Nontouching Nanoparticle Diclusters Bound by Repulsive and Attractive Casimir Forces

Abstract

We present a scheme for obtaining stable Casimir suspension of dielectric nontouching objects immersed in a fluid, validated here in various geometries consisting of ethanol-separated dielectric spheres and semi-infinite slabs. Stability is induced by the dispersion properties of real dielectric (monolithic) materials. A consequence of this effect is the possibility of stable configurations (clusters) of compact objects, which we illustrate via a "molecular" two-sphere dicluster geometry consiting of two bound spheres levitated above a gold slab. Our calculations also reveal a strong interplay between material and geometric dispersion, and this is exemplified by the qualitatively different stability behavior observed in planar versus spherical geometries.

Casimir microsphere diclusters and three-body effects in fluids

“…3 shows the isolated Si-PS and TefSi sphere-sphere equilibrium separation d e as a function of the radius R of the spheres. As a consequence of its material dispersion (similar to phenomena observed in [4]), the Si-PS combination exhibits a bifurcation at R ≈ 55 nm where the stable and unstable equilibria, such that there is no equilibrium for larger R (the interaction is purely attractive). The Tef-Si combination exhibits no such bifurcation (even if we extend the plot to R = 300 nm), because it has no unstable equilibrium: the interaction is purely repulsive for small separations and attractive for large separations.…”

“…To begin with, let us consider sphere radii on the order of 10 2 nm, as in our previous work [4]. We wish to make a bound dicluster, at some separation d, of two spheres (Si and teflon) that are suspended above a gold substrate by Casimir repulsion in balance with gravity.…”

“…Such micro-sphere interactions are predicted to possess a variety of unusual Casimir effects, including repulsive forces, [6][7][8] a strong interplay with material dispersion [4], and strong temperature dependences [9], and may have applications in microfluidic particle suspensions [10,11]. A typical situation considered in this paper is depicted in Fig.…”

“…Alternatively, for a sphere-plate geometry in which the sphere is pulled downwards by gravity, a purely repulsive Casimir force (which dominates at small separations) will also lead to a stable suspension. These basic ideas were exploited in our previous work [4] to design sphere-sphere and sphere-plate geometries exhibiting a stable non-touching configuration. The effects of material dispersion are further modified by an interplay with geometric effects (which set additional length-scales beyond that of the separation), as well as by nonzerotemperature effects which set a Matsubara length-scale 2πkT / [14] that can further interact with dispersion in order to yield strong temperature corrections [9].…”

“…In this paper, we investigate the influence of nonadditive/three-body and nonzero-temperature effects on our earlier prediction that the Casimir force (which arises from quantum electrodynamic fluctuations [1][2][3]) can enable dielectric objects (micro-spheres) with certain material dispersions to form stable non-touching configurations (diclusters) in fluids [4,5]. Such micro-sphere interactions are predicted to possess a variety of unusual Casimir effects, including repulsive forces, [6][7][8] a strong interplay with material dispersion [4], and strong temperature dependences [9], and may have applications in microfluidic particle suspensions [10,11].…”