Hamaker coefficient negative

Sampling frequencies 25 If I take that formula seriously, then two bubbles or even two pockets of vacuum will attract across a material body. How can two nothings do anything 26 Does charge-fluctuation resonance translate into specificity of interaction 26 How does retardation come in 27 Can there ever be a negative Hamaker coefficient and a positive charge-fluctuation energy 28... 

Because the van der Waals interactions go as the negative of the Hamaker coefficients, the sum of the (negative) A — A and B — B interactions at a given separation is more negative than two A — B interactions at that same separation. 

Can there ever be a negative Hamaker coefficient and a positive charge-fluctuation energy ... 

It has been shown that the free energy of adhesion can be positive, negative, or zero, implying that van der Waals interactions can be attractive as well as repulsive [130,133,134]. While Eq. (14) can, strictly speaking, be expected to hold only for systems that interact by means of dispersion forces only, there are no restrictions on Eq. (15). Since this equation describes very well the fundamental patterns of the behavior of particles, including macromolecules, independent of the type of molecular interactions present, it was found to be convenient to define an "effective Hamaker coefficient that reflects the free energy of adhesion [130],... 

Visser Jan. The concept of negative Hamaker coefficients. 1. History and present status. Adv. Colloid Interface Sci. 15 no. 2 (1981) 157-169. 

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