My question may be pretty basic, but I feel it is important to ask this as I've gone through several texts and none offer me the clarity I seek.
The question is: What is a fluid? What is flow? If we say that a fluid is something that flows, the next right question to ask would be what flow is. To my surprise and disappointment, there is no clear distinction between various definitions, which I present in the form of questions –
- Is a fluid simply something that can flow?
- Is a fluid, an object that can be continuously deformed, as a result of shear forces? (fluids can't sustain tangential stress)
- What is flow? Does it refer to the motion of fluid elements relative to one another, or does it refer to the motion of the fluid as a whole with respect to the container it is contained in? or, is it just the continuous sliding/deformation of fluid layers, which texts refer to as flow?
So, what properties really define a fluid? (Something that brings up a clear distinction between fluids and non-fluids)
A detailed explanation would be great. Thanks a lot.
Best Answer
There is no standard definition of the word fluid. It is a somewhat imprecise term used in various ways by different people.
Indeed, in real life there is no simple example of a fluid. There is a spectrum from superfluids at one end, through non-Newtonian fluids all the way to crystalline solids. I speak as an (ex) industrial colloid scientist who has spent many happy hours studying the flow properties of many vaguely fluid systems.
The practical definition widely used by colloid scientists is that a fluid is something that has a measurable viscosity. That is, if subject to a constant shear stress (typically in a rheometer) it has a constant strain rate (note that non-Newtonian fluids may take a long time to equilibrate to a constant strain rate).
The problem with this is that if you carry out your measurement for long enough even apparently solid materials like pitch will flow. I have heard rheologists claim that on a long enough timescale everything is fluid, though these claims tend to be reserved for the bar rather than in refereed publications. Where you draw the line between a fluid and a solid depends on the application and to an extent personal preference.