Top: sinusoidal signal used in standard oscillatory rheological measurements. Center: chirp signal without optimization, which reduces the experimental time but is not precise. Bottom: optimally windowed chirp signal optimized by the researchers for fast and precise measurements. Image courtesy of the researchers.
Top: sinusoidal signal used in standard oscillatory rheological measurements. Center: chirp signal without optimization, which reduces the experimental time but is not precise. Bottom: optimally windowed chirp signal optimized by the researchers for fast and precise measurements. Image courtesy of the researchers.

Scientists tend to use various instruments to stretch, squeeze and twist soft materials to precisely characterize their strength and elasticity. Typically, such experiments are carried out sequentially, which can be time-consuming.

Now, inspired by the sound sequences used by bats and dolphins in echolocation, engineers at Massachusetts Institute of Technology (MIT) have devised a new technique that vastly improves on the speed and accuracy of measuring soft materials' properties. The technique can be used to test the properties of drying cement, clotting blood or any other ‘mutating’ soft materials as they change over time. The researchers report their results in a paper in Physical Review X.

"This technique can help in many industries, [which won't] have to change their established instruments to get a much better and accurate analysis of their processes and materials," says Bavand Keshavarz, a postdoc in MIT's Department of Mechanical Engineering.

"For instance, 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," adds graduate student Michela Geri. "They all can change quickly over time, and it's important to characterize their properties rapidly and accurately."

Geri and Keshavarz are co-authors on the paper. Other authors include: Gareth McKinley, a professor of mechanical engineering at MIT; Thibaut Divoux of the CNRS-MIT joint laboratory; Christian Clasen of KU Leuven in Belgium; and Dan Curtis of Swansea University in the UK.

The group's new technique improves and extends the deformation signal that's captured by an instrument known as a rheometer. Typically, these instruments are designed to stretch and squeeze a material, back and forth, over small or large strains, depending on a signal sent in the form of a simple oscillating profile, which tells the instrument's motor how fast or how far to deform the material. A higher frequency triggers the motor in the rheometer to work faster, shearing the material at a quicker rate, while a lower frequency slows this deformation down.

Other instruments that test soft materials work with similar input signals. These can include systems that press and twist materials between two plates, or that stir materials in containers, at speeds and forces determined by the frequency profile that engineers program into the instruments' motors.

To date, the most accurate method for testing soft materials has been to conduct the tests sequentially over an extended period. During each test, an instrument may, for example, stretch or shear a material at a single low frequency, or motor oscillation, and record its stiffness and elasticity before switching to another frequency. Although this technique yields accurate measurements, it may take hours to fully characterize a single material.

In recent years, researchers have looked to speed up the process of testing soft materials by changing the instruments' input signal and compressing the frequency profile that is sent to the motors.

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 – and, very broadly, in some vocalizations of birds and bats. The chirp profile can significantly speed up an experimental test run, allowing an instrument to measure in just 10–20 seconds a material's properties over a range of frequencies or speeds that traditionally would take about 45 minutes.

But when analyzing these measurements, researchers have found artifacts known as ringing effects in the data from normal chirps, meaning the measurements aren't sufficiently accurate. The measurements seemed to oscillate or ‘ring’ around the expected or actual values of stiffness and elasticity of a material, and these artifacts appear to stem from the chirp's amplitude profile, which resembles a fast ramp-up and ramp-down of the motor's oscillation frequencies.

"This is like when an athlete goes on a 100m sprint without warming up," Keshavarz says.

Geri, Keshavarz and their colleagues looked to optimize the chirp profile to eliminate these artifacts and therefore produce more accurate measurements, while keeping to the same short test timeframe. They studied similar chirp signals in radar and sonar – fields originally pioneered at MIT Lincoln Laboratory – with profiles that were originally inspired by chirps produced by birds, bats and dolphins.

"Bats and dolphins send out a similar chirp signal that encapsulates a range of frequencies, so they can locate prey fast," Geri says. "They listen to what [frequencies] come back to them and have developed ways to correlate that with the distance to the object. And they have to do it very fast and accurately, otherwise the prey will get away."

The scientists analyzed the chirp signals and optimized these profiles in computer simulations, then applied certain chirp profiles to their rheometer in the lab. They found the signal that reduced the ringing effect most was a frequency profile as short as the conventional chirp signal – about 14 seconds long – but which ramped up gradually. Compared with the original chirp profiles that other researchers have been using, their chirp profiles had a smoother transition between the varying frequencies.

They call this new test signal an ‘Optimally Windowed Chirp’ (OWCh) for the resulting shape of the frequency profile, which resembles a smoothly rounded window rather than a sharp, rectangular ramp-up and ramp-down. Ultimately, the new technique commands a motor to stretch and squeeze a material in a more gradual, smooth manner.

The team tested their new chirp profile in the lab on various viscoelastic liquids and gels, starting with a laboratory-standard polymer solution that they characterized using the traditional, slower method, the conventional chirp profile and their new OWCh profile. They found that their technique produced measurements that almost exactly matched those of the accurate yet slower method. Their measurements were also 100 times more accurate than those produced by the conventional chirp method.

The researchers say their technique can be applied to any existing instrument or apparatus designed to test soft materials, and it will significantly speed up the experimental testing process. They have also provided an open-source software package that researchers and engineers can use to help them analyze their data for quickly characterizing any soft, evolving material, from clotting blood and drying cosmetics to solidifying cement.

"A lot of materials in nature and industry, in consumer products and in our bodies, change over quite fast timescales," Keshavarz says. "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."

This story is adapted from material from MIT, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.