Crossposted at Politicook.net
It has been a while since I added to this series. For this discussion, only the "classic" phases of matter (solid, liquid, and gas) will be covered. That gives, for binary systems, the following possibilities:
Solid in solid suspensions
Solid in liquid suspensions
Solid in gas suspensions
Liquid in solid suspensions
Liquid in liquid suspensions
Liquid in gas suspensions
Gas in solid suspensions
Gas in liquid suspensions
Gas in gas suspensions
A suspension is defined as a material, on a level larger than single molecules, ions, or atoms, dispersed in a homogeneous manner in another material. Thus they differ from solutions, where the material dispersed in on a molecular or smaller scale.
We can eliminate the last entry, gas in gas suspensions, because these are by definition occurring at the atomic or molecular level, and thus are true solutions. The rest exist, and some are extremely common, others very rare.
On a microscopic level, suspensions occur because aggregates of the lesser substance interacting with the major substance are more stable than individual molecules of the lesser substance interacting with the major substance. In other words, it costs less energy in these systems for the minor substance to remain in somewhat of a cohesive form than dissociated.
Suspensions generally settle due to the force of gravity, but in some cases this can take a long time, especially when the major component is a solid, because of rigidity of the matrix. In those cases, they are pretty much inert.
Let us consider some examples of each type of suspension.
Solid in solid suspensions happen all of the time. Some alloys are such (others are true solutions). Granite comes to mind, since it is a suspension of very small crystals of quartz, tourmaline, and mica, with other materials that alter its characteristics. Since all of the components are solid, it essentially never settles, so is unchanging, unless heated to the point that it becomes semifluid. Then gravity has its way.
Solid in liquid suspensions are best known by muddy water. Given time, and without agitation, most of the mud will settle, but some small particles are just right to stay suspended for a long time. Other examples include the so called "colloidal silver" hawked as a cure all, and Jello.
Solid in gas suspensions are too familiar to the inhabitants of Los Angeles. Smog is the bad actor there, formed when small particles of hydrocarbons are expelled from exhaust pipes from automobiles. This forms an easily observed phenomenon, and smoke is another common solid in gas suspension.
Liquid in solid suspensions are less common, but interesting. The semiprecious gem opal is a suspension of water in a silica matrix. The unique appearance of opal is a result of the relatively large water particles dispersing light. There are other examples, I am sure, but I can not think of one off of the top of my head.
Liquid in liquid suspensions are pretty common. The one that most of us are familiar with is the lowly mayonnaise. This is a suspension of water (in the form of vinegar or lemon juice) in vegetable oil. We cheat a little on this one, because we add some egg as a stabilizer, and it makes the suspension last for a very long time. This is called a protective colloid, and stabilizes the interface between oil and water. Another common liquid in liquid suspension is homogenized milk. For those of you from the farm, you know that cream (essentially an oil) rises from the milk (essentially water). By running a mixture of milk and cream through a very efficient blender, the cream globules are made so small as not to rise. If you look very carefully at the top of an unopened milk jug that is past its prime, you are apt to see some cream reform due to gravity.
Liquid in gas suspensions are not all that common, but one example is. We call it fog, and it is different than water vapor, which is a gas in gas solution. Fog is extremely tiny droplets of water, light enough to stay suspended in air for some time. Another example is that we call "steam" from a coffee pot or cooking vessel. It goes away as the small droplets of water evaporate and become a true solution with the air. There are other ones that exist, but not in many common experiences.
Gas in solid suspensions are difficult to run down, but one could argue that very fine grained pumice stone would qualify. I personally think that the cell structure is too large, and that there is exchange, so that the air trapped is not really suspended, just in a chamber. Cork might be a better example, because the air trapped in it stays put, essentially forever.
Gas in liquid suspensions are not common, but do exist. The most common one is carbonated water. In cold storage, the carbon dioxide in carbonated drinks is in a true solution, under pressure. But when opened, the carbon dioxide tends to evaporate, and make the "fizz". If you take a microscope, you can see that some of those bubbles just stay suspended in the liquid, so they qualify as a suspension. I take back saying that they are not common: if you pour a Guiness carefully into a clean glass, you will see the big bubbles expanding and reaching the top of the glass to make a proper head. But, looking carefully, and perhaps using a lens, you can see much smaller ones moving down. This has to do with boyancy, and it turns out that small bubbles are less boyant that large ones. The large ones reduce the density of the entire glass of stout, so the small ones tend to fall.
As I said before, it is not possible to have a gas in gas suspension, because they are on a molecular level in the first place.
The implications are huge. For example, blood is a suspension of red blood cells, white ones, platelets, and other components is water. In turn, many of the subcomponents of those materials are suspensions themselves. Suspensions are essential for life as we know it.
I will stick around for a bit to answer questions, comments, and whatever. Warmest regards, Doc.