Attractive forces between anisotropic inclusions in the membrane of a vesicle

The fluctuation-induced interaction between two rod-like, rigid inclusions in a fluid vesicle is studied by means of canonical ensemble Monte-Carlo simulations. The vesicle membrane is represented by a triangulated network of hard spheres. Five rigidly connected hard spheres form rod-like inclusions that can leap between sites of the triangular network. Their effective interaction potential is computed as a function of mutual distance and angle of the inclusions. On account of the hard-core potential among these, the nature of the potential is purely entropic. Special precaution is taken to reduce lattice artifacts and the influence of finite-size effects due to the spherical geometry. Our results show that the effective potential is attractive and short-range compared with the rod length L. Its well depth is of the order of , where is the bending modulus. Received 5 February 1999 and Received in final form 14 May 1999     

Attractive and repulsive Casimir vacuum energy with general boundary conditions

The infrared behaviour of quantum field theories confined in bounded domains is strongly dependent on the shape and structure of space boundaries. The most significant physical effect arises in the behaviour of the vacuum energy. The Casimir energy can be attractive or repulsive depending on the nature of the boundary. We calculate the vacuum energy for a massless scalar field confined between two homogeneous parallel plates with the most general type of boundary conditions depending on four parameters. The analysis provides a powerful method to identify which boundary conditions generate attractive or repulsive Casimir forces between the plates. In the interface between both regimes we find a very interesting family of boundary conditions which do not induce any type of Casimir force. We also show that the attractive regime holds far beyond identical boundary conditions for the two plates required by the Kenneth–Klich theorem and that the strongest attractive Casimir force appears for periodic boundary conditions whereas the strongest repulsive Casimir force corresponds to anti-periodic boundary conditions. Most of the analysed boundary conditions are new and some of them can be physically implemented with metamaterials.     

Tailoring repulsive optical forces in nanophotonic waveguides

We describe a mechanism and propose design strategies to selectively tailor repulsive-gradient-optical forces between parallel, nanophotonic waveguides via morphology augmented by slow-light band-edge modes. We show that at small separation lengths, the repulsive force can be made nearly 2 orders of magnitude larger than that of standard dielectric waveguides with a square cross section. The increased coupling interactions should enable a wider dynamic range of optomechanical functionality for potential applications in sensing, switching, and nanoelectromechanical systems.     

Attractive Casimir forces in a closed geometry

We study the Casimir force acting on a conducting piston with arbitrary cross section. We find the exact solution for a rectangular cross section and the first three terms in the asymptotic expansion for small height to width ratio when the cross section is arbitrary. Though weakened by the presence of the walls, the Casimir force turns out to be always attractive. Claims of repulsive Casimir forces for related configurations, like the cube, are invalidated by cutoff dependence.     

Measurement of long-range repulsive forces between charged particles at an oil-water interface

Using a laser tweezers method, we have determined the long-range repulsive force as a function of separation between two charged, spherical polystyrene particles (2.7 microm diameter) present at a nonpolar oil-water interface. At large separations (6 to 12 microm between particle centers) the force is found to decay with distance to the power -4 and is insensitive to the ionic strength of the aqueous phase. The results are consistent with a model in which the repulsion arises primarily from the presence of a very small residual electric charge at the particle-oil interface. This charge corresponds to a fractional dissociation of the total ionizable (sulfate) groups present at the particle-oil surface of approximately 3 x 10(-4).     

On the dispersion forces between molecules in an active medium

The theoretical analysis by Fröhlich and by Genzel of a medium's influence on the dispersion forces between molecules is criticized. A corrected treatment is given for some model systems. No anomalies are apparent.     

Crossover from attractive to repulsive Casimir forces and vice versa

Systems described by an O(n) symmetrical varphi;{4} Hamiltonian are considered in a d-dimensional film geometry at their bulk critical points. The critical Casimir forces between the film's boundary planes B_{j}, j=1,2, are investigated as functions of film thickness L for generic symmetry-preserving boundary conditions partial differential_{n}phi=c[over composite function]_{j}phi. The L-dependent part of the reduced excess free energy per cross-sectional area takes the scaling form f_{res} approximately D(c_{1}L;{Phi/nu},c_{2}L;{Phi/nu})/L;{d-1} when d<4, where c_{i} are scaling fields associated with the variables c[over composite function]_{i} and Phi is a surface crossover exponent. Explicit two-loop renormalization group results for the function D(c_{1},c_{2}) at d=4- dimensions are presented. These show that (i) the Casimir force can have either sign, depending on c_{1} and c_{2}, and (ii) for appropriate choices of the enhancements c[over composite function]_{j}, crossovers from attraction to repulsion and vice versa occur as L increases.     

An experiment concerning the dispersion forces between very small metal spheres

The London-Van der Waals attraction between very small spheres is determined in an experiment with colloidal particles. The measured value of the attraction constant is a function of the particle size. It is ten times smaller than values calculated with the macroscopic theory of dispersion forces on the basis of data for a bulk material.     

Surface polariton response function approach to dispersion forces between macroscopic bodies

A new method for the calculation of dispersion forces between macroscopic bodies is developed. The basis idea here is to use the surface polariton response function and the fluctuation-dissipation theorem.     

Appropriate PEG compounds for dispersion of single wall carbon nanohorns in salted aqueous solution

Aiming at improving the dispersion state of single wall carbon nanohorns (SWNHs) in highly salted aqueous environment for potential biological applications, we compared the dispersion ability of different PEG-based dispersants in this work. We have found that ceramide-conjugated poly(ethylene glycol) (CPEG) dispersed SWNHs better than phospholipid-conjugated PEG (DPEG) in both water and phosphate buffer saline (PBS), which was evidenced by the measurements of particle sizes and dispersion stability. The large zeta potentials, according to the high surface charges, did not fully explain the high dispersion ability of CPEG for SWNHs. We propose that the more neutral linker group between alkyl and PEG chains in CPEG, in contrast to the charged phosphoethanolamino group in DPEG, resulted in the larger number of CPEG molecules attaching on SWNHs, and those PEG molecules enhanced the dispersion of SWNHs in water and PBS. In addition, for the first time we reported the adverse effect of the amino terminal group of PEG chain on the dispersion ability, compared with the methyl terminal group.     

Possible role of dispersion forces in fracture

Dispersion (London) forces act between any two solid bodies. These forces are small except when the bodies are close together. They decrease rapidly as the distance between the bodies increases. Near the tip of a crack one body separates into two, and the two pieces are close together for a short distance. Thus, dispersion forces may play a role in the behaviour of crack tips. They cannot change the intrinsic surface energy, but they can affect the tip's width, and therefore, the time needed (at a given velocity) to complete the separation process. This in turn may affect the time available for plastic deformation; and therefore the extrinsic fracture surface energy. This may be one reason that polarizability affects fracture. Also, it may relate to the correlation between fracture surface roughness and energy.     

Van der Waals forces and spatial dispersion

A version of the Green’s functions theory of the Van der Waals forces which can be conveniently used in the presence of spatial dispersion is presented. The theory is based on the fluctuation-dissipation theorem and is valid for interacting bodies, separated by vacuum. Objections against theories accounting for the spatial dispersion are discussed.     

Scaling of optical forces in dielectric waveguides: rigorous connection between radiation pressure and dispersion

We show that eigenmodes of dielectric optical waveguides exert surface dilation forces on waveguide boundaries owing to radiation pressure, and we develop an exact scaling law relating modal dispersion of an arbitrary dielectric waveguide to the magnitude of optical forces generated by radiation pressure. This result points to highly dispersive waveguides as an optimal choice for the generation of large optical forces in nano-optomechanical systems. Exact agreement with ab initio calculations is demonstrated.     

The non-pair forces and phonon dispersion in heavy alkali metals

Two types of the non-pair forces, one from the Born-Mayer and other from the Morse potential, are derived to discuss the response of the electrons in heavy alkali metals, i.e. rubidium and caesium. These potentials are added to the two-body potential of Morse to account for the ion-ion interactions as well. The potentials so obtained are employed to predict the phonon dispersion relations in the bcc metals, which are also compared with the recent precise neutron scattering data.