Origins and applications of London dispersion forces and Hamaker constantsin ceramics

The London dispersion forces, along with the Debye and Keesom forces, const itute the long-range van der Waals forces. London's and Hamaker's work on t he 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 interban d electronic structures of materials. The force law scaling constant of the dispersion force, known as the Hamaker constant, can be determined from sp ectral or parametric optical properties of materials, combined with knowled ge of the configuration of the materials. With recent access to new experim ental and ab initio tools for determination of optical properties of materi als, dispersion force research has new opportunities for detailed studies. Opportunities include development of improved index approximations and para metric representations of the optical properties for estimation of Hamaker constants. Expanded databases of London dispersion spectra of materials wil l permit accurate estimation of both nonretarded and retarded dispersion fo rces in complex configurations. Development of solutions for generalized mu ltilayer configurations of materials are needed for the treatment of more-c omplex problems, such as graded interfaces. Dispersion forces can play a cr itical 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 m akes them a factor in a wide spectrum of problems; they have been in eviden ce since the pioneering work of Young and Laplace on wetting, contact angle s, end surface energies. Additional applications include the interparticle forces that can be measured by direct techniques, such as atomic force micr oscopy. London dispersion forces are important in both adhesion and in sint ering, where the detailed shape at the crack tip and at the sintering neck can be controlled by the dispersion forces. Dispersion forces have an impor tant role in the properties of numerous ceramics that contain intergranular films, and here the opportunity exists for the development of an integrate d understanding of intergranular films that encompasses dispersion forces, segregation, multilayer adsorption, and structure. The intrinsic length sca le at which there is a transition from the continuum perspective (dispersio n forces) to the atomistic perspective (encompassing interatomic bonds) is critical in many materials problems, and the relationship of dispersion for ces and intergranular films may represent an important opportunity to probe this topic.The London dispersion force is retarded at large separations, w here the transit time of the electromagnetic interaction must be considered explicitly. Novel phenomena, such as equilibrium surficial films and binod al wetting/dewetting, can result in materials systems when the characterist ic wavelengths of the interatomic bonds and the physical interlayer thickne sses lead to a change in the sign of the dispersion force. Use of these nov el phenomena in future materials applications pro,ides interesting opportun ities in materials design.