I know about the 3 clasical states of matter: Solid, Liquid and Gas. But I've also observed how between Solid and Liquid there are some materials that will have an "in-between" phase state: Liquids that get thicker as the temperature decreases until they freeze, or solids that become more malleable when the temperature raises until they melt. See for instance glass (silicon dioxide), or metals (iron, aluminium, sodium, etc.)
But I've never seen such behaviour in between liquid to gas. I've always seen all transitions between these two states as binary – the state is either gas or liquid. The closer thing I can think of is Clouds, but AFAIK they are actually made of tiny droplets of liquid water that are just on suspension in the air. Gas can get its density drastically changed by heating it up, but it always makes a big jump when going from gas to liquid.
Which makes me wonder – Why? Why do we have a in-between state between solid and liquid, but not between liquid and gas?
Best Answer
I would argue that you do see certain temperature-dependent responses in a liquid relative to a gas analogous to those that you see relative to a solid.
The two examples you give are the temperature-dependent viscosity of a liquid and the yield stress of a solid decreasing with increasing temperature.
For the first example, in the same way that a liquid's viscosity increases with cooling (towards a solid's high viscosity associated with its creep rate), it correspondingly decreases with heating (towards the low viscosity of the gas phase).
In the second example, the yield strength of a solid (which is one of its material properties) decreases with increasing temperature, approaching the limiting condition of a fluid, which has no yield strength. (Put another way, liquids and gases cannot sustain a shear force.)
But as I heat a liquid, its surface tension (which is another material property) decreases, approaching the limiting condition of a gas, which has no positive surface tension.
So I don't see the situation as being quite as one-sided as you describe.