Taking Silly Putty seriously

Sept. 1, 2005
The schematic on the right shows the silicon common plate suspended above two gold capacitor pads deposited on a Pyrex substrate (the contacts and leads to the two pads are gold-colored in the image on the left).

The capacitor gap is etched into the Pyrex. These two are then anodically bonded to form the sensor. The "teeter-totter" common plate is suspended by torsion bars above the pads with a gap of about 6 μm. A tip of virtually any solid material is attached to the teetertotter (black dot). The dimensions of the finished sensor are 1 x 1 cm.

A researcher at Sandia National Laboratories has demonstrated that a special microscope quick enough to measure changes in Silly Putty may be the most effective tool for gauging the plastic behavior of more widely used materials.

Materials made of matrices holding embedded particles are found in such widely varying products as golf club shanks, solid rocket fuel, polymers mixed into cement, and rubber bands. Over time, however, golf clubs become creaky, cement weakens, and rubber bands lose their elasticity.

Of all these, Silly Putty possesses one of the broadest ranges of plastic behavior. A ball of it left for a few minutes on a table flattens to a thin disk. Or to express the behavior more technically, the polymeric stuff creeps forever under a static load. The problem with measuring it, though, is that adhesion coupled with creep causes the conditions of any experiment to continually change.

A tool that could measure the plastic behavior of Silly Putty—the champion shape shifter—could characterize matrix deterioration of many similar substances before the material fails. In a paper accepted for publication in the Journal of Polymer Science B (Physics), Sandia researcher Jack Houston reports that the Interfacial Force Microscope (IFM) was able to achieve such consistent measurements before the Silly Putty could shift enough to invalidate the data.

The IFM is unique in being able to obtain quantitative data of a material's time-dependent mechanical properties. It's like the atomic force microscope, but that technique suffers from being mechanically unstable, says Houston. It snaps in and out of contact, like two magnets brought together, one in each hand. "It can't be done!" he says.

The IFM, on the other hand, has a tip at one end of a very small "teeter-totter," which is supported by torsion bars above two tiny capacitor pads. When a sample is brought near the tip, the force of attraction between the tip and sample causes the teeter-totter to rotate slightly. This change raises the capacitance of one pad and reduces that of the other. A feedback system places the proper voltages on the capacitor pads to drive the rotation back to zero. Forces can be measured quantitatively by the amount of voltage necessary to achieve that balance.

A probe deforms a target when it pushes on it. In return, the force against the probe tip changes with time and depends on the nature of the material. Suddenly advancing the tip into Silly Putty causes a springy deformation and a correspondingly large initial that rapidly decays as the material creeps away in a viscous flow. The voltage induced on the capacitor pads that keep the tip stable measures this response. The results can be translated into the material's frequency response. The microscope measures this stress response in a few seconds, with results that match frequency tests by a classical rheometer.

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