A study of finite-size, non-perturbative and anisotropic effects on the Lifshitz-van der Waals forces and torque with material dielectric responses from first-principles calculations

Abstract: The van der Waals and the Casimir-Lifshitz forces are forces of attraction that exist between neutral polarizable bodies due to quantum fluctuations. They can be repulsive depending on the material properties and the geometry of the system. Since they are pervasive in nature, they encompass a great deal of relevance in the study of interaction between bodies in several different scenarios and background geometrical settings. This doctoral thesis first addresses the important aspects of finite-size and the non-perturbative effects of the van der Waals interactions between two atoms or molecules. Going beyond the usual assumption of atoms and molecules as point particles and adopting a description of finite size, the divergence inherent in such interaction energies in the limit of zero separation distance between the two interacting atoms or molecules is removed. The attainment of finite interaction energy at such close separation distance facilitates the estimation of van der Waals force contribution to the binding energy of the molecules, and towards surfaces. This is particularly important for noble atoms. The interaction between a pair of helium (He) atoms and krypton (Kr) atoms, and between a pair of methane (CH$_4$) molecules considering its environmental relevance, is investigated in detail. The application of finite size further leads to finite self energies of the atoms. The full expression of the interaction energy, as is discussed in detail in this thesis, typically contains a logarithmic factor of the form $\ln(1 \pm x)$. Formerly, in evaluating the interaction energies, this factor is customarily series-expanded and truncated in the leading order with certain assumptions. This thesis explores the effect of using the full expression, which is referred herein as the non-perturbative (or, the non-expanded) theory, analytically wherever possible as well as numerically. The combined application of the finite-size theory and the non-perturbative theory results in as much as 100\,\% correction in the self energy of atoms in vacuum. This may give rise to significant physical consequences, for example, in the permeabilities of atoms across dielectric membranes. The thesis next addresses the aspect of anisotropy in the Casimir-Polder interaction between a completely polarizable molecule and a dielectric slab polarizable in the normal direction. The formalism is applied to the study of preferential adsorption in the specific case of carbon dioxide (CO$_2$) and methane (CH$_4$) molecules interacting with amorphous silica slabs and thin gold films. Owing to its greater polarizability, the linearly polarizable CO$_2$ molecule is found to attract more towards the surfaces than the isotropically polarizable CH$_4$ molecule. In addition, the stable orientation of the CO$_2$ molecule with respect to the surface is determined to be the one in which the long, linear axis of the molecule is perpendicular to the surface. Further, the feature of Casimir torque which is a consequence of anisotropy in the interacting dielectric slabs is explored in the case of biaxial materials, in particular the bulk black phosphorus and its novel 2D counterpart phosphorene. The torque between a pair of phosphorus slabs, one face rotated with respect to the other, is observed to change sign at a particular separation distance which is determined by the crossing frequency of its planar dielectric components. This distance-dependent reversal of the sign of torque has never been observed before. The observation is verified with several other biaxial materials. This finding will help assist in the experimental detection of the Casimir torque, and can potentially be exploited in the future for designing nanodevices. Another remarkable effect that is uncovered is the submersion of ice microcrystals under water governed by the balance of repulsive Lifshitz force from the vapor-water interface and the buoyant force of water. The repulsive effect is found to be enhanced by the presence of salt ions in the system. An exclusion zone ranging from 2 nm to 1 ${\mu}$m devoid of small ice particles is formed below the water surface. As the ice sphere grows in size, the buoyant force overcomes the Lifshitz force, and the ice sphere starts to float with a fraction of its volume above the water surface in accordance with the classical Archimedes principle. The combined impact of Lifshitz forces and double-layer interactions is further investigated in ice-water-CO$_2$ and vapor-water-CO$_2$ systems  employing different models of effective polarizability for ions, {\it viz.} the hardsphere model and Onsager's model. The CO$_2$ bubble is found to be repelled by the vapor-water interface and attracted towards the ice-water interface. The equilibrium thin film of water formed between vapor and ice surfaces varies in thickness depending on the model of effective polarizability and the type of salt present in the system. Further studies of the interaction energy in geometries comprising two molecules near an interface and molecule in a three-layer geometry are conducted which may be relevant for potential energy storage applications. The density functional theory (DFT) is employed to generate the frequency-dependent dielectric functions required for Lifshitz energy and force calculations. Summing up, in the numerous contexts outlined above, the importance of the van der Waals and Lifshitz forces has been demonstrated. The studies in this thesis enable significant predictions related to these forces which may be verifiable by experiments.