Here’s a quick lesson about fluids and a demonstration of how you can walk on water! You know that different fluids have different viscosities, some are runny whereas others are thick and flow slowly. But did you know that fluids can have variable viscosities? This is key to walking on water.
Here’s one example. Paint that you’d apply to your wall is a shear-thinning liquid. When it’s sitting in the paint can or on your brush it’s about as thick as syrup, but when it’s being applied to your wall it’s about as thin as water. The liquid gets runnier when you apply force to it (i.e. pressing it against the wall) but as soon as you remove the force it thickens up. This allows it to spread easily, but not just drip down the wall like water would; you want it to hold in place until it dries.
Likewise, there are shear-thickening fluids. Silly Putty is the most well known example. It’s normally soft and pliable, it will even drip between your fingers if you spread them out, but as soon as you exert pressure it becomes very thick. This is why you can roll it up in a ball and smash it at the ground, it bounces with barely a dent. But once you’ve caught it again you can effortlessly push your finger through it.
A mixture of cornstarch and water is a shear-thickening fluid. Fill a pool with it and you can run across it!
HT to Richard Yoo for this video
One practical application of shear-thickening fluids is in protective equipment. Take soccer shin guards as an example. Typically you have a plastic plate in the shin guard to protect you. Instead, if you fill it with shear-thickening fluid, then it’s soft and pliable most of the time so it’s much more comfortable, but at the moment of impact it hardens up to protect you right when you need it. I found a company, d3o, that creates this material for use in a wide variety of products.
This is a video of shear-thickening fluid placed on a vibrating surface so it’s alternating between being thin and thick, it does weird stuff!
You can read more about non-Netonian fluids on wikipedia.