If you dropped a water balloon on a bed of nails, you’d expect it to burst spectacularly. And you’d be right – some of the time. Under the right conditions, though, you’d see what a high-speed camera caught in the animation above: a pancake-shaped bounce with nary a leak. Physically, this is a scaled-up version of what happens to a water droplet when it hits a superhydrophobic surface.
Water repellent superhydrophobic surfaces are covered in microscale roughness, much like a bed of tiny nails. When the balloon (or droplet) hits, it deforms into the gaps between posts. In the case of the water balloon, its rubbery exterior pulls back against that deformation. (For the droplet, the same effect is provided by surface tension.) That tension pulls the deformed parts of the balloon back up, causing the whole balloon to rebound off the nails in a pancake-like shape. For more, check out this video on the student balloon project or the original water droplet research. (Image credits: T. Hecksher et al., Y. Liu et al.; via The New York Times; submitted by Justin B.)
This is what happens when a carrot is fired at 300 km/hour at an egg, through two sheets of cardboard.
This is what happens if you separate out the two sheets:
The egg survives! This shows how a Whipple shield works, and is what spacecraft use to protect themselves from micrometeoroid impacts in space. When the projectile (in this case a carrot, but in space it could be a speck of paint, a piece of an old satellite, or a bit of space rock) hits the first layer, it’s moving so fast that it starts to vaporise, because the energy of the collision is enough to break almost every bond in the substance.
It then sprays outwards, spreading the force of impact across a much wider area, meaning the second layer can stop it going any further, keeping your egg (or astronauts) safe.
Electron excitation is the transfer of a bound electron to a more energetic, but still bound state. This can be done by photoexcitation (PE), where the electron absorbs a photon and gains all its energy or by electrical excitation (EE), where the electron receives energy from another, energetic electron.
When an excited electron falls back to a state of lower energy, it undergoes electron relaxation. This is accompanied by the emission of a photon (radiative relaxation) or by a transfer of energy to another particle. The energy released is equal to the difference in energy levels between the electron energy states.