In the field of engineering, there are several forces that scientists have to deal with when building structures or creating devices. One such force, adhesion— the attraction between two surfaces that can lead them to stick together — is particularly important on the microscale level. Because surfaces can be extremely small, engineers are often faced with the issue of these surfaces unintentionally adhering to one another. To try to alleviate this adhesion problem, University researchers conducted experiments to measure the work of adhesion, which is essentially the amount of energy required to separate a unit area of two surfaces stuck together.
When an object is next to a surface, forces act between this object and the surface, said Joyce Mok GS, who is studying solid mechanics in the School of Engineering. But if this object is large enough, gravity overpowers these other forces. “When you get to the point where (the object) is so small, however, this force of gravity is not going to matter as much… and (the other forces) do start to matter. And so that same principle is the reason why adhesion … is such a big issue for (microelectromechanical devices).”
Microelectromechanical systems are essentially systems consisting of microscaled, moving devices, Mok said. “You can use it in a lot of different fields,” she added, listing biology research and electronics as examples. Because many of these devices are so small, they require surfaces that are close together for efficient operation. “But then naturally, of course, this makes the problem (of adhesion) bigger,” she added.
The most common method that has been used to measure the work of adhesion involves the use of a microbeam, an extremely narrow beam of radiation, and its tendency to stick to a surface when placed near it. “Previous research related the work of adhesion for an adhered microbeam to the beam’s unadhered length, and as such, interferometric techniques were developed to measure that length,” wrote Mok and Wenqiang Fang GS, who is also studying solid mechanics, in a jointly-written article.
But directly finding that critical length at which the beam sticks to the surface is not easy, Mok said. What could be easier is using the natural frequency property of the beam, which would help determine this critical length and ultimately relate to the work of adhesion.
“Every structure has a frequency that it vibrates at when it’s resting,” Mok said. Finding that frequency can be done by shining a laser at one point on the structure with something like an atomic force microscope, she added.
In a standard AFM system, a microbeam with a sharp needle tip moves across a surface, and a laser within that beam measures its small movements. The beam’s movements across the surface could then be used to determine the surface’s properties.
Mok and Fang modified this system by removing the needle tip of the beam. Instead, the microbeam is now lowered onto the test surface and then raised back up to result in part of it becoming stuck to the surface. The remaining unstuck portion slightly vibrates. By measuring the vibration, scientists are able to find the work of adhesion more easily.
“The main advantage of our technique is that it is simpler to implement than the old methods,” Fang said. With older techniques based in interferometry, a large number of different samples are needed to gather many data points, which the new technique does not require. Their vibration-based technique is also “known to have high sensitivity and repeatability and is easy to use on a MEMS chip,” he added.
Determining a material’s work of adhesion has large implications in the field of MEMS, Fang said. “This new capability … will help alleviate the stiction failure of MEMS devices, (ultimately aiding) in the development of the MEMS field.” Development of more reliable microdevices, such as accelerometers and gyroscopes, “will improve our lives in many different ways,” he added.
