Sign of the Casimir-Polder interaction between atoms and oil-water interfaces: Subtle dependence on dielectric properties

Abstract

We demonstrate that Casimir-Polder energies between noble gas atoms (dissolved in water) and oil-water interfaces are highly surface specific. Both repulsion (e.g., hexane) and attraction (e.g., glycerine and cyclodecane) is found with different oils. For several intermediate oils (e.g., hexadecane, decane, and cyclohexane) both attraction and repulsion can be found in the same system. Near these oil-water interfaces the interaction is repulsive in the nonretarded limit and turns attractive at larger distances as retardation becomes important. These highly surface specific interactions may have a role to play in biological systems where the surface may be more or less accessible to dissolved atoms. The Casimir-Polder interaction [1] between polarizable particles and a wall has received intense attention in recent decades. The theoretical framework for this interaction was worked out a long time ago [1][2][3][4][5], yet it remains a topic of great interest due to its many applications in biological, chemical, and atomic systems. For reviews, cf., e.g., [6][7][8].An interesting aspect of the Casimir-Lifshitz and CasimirPolder forces is that according to theories these forces can either be attractive or repulsive. Anderson and Sabiski performed experiments on films of liquid helium on calcium fluorite, and similar molecularly smooth surfaces [9]. The film thicknesses ranged from 10 to 200Å [9]. In these measurements the repulsive van der Waals potential opposed the gravitational potential [9]. A good agreement was found [10] between experimental data and Lifshitz theory [3]. In a set of experiments that inspired the present work, Hauxwell and Ottewill [11] measured the thickness of films of oil on water near the alkane saturated vapor pressure, an asymmetric system (oil-water-air) in which the surfaces are molecularly smooth. For this system n-alkanes up to octane spread on water. Higher alkanes do not spread. The phenomenon depends on a balance of van der Waals forces against the vapor pressure [11,12]. The net force, as a function of film thickness, depends on the dielectric properties of the oils [13]. As demonstrated [12] it involves an intricate balance of repulsive and attractive components from different frequency regimes. When the ultraviolet components are exponentially damped by retardation, the opposing (repulsive) infrared and visible components take over [14,15]. Other studies discussing the transition between attractive and repulsive interactions are found in Refs. [16][17][18][19].Surfaces of interest in biology and biotechnology may involve alkane molecules creating an oil-water interface. In this Brief Report we will demonstrate that the Casimir-Polder interaction between dissolved atoms and different oil-water * mabos@ifm.liu.se † Drew.Parsons@anu.edu.au ‡ bos@ifm.liu.se interfaces may be either repulsive or attractive, or as we will see for some of the alkanes, it may change from repulsion to attraction as the distance to the interface increases. The relevant geometry is sketched in Fig. 1. The n...

Nontrivial retardation effects in dispersion forces: From anomalous distance dependence to novel traps

“…Here, we demonstrate that a dispersion potential alone, even in the absence of a layered surface, has the ability to trap particles without the presence of any balancing forces. In contrast to previous works, where for instance retardation effects of the Casimir force have been predicted [22] or measured [23], we demonstrate that the thermal limit is more important for medium-assisted dispersion forces. Furthermore, we demonstrate, the crossings of dielectric function also may yield a breakdown of the retarded asymptote, leading to unexpected potential behaviours.…”

Electromagnetic fluctuation-induced interactions in randomly charged slabs

“…We now consider a situation where ǫ 1 (ıξ) < ǫ m (ıξ) < ǫ 2 (ıξ) and again only the two semi-infinite slabs carry disorder charges (g m = 0). In this case, the pure Casimir-vdW force is known to be repulsive [21,36,37] but the disorder force may be repulsive or attractive depending on the static dielectric constants [18]. In Fig.…”

Finite-size–dependent dispersion potentials between atoms and ions dissolved in water

“…They were presented recently by Boström, Parsons, and co-workers. [13] Dynamic polarisabilities of noble gas atoms in vacuum were calculated using Molpro [14] at a coupled cluster singles and double (CCSD) level of theory. An aug-cc-pV6Z basis set [15,16] was used for a selection of noble gas atoms (He, Ne and Ar while aug-cc-pV5Z was used for Kr [17]).…”

“…The excess polarisabilities (α * (iξ)) and atomic radii (a) for helium and other noble-gas atoms dissolved in water were derived as in several papers by Parsons and Ninham [11,12]. They were presented recently by Boström, Parsons, and co-workers [13]. Dynamic polarisabilities of noble-gas atoms in vacuum were calculated using Molpro [14] at a coupled-cluster single and double (CCSD) level of theory.…”