At the scale of bridges or buildings, the most important force that engineered structures need to deal with is gravity. But at the scale of microelectro-mechanical systems (MEMS) — devices like the tiny accelerometers used in smartphones — the relative importance of gravity decreases and adhesive forces become more important.
A new method of measuring the stickiness of microscale surfaces was devised. The technique could be useful in designing and building MEMS by measuring a quantity known as “work of adhesion” that roughly translates into the amount of energy required to separate a unit area of two adhered surfaces.
The key development is that thermal vibrations of a microbeam can be used to calculate work of adhesion. That insight suggests a method in which a slightly modified atomic force microscopy (AFM) system can be used to probe adhesive properties.
Standard AFM works a bit like a record player. A cantilever with a sharp needle moves across a target material. A laser shown on the cantilever measures the tiny undulations it makes as it moves along the material’s contours. Those undulations can then be used to map out the material’s surface properties. Adapting the method to measure adhesion would require simply removing the metal tip from the cantilever, leaving a flat microbeam. That beam can then be lowered onto a target material, where it will adhere. When the cantilever is raised slightly, some portion of the beam will become unstuck, while the rest remains stuck.
The unstuck portion of the beam will vibrate slightly. Researchers found a way to use the extent of that vibration, which can be measured by an AFM laser, to calculate the length of the unstuck portion, which can in turn be used to calculate the target material’s work of adhesion.
The technique could be useful in assessing new material coatings or surface textures aimed at alleviating the failure of MEMS devices through sticking. The technique for measuring the material’s work of adhesion provides a systematic way of evaluating the methods to get the level of adhesion needed for a particular application. The method does not require changing a standard AFM setup. Previous methods based on interferometry are labor-intensive and may require many data points to be taken.