Viscosity of a liquid

Rheology is a science about deformations and fluidity of substance. studying of biophysical features of blood as viscous liquid is rheology of blood (gemorheology). In a real liquid between molecules the forces of a mutual attraction, causing internal friction operate. Internal friction, for example, causes force of resistance at mixing of a liquid.

In a real liquid of a molecule are drawn to each other and create internal friction. It is shown as force of resistance at hashing a liquid. At laminar current the liquid shares on layers, moving in parallel with various speeds. Newton has established, that force of internal resistance between two layers depends by nature liquids and is directly proportional to the area of adjoining layers and change of speed at transition from a layer to a layer dv/dz

F= ηS dv/dz,

where η - factor of viscosity of a liquid or simply viscosity. Unit of measure of factor of viscosity in SI is Pascal-second. The liquid in which on a layer the area 1m operates force of internal friction 1 Newton at a gradient of speed 1m/sec has such viscosity.

However at measurements use other unit - poise is more often. 1 poise - there is a viscosity of a liquid, if on a layer 1 cm at a gradient of speed 1N/sec force 1 dyne operates. Other unit of viscosity is N∙s/m².

The viscosity is depended on condition and molecular properties of liquids (or gase). For the characteristic of viscosity of a liquid in practice relative viscosity of a liquid is used η_r= η/η₀, η- viscosity of a liquid η₀ - viscosity of water at identical temperature. At the majority of liquids (the water, the fused metals and their salts low-molecular connections) viscosity depends by nature liquids and temperatures. Such liquids refer to Nuton's liquid. At some liquids (solutions of polymers) or dispersive systems (suspensions) viscosity depends from also on a mode of current of a liquid - pressure and a gradient of speed. Such liquids refer to nonNuton's liquid.

Distinguish laminar and turbulent current of a liquid At laminar current a liquid it is divided into molecular layers which move with various speeds do not mix up Such character of current of a liquid has at slow movement of a liquid on a pipe the layer of the molecules adjoining to a wall of a pipe, sticks to it and does not move. The following layer of molecules is displaced under action of force of pressure and force of internal friction. Each following layer of molecules is displaced concerning previous and speed of each layer increases. Thus, speed of molecular layers of a liquid is maximal in the center of a pipe.

If speed of particles in a liquid randomly varies, particles pass from one layer in another such movement of a liquid refers to turbulent.

If the liquid flows on a horizontal pipe its potential energy is spent for work on overcoming internal friction, therefore static pressure gradually decreases.

The blood is suspension of formed elements in protein's solution-plasma. Therefore it must be nonNewton liquid. In addition at movement of blood in vessels the concentration of formed elements increases in central part of stream wherein consequence of the viscosity increased. But so far as the blood's viscosity is not so great these phenomenon are neglected and blood's coefficient of viscosity is considering being constant. The relative viscosity of blood is completed in norm 4,2-6. Under pathology it can decrease till 2-3 (under anaemia) or increase till 15-20 (under polycetemia). The viscosity of blood's serum is in standard 1,64-1,69 and under pathology -1,5-2,0.

Change of viscosity of blood is one of the reasons of change of speed of subsidence erythrocytes. Viscosity of blood has diagnostic value. Some infectious diseases increase viscosity, others, for example a belly typhus and a tuberculosis - reduce.

Relative viscosity serum of blood in norm 1,64-1,69 and at a pathology 1,5-2,0. As well as at any liquid, viscosity of blood increases at decrease of the temperature. At increase of rigidity of erythrocytic membranes, for example at an atherosclerosis, viscosity of blood also increases, that leads to increase in loading at heart. Viscosity of blood different in wide and narrow vessels, and influence of diameter of a blood vessel on viscosity starts to affect at a gleam less than 1 mm. In vessels 0,5 mm viscosity decreases in linear response to reduction of diameter, because erythrocytes are built along an axis of vessels in a chain like “snake” and are surrounded by a layer of the plasma isolating “snake” from a vascular wall.

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