You know those videos: someone sprints across a pool of cornstarch and water, defying gravity and basic physics. It’s a party trick, sure, but it’s also a scientific enigma. These “non-Newtonian fluids” — think ketchup, quicksand, or that cornstarch concoction — change their thickness based on how much force you apply. But how they do it, especially under extreme conditions, has been a bit of a mystery.
Enter Xiang Cheng and his team at the University of Minnesota, who decided the best way to understand these fluids was to smash tiny droplets of them onto a metal plate at high speed. Because apparently that’s where we are now.
When Liquid Acts Like a Solid (Then Liquid Again?)
Most liquids are pretty chill. Their viscosity (thickness) stays the same, no matter what. But non-Newtonian fluids are the rebels. Hit a cornstarch-water mix, and it momentarily becomes an almost solid mass. This is called “shear-thickening,” and it’s why you can run across it without sinking (for a few glorious seconds, anyway).
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Start Your News DetoxCheng’s team made cornstarch-water mixtures ranging from mildly thick to really thick (30% to 43% cornstarch, for the science nerds keeping score). Then, they dropped these tiny liquid bullets onto a plate, all while filming with a high-speed camera and measuring the force. The goal: create the most extreme shear thickening possible and see what happens.
What happened was… not what they expected. They saw three distinct behaviors:
- Low cornstarch, low speed: The droplets acted like regular liquids, spreading out as expected.
- High cornstarch, slower speed: The droplets behaved like solid balls, which was also expected. This is your classic “running on water” scenario.
- High cornstarch, high speed: This was the curveball. Under the most intense forces, the droplets started like a liquid, spreading out. But then, as the force lessened while they were still spreading, they suddenly became solid-like. It was like they couldn't make up their minds. Previous theories suggested stronger forces would always lead to a more solid response, so this was a genuine head-scratcher.
To make sense of this liquid-solid-liquid identity crisis, the team developed a new model. It combines existing drop-impact theory with how the cornstarch particles physically push fluid out of their network when stressed. Which, if you think about it, is both impressive and slightly terrifying.
This new understanding isn't just for showing off at parties. It could lead to better body armor (materials that instantly stiffen on impact) and more responsive soft robotics. Because who doesn't want a robot that can change its mind about being solid?
