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"Everything Flows" : The Physics of Rheology

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 We come across numerous different types of materials which behaves quite surprisingly. For instance behavior of water, oil, ketchup, blood, peanut butter, curd, cosmetics, cement paste etc. is very different. A material can flow at one instance and the same material will act as a solid at the other instance. These are general materials which we use in our mundane life, but we fail to appreciate their amazing properties and behavior. You might wonder if I make a statement that, "Everything you look around has a potential to flow, yeah right... Everything!!" 

The Science Behind Why Ketchup Is So Hard to Pour


Whenever we study the flow behavior of any material then the most important thing to look forward is timescale. The timescale of occurrence of the process to the timescale of our observation governs the flow interpretation. This ratio is scientifically termed as the Debroah Number (De). Each material has it's own timescale of response to an applied deformation and that depends on the structural complexities. When the timescale of observation is small, the structure may have shown no changes and hence the material behave as an elastic solid i.e De is high. At longer timescales, the structure experiences continuous deformation which in-turn leads to liquid like viscous behavior i.e De is low. So, the bottom line is if you observe something at smaller timescales then essentially the material will act more like an elastic solid while if your observational timescales are higher then  the material will be liquid like. 

Mostly, our interpretation of the nature of material are based on smaller timescales of observation. But if we increase our observation timescales crazily like say years which generally happens in geological timescales, then your interpretation of the material can be completely different. In geological timescales you can say even Earth is flowing!!

There are few different classes of materials which has dependency on various factors. Some materials initially pose as solid then after certain fixed amount of deformation (yield stress) they eventually begin to flow like liquids. Some material are also known to show time dependent structural changes. Some materials becomes thin upon deformation like ketchup, while others thickens like peanut butter.

Many materials response to force non-linearly, i.e their apparent thickness changes on how hard (Force), how long (Time) and how fast (Speed) deformation is applied. Materials can be non-linear in one or in combination of these factors. For e.g. flow of Ketchup from a bottle depends on the force applied and also the time. Either you apply significant force or wait for a longer time with small amount of applied force to get ketchup out. 

Well the complexity of the behavior of these materials is very huge and that renders them with such astonishing properties which makes them unique for various end applications. 

It's just the matter of time, else Everything Flows!!




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