These forces arise due to dipole–dipole interactions (induced/permanent) between molecules of bilayers. As molecules come closer, this attractive force arises due to the ordering of these dipoles; like in the case of magnets that align and attract each other as they approach. This also implies that any surface would experience a van der waals attraction. In bilayers, the form taken by van der Waals interaction potential V is given by
where H is the Hamaker constant and D and z are the bilayers thickness and the distance of separation respectively.