![]() When this happens, the field returns to find that the direction of the instantaneous dipole of the first atom is now totally different from the original and is less favorably disposed to an attractive interaction. the distance traveled by light during one rotation of a Bohr atom electron is c/ ν = 3 × 10 8 m s −1/3 × 10 15 s −1 ≈ 10 −7 m or 100 nm). When two atoms are an appreciable distance apart, the time taken for the electric field of the first atom to reach the second and return can become comparable with the period 1/ ν of the fluctuating dipole itself (cf. Israelachvili, in Intermolecular and Surface Forces (Third Edition), 2011 6.9.3 Retardation Effects This would result in the interaction potential falling as 1/ r 7, rather than as 1/ r 6, as occurs in the non-retarded case for atoms very close to one another. As a result, the interactions would have to be calculated assuming retardation effects, when the distances are comparable or larger than the wavelength. The resulting interaction between the two atoms would not be between dipoles as they presently exist, but, rather, as they previously existed. By the time an electromagnetic wave originating at one atom reaches another atom located this distance away from the first, the position of the electron would have measurably changed. ![]() It should be noted that this frequency is in the UVpart of the spectra and has a corresponding wavelength of approximately 90 nm. This is the energy needed to ionize the atom and is often referred to as the first ionization potential I. This corresponds to an energy of approximately 2.2 × 10 −18 J. The frequency of oscillation v of an electron in the first Bohr orbit is approximately 3.3 × 10 15 s −1. Even for many polymers, the surface energy is dominated by dispersion forces, as estimated by measuring the contact angle made with polar and non-polar liquids such as water and diiodomethane, respectively. ![]() Similarly, semiconductors, which are still fairly polarizable, also tend to have high surface energies, although not as high as metals. As a result of its high polarizability, such a material typically has a very high surface energy. As an example of this type of interaction, consider a metal that has a clean surface and lacks permanent dipole moments. Often referred to as ‘London’ or ‘dispersion’ forces, this type of interaction is frequently the most significant contributor to van der Waals forces and it is often assumed that van der Waals forces arise solely from these interactions. The third type of interaction that contributes to van der Waals interactions arises from the formation of an instantaneous dipole in one molecule inducing an instantaneous dipole in a neighboring molecule. QUESNEL, in Adhesion Science and Engineering, 2002 6.3 Dispersion forces ![]()
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