Malcolm J. Grimson

Hysteresis and the dynamic phase transition in thin ferromagnetic films.

Hysteresis and the nonequilibrium dynamic phase transition in thin magnetic films subject to an oscillatory external field have been studied by Monte Carlo simulation. The model under investigation is a classical Heisenberg spin system with a bilinear exchange anisotropy Lambda in a planar thin film geometry with competing surface fields. The film exhibits a nonequilibrium phase transition between dynamically ordered and dynamically disordered phases characterized by a critical temperature T(cd), whose location is determined by the amplitude H0 and frequency omega of the applied oscillatory field. In the presence of competing surface fields the critical temperature of the ferromagnetic-paramagnetic transition for the film is suppressed from the bulk system value T(c) to the interface localization-delocalization temperature T(ci). The simulations show that in general T(cd)<T(ci) for the model film. The profile of the time-dependent layer magnetization across the film shows that the dynamically ordered and dynamically disordered phases coexist within the film for T<T(cd). In the presence of competing surface fields, the dynamically ordered phase is localized at one surface of the film.

Short range solvation forces in fluids

A formalism based on a linearization procedure is used to obtain an expression for the thermodynamic potential of a system of fluid particles in terms of the local particle number density and bulk structure factor. For the case of adsorption of particles at a single impenetrable interface, minimization of the thermodynamic potential yields an integral equation for the number density identical to that obtained by Percus. From the thermodynamic potential for our model we obtain the force/unit area between two plates which is simply related to the surface density of fluids at the plates. The equations are solved numerically for the one-dimensional hard rod fluid and the three-dimensional Percus-Yevick fluid. The resulting density and force show pronounced oscillations in agreement with recent Monte Carlo calculations. The magnitude of the force is comparable to van der Waals interactions and can thus modify significantly forces that exist between colloid particles.

Linear and non-linear theories of solvation forces in fluids

From an exact expression for the free energy of a non-uniform classical fluid, due to Saam and Ebner, a closure is used to develop a non-linear theory for the density and solvation force between two planar walls. In the linear limit these expressions reduce to ones used successfully elsewhere. Numerical solution of the equations for a hard sphere fluid shows that while the density profiles predicted by the two theories are markedly different, the solvation forces are similar.

Exchange anisotropy and the dynamic phase transition in thin ferromagnetic Heisenberg films

Monte Carlo simulations have been performed to investigate the dependence of the dynamic phase behavior on the bilinear exchange anisotropy of a classical Heisenberg spin system. The system under consideration is a planar thin ferromagnetic film with competing surface fields subject to a pulsed oscillatory external field. The results show that the films exhibit a single discontinuous dynamic phase transition (DPT) as a function of the anisotropy of the bilinear exchange interaction in the Hamiltonian. Furthermore, there is no evidence of stochastic resonance associated with the DPT. These results are in marked contrast to the continuous DPT observed in the same system as a function of temperature and applied field strength for a fixed bilinear exchange anisotropy.

Solvation forces in charged fluids: II. A non-linear theory

A formalism based on linear response theory is used to obtain an expression for the free energy of a non-uniform charged fluid in terms of the local ion number density and the bulk direct correlation functions. When the fluid is a restricted primitive model electrolyte the free energy splits into two independent parts, the minimization of which leads to expressions for the equilibrium charge and density distributions. From the free energy an expression for the force between two thick plates immersed in an electrolyte is obtained. In the limit of point ions, the expressions we obtain reduce to those of the Debye-Huckel theory of electrolytes. The equations are solved numerically and at low bulk electrolyte concentrations the monotonically decaying repulsive force of the classic Verwey and Overbeek results is found. But at higher concentrations and larger inverse Debye screening lengths the force displays pronounced oscillations. Correspondingly, the electric potential displays oscillations which have conse...

Small-angle scattering from colloidal dispersions

A theory of the small-q behaviour of the static structure factor S(q) for monodisperse colloidal dispersions is presented in which the long-range forces between the colloid particles are treated in the random phase approximation. By using a hard-sphere reference fluid, explicit results for the coefficients to (q4) in a small-q expansion of c(q), the Fourier transform of the direct correlation function, are obtained.The theory gives an expression for the osmotic compressibility of a dispersion that is equivalent to an empirical relation used to interpret light-scattering experiments on microemulsions. When applied to charge-stabilised dispersions the theory predicts the existence of a phase diagram closely analogous to one normally observed in molecular systems.

Forces between surfaces in electrolyte solutions

Abstract Density functional theories of solvation forces in charged fluids are extended to treat electrolytes consisting of finitesized ions and neutral solvent particles. The resulting forces display pronounced oscillations whose magnitude is a strong function of the bulk density of the neutral species.

Solvation forces in fluids: II. Ornstein-Zernike and Percus-Yevick temperature dependent fluids

Abstract The theory of Hamaker and its extension via Lifshitz theory is well known and frequently used to calculate the van der Waals forces between colloidal particles and surfaces. in these theories, the solvent is treated as a uniform dielectric continuum. Restructuring of the solvent in the interfacial region due to the presence of the boundary is not included in a self consistent way. This means such results are not valid at short separations which determine adhesion and friction. However, recent statistical mechanical approaches show how such an effect can be systematically incorporated into unified theory that yields the long range component of the forces (Hamaker/Lifshitz) and the short range force that arises essentially due to this restructuring. We survey developments in this area and consider trends for the future.

Scattering from polyelectrolyte solutions

The small-angle scattering from salt-free polyelectrolyte solutions has been studied as a function of the chain concentration along with the effects of added salt on the system. The theory is based on a multicomponent generalisation of the single-contact approximation that gives equal emphasis to both the intrachain correlations of the polyion and the interparticle correlations of all ionic species present in solution. Two models are introduced. One treats the polyions as macroparticles, while the second considers direct monomer correlations. The interparticle correlations are calculated using the restricted primitive model electrolyte familiar from liquid-state physics. Both models are able to reproduce the qualitative features shown by solutions of biopolymers and synthetic polyelectrolytes.

Linear viscoelastic behaviour of sterically stabilized oil–water emulsions

Shear moduli of several soya oil–water emulsions stabilized by xanthan and of comparable xanthan solutions have been studied. Comparison of the xanthan values and the emulsion values indicates the presence of strong interactions between the xanthan microgel and emulsion droplets in the emulsions. Analysis of the data suggests that most of the contribution to the shear modulus arises from the repulsive forces between the emulsifier molecules, the contribution from attractive forces being negligible. The rheological behaviour of the emulsions is similar to that of highly concentrated emulsions with deformable droplets, even at relatively low disperse phase concentrations of 20%. The results are interpreted with a model that treats the emulsions droplets as an assembly of soft spheres. The form of the interaction is found to be consistent with a system where the principal forces between the droplets arise from undulating membranes rather than rigid bodies.