Fluid mechanics is the branch of classical physics and mathematics concerned with the response of matter that continuously deforms (flows) when subjected to a shear stress. The subject can be divided into fluid statics - the study of fluids at rest, and fluid dynamics - the study of the effect of forces on fluid motion. Fluid flows impact transport and mixing (of materials or properties) in the environment and in industrial processes, and are hence of interest across a wide range of fields - including engineering, physics, mathematics and biology. Fluid mechanics is the subject of active international research, and features some of the most important unsolved problems of classical physics.
The formula for measuring viscosity is fairly simple:
viscosity = shear stress / shear rate
The result is typically expressed in centipoise (cP), which is the equivalent of 1 mPa s (millipascal second).
Shear stress is the force per unit area required to move one layer of fluid in relation to another.
Shear rate is the measure of the change in speed at which intermediate layers move with respect to one another.
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Newtonian vs. Non-Newtonian Fluids
Isaac Newton, the man to discovered this formula, thought that, at a given temperature and shear stress, the viscosity of a fluid would remain constant regardless of changes to the shear rate.
He was only partly right. A few fluids, such as water and honey, do behave this way. We call these fluids Newtonian fluids. Most fluids, however, have viscosities that fluctuate depending on the shear rate. These are called Non-Newtonian fluids.
There are five types of non-Newtonian fluids: thixotropic, rheopectic, pseudoplastic, dilatant, and plastic. Different considerations are required when measuring each of these fluid types.
Who Does Viscosity Testing?
The following are examples of how viscosity measurements are used:
Food
Viscosity measurements are used in the food industry to maximize production efficiency and cost effectiveness. It affects the rate at which a product travels through a pipe, how long it takes to set or dry, and the time it takes to dispense the fluid into packaging. The production process has to be designed with the viscosity of the product in mind, making sure that pipes are angled to optimize flow, or that dispensers provide the right amount of force to induce flow, but not so much that the packaging will overfill.
Viscosity is also a characteristic of the texture of food. The product’s viscosity must be measured and monitored during production to ensure that each batch is consistent and will go through the production process efficiently.
Adhesives
To choose the right viscosity for an adhesive, consider how the adhesive must flow, or not flow, on a part after the adhesive is applied. Low-viscosity adhesives will flow more readily than high-viscosity ones, so if you want an adhesive to stay exactly where you put it after dispensing – rather than spread out – a high-viscosity adhesive is ideal.
Other Industries
Petroleum: Viscosity measurements project the effectiveness of lubricating oil and determines design elements of pipelines.
Concrete: Viscosity determines the self-leveling and pumping behavior of a mix.
Cosmetics: Viscosity should be considered when designing the feel and flow of cosmetic products.
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