Sequential stretching, squeezing, and twisting is the usual way to characterize the physical properties of soft materials. But, taking inspiration from bat and dolphin echolocation, engineers at Massachusetts Institute of Technology have developed a much quicker approach [Keshavarz, B. et al., Phys. Rev. X (2018) in press; arXiv:1804.03061v1] The same technique could be used to characterize everything from viscous bodily fluids, such as saliva to solidifying cement.
The new technique improves and extends the deformation signal that is captured by a rheometer. Typically, a rheometer stretches or squeezes a sample back and forth. Researchers have previously investigated how they might improve testing by changing the instrument's input signal and compressing the frequency profile. Chirping, a shorter, faster, and more complex frequency profile is now commonly used to allow tests to be carried out in 10 to 20 seconds rather than close to an hour. However, the data from normal chirps, contains artifacts known as ringing effects, which reduce accuracy. The MIT team hoped to damp these ringing effects without extending the timeframe of the tests. They explain that bats and dolphins send out a similar chirp signal that encapsulates a range of frequencies, allowing them to locate prey quickly.
The team analyzed their chirp signals and optimized these profiles in computer simulations, then applied certain chirp profiles to their rheometer in the laboratory. They found the signal that reduced the ringing effect most was a frequency profile that was still as short as the conventional chirp signal - about 14 seconds long - but that ramped up gradually, with a smoother transition between the varying frequencies, compared with the original chirp profiles that other researchers have been using where they hit full speed too quickly leading to the artifacts.
The team refers to this new test signal as an "Optimally Windowed Chirp." The frequency profile resembles a smooth, rounded window rather than a sharp, rectangular ramp-up and ramp-down. Essentially, the new technique commands the rheometer's motor to stretch and squeeze a material in a more gradual, smooth manner.
They have demonstrated proof of principle testing several viscoelastic liquids and gels, such as a laboratory standard polymer solution which they characterized using the traditional, slower method, the conventional chirp profile, and their new profile. The measurements from the new profile matched almost exactly those obtained with the conventional, long-winded method. Moreover, they were one hundred times more accurate than those available using a conventional chirping method.
"A lot of materials in nature and industry, in consumer products and in our bodies, change over quite fast timescales," MIT post-doctoral researcher Bavand Keshavarz explains. "Now, we can monitor the response of these materials as they change, over a wide range of frequencies, and in a short period of time."
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