Animal chatter speeds up material testing

Materials World magazine
6 Dec 2018

Inspired by the way dolphins and bats use sonar to explore their environment, with a high degree of accuracy, scientists applied this theory to materials testing.

Soft material properties can vary widely, with one single sample capable of demonstrating high tensile and compressive strength for stretch and brittle snapability, and varying levels of viscosity. Cataloguing the whole range of material properties takes a great deal of time, as tests tend to be carried out sequentially. But sonar could fasttrack this.

Scientists at MIT, USA, found that their method of characterising soft materials using electrical signals improved accuracy, was faster, and enabled them to track the material over time, recording as it changed.

Dr Bavand Keshavarz at MIT’s Department of Mechanical Engineering said this technique could help many industries and they won’t have to change their established instruments to get 'a much better and accurate analysis of their processes and materials’.

Ringing true

During testing, instruments continuously squeeze, stretch, twist, press or stir the sample, at designated speeds and strain loads, according to the signals they’re receiving. So the team improved the deformation signal captured by the rheometer device and compressed the frequency profile sent to the motors.

MIT says, ‘Scientists refer to this shorter, faster and more complex frequency profile as a “chirp,” after the similar structure of frequencies that are produced in radar and sonar fields […] The chirp profile significantly speeds up an experimental test run, enabling an instrument to measure in just 10 to 20 seconds a material’s properties over a range of frequencies or speeds that traditionally would take about 45 minutes.’

This original technique proved very fast and erratic, but after closer examination of the complexity of dolphin and bat chirps, the signal frequencies were adjusted, and the accuracy without ringing was improved and the test time set at 14 seconds. This was named the Optimally Windowed Chirp, or OWCh.

The team tested the new chirp profile on various viscoelastic liquids and gels, starting with a lab standard polymer solution which they characterised using the traditional, slower method, the conventional chirp profile, and then the OWCh profile. ‘They found that their technique produced measurements that almost exactly matched those of the accurate yet slower method,’ said MIT, but that ‘their measurements were also 100 times more accurate than what the conventional chirp method produced’.

‘This protocol can be used for a wide range of soft materials, from saliva, which is viscoelastic and stringy, to materials as stiff as cement,’ said MIT graduate student Michela Geri. ‘They all can change quickly over time and it’s important to characterise their properties rapidly and accurately.’ 

More information can be found on the MIT website here, and the full study will be published in Physical Review X 

Image: High-res E-SEM image of the milk protein network in the casein gel (yogurt) characterised using OWCh technique during the entire gelation process. Credit: MIT.