As previously mentioned that rheology covers all materials, from very low viscosity gases, medium viscosity liquids to very high viscosity solids. There are specific terms that describe all these categories in rheology. Let's ignore the extremes; the very low and very high viscosities materials. It means that I don't want to include gas and some solids that do not flow in years (stone, glass etc..). I am interested, or perhaps most of the rheologists are interested, only in the middle range. I am talking about liquids to semi-solid materials.
In rheology, the most interesting and demanding area involves changes that can be observed immediately or within a reasonable observation period of time (Let you imagine what is the reasonable observation period of time). Gases respond to stress immediately right? (sorry I am not going to cover gas here). I am going to mention viscous liquids such as water, coffee, teh tarik etc.. and elastic materials such as spring made of metal. In between viscous and elastic is a cleverly defined term: viscoelastic (I bet a child 3 years old can do that!). This will be covered too.
Apparently, according to an article found in wikipedia, visco means 'creep' (will come back to this next time). As elastic means that a material can recover after the stress is removed. Spring for instance, elongates when pulled at both ends but returns to its original length after the stress is removed. Metal spring does that immediately but elastomer such as rubber will take some time to recover. I will discuss about rubber in the future.
Viscous liquids are also called Newtonian liquids. Examples given already.
Viscoelastic materials are non-Newtonian. Note this category covers anything in between viscous and elastic materials so expect a wide range of materials.
Elastic materials obey Hooke's law.
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