viskozita

Viskozita is a property of fluids that describes their internal resistance to flow. It reflects how easily layers of a liquid slide past one another and is a key factor in determining how a fluid behaves when poured, pumped, or sheared.

There are two common measures. Dynamic (or absolute) viscosity, denoted mu, expresses the internal friction between adjacent layers and has units of pascal-seconds (Pa·s) in SI. Kinematic viscosity, denoted nu, relates mu to fluid density (nu = mu / rho) and has units of square meters per second (m^2/s). The viscosity of a fluid generally decreases as temperature rises, although behavior can vary among mixtures and suspensions.

Fluids are further categorized by how viscosity depends on shear rate. Newtonian fluids maintain a constant viscosity regardless of the applied shear rate (e.g., water, mineral oil). Non-Newtonian fluids exhibit viscosity that changes with shear rate, showing phenomena such as shear thinning or thickening, thixotropy, or rheopexy.

Measuring viscosity involves viscometers or rheometers. Capillary viscometers (like Ubbelohde or Ostwald) and rotational viscometers are common. Temperature control is essential, as viscosity is highly temperature dependent. Units and reference values vary widely: water at room temperature has about 1.0 mPa·s (0.001 Pa·s); honey can reach many pascal-seconds; motor oils typically range from ~0.1 to a few tenths of a Pa·s.

Applications span engineering, lubrication, hydraulic systems, polymer processing, food science, and geology. Understanding viscosity helps predict flow in pipes, design pumping equipment, and optimize mixing and heat transfer processes. The term derives from Latin viscosus meaning sticky; historically, viscosity has been linked with Poiseuille’s studies and the broader development of rheology.