During the winter, temperatures at the north and south poles can drop to -100 F. No wonder they're covered in ice! These ice sheets—which may be many kilometers thick and up to 200 million football fields in size—harbor hidden, ancient mountains and even buried lakes of freshwater. But they also have another secret, hiding in plain sight: this ice isn’t static like most solids. Though it may seem odd to think of them as fluids, glaciers are actually "rivers of ice," flowing very slowly over hundreds and thousands of years.
By treating glaciers as fluids, scientists can measure their flow speed, predict their behavior, and even anticipate their contributions to sea-level rise as they form ice streams and ice shelves that break up in the ocean. But how can we understand a glacier's flow when it can only be observed over long time intervals? Using computers to model glacial flow is one approach, but we can also study other “weird” fluids that resemble glaciers in their flow properties.
One such material is Flubber. Made from school glue, borax, and water, Flubber is a viscoelastic material, meaning it exhibits both viscous behavior (flowing like a liquid) and elastic behavior (returning to its original shape when deformed). Flubber gets its unique properties from long molecules in the glue called polymers which, when mixed with borax, form cross-linked networks that can stretch and flow. Using Flubber, we can build intuition for how real glaciers behave and have some fun along the way!
- Elmer's Glue (1 cup)
- Borax (2 teaspoons)
- Warm Water (5/4 cups total, divided)
- Marker (optional)
- Pebbles or beads (optional)
Warm 296 ml (5/4 cup) of water in the microwave or on the stove (it doesn’t need to be boiling, just warm to the touch).
In a large mixing bowl, combine 177 ml (3/4 cup) of the warm water with 237 ml (1 cup) of glue. Stir until smooth.
In a separate container, dissolve 2 teaspoons of borax in 118 ml (1/2 cup) of warm water, stirring until fully dissolved.
Slowly pour the borax solution into the glue mixture. Stir and then knead with your hands until it reaches a mozzarella-like texture and appearance. Let the Flubber sit for about 5 minutes to absorb any excess moisture.
Now, let's play! Place your Flubber on a smooth surface, like a large cutting board or a baking sheet. Tilt the surface slightly to mimic a glacier flowing downhill. What happens when you increase or decrease the slope?
Use a marker to draw straight lines on the Flubber. What happens to the lines as the Flubber flows?
Place a small ball of Flubber and a large one next to each other on your inclined surface. Does size affect the Flubber's flow? If so, how?
Experiment by adding "rocks" (small pebbles or beads) to simulate how debris is carried by a glacier.
To better understand how Flubber flows, we can differentiate three types of fluids based on how they respond when a force is applied to them. First, imagine a bottle of honey: when you squeeze it, the honey flows at a certain speed, and if you squeeze the bottle twice as hard, the honey flows twice as fast. This is how a normal fluid (what physicists call a "Newtonian" fluid) behaves. Other fluids change their viscosity, or their resistance to flow, when a force is applied. If you've ever played with a corn-starch-and-water mixture called oobleck, you've experienced a shear-thickening fluid—the more force you apply to it, the less it wants to flow, behaving more like a solid than a liquid. The opposite is true of ketchup, which is pretty resistant to flowing until you agitate the bottle, when it may suddenly flow quite quickly. Ketchup is a shear-thinning fluid, as are Flubber and glaciers, which flow more easily under greater stress.
While it might seem silly to use a children's toy to stand-in for a real glacier, Flubber shares some important properties with glaciers. If you form it into a ball, it might initially retain its shape like a solid, but it will gradually start to deform and ooze like a liquid. The main force acting on glaciers is gravity, which can place more or less stress on a glacier depending on its mass and angle of incline. Flubber flows under its own weight, just like a real glacier!