNA origami can be used to develop drug delivery systems and sensors.

For DNA origami, researchers need to understand the elasticity and binding properties of these structures. So, researchers from Basel have used cryo-force microscopy to characterize DNA molecules.

Cryo-force microscopy is based on AFM. The difference is that the AFM process takes place in cryogenic conditions (5 K), according to researchers. It is somewhat related to cyro-electron microscopy (cryo-EM). Cyro-EM is often used in structural biology. In one application, a cryo-EM is used to freeze biomolecules mid-movement. Then, the structure is imaged at atomic resolutions. The system allows researchers to produce films that reveal how molecules interact with each other.

Cryo-force spectroscopy is also promising. Researchers from the University of Basel have demonstrated the ability to characterize a single-strand DNA, 20-cytosine oligomer down to the sub-nm level. “As with cryogenic electron microscopy, we take a snapshot with cryo-force spectroscopy, which gives us an insight into the properties of DNA,” said Ernst Meyer, a professor from the Swiss Nanoscience Institute and the University of Basel’s Department of Physics. “In future, we could also make use of scanning probe microscope images to determine nucleotide sequences.”