Definition of Viscosity

Viscosity is an important physical property of fluids that determines resistance to flow and can be used to characterize a variety of materials, from liquids like oil and water to gases like air. Likewise, viscosity can be used as an indicator of how well a material will perform in certain applications, such as lubrication or sealing.

The viscosity of a fluid is determined by its molecular structure; larger, more complex molecules tend to be more viscous than smaller ones. Temperature can also affect viscosity; higher temperatures tend to reduce it, while lower ones increase it. Additionally, pressure affects viscosity, as increased pressure causes the fluid molecules to move closer together, resulting in increased resistance against movement within the fluid itself.

In general, understanding how different fluids behave under variable conditions helps us to better use them for specific purposes, such as manufacturing processes or engineering projects, where precise control of their properties is essential for the proper selection of the fluid to use.

Oils and other industrial substances, such as paints and resins, are classified considering, among other parameters, their viscosity.

Types of viscosities

Kinematic and dynamic viscosities are two physical properties of fluids that are related to resistance to their flow. Kinematic viscosity is a measure of the internal resistance of the fluid, while dynamics is an indicator of the external force required to keep the fluid moving.

Both types of viscosities vary with respect to different temperatures, which can have important effects in many industrial and commercial situations.

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Honey from bees is a fluid that has a higher viscosity than water, since, as can be seen, it presents greater resistance to flow over a surface.

First of all, it should be noted that generally the kinematic and dynamic viscosities decrease when the temperature rises: studies have almost always shown a linear decrease between the variations depending on the degrees Celsius. This would mean, for example, for food or pharmaceutical producers the use of less amount of lubricating substance during processing to facilitate the flow of products at working temperature.

On the other hand, due to the aforementioned linear descent, there are certain situations where the heat exceeds the systems controlled by a viscous fluid (such as the engine), so they can deteriorate due to their increase in working speed and cause the increase in revolutions and engine temperature, beyond the values ​​established by the manufacturer, causing significant long-term damage to its operation and operational performance.

Viscosity determination

If we assume two parallel flat plates between which is a fluid, and one of the plates remains at rest, while the other moves parallel to the other plate; experimentally it is known that the layers of the fluid that are closest to the moving surface will experience greater velocity than those that are further away from it (velocity profile represented in the figure with orange color).

In turn, suppose that the separation between the two plates is very small (dy). Under these premises, it is known, from Newton’s law of viscosity, that for many fluids (Newtonians), the tangential force applied to the surface plate “A” is directly proportional to the speed of the plate ” v”, and inversely proportional to the distance (dy), that is:

Where:
τ: shear stress (N/m2)
dv/dy: is known as shear cut or speed differential
μ: absolute viscosity and represents the proportionality factor in the equation

From this expression, it can be deduced that the fluids that obey this relationship are known as Newtonian fluids, in which the viscosity remains constant, although the shear stress varies. On the contrary, in non-Newtonian fluids, the viscosity does not remain constant with changes in shear stress.

When corn starch is mixed with water, it forms a non-Newtonian fluid, so when the shear stress varies, its viscosity changes. On the contrary, liquid milk behaves like a Newtonian fluid.

From the previous expression it can also be deduced that the absolute viscosity, or also called dynamic, has in the International System, units of Ns/m2, and in the CGS (Cegesimal) system, it is expressed in dynas.s/s2, which is equivalent to 1 Poise (P) = 100 cP (centiPoise)

It is also common to refer to viscosity as the relationship between the absolute viscosity (μ) between the density of the fluid (ρ), and this ratio is called kinematic viscosity (φ). Whose units in the International System are m2/s or in the CGS cm2/s = stoke (st)

1 st = 100 cst

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