What are foams?
Foams are made when gas bubbles are trapped inside of a liquid or solid. In many foams, the gas could take up 60-97% of the total volume. To make a liquid foam, one needs to work the liquid, add surfactants, and make sure the foam forms more quickly than it can break down. Working the liquid can include injecting a gas into it or shaking or whipping it. The mechanical stress will open pockets of liquid for gas to enter, while the surface tension will form liquid films enclosing the gas (i.e., it makes bubbles). Surfactants are chemicals which reduce the surface tension of the liquid into which it is added. They make it easier to work the liquid but also prevent the final foam bubbles from joining together. In short, they make foams more stable. This stability is also caused by Van der Waals forces and the Marangoni effect. Van der Waals forces are intermolecular forces in the liquid itself, but also form between the liquid and surfactant molecules. The Marangoni effect, or Marangoni convection, is the tendency for mass and heat to travel towards areas of higher surface tension. This mass transfer keeps the films thick enough to prevent bubbles from bursting or combining with each other. If you’ve heard of wine having “tears,” it’s because of this effect.
Let’s get into solid foams. Some solid foams include polystyrene (Styrofoam), sponges, cheese, and pumice stone. Solid foams are either closed cell or open cell. On the left of the image is an open cell foam, where the volumes of gas are connected by pores. A common example of an open cell foam is a sponge. On the right, there is a closed cell foam, where the volumes of gas are enclosed by the solid. An example of a closed cell foam is polystyrene.
- Low density/light-weight
- Most of it is air.
- Thermal insulation
- Heat needs to go through a long path from one end to the other. Air is a very poor conductor of heat, so the more air the heat needs to pass, the more it disperses throughout the material. Therefore, very little heat travels from one end of the foam to the other.
- Flexible and soft (many, but not all)
- Cushions are designed this way, but packaging foams and those made of metal are not as flexible.
- High energy absorption
- Open cell foams are more flexible and can handle compression better by essentially pushing out the air within it. The smaller the cells, the tougher it is. When the pressure is released, it can suck in air and reform back to its original shape. It will be weaker, however.
- Packaging foams take impacts well, as they easily deform under the stress. Instead of the energy going into the product, it is dispersed in the foam by deforming it.
- Metal foams need a higher density, i.e., contain less air, to be stronger. They are typically anisotropic, in that they handle stress better along the direction in which they were formed.
- Acoustic insulation and vibration damping
- This is due to their high energy absorption. Primarily found in flexible foams.
- If one rod in a cell of a metal foam fails, it will not affect the rest of the foam; one failure will not weaken the entire material.
- Flotation devices
- Shoes (soles)
- Thermal/acoustic insulation
- Home insulation
- Optics (waveguides)