Viscosity

VISCOSITY, or internal friction, is the resistance that manifests itself when some particles of a substance move in relation to others. The concept of "internal friction" is applicable to liquid, solid, and gaseous substances alike, whereas the term viscosity is used predominantly in relation to liquids. In a biological respect, of the greatest significance is precisely the viscosity, or internal friction, of liquid and semi-liquid substances, particularly protoplasm. This significance of viscosity lies in the fact that changes in it substantially affect both the speed of purely chemical reactions proceeding within the system and a number of physical phenomena of paramount importance for vital processes in the cell. Thus, with an increase in viscosity, the speed of diffusion decreases, electrical conductivity drops, and Brownian motion (see) slows down up to a complete stop. Furthermore, the mechanical resistance experienced by a liquid when passing through narrow spaces (capillaries, intercellular spaces) changes sharply. The dependence of this resistance on viscosity is often taken as the basis for measuring viscosity; according to the so-called Poiseuille law, this dependence can be expressed by the following formula: where v is the volume of liquid flowing during time t through a capillary with cross-section q and length l, at constant pressure p and liquid viscosity. While the viscosity of dispersions and suspension colloids hardly differs from the viscosity of the liquid part of the suspension, the presence of emulsion (lyophilic) colloids has a sharp effect on viscosity. This is caused by the general rule according to which viscosity is higher the more strongly the suspended particles are hydrated (more precisely, solvated, i.e., the more closely they are bound to the solvent molecules). Therefore, all factors influencing the degree of hydration of a colloid sharply alter its viscosity. From this follows an important consequence: the possibility of judging the degree of swelling or hydration of a colloid from its viscosity. In colloids, and particularly in the constituent parts of the cell, true viscosity often passes, through completely imperceptible gradations, into elasticity, which is already predominantly a property of a solid body. Therefore, one often speaks of elastic, or structural, viscosity. The difference is most clearly seen in an example: with true viscosity, a particle of iron located in a given medium under the influence of an electromagnet will move slowly and, when the current is turned off, will remain in the very place where it was at the last moment. In the presence of elastic forces, however, such a particle can be displaced from its initial position, and when the electromagnet current is turned off, it will return back again. The viscosity of protoplasm fluctuates within extremely wide limits; thus, for instance, Heilbronn found values of 1-2 for the eggs of certain echinoderms (water = 1), and Fetter found 8.726 for Paramecium. It reaches a maximum at 15°. Under the influence of narcotics, the viscosity of protoplasm decreases. To measure the viscosity of protoplasm, one usually uses the measurement of the falling speed of intracellular inclusions (for example, starch grains), and to increase the effect of gravity, centrifugation is resorted to. A more perfect method is the application of micrurgy (see): a microscopic piece of iron is introduced into the cell and, acting upon it with an electromagnet, the current strength necessary for a certain movement of this particle in the cell protoplasm and in pure water is compared. (For the methodology of measuring the viscosity of ordinary liquids, see Viscosimetry.)