Background Essay: Mystery Mud: Exploring Changes in States of Matter

On Earth, matter exists naturally in three physical phases, or states: solid, liquid, and gas. The state of a substance is the result of the movement of its individual particles -- atoms and molecules -- and the interaction between them. In general, the more energy particles have, the more they can overcome the attractions between them, and the more freely they move. For example, the particles that make up solids have low energy and cannot overcome the attraction between them. They are therefore packed tightly together, often in regular geometric patterns. The particles of liquids have more energy (compared to solids) relative to their attraction for one another and, thus, move more freely. Gas particles have enough energy to completely overcome the attraction between them, and, if allowed to, will disperse.

The most common way to change a substance's state is through a change in temperature. Temperature is a measure of the average kinetic energy of the particles in a substance -- the speed at which the particles move. By heating or cooling a substance, we raise or lower the energy of its particles, causing them to move faster or slower. When this change in energy is sufficient, the substance changes from one state to another.

Changing state does not always require a change in temperature. The magnetic fluid featured in this video segment changes state when subjected to the force of magnetism. In the presence of a magnet, microscopic iron particles suspended in the fluid link up along magnetic field lines to create a geometric pattern similar to the patterns of particles in other solids. In this type of phase change, the magnet has no effect on the energy level of the particles. Instead, it increases the attractive force between them. When the magnetic field is strong enough, the attractive force counteracts the particles' kinetic energy and transforms the fluid into a solid.

Engineers are capitalizing on the unique type of phase change seen in such magnetic fluids, also known as magnetorheological fluids. In about 20-thousandths of a second this substance can change from a liquid to a solid, or the reverse. More importantly, the transformation can be highly controlled by turning electro-magnets on or off. Using this fluid in place of a standard hydraulic fluid, engineers have created many different highly tunable shock absorbers, including those for race cars, and huge "seismic dampers" at the foundations of buildings to reduce damage caused by earthquakes. Other research teams are making clothing woven from hollow fibers containing magnetic fluid. Their goal is to provide military and police personnel with soft and pliable garments that can instantly be made stiff and impenetrable to bullets and shrapnel.