Role Of Long-range Forces Research Articles

Cold and controlled chemical reaction dynamics.

The prospect of studying state-to-state chemical reaction dynamics, with full control over all of the reaction parameters, is becoming a reality for a small number of systems. Thanks to the rapid development of new experimental techniques (alongside novel combinations of existing methods), an increasingly diverse range of reactants can be prepared under cold conditions and manipulated with external fields. These tools are enabling the study of reactions at previously inaccessible collision energies; the role of long-range forces and quantum effects are beginning to be experimentally probed-challenging the accuracy of theoretical predictions and fundamental models of reactivity. In this perspective article, we outline the key methodologies that are adopted for the study of cold and controlled reaction dynamics. We discuss the motivation for these studies, detail the progress made to date, and highlight the future prospects for the field.

The Role of Long-Range Forces in the Determination of Translational Kinetic Energy Release. Loss of C4H4+ from the Benzene and Pyridine Cations

Kinetic energy release distributions (KERDs) for the benzene ion fragmenting into C 4H 4 (+) and C 2H 2 have been recorded by double-focusing mass spectrometry in the metastable energy window and by a retarding field experiment up to an energy of 5 eV above the fragmentation threshold. They are compared with those resulting from the HCN loss reaction from the pyridine ion. Both reactions display a similar variation of the kinetic energy release as a function of the internal energy: the average release is smaller than statistically expected, with a further restriction of the phase-space sampling for the C 5H 5N (+) dissociation. Ab initio calculations of the potential-energy profile have been carried out. They reveal a complicated reaction mechanism, the last step of which consists in the dissociation of a weakly bound ion-quadrupole or ion-dipole complex. The KERDs have been analyzed by the maximum entropy method. The fraction of phase space effectively sampled by the pair of fragments has been determined and is similar for both dissociations. Both reactions are constrained by the square root of the released translational energy, epsilon (1/2). This indicates that in the latter stage of the dissociation process, the reaction coordinate is adiabatically decoupled from the bath of the bound degrees of freedom. For the C 6H 6 (+) fragmentation, the analysis of the experimental results strongly suggests that, just as for a spherically symmetric interaction potential, the translational motion is confined to a plane. For the dissociation of the pyridine ion, the main dynamical constraint is also a restriction to a two-dimensional subspace. This dimensionality reduction of the translational phase space is due to the fact that the Hamiltonian of both weakly bound complexes contains a cyclic coordinate.

The Role of Long-Range Forces in Porin Channel Conduction

A fully self-consistent 3D Poisson P3M Brownian dynamics solver is used to investigate the role of the long-range electrostatic forces on the selectivity and conductivity of OmpF porin. Our simulations show that even with zero applied bias, the long-range interactions are an important component of the total potential energy. In addition, the long-range force due to mobile carriers is shown to play a role in facilitating the flow of anions through the OmpF channel by screening the effect of the negative fixed charge of the protein.

Measurement of density distributions for colloidal β-FeOOH rods in suspensions exhibiting phase separation: The role of long-range forces in smectic ordering

We prepared monodisperse colloidal beta-FeOOH rods with length-to-width ratios L/W of 3.6-7.0 (L=210-330 nm and W=40-58 nm). Density gradients of the rods occurred in the suspensions by gravity, inducing a phase separation. The denser phase showed smectic (Sm) liquid crystalline structures exhibiting iridescent colors in a wide range of pH from 1.2 (at which the rods interact attractively) to 4.7 (repulsively). The lower density phase was disordered, but frequently emitted diffuse colors locally (at pH>2.6), implying the occurrence of short-range order. The nematic phase was not observed in the beta-FeOOH systems, being consistent with theoretical predictions. The particle density distributions were measured over the whole region of the suspensions (separated into two phases) at various pH values using a rapid freezing method. A phase diagram was determined thereby, where the critical (minimal) packing fraction of the particles for the Sm phase showed a nonlinear decrease from 0.43 to 0.12 with increasing pH. Rod-rod spacings in the Sm phase estimated experimentally at various pH were well explained using Derjaguin-Landau-Verwey-Overbeek (DLVO) type pair potentials. It is suggested that Sm ordering can be induced by attractive minima at pH<2.2, while driven by soft repulsions at pH>2.6. The former Sm ordering is expected to be the condensation-type phase transition and the latter the disorder-order transition.

Self-destruction and dewetting of thin polymer films: the role of interfacialtensions

We present real-time optical microscopy observations of the pattern evolution inself-destruction and subsequent dewetting of thin polymer films based on experimentswith polydimethylsiloxane films sandwiched between silicon wafers and aqueoussurfactant solutions. A clear scenario consisting of four distinct stages has beenidentified: amplification of surface fluctuations, break-up of the film and formationof holes, growth and coalescence of holes, and droplet formation and ripening.Besides a linear dependence on film viscosity and surface tension, the time τfor film rupture varied significantly with film thicknessh (τ ∼ h5),as expected from theory. While the role of long-range forces is dominant only inthe first stage, the later stages are controlled by the combination of interfacialtensions resulting in the contact angle characterizing the three-phase contact line.During the first stage, the characteristic distance of the pattern remains constant,represented by a time-independent wavevector. In all subsequent stages,this wavevector decreases with time as a consequence of hole opening,coalescence, and ripening on droplets. The later stages of evolution are afunction of the contact angle at the three-phase contact line. Only a cleardistinction between stages before and after film break-up allows a correctinterpretation of the observed pattern evolution in unstable thin films.

The Role of Long Range Forces in the Stability of Nematic Films

We present an experimental study of the wetting of 5CB in the nematic phase on silicon wafers. A nematic droplet of 5CB wets completely the surface. However a characteristic structure is visible in the mesoscopic range on the thickness profile. The corresponding thickness, typically 200 Å, increases with temperature and “diverges” at the Nematic/Isotropic transition. This is characteristic of a wetting transition: a 5CB droplet in the isotropic phase does not wet the surface. In the discussion, we propose a simple model which takes into account long range interaction and we show that the balance between elastic, anchoring and van der Waals energies may fix the wetting condition.

Role of long-range forces in H sticking to liquid He.

An analysis is presented of the problem of H sticking to liquid He at low temperature. For the bulk liquid, the sticking coefficient is computed to be less than 0.01 at T${10}^{\mathrm{\ensuremath{-}}4}$ K, varying as ${\mathit{T}}^{1/2}$ for lower T. While apparently contradicted by the recent data of Doyle et al., compatibility is found if cognizance is taken of the fact that the He exists as a film in the experiments. In this case, the underlying substrate greatly enhances the attraction experienced by the H atom. The calculations demonstrate that the effects of retardation and corrections to the distorted-wave Born approximation are significant.

Brownian coagulation in aerosols—the role of long range forces

A recent solution to Brownian coagulation in a field of force (M.G. Sceats, J. Chem. Phys. 84, 5206 (1986)) is applied over the entire range of relative particle size for neutral particles interacting through dispersion forces at arbitrary bath gas density. The long range forces manifest themselves through two stability factors W(0) and W(∞), which are readily evaluated for any interaction potential.

Double many-body expansion of molecular potential energy functions and the role of long-range forces in the rates of chemical reactions

A review is given of the double many-body expansion (DMBE) of molecular potential energy functions and of the role of long-range forces - these being properly described in the DMBE - in the rates of chemical reactions. The O+OH→O 2+H and H+H 2→H 2+H reactions are discussed. New thermal rate coefficients for the former reaction are also reported from the assumption of loose transition states.

Spreading of non volatile liquids on smooth solid surfaces : role of long range forces

Spreading of non volatile liquids on smooth solid surfaces : role of long range forces

Role of long-range forces in heterogeneous nucleation

We analyzed the structure of the contact line between a van der Waals fluid and a flat solid, in the regime where the contact angle θ is small—which allows for a continuum theory of the long-range forces. We computed the size ( χ m) of the perturbed region near the line: χ m ∼ a/ θ 2 (where a is a molecular length), and the line energy τ. We then discussed the structure of the droplets which occur in heterogeneous nucleation, with special emphasis on the case of relatively large sursaturations Δ, where the droplets are small. We found, surprisingly, that an approach based on a constant τ remains qualitatively valid even for drops smaller than the line core size χ m: τ is only logarithmically sensitive to the overall size. We also extended these results to long-range forces other than those of van der Waals, leading to disjoining pressures Π d vs thickness e of the form Π d ∼ e − n with an arbitrary exponent n.

Experimental evidence for the role of long range forces in fibroblast-substrate interaction

Reflection interference microscopy was used to demonstrate that reducing the electrolyte concentration of the medium (sucrose was added to the medium to keep it isotonic) resulted in large areas of the ventral surface of the avian fibroblast moving further away from the substrate. Thus in cells 3 h after plating out the grey reflection image changed to white and in cells 24 h after plating out the predominantly white image was lost as the ventral surface moved even further away. These changes were reversible on replacing the low ionic strength solutions with 199 medium or 145 mM NaCl. This offers direct experimental evidence for the role of long range electrostatic forces in cell-substrate interactions of fibroblasts.

The role of long range forces in ordered arrays of tobacco mosaic virus

We examine the role of electrostatic repulsion and van der Waals' attraction in determining interviral spacings in 'equilibrium' gels of tobacco mosaic virus. The observed spacings are inconsistent with a force-balance model. Experimental probes of the role of forces are suggested.

Hydrodynamic model of steady movement of a solid/liquid/fluid contact line

Movement of the contact line over a solid surface violates the adherence, or “no slip,” boundary condition which is otherwise obeyed by flowing liquids. A flat fluid interface moving steadily over a flat solid is modeled with the creeping flow approximation, which turns out to be self-consistent. Adherence is required except at the contact line itself. Though the velocity field appears to be realistic, stresses and viscous dissipation are found to increase without bound at the contact line. The way the hydrodynamic model breaks down suggests that in reality there may be steep gradients, rheological anomalies, and discontinuous processes around the contact line. Slip and the role of long-range forces are explored with the aid of the lubrication flow approximation.