I was just looking at a bottle of aloe vera gel (for sunburns). It is a green, viscous fluid-so thick that bubbles in it never rise. Are there substances like this that behave like both solids and liquids?
Glass is supposed to be a supercooled liquid-but it has no measurable flow rate at room temperature.
Is aloe vera gel considered a solid?
AFAIK, technically, a typical fluid has a freezing and a boiling point. You can measure were both are by heating (or cooling) and noticing that at the transition points, the fluid stays at the same temperature while you keep adding or removing heat (until all of the substance has transitioned to solid or gas form). Anything in between those temperature points is a fluid.
[nitpick]Anything above the boiling point is a fluid as well. “Fluid” applies to both liquids and gases, basically anything that flows.[/nitpick]
The aloe vera gel is an example of a non-Newtonian fluid.
When Do Fluids End and Solids Begin?
Usually at about 4-6 months. Depends on the child.
shakes fist Beaten again!
Disclaimer: I am just a college student who has done some minor work in the field of rheology a year ago. If contradicted anywhere, believe that other person.
How a material reacts under stress is dependent on if it’s a solid or a liquid, so looking at that might be helpful for you. Liquids shear based on the strain rate, while solids shear is just based on the strain. Nothing is perfect, however - at high enough rates, liquids don’t behave as nicely as they do at low rates, and there are some materials that are a sort of “in-between” type. These are sometimes just called gels, when they’re not quite solid, and not quite liquid.
My head will explode if I don’t let those things out.
For that matter, there are all sorts of observable step changes in lots of physical properties when there is a proper phase change. It’s not just “hm, it used to be solid but now it flows. That’s a phase change!”
For example, you can look up all sorts of interesting physical properties on the NIST Chemistry Webbook. Let’s look at water.
Here’s a table of a bunch of physical properties of water at 1 atm, temperature varying from 80 to 120 C.
Notice the dramatic changes at 99.974 C in all sorts of properties? That’s a phase transition.