The viscosity of simple liquids,
depends only on composition, temperature, and pressure.
It increases moderately with increasing pressure and markedly with decreasing temperature
. This is because pressure and temperature affect the intermolecular forces between the molecules of the liquid. At lower temperatures, the molecules move more slowly, allowing stronger intermolecular forces to take hold and increase viscosity. Similarly, increased pressure forces the molecules closer together, also increasing the strength of intermolecular forces and viscosity.
However, the viscosity of complex liquids, such as polymers, suspensions, or colloids, can also be influenced by factors such as molecular weight, branching, crosslinking, and concentration. These factors can cause the viscosity of these liquids to exhibit non-Newtonian behavior, meaning their viscosity changes with applied stress or rate of deformation.