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Demonstrations with Complex Fluids and Soft Materials
You can also see all our movies on the Complexfluids channel at YouTube.

Making capsules.
Movie created by Jae-Ho Lee and Sarah Everett.

This movie shows how to make "capsules". A polymer solution , colored with a blue dye is added dropwise to a beaker filled with soapy water. As soon as the blue drops hit the water, they "harden" on the outside, creating soft capsules with a liquid core. Such capsules may be used to deliver drugs or other medicines.

Rod climbing.
Movie created by Gokul Kalur.

What happens when you stir a rod in a liquid? If the liquid is water, not much. But if the liquid is a polymer solution, it will appear to climb up the rod as you stir, as shown by this movie. The faster you stir, the higher the liquid will climb. The reason for this is that polymers are spaghetti-like chains that tend to get uncoiled when you stir the liquid.

Adding salt to milk.
Movie created by Rakesh Kumar, Joon-Hyuk Yang and Lessan Seifu for ENCH 648C.

What happens when you add salt to milk? As shown by this movie, the milk curdles. In other words, it forms clumps that separate out into a top layer leaving a watery fluid at the bottom. Why does salt have this effect? Milk is a stable emulsion of microscopic fat droplets suspended in water. Adding salt induces the droplets to aggregate and thus makes the emulsion unstable.

Superabsorbent gel beads.
Movie created by Hsin-Ling Cheng, Joongjin Han and Lisa Eckman for ENCH 648C.

Why are disposable diapers so efficient at soaking up liquid? The secret lies in the tiny beads of a polymer gel that are inside the material. Here, the gel beads are removed from a diaper and the movie shows how the beads swell up when contacted with water. The swelling occurs because the polymer gel "really likes" the water.

Moving pepper flakes.
Movie created by Jae-Ho Lee, Bani Cipriano and Tanner Davies for ENCH 648C.

Water is a fluid with a high surface tension. When a soap solution is added, it reduces the surface tension locally and this can have dramatic results. Here, a drop of soap solution is added to the center of a bowl of water containing pepper flakes sprinkled on the surface. The pepper flakes swiftly move towards the edges of the bowl. This phenomenon is called the Marangoni effect.

Self-propelled toy boat.
Movie by Xuezheng Wang, Yechun Wang and Sadia Rafique for ENCH 648C.

This movie is another demonstration of the Marangoni effect. Here, a toy boat is placed on the water and a drop of dishwashing soap is added on one end of the boat. This causes the boat to move in the opposite direction. The motion of the boat is propelled by surface tension forces caused by the lowering of surface tension at the point where the soap is added.

Chaotic motion on water surface.
Movie by Xuezheng Wang, Yechun Wang and Sadia Rafique for ENCH 648C.

A third demonstration of the Marangoni effect. Here, small shapes are cut out of cardboard and placed on the surface of water. A volatile organic liquid is added to the water. After some time, you observe the cardboard pieces moving chaotically on the water surface. This effect is again due to surface tension gradients that arise once the organic liquid begins to evaporate.

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© 2007 Srinivasa Raghavan |
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