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Force acting on egg
Rheology aims at measuring those properties of materials that control their deformation and flow behaviour when subjected to external forces. Thus, rheology is mainly concerned with the relationship between strain, stress, and time (so we use terms such as strain rate, flow rate, shear rate, etc.). In the context of rheology, stress is related to force whereas strain is related to deformation (change of shape or dimension as a result of the applied force).
Flow behaviour and viscosity
Viscosity is a measure of a fluid's ability to resist motion when a shearing stress is applied. Imagine yourself pouring a tomato sauce from the bottle. Can you pour the toothpaste from the tube? You can't, you have to squeeze the tube. So we say that the tomato sauce and the toothpaste exhibit different resistance to motion (flow). They have different viscosity!
More about viscosity
It is important to recognize that different fluids flow at different rates, as I have mentioned earlier. It turn out that when we measure the fluid flow in a suitable instrument, and plot the data in the form of shear stress vs shear rate, we obtain either a straight line through the origin, or a curve. We call such a plot as a flow curve (although in the case of Newtonian fluid, it’s a straight line). A flow curve can also be presented in the form of viscosity (apparent viscosity) vs shear rate (or shear stress). This plot is better in a sense that we can see clearly how viscosity changes as a function of shear rate (i.e., when we increase or decrease the shear rate).
Word game - General knowledge on physical properties of food
It is important to appreciate that physical properties such as colour, texture, flow behaviour, surface property, phase transition (e.g., crystallization) are important parameters to consider, together with the chemical properties and reactions in food systems.
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