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, technically a suspension, is generally controlled by the size, shape and amount of the solid particles. The researchers started out with tiny hydrophilic glass beads in an organic solvent. Then they added just one percent weight of water. And like when you add water to sand and it clumps together, the mix turned into almost a gel. Microscopic 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 droplets, as this formation minimizes each beads contact with water. The research was published in the journal Science. [Erin Koos and Norbert Willenbacher, "Capillary Forces in Suspension Rheology"—link to come]
The scientists say this effect could be used to keep solids from separating out of liquids, or to fine-tune the viscosity of a liquid in engineering. Because sometimes, to make a mixture thicker, just add water.
—Cynthia Graber
