Roger H. French
DuPont Company, Central Research, E356-384 Experimental
Station, Wilmington DE 19880.
Materials Science Department, University of Pennsylvania,
Philadelphia PA, 19104
The London dispersion forces, along with the Debye and Keesom
forces, constitute the long-range van der Waals forces. London and Hamaker’s
work on the point to point dispersion interaction and Lifshitz’s development
of the continuum theory of dispersion are the foundations of our understanding
of dispersion forces. Dispersion forces are present for all materials and are
intrinsically related to the optical properties and the underlying interband
electronic structure of materials. The force law scaling constant of the
dispersion force, known as the Hamaker constant, can be determined from spectral
or parametric optical properties of materials combined with knowledge of the
configuration of the materials. With recent access to new experimental and ab
initio tools for determination of optical properties of materials,
dispersion force research has new opportunities for detailed studies.
Opportunities include development of improved index approximations and
parametric representations of the optical properties for estimation of Hamaker
constants. Expanded databases of London dispersion spectra of materials will
permit accurate estimation of both non-retarded and retarded dispersion forces
in complex configurations. Development of solutions for generalized multilayer
configurations of materials are needed for the treatment of more complex
problems such as graded interfaces.
Dispersion forces can play a critical role in materials
applications. Typically they are a component with other forces in a force
balance, and it is this balance that dictates the resulting behavior. The
ubiquitous nature of the London dispersion forces makes them a factor in a wide
spectrum of problems; they have been in evidence since the pioneering work of
Young and Laplace on wetting, contact angles, and surface energies. Additional
applications include the interparticle forces which can be measured by direct
techniques such as atomic force microscopy. London dispersion forces are
important in both adhesion and in sintering where the detailed shape at the
crack tip and at the sintering neck can be controlled by the dispersion forces.
Dispersion forces have an important role in the properties of numerous ceramics
which contain intergranular films, and here the opportunity exists for the
development of an integrated understanding of intergranular films which
encompasses dispersion forces, segregation, multilayer adsorption, and
structure. The intrinsic length scale at which there is a transition from the
continuum perspective (dispersion forces), to the atomistic perspective
(encompassing interatomic bonds), is critical in many materials problems, and
the relationship of dispersion forces and intergranular films may represent an
important opportunity to probe this topic.
The London dispersion force is retarded at large separations,
where the transit time of the electromagnetic interaction must be considered
explicitly. Novel phenomena, such as equilibrium surficial films and bimodal
wetting / dewetting, can arise in materials systems when the characteristic
wavelengths of the interatomic bonds and the physical interlayer thicknesses
lead to a change in the sign of the dispersion force. Use of these novel
phenomena in future materials applications raises interesting opportunities in
materials design.
