Viscosity is a quantity that describes fluid's resistance to flow. It resists the relative motion of an object immersed in it and also the relative motion of layers with differing velocities. This property of fluid in broad sense depends on the cohesive forces which describe how molecules interact with each other. The forces between the molecules of fluid are in specific called intermolecular forces and the viscosity depends on the strength of the intermolecular forces. Stronger the intermolecular forces, more viscous the fluid will be. It also perhaps depend on molecular structure and the length of the molecule. Then the question comes what controls the strength of these inter molecular forces? Why does water have stronger intermolecular forces than methane, even though they have about the same mass? This is due to differing charges on adjacent molecules that lead to electrostatic attractions which obey Coulomb's Law. All the inter molecular forces are categorized by the origin of their charges and the origin of charges differ for polar and non polar molecules.

1. Dipole-Dipole forces
2. Dipole-induced dipole forces
3. Hydrogen bonding
4. Induced dipole-induced dipole forces

F = qm/Wr3