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Just how hard is it to measure stiffness?

We effortlessly measure objects’ tautness every day. But for scientists, figuring out the mechanical properties of an object can be quite challenging.

For us, figuring out how hard something is requires only a simple touch. From the stiff surface of a stone to the fluffy delight of snow – we effortlessly measure objects’ tautness every day. But for scientists, figuring out the mechanical properties of an object, such as how stiff or soft something is, can be quite challenging. 

Photo of Sarah Heilshorn
ChEM-H faculty fellow Sarah Heilshorn co-authored the study.

Fortunately, researchers at Stanford have found a way to measure these properties with relatively simple equipment. In a recent article in ACS Central Science, they introduced a technique that can measure a material’s stiffness based on how small particles added to the material spread out light. “We developed a technique that allows us to test the mechanical properties of a material just by looking at how light bounces off of it,” says the paper’s lead author, Brad Krajina. Krajina is a PhD student in the labs of Andy Spakowitz, an associate professor of chemical engineering and of materials science and engineering, and Sarah Heilshorn, an associate professor of materials science and engineering and a faculty fellow of Stanford ChEM-H. 

Usually, to measure the mechanical properties of a material, researchers use machines to flatten, extend, or twist a material and see how it much it squishes, stretches, or deforms. Unfortunately, the material is sometimes too small or too delicate to yield accurate measurements. Additionally, some biological samples, such as tissues, may be too sparse or too sensitive to test this way.

Instead of actively moving the sample, the team came up with the idea of adding particles to the material and watching how the particles move. Since the particles are too small to be seen directly, light shined through the sample is measured instead. How the light gets spread out by the movement of the particles is then used to calculate the mechanical properties of the material. 

The nature of the technique allows researchers to test tiny or limited samples, such as mucus from the gut of mice. Krajina and the team showed that in a model of an unhealthy intestine, the mucus layer is softer and disorganized. Their work suggests that the physical integrity of the mucus layer is important for its role in guarding against infection.

 When testing a material’s properties, the time scale matters—measurements taken over a long period of time may give different results than measurements taken over a short period. Just think of a piece of taffy – slow tugs will stretch it out, but fast, sudden pulls will rip the taffy apart. Machines that physically manipulate materials can only move so fast or so slow, which limits how much you can learn about the material. But the new technique can measure the material’s properties over a wide range of time scales, without the need for extremely precise or advanced equipment.

Krajina and his colleagues used this approach to examine the mechanics of DNA. “DNA actually bends at short time scales and flows at longer time scales,” said Krajina. They observed how the bending behavior is due to the stiffness of individual DNA molecules, while the flow behavior is caused by many DNA molecules rearranging themselves. By seeing how the mechanical properties of DNA can change when measured at different ranges of time, researchers can learn about the fundamental processes that control DNA mechanics. 

This wealth of information can be used to learn more about the underlying physics and structure of the material. “Hopefully,” Krajina says, “this can help us learn why materials behave how they do and potentially even help us design new materials.”