INTERFACIAL CONTINUUM MECHANICS GROUP

STAFF MEMBERS

Professor Lee R. White
Dr Derek Y.C. Chan (Reader)
Dr Steven L. Carnie (Senior Lecturer)
Dr Barry D. Hughes (Senior Lecturer)
Dr Kerry A. Landman (Senior Lecturer)
Dr Maria Eberl (Research Fellow)
Dr Jonathan Ennis-King (Research Fellow)
Dr Kurt Liffman (Research Fellow)
Dr John Sader (Research Fellow)

POSTGRADUATE STUDENTS

James Gunning
Warwick Holt
Christine Mangelsdorf
Jim Stankovich
Aris Theocharides
Michael Yudin

POLYMERS AND POLYELECTROLYTES (S.L.C., L.R.W., D.Y.C.)

We study the properties of both electrically neutral and charged polymer chains in solution, using both computer simulation techniques and analytic approximations based on mean-field concepts. Recent work has included Monte Carlo studies of the internal distribution of isolated neutral polymer chains and the conformational properties of isolated polyelectrolyte chains. The results of a model where the charged beads interacted via screened Coulomb potentials were compared with a model explicitly containing salt ions and where all Coulomb interactions are included. This work has been extended to investigate interchain effects in semi-dilute polyelectrolyte solutions. We are currently investigating the feasibility of calculations for polyelectrolyte chains adsorbed onto a surface, of relevance to the use of polyelectrolytes as flocculating agents.

COLLOIDAL INTERACTIONS (S.L.C., D.Y.C., J.S.)

We have developed software to calculate the interaction free energy and force between charged colloidal spheres, as described by the linearized and non-linear Poisson-Boltzmann calculations. The spheres can be of unequal size and can satisfy constant charge, constant potential or charge regulation boundary conditions. The energy and force between a sphere and a plate can also be calculated. The validity of the linearized Poisson-Boltzmann equation has been tested by comparing with the forces calculated from the nonlinear Poisson-Boltzmann equation using either bicubic Hermite collocation or multigrid methods. Finally, an approximate analytic expression for the energy and force has been found and tested against the numerical solutions mentioned above, with errors of typically a few percent over a wide range of particle sizes, separation and surface potentials.

GRANULAR MATERIALS (K.L., D.Y.C., B.D.H., L.R.W.)

The force distribution in a pile of dry powder is studied by computer simulation techniques and analytic methods, leading to predictions of the normal and shear stresses beneath the pile and the contours of maximum stress within the pile. This work is not only relevant to the storage and handling of bulk materials in the mineral processing industry and in agriculture, but is also of general physical interest because of the counter-intuitive dip in the normal stress underneath the highest section of the pile. This puzzling phenomenon, and our endeavour to explain it, have been reported in New Scientist (14/12/91) and are expected to be featured in an upcoming edition of the television program Beyond 2000.

* FLUID FLOW IN POROUS MEDIA (D.Y.C., B.D.H.)

(i) We study the flow of fluids through porous materials. Of particular interest is the interface instability generated when a fluid of low viscosity (such as air or water) displaces a fluid of higher viscosity (such as crude oil). This phenomenon ("viscous fingering") limits the effectiveness of certain enhanced oil recovery strategies. Our work focuses on the role of porous media microstructure in controlling the fingering process and has quantified the observed similarity between viscous fingering in real porous materials and aggregation processes. (ii) We study transient gas flow in porous media, modelling the flow of methane from a coal seam into a borehole, and we calculate the stresses induced in the seam by the flow, to investigate fracture of the borehole environs. This work is relevant to the efficient recovery of Australia's large coal-seam methane resource.

FAILURE AND FRACTURE IN RANDOM AND COMPOSITE SYSTEMS (D.Y.C., B.D.H., L.R.W.)

Fracture patterns in geomechanics and in man-made structures are frequently irregular in shape and are often branched rather than linear. We investigate the effect of microstructure variation on fracture and failure patterns, using computer simulations. For the canonical example of a network of electrically conducting elements, each element associated with a fuse rating, we find that random variation in fuse ratings has a more dramatic effect on failure patterns than random variation in local electrical resistance. By appropriately varying the statistical distribution of microstructure, one may simulate a wide range of fracture patterns, from brittle fracture with varying degrees of crack tortuosity, to ductile failure with widely distributed damage.

* COLLOIDAL ELECTROKINETICS (L.R.W.)

We study the transport properties of colloidal systems when subjected to applied fields (electrical, gravitational, sound pressure, shear) as functions of the state of charge on the particle surface. Numerical schemes for the computation of these properties are also under investigation. Recent work has focussed on high frequency electric fields and the corresponding electrophoretic mobility and induced dipole strength of the particles. The effect of relaxing the classically accepted boundary conditions that ions do not penetrate the slipping plane surface of the particle is presently under investigation along with extensions of the theory to high volume fraction systems.

* SUSPENSION RHEOLOGY (K.A.L., L.R.W.)

The concentration and isolation of fine particles dispersed in liquids are important in the chemical and mineral processing industries. In spite of this, the procedures available for the selection of the most appropriate method and for the prediction of its performance remain crude. This work develops a generalized approach to understanding and prediction of solid-liquid separation methods based on the measurement of fundamental material properties such as a compressional yield strength. Gravitational thickening and pressure filtration studies have been undertaken. This will be of value in designing more efficient methods and ultimately in optimizing the performance of solid-liquid separation methods.

MECHANICAL PROPERTIES OF AGGREGATES (K.L., D.Y.C., L.R.W., B.D.H., W.H., S.L.C.)

(i) We use computer simulation methods to study the mechanical properties of aggregates produced by precipation of suspensions. These aggregates possess self-similar internal structure and because of the consequent presence of a broad distribution of characteristic defect sizes can exhibit a dramatic response to externally imposed stress.

(ii) A cell model for colloidal interation has been developed to model the compression of low volume fraction dispersions in ultra filtration experiments. This work has implications in determining the correct ionic composition in colloidal systems that have been prepared by this method.

ADHESION AND FRACTURE (B.D.H.)

We are investigating the role of short-range, distance-dependent surface interactions in adhesion and fracture of deformable solids, with particular reference to the subtleties of the concept of surface energy of a solid. This work produces interesting questions in the area of mixed boundry values of potential theory, variational methods, and asymptotics.