Hamaker constant

In molecular physics, the Hamaker constant (denoted $A$ ; named for H. C. Hamaker) is a physical constant that can be defined for a van der Waals (vdW) body–body interaction:

A = π^{2} C ρ_{1} ρ_{2},

where $ρ 1, ρ 2$ are the number densities of the two interacting kinds of particles, and $C$ is the London coefficient in the particle–particle pair interaction.^[1]^[2] The magnitude of this constant reflects the strength of the vdW-force between two particles, or between a particle and a substrate.^[1]

The Hamaker constant provides the means to determine the interaction parameter $C$ from the vdW-pair potential,

w (r) = \frac{- C}{r^{6}} .

Hamaker's method and the associated Hamaker constant ignores the influence of an intervening medium between the two particles of interaction. In 1956 Lifshitz developed a description of the vdW energy but with consideration of the dielectric properties of this intervening medium (often a continuous phase).^[3]

The Van der Waals forces are effective only up to several hundred angstroms. When the interactions are too far apart, the dispersion potential decays faster than $1 / r^{6};$ this is called the retarded regime, and the result is a Casimir–Polder force.

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