SSS 2011-02-17

（单词翻译）

This is Scientific American’s 60-Second Science. I’m Cynthia Graber. This will just take a minute.

Usually, we trust common sense to solve problems. Is a runny mixture too thick? Add some fluid. That’ll make it runnier. Except when the fluid makes it thicker. Scientists now think they know why.

The flow of a runny mixture, technically¹ a suspension, is generally controlled by the size, shape and amount of the solid particles. The researchers started out with tiny hydrophilic glass beads² in an organic solvent³. Then they added just one percent weight of water. And like when you add water to sand and it clumps⁴ together, the mix turned into almost a gel. Microscopic⁵ analysis showed that a few beads stick tightly to each water drop.

Then the scientists made the glass beads hydrophobic. But the addition of water still made the mixture firmer. Turns out that in this case the beads form loose rings around the water droplets⁶, as this formation minimizes each beads contact with water. The research was published in the journal Science.

The scientists say this effect could be used to keep solids from separating out of liquids, or to fine-tune the viscosity⁷ of a liquid in engineering. Because sometimes, to make a mixture thicker, just add water.

Thanks for the minute. For Scientific American’s 60-Second Science, I’m Cynthia Graber.