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Ron Dippold
San Diego, California, US
Yes! To expound on that, a foam is a “colloid”, which is a small amount of liquid or solid wrapped around a lot of gas. For instance, if you froth up some water with soap, the foam is a liquid around gas. Packing foam is a solid (usually polystyrene or polyethylene) wrapped around gas.
The four classic states of matter are solid, liquid, gas and plasma. But there are dozens of more exotic states, and very many common materials exist outside the four classic states, or are at least a combination of them. Colloids are one example. Glass is another material that is solid but looks like a liquid if you examine it closely – an “amorphous solid”. And then, as implied by the name, liquid crystals (used in LCD displays) flow like liquids but show long-term crystallisation like a solid.
One of the most fun things about science is that whenever you try to nail down “Is it A or B or C?”, the answer is usually always “It is A and B and C and you didn’t even think of these other neat things!”. Anything that can happen will happen.
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If you froth up some water with soap, the foam is a liquid around gas. Packing foam is a solid wrapped around a gas
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Peter Borrows
Amersham, Buckinghamshire, UK
Foam is an intimate mixture of tiny gas bubbles (nanoparticles) with either a solid (e.g. pumice) or a liquid (e.g. whipped cream). It is a colloid, which means that one of the two components can be very small (the disperse phase) but cannot mix perfectly with the other component (the continuous phase). There are therefore eight types of colloid in all: solid nanoparticles dispersed in a solid, liquid or gas continuous phase; liquid nanoparticles dispersed in a solid, liquid or gas continuous phase; and gaseous nanoparticles dispersed in a solid or liquid continuous phase.
You cannot have the ninth type of colloid, gaseous nanoparticles dispersed in a gaseous continuous phase, because gases are perfectly miscible in all proportions. Some colloids are quite stable, like pumice, but others are quite easy to separate out, such as when muddy river water runs into a salty sea and deposits its silt. We deliberately try to stabilise some colloids. For instance, egg yolk is added to oil and vinegar to stabilise the resultant liquid in liquid colloid.
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