W. van Megen

Bio: W. van Megen is an academic researcher from RMIT University. The author has contributed to research in topics: Hard spheres & Scattering. The author has an hindex of 18, co-authored 28 publications receiving 1155 citations.

Solvation forces in simple dense fluids. I

Abstract: The grand canonical ensemble Monte Carlo method is used to calculate the density profile of a simple dense liquid, under conditions close to the vapor line, between two solid bodies and also the solvation force between the solids due to the simple fluid. The force is large compared with the van der Waals force at moderate surface separations, h, but is an oscillatory function of h. At small values of h the solvation force is strongly repulsive.

Tracer diffusion in concentrated colloidal dispersions. III. Mean squared displacements and self‐diffusion coefficients

Abstract: The incoherent dynamic structure factor Fs has been measured on concentrated dispersions of particles with interactions approximating hard spheres The particle mean squared displacements and self‐diffusion coefficients are obtained from Fs at times and scattering vectors where the Gaussian approximation (for Fs ) is expected to be reasonable

Detection of small polydispersities by photon correlation spectroscopy

Abstract: We consider dilute suspensions of homogeneous polydisperse spherical particles for which the Rayleigh–Gans–Debye (RGD) approximation is valid. For two model particle size distributions we calculate the dependence on scattering vector Q of the average scattered intensity I(Q) and the effective diffusion coefficient De(Q) obtained from the first cumulant measured by photon‐correlation spectroscopy. If the mean particle radius R is large enough (≳170 nm) that the intensity form factor P(QR) shows at least one minimum in the accessible range of Q, we find that De(Q) exhibits a characteristic variation with Q which is very sensitive to the sample polydispersity. Under favorable conditions it should be possible to measure polydispersities (standard deviation/mean size) as small as 0.01. These theoretical considerations are supported, at least qualitatively, by experiment. We also discuss briefly the effect of relaxing the RGD approximation and the implications of this work for the more common PCS probes of polydispersity such as Laplace transformation of the light scattering correlation function.

Solvation forces in simple dense fluids. II. Effect of chemical potential

Abstract: The grand canonical ensemble Monte Carlo method is used to calculate the distribution of liquid molecules and the solvation force between two parallel solid surfaces separated by a simple liquid. Neither this distribution nor the solvation force are very sensitive to changes in the chemical potential (i.e., bulk density) of the interstitial liquid.

Phase behaviour of concentrated suspensions of nearly hard colloidal spheres

Abstract: Suspensions of spherical colloidal particles in a liquid show a fascinating variety of phase behaviour which can mimic that of simple atomic liquids and solids. ‘Colloidal fluids’1–4, in which there are significant short-range correlations between the positions of neighbouring particles, and ‘colloidal crystals’5–7, which have long-range spatial order, have been investigated extensively. We report here a detailed study of the phase diagram of suspensions of colloidal spheres which interact through a steep repulsive potential. With increasing particle concentration we observed a progression from colloidal fluid, to fluid and crystal phases in coexistence, to fully crystallized samples. At the highest concentrations we obtained very viscous samples in which full crystallization had not occurred after several months and in which the particles appeared to be arranged as an amorphous ‘colloidal glass’. The empirical phase diagram can be reproduced reasonably well by an effective hard-sphere model. The observation of the colloidal glass phase is interesting both in itself and because of possible relevance to the manufacture of high-strength ceramics8.

Motions and relaxations of confined liquids

Abstract: When a liquid is confined in a narrow gap (as near a cell membrane, in a lubricated contact between solids, or in a porous medium), new dynamic behavior emerges. The effective shear viscosity is enhanced compared to the bulk, relaxation times are prolonged, and nonlinear responses set in at lower shear rates. These effects are more prominent, the thinner the liquid film. They appear to be the manifestation of collective motions. The flow of liquids under extreme confinement cannot be understood simply by intuitive extrapolation of bulk properties. Practical consequences are possible in areas from tribology and materials processing to membrane physics.

Colloidal Suspension Rheology

Abstract: 1. Introduction to colloid science and rheology 2. Hydrodynamic effects 3. Brownian hard spheres 4. Stable colloidal suspensions 5. Non-spherical particles 6. Weakly flocculated suspensions 7. Thixotropy 8. Shear thickening 9. Rheometry of suspensions 10. Suspensions in viscoelastic media 11. Advanced topics.

Theory Of Simple Liquids

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Direct measurement of structural forces between two surfaces in a nonpolar liquid

Abstract: The force as a function of separation has been measured between two molecularly smooth surfaces immersed in the liquid octamethylcyclotetrasiloxane (OMCTS) whose molecules are quasispherical of diameter ∼1.0 nm. The force is an oscillatory function of distance, varying between attraction and repulsion, with a periodicity equal to the size of the liquid molecules to within the experimental resolution of ∼0.1 nm. The oscillations decay rapidly with distance: their measurable range is 6–10 molecular diameters and their magnitude exceeds that of conventional van der Waals forces at small distances. The magnitude of the oscillations is insensitive to changes in temperature, but sensitive to the chemical nature of the surfaces, and very sensitive to the presence of water. The results are considered, qualitatively, within the context of current theories of the liquid state near solid interfaces, and some implications for surface chemistry and colloid science are discussed.