Brief introduction to the field of Rheology - A treat to flow enthusiasts
In everyday life, we come across several materials ranging from water, toothpaste, cosmetics, food products to the biological fluids we are made up of. We get so drowned in our mundane activities that we fail to appreciate some of the amazing and interesting properties they possess. The science behind their flow behavior is fascinating and as a rheologist, let me take you to a stroll along this road.
Have we ever wondered why?
1. Water flows out easily from an open bottle while a ketchup or toothpaste needs some push or shake to do the same
2. We chew semi-solid food products like cheese, bread, rice or pulses sufficient enough to flow like a liquid so that they can be easily swallowed
3. It is easier to walk on the wet sand than on the dry sand
4. Fun toys like silly putty or slime bounce back like a rubber ball when thrown quickly while they flow and settle like a liquid when left undisturbed
5. Paints flow like liquids during coating process and slowly come to rest sometime after the application
6. Difference in the top surface of honey (rapid recovery back to its initial state) and butter (retains the shape carved in it by a knife) after serving
and so on……
All the above can be answered when we carefully compare the timescale of occurrence of these processes with the timescale of our observation. This ratio is scientifically termed the ‘Deborah Number (De)’. Each material has its own timescale of response to an applied deformation which is attributed to the complexities in their structure and how they rearrange or breakdown throughout. At shorter times of observation, the structure may still be at rest and therefore the material behaves as an elastic solid (De is high). At longer times, the structure undergoes continuous deformation leading to viscous liquid-like behavior of the material (De is low). Some materials that initially pose as solids eventually yield and begin to flow like liquids upon significant deformations. On the other hand, some materials undergo inherent time dependent structural changes during deformation or at rest. For such systems, it is essential to additionally compare the timescale of our observation to the timescale of relevant property change during our observation. This ratio is scientifically termed the ‘Mutation number (Mu)’. For thixotropic systems, this ratio is higher.
Therefore, the most incredible truth is 'a majority of real systems with complex structures are Thixo-Elasto-Visco-plastic (TEVP)'
The applied deformation is generally ‘strain’ or the ‘rate of strain’ while response of the material is the ‘stress’. From each of the above processes, one can decipher the ‘stress-strain / strain rate’ dependence and establish the corresponding structure-property relationships.
Therefore the famous saying ‘mountains flowed before the lord’ is indeed true !
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