Re: What is the difference between an amorphous solid and a non-Newtonian fluid

To understand the biggest difference, leave out all of the qualifying words. The difference between a solid and a fluid is that a solid is rigid, and holds its shape, but a fluid moves and flows around and changes its shape (if it has one. Gases are also fluids, and they do not have a shape!) And there is your answer: an amorphous solid is a SOLID; a non-Newtonian fluid is a FLUID.

Once that is clear, you will understand that statements like 'glass is really a liquid', that can sometimes be found in otherwise respectable books, are really nonsense. If it were the case, all of the glass windows in the mediaeval cathedrals would have lost their panes, or at least have had panes that were thicker at the bottom than at the top by now!

But there are substances that really do sit on the borderline. At the University of Queensland, I am told, there is a funnel of pitch that has been set up for nearly a century. About once every 10 years it 'drips', and the date is duly recorded. So that is a case of something that is really a liquid, but you would think it was solid if you were not very patient. And something like rubber is not really either solid or liquid, but in a special category all of its own.

Now, let us look at the qualifying words. In a solid, the atoms or molecules it is made up of are held at fixed locations. They do move, but only to the extent of vibrating or oscillating about these fixed positions. If these fixed positions form a regular array, the solid forms crystals, and is said to be crystalline. Most solids are like this. But some solids - amorphous (= without shape) solids - have their atoms or molecules fixed in position in an irregular array. There are several reasons why this might happen. By far and away the most usual one is that the molecules have a complicated and awkward shape that makes it difficult for them to find their positions in a regular array. So it is often giant molecules (polymers and biological molecules) that form amorphous solids - but not only giant molecules: ordinary alcohol (ethanol) tends to form a glass rather than crystals when it freezes. Also, paraffin wax is an amorphous solid.

With liquids, most simple liquids flow in a normal fashion. A 'non- Newtonian' liquid is one with unusual flow properties. There are at least half a dozen different behaviours that might cause a liquid to be described as 'non-Newtonian'. Many of these behaviours are again usually associated with giant molecules. But that is really about as far as the link goes. Ethanol and paraffin oil are both, for example, Newtonian liquids.

I will conclude by briefly describing some non-Newtonian behaviours.

1. If you whisk a bowl of water with a rotating egg-beater (you'll have to excuse my Australian; I do not really know how to say it in American), the water is thrown out to the sides of the bowl, and there is a depression in the middle where the beater is working. But if you do it with egg-white, the egg-white will be sucked in to the middle, and wrap itself around the beaters.
2. You can buy little containers of Silly Putty, which you can mould or stretch like treacle or chewing gum, but which will bounce off the floor if you mould it into a ball.
3. Plastic (water-based) house paint is usually designed so that it is quite thick and sticky if you leave it to sit, but goes very thin and runny if you stir it or work it with the brush. This helps it to spread evenly and smoothly when you are actually applying it, but stops it from dripping so much.

Admin Note: The file, Is glass a liquid or a solid?, on our site has further specifics about the amorphous nature of glass.

-L. Bry, MadSci Admin