Intermolecular forces like the ones geckos use to stick to the ceiling are responsible for all kinds of other behavior, too. For example, intermolecular forces tend to make the molecules of a substance line up neatly, but thermal energy makes them vibrate and move around. Organized, So phase transitions—when a substance changes from a gas to a liquid, or a liquid to a solid—happen when the thermal energy gets too low to overcome the intermolecular forces.

For more detail, and some visuals, check out this video—the producer compares molecules to people at a concert.

You can learn a lot about the strength of the intermolecular forces in a particular substance by studying its melting and boiling points. For example, the intermolecular forces in water are stronger than the intermolecular forces in ethanol. That means it takes more thermal energy to overcome them. The result is that the melting and boiling points of water are higher than those of ethanol: 0° C and 100° C vs. -114° C and 78° C. This is why, alcohol can be stored in a freezer without turning to ice, and it easily boils off when you cook with it. So, if you come across a substance that melts or boils at really low temperatures, you know that its intermolecular forces aren’t that strong.

Here’s another example: we know that water is a liquid at room temperature. But methane, CH₄, is a gas at room temperature. So we can deduce that the intermolecular forces in water are stronger—which makes sense, because the electronegative oxygen atom creates an uneven electron distribution, with positive and negative patches that are attracted to each other. Carbon is only a little bit more electronegative than hydrogen, so its electrons are more evenly distributed—and therefore, its intermolecular forces are weaker.

But wait: octane is made of just carbon and hydrogen, too (it’s C₈H₁₈), and it’s a liquid at room temperature. And icosane (C₂₀H₄₂) is a solid at room temperature. What’s going on here? Well, the distribution of electrons isn’t only controlled by the electronegativity of a molecule’s atoms; it also depends on the molecule’s polarizability: basically, the “squishiness” of its cloud of electrons. Small, compact electron clouds, like the one around methane, aren’t very polarizable, so its intermolecular forces are weaker. But bigger molecules like octane and icosane have bigger, more polarizable electron clouds. The electrons have more room to move around, so they can become distributed very unevenly—leading to strong intermolecular forces, even without a very electronegative atom (for more, see here). It even depends on the shape of the molecule: long, skinny molecules are more polarizable than dense, branched ones.

As you can see, it gets complicated—but fortunately, the gecko doesn’t have to think about it! For more on sticky gecko feet, read this article! Then watch the TED Talk ” Learning from the gecko’s tail” or “The sticky wonder of gecko feet ” both by biologist Robert Full. How does the gecko release its sticky feet? What exact angle causes it to lose its grip? Listen to BBC’s David Attenborough and find out.

For more on innovations inspired by nature, watch this video. Then, listen to Janine Benyus and her TED Talks on Biomimicry: Biomimicry in action and Biomimicry’s surprising lessons from nature’s engineers. What else can we invent with nature’s help? Make some observations and begin to brainstorm!

Next Section »