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Viscosity: examples, causes, units, types

Some examples of sticky or viscous substances are honey, oil, toothpaste, hair gel or shampoo, pharmacy syrups, and some chemicals such as mercury.

Viscosity is a level of the “thickness” of a fluid. There are liquids that flow very easily, for example, water, which is why they are called “light” fluids. Meanwhile, we have liquids that flow slowly, such as ketchup (tomato paste), which are called “thick.” In this sense, fluidity would be the opposite of viscosity.

Another way to express the viscosity of a substance is in relation to its penetrability. Then, the viscosity is also a measure of the resistance that a fluid presents to the movement of a body or object through them.

It is observed, for example, that some corn kernels sink without difficulty in water: a low viscosity liquid. However, they do so very slowly in honey – a viscous liquid.

Compared to water, air has a viscosity of 1,983 · 10 -5 Pa · s; that is, about 100 times less. While a solid, such as glass, has a very high viscosity, in the order of 10 40 Pa · s.

Causes of viscosity

When a liquid moves at low speed, it breaks down into sheets that move relative to each other. The sheets in contact with the walls of the duct move with less speed, since they experience greater friction.

However, the sheets of liquid that are located towards the central part of it, move with greater speed, since they experience less friction. This type of flow is called laminar. But when the velocity of the liquid reaches a critical velocity, it becomes a turbulent, higher viscosity flow.

Definition and units

Viscosity is the force that must be supplied to a sheet of liquid to overcome the friction force it experiences with a neighboring sheet, which moves relative to it at a certain speed.

Instead, the most commonly used unit in these writings, based on the cgs (centimeter-gram-second), is the dyne s / cm 2 , which is called poise (P), in honor of the French physiologist Jean Poiseuille.

Equivalences

10 poise equals 1 Pa · s.

For practical reasons, viscosity is usually expressed in centipoise (cP); that is, 10 -2 P or one hundredth of a poise. Thus, the viscosity of water is 1 cP.

In kinematic viscosity the unit m 2 / s is used, but the one commonly used is called Stoke (st), whose equivalence is:

1 St = 10 -4 m 2 / s = 1 cm 2 / s

And in turn, 1 stoke equals 10 Poise.

Viscosity formula

The viscosity formula is expressed using Newton’s equation for fluids:

F / A = η dv / dr

Where F / A is the force applied per unit area to produce the displacement of a laminar portion, η the viscosity, and dv / dr the variation of the speed.

The similarity of this Law with Newton’s second law for the motion of bodies can be observed:

F = m a

Being able to equate the mass with the viscosity.

Viscosity types

There are two types of viscosity: the absolute or dynamic viscosity, and the kinematic viscosity.

Absolute or dynamic viscosity

Absolute viscosity is the force that must be applied per unit area to move a sheet of liquid horizontally relative to another sheet of liquid at one unit of speed. References to viscosity can be extended to absolute viscosity as they are the same.

Kinematic viscosity

It is the relationship between the absolute viscosity and the density of the liquid:

ν = η / ρ

Where ν is the kinematic viscosity (m 2 / s), η the absolute or dynamic viscosity (N · s / m 2 ), and ρ the density (Kg / m 3 ). Kinematic viscosity is also represented by the symbol µ.

The viscosity of a liquid is influenced by temperature, so reference temperatures are used: 100 ºC for residual liquids, and 40 ºC for diluted liquids.

Kinematic viscosity, like absolute viscosity, decreases as temperature increases.

Water viscosity

The viscosity of liquid water varies as a function of temperature. For example: its viscosity is 1,308 cP, at 20 ºC, and 0.4658 at 60 ºC. This decrease occurs because the increase in temperature decreases the cohesion between the water molecules. Therefore, its liquid sheets slide past each other more easily and quickly.

Examples of viscous substances

Oil

Oil is any liquid and fatty substance that is obtained after the processing of different seeds and fruits. On the other hand, fuel oils are obtained from oil refining .

Each of these oils has specific characteristics and uses. However, one of the elements that makes both types oils is precisely their viscosity, fuel oils tend to have a higher degree of viscosity than edible ones.

Honey

It is a fluid that bees produce from the nectar of flowers or excrement of living parts of plants . It can be said that honey is one of the most viscous and sweetest animal fluids.

However, honey can sometimes have varying degrees of viscosity due to post-capture processing. Sometimes they mix honey with sugar and other elements to market it and this causes it to lose viscosity.

Toothpaste

Toothpaste is a perfect example of a highly viscous fluid that we find every day in our chores. It is made up of water, abrasives, foam, colorants, fluorine, and other chemicals. It is also called toothpaste or toothpaste.

Hair gel

It is one of the most particular cases because its structure resembles that of solids but it is a liquid with a high volume of viscosity.

Some gels can go from one state to another depending on how they are used. When they are agitated they are liquid and when they remain immobile they become solid.

Glycerin

It is a kind of alcohol with three hydroxyl groups. It is shown in liquid form at an ideal temperature of about 25ºC. It has a high coefficient of viscosity and a sweet taste. Glycerin is found in all animal fats and oils.

Its applications are diverse in the commercial plane, it is used for the production of cosmetics, soaps, detergents, humectants, antiseptics, solvents, lubrication and much more.

Ethyl alcohol

It is a normal colorless and flammable alcohol whose boiling point is 78.4ºC. It is the main product for the production of alcoholic beverages such as whiskey, wine, beer, rum and brandy. Viscosity grades in alcohols are generally very low.

Bitumen

Also called bitumen, it is a mixture of black organic substances that contain a high density that is completely soluble in “carbon disulfide” and is composed primarily of hydrocarbons. It is one of the most viscous organic substances that can be found in nature.

Said viscosity is caused by the elements that compose it: sulfur, metals, vanadium, lead, mercury, arsenic and selenium; heavy and viscous elements that when integrated form an even more viscous compound.

Syrups

They are a type of liquid medicine made up of chemicals. Syrups have high viscosity because among their main components is sugar.

Sugar fulfills a double function, on the one hand, that of adding viscosity to the fluid and, on the other, it is a sweetener that gives the mixture a sweet flavor.

It is one of the most used components in pediatrics and can be ingested by both children and adults.

Pitch

It is a dark reddish substance that is obtained by distilling the wood of various types of trees on fire. Its degree of viscosity is very high due to the confluence of compounds that melt.

The pitch can be used as an antiseptic, for machine maintenance. Some basketball players smear it on their hands to better hold the ball.

Mercury

It is a chemical element that uses the symbol Hg. Mercury is the only metallic element that is liquid under standard conditions. Because it is heavy and due to its chemical structure, mercury has a high degree of viscosity.

Today it is common to use mercury for research purposes, it is also used to make fluorescent lights and dental amalgams.

Viscous substances appear both in everyday substances and in elements necessary to carry out industrial processes. Without this property of fluids, surely life would be very different.

The blood

Blood has a viscosity between 3 and 4 cP for a hematocrit of 45%. But as the hematocrit increases, the viscosity of the blood increases exponentially. This circumstance increases cardiac work.

Motor oil

The viscosity of the oil varies with its type. But in general, the viscosity of a motor oil is greater than 100 cP, explained by the size of the oil molecules.

The gas

Gasoline is a mixture of hydrocarbons obtained by distillation of petroleum. It has a viscosity lower than that of water and with a value, depending on the type, oscillating between 0.40 and 0.88 cP. In part, this is because, while their molecules are heavier, their intermolecular interactions are weaker.