The magnitude of this attractive force is mostly determined by three factors: polarizability, the number of electrons and the distance between the two bodies. However, if molecules are close together this induces a dipole in the neighboring molecule, resulting in a small attractive force between these two dipoles: the London dispersion force ( Fig. A conceptually easier interpretation is that molecules form random dipole moments in every direction all the time. The probability of an instantaneous dipole in any direction remains the equal, however, if ψ 1 were to collapse with a dipole in a certain direction, the probability of ψ 2 also collapsing with a dipole in that direction is increased the dipole resulting from ψ 1 induces a dipole in ψ 2 leading to a small attractive force. 12 Two molecules at infinite distance have their own individual wavefunctions (ψ 1 and ψ 2), however, if they move closer together the wavefunctions will influence each other. London dispersion forces are the result of electron correlation the movement of a single electron being influenced by the presence of other electrons. More details on the dispersion forces in cement suspension can be found in Flatt (2004). This screening effect can be taken into account by calculating a screened (retarded or non-retarded) Hamaker constant. The intervening electrolyte can reduce the attractive force by interacting with the fluctuating electromagnetic field. The Hamaker constant depends on the electrolyte concentration. Hence for the size range of interest to cement suspension, the microscopic theory can be used to estimate the magnitude of the dispersion forces. Even though this separation can be increased by the presence of adsorbed polymers, the values remain small. In cement suspensions the maximum attractive force, which is relevant for yield stress calculations, occurs at a very small separation. Retardation of the Hamaker constant is taken into account in the continuum theory formulated by Lifshitz (1956). However, beyond a certain distance (>5–10 nm) the dipoles of the particles may no longer be correlated, so the value of the Hamaker constant decreases with increasing separation distance because of this effect, known as retardation. In the microscopic theory, the Hamaker constant A is considered independent of the separation distance.
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