Fluid viscosity Methods for determining fluid viscosity
In industry, research activities often need to calculate the viscosity of a fluid. Working with ordinary or dispersed media in the form of aerosols, gas emulsions requires knowledge of the physical properties of these substances.
What is fluid viscosity?
Even Newton marked the beginning of such a science as rheology. This industry is studying the resistance of a substance in motion, i.e., viscosity.
In liquids and gases, continuous interaction of molecules occurs. They hit each other, repel, or simply fly past. As a result, the layers of matter interact with each other, giving speed to each of them. The phenomenon of such interaction of molecules of liquids / gases is called viscosity, or internal friction.
To better consider this process, it is necessary to demonstrate the experience with two plates, between which there is a liquid medium.If you move the upper plate, then the layer of liquid “stuck” to it will also begin to move with a certain speed v1. After a short period of time, we notice that the underlying liquid layers also begin to move along the same trajectory at a speed of v2, v3 ... vn, etc., with v1> v2, v3 ... vn. The rate of the lowest one remains zero.
By the example of gas, such an experiment is practically impossible to carry out, since the forces of interaction of molecules with each other are very small, and it will not be possible to register visually. Here they also speak about layers, about the speed of movement of these layers, therefore in gaseous media there also exists viscosity.
Newton and non-Newtonian media
Newtonian fluid is a fluid whose viscosity can be calculated using Newton's formula.
Such environments include water and solutions. The viscosity of a fluid in such media may depend on factors such as temperature, pressure, or the structure of an atom of a substance, but the velocity gradient will always remain the same.
Non-Newtonian fluids are such media in which the above-mentioned value may change, which means that the Newton formula will not act here.Such substances include all dispersed media (emulsions, aerosols, suspensions). This also includes blood. We will discuss this in more detail below.
Blood as the internal environment of the body
As you know, 80% of the blood is plasma, which has a liquid state of aggregation, and the remaining 20% is red blood cells, platelets, white blood cells and various inclusions. Human erythrocytes have a diameter of 8 nm. In a stationary state, they form aggregates in the form of coin columns, while significantly increasing the viscosity of the liquid. If the blood flow is active, these “constructions” disintegrate, and the internal friction, respectively, decreases.
Medium viscosity ratios
The interaction of the layers of the medium at each other affects the characteristics of the entire system of a liquid or gas. Viscosity is one example of a physical phenomenon such as friction. Thanks to it, the upper and lower layers of the medium gradually equalize the speeds of their current, and in the end it is equal to zero. Viscosity can also be characterized as the resistance of one layer of the medium to another.
To describe such phenomena, there are two qualitative characteristics of internal friction:
- dynamic coefficient of viscosity (dynamic viscosity of the fluid);
- kinetic coefficient of viscosity (kinetic viscosity).
Both quantities are related by the equation υ = η / ρ, where ρ is the density of the medium, υ is the kinetic viscosity, and η is the dynamic viscosity.
Methods for determining fluid viscosity
Viscometry is a measure of viscosity. At the present stage of development of science, it is possible to find the value of the viscosity of a liquid in practical ways in four ways:
1. Capillary method. For this purpose it is necessary to have two vessels connected by a glass channel of small diameter of known length. You also need to know the pressure values in one vessel and in the other. The liquid is placed in a glass channel, and for a certain period of time it flows from one flask to another.
Further calculations are performed using the Poiseuille formula to find the value of the fluid viscosity coefficient.
In practice, liquid media can be mixtures heated to 200-300 degrees. An ordinary glass tube in such conditions would simply be deformed or even burst, which is unacceptable. Modern capillary viscometers are assembled from high-quality and resistant material that easily endures such loads.
2. Medical method according to Hesse.To calculate the viscosity of a liquid in this way, it is necessary to have not one, but two identical capillary installations. In one of them put the environment with a previously known value of internal friction, and in the other - the test liquid. Next, measure two time values and make up the proportion by which they reach the desired number.
3. Rotational method. For its implementation it is necessary to have a design of two coaxial cylinders. This means that one of them must be inside the other. In the interval between them pour liquid, and then give the speed of the inner cylinder. This angular velocity is also reported to be fluid. The difference in force of the moment allows us to calculate the viscosity of the medium.
4. Determination of fluid viscosity by the Stokes method. To carry out this experience, it is necessary to have a Geppler viscometer, which is a cylinder filled with liquid. Before the start of the experiment, two marks are made on the cylinder and the length between them is measured. Then take a ball of a certain radius R and lower it into a liquid medium. To determine the speed of its fall, find the time of movement of the object from one label to another.Knowing the speed of the ball, you can calculate the viscosity of the fluid.
Practical application of viscometers
The determination of fluid viscosity is of great practical importance in the oil refining industry. When working with multiphase dispersed media, it is important to know their physical properties, especially internal friction. Modern viscometers are made of durable materials, in their production they use advanced technologies. All this together allows you to work with high temperature and pressure without harm to the equipment itself.
Fluid viscosity plays a large role in industry because the transportation, processing and production of, for example, oil depends on the values of the internal friction of the fluid mixture.
What role does viscosity play in medical equipment?
The flow of the gas mixture through the endotracheal tube depends on the internal friction of this gas. The change in the viscosity of the medium here is differently reflected in the penetration of air through the apparatus and depends on the composition of the gas mixture.
The introduction of drugs, vaccines through a syringe is also a vivid example of the action of viscosity of the medium.We are talking about pressure drops at the end of the needle when injecting a liquid, although it was initially believed that this physical phenomenon can be neglected. The occurrence of high pressure on the tip is the result of internal friction.
The viscosity of the medium is one of the physical quantities, which has a great practical application. In the laboratory, industry, medicine - in all these areas the concept of internal friction appears very often. The work of the simplest laboratory equipment may depend on the degree of viscosity of the medium, which is used for research. Even the processing industry can not do without knowledge in the field of physics.