Viskoosi

Viskoosi describes a property of fluids that resist flow due to internal friction between their layers. In science, viscosity quantifies this resistance. There are two related quantities: dynamic viscosity (mu) and kinematic viscosity (nu). Dynamic viscosity is defined as the ratio of shear stress to shear rate (mu = tau / gamma_dot) and has units of pascal-seconds (Pa·s). Kinematic viscosity is dynamic viscosity divided by density (nu = mu / rho) and has units of square meters per second (m^2/s).

Fluids can be Newtonian, with a constant viscosity independent of shear rate (for example, water or light mineral oils at a fixed temperature), or non-Newtonian, where viscosity changes with shear rate or time. Non-Newtonian behaviors include shear-thinning (viscosity decreases with higher shear, as in ketchup), shear-thickening (viscosity increases with shear, seen in some starch suspensions), and Bingham plastics (a yield stress must be exceeded before flow begins).

Viscosity depends strongly on temperature and, to a lesser extent, pressure. For liquids, increasing temperature generally reduces viscosity; for gases, viscosity can increase with pressure under certain conditions. Viscosity is measured with viscometers and rheometers. Capillary viscometers assess flow through narrow tubes, while rotational viscometers impose shear via a rotating spindle and measure torque. Rheometers explore how viscosity changes with shear rate, time, and temperature.

Applications are broad, including lubrication, hydraulics, food and beverage processing, cosmetics and pharmaceuticals, polymer processing, and geophysical and biological contexts. Typical room-temperature values show water around 0.001 Pa·s, olive oil about 0.1–0.2 Pa·s, glycerol near 1–2 Pa·s, and honey in the range of 2–10 Pa·s, illustrating the wide range of viscoelastic behavior across liquids.