How would you define dispersion force?
London dispersion forces are considered the weakest intermolecular type of Van der Waals forces.
They are present in an instantaneous electric dipole moment in non-polar as well as polar molecules.
This type of force becomes stronger as the atom enlarges, and to a smaller degree for large molecules. This is due to the increased polarizability of molecules with more dispersed electron clouds where free electrons can be found.