rheology_tee

From Greek “Rheos” means to flow, “logy” means study……so rheology is the study of deformation and flow (no matter how unlikely).
 * What is Rheology? **


 * “Everything flow” if you wait long enoughuser:LeeYing90**

This means that not only liquid and gases will flow, but **SOLID** too. It maybe sounds strange a bit. Does solid really can flow? But if under the factor of time, stress and strain, the solid will gradually flow like liquid.

Take a look at the examples I given below,

Mayonnaise in jar looks solid in a jar, but can be spread with a knife, just like honey.

Amorphous solid such as gels and beauty cream in our bathroom are capable of flowing if force is applied. The force may be squeezing, pumping or pouring out the cream. These are related to the yield stress where the force applied has to exceed one critical point to start the flow.

“Everything will flow if you wait long enough” After several centuries, the solid glass shown thickening at its base. This is a rheological event. This may be due to the materials inside the glass had experienced the gravitational force. Because the gravitational force is very small, so the viscous flow is very slow, only after we wait long as several centuries, then we can saw the thickening of glass window. These reflect the study of rheology- deformation and flow, no matter how unlikely.

**STRESS** and **STRAIN** are two important terms in rheology study. What is the different between them? In a simple way, stress is defined as the force intensity, while strain is the deformation intensity. For example, when we spread (stress) the peanut butter using knife, the peanut butter will deformed from thick layer to thin layer on the bread.

**SHEAR RATE** is the differentiation of velocity with respect to the distance (dv/dy). It describes how quickly one portion of the fluid in a flow slides over another portion. The processes of mixing, chewing, and swallowing all are associated with specific shear rates. It is important to know the typical shear rate for a particular process.This is because when we want to measure the viscosity of a non-Newtonian fluid, we have to choose an appropriate of shear rate to determine the viscosity.

For example, if you were to spray dry milk to milk powder, and you wanted to know how the milk will flow through the atomizer (flow behavior), then you would measure the viscosity within the range of shear rate relevant to the spray drying process (105 - 106 s-1).

For example, honey is more viscous than water, thus honey flow slower than water. Different brands of tomato ketchup would also have different viscosity. The viscosity of most fluids will decrease as the temperature of fluid increases.
 * VISCOSITY ** is a measure of fluid's ability to resist motion when a shearing stress is applied. It is important to recognize that different fluids flow at different rates.



- Dynamic or absolute viscosity is the ratio of shear stress to shear rate.

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- Apparent viscosity is the viscosity for non-Newtonian fluids. It is the viscosity that measured at a single shear rate. Apparent viscosity is useful if it is determined at a relevant shear rate for a particular process. For example, if the oral evaluation of food having a high resistance to flow is to be examined, the apparent viscosity determined at a shear rate of 10s-1would give a good indication of sensory acceptability. On the other hand, if the food is to be examined by stirring with a spoon, then the apparent viscosity determined at a nearly constant shear rate of 90-100s-1 would be more approperiate. If the viscosity of a high consistency food is to be examined by tilting the container, the shear rate developed (0.1-40 s-1) would be the critical factor. =====