Viscosity

Viscosity measures the internal friction within a flowing liquid -- how strongly its layers resist sliding past one another. When a liquid flows through a tube in an orderly, layer-by-layer way (laminar flow), the layer touching the tube's inner wall is essentially stationary due to friction, while the central layer, furthest from the wall, moves fastest; this sets up a velocity gradient across the tube's cross-section. Strong intermolecular forces resist the layers slipping past each other, producing friction between the layers -- this internal friction is viscosity, formally defined via the coefficient of viscosity $\eta$ (eta): the tangential frictional force per unit area, per unit velocity gradient, needed to keep the flow laminar. Its SI unit is N s m$^{-2}$; the older CGS unit is the poise, where $1$ poise $= 1$ g cm$^{-1}$ s$^{-1} = 10^{-1}$ kg m$^{-1}$ s$^{-1}$. Viscosity is temperature-dependent, roughly $\eta \propto 1/T$ -- it falls as temperature rises, because increased thermal motion partly overcomes the intermolecular attraction resisting flow. Viscosity also depends on molecular size and shape: larger molecules are generally more viscous, while more compact, spherical molecules offer less resistance to flow and are therefore less viscous than long, spread-out ones of similar mass; the greater a liquid's viscosity, the more slowly it flows. Practical illustrations include the grading of lubricating oils by viscosity (a good lubricant keeps a steady viscosity across a temperature range), raised blood viscosity as a marker of car …