Search

A formally exact effective boundary potential is defined that can be combined with hard-wall boundary conditions to allow computer simulations to be performed that mimic an infinite system. The effective potential accounts for the interactions that would be present across the boundary if the system were truly infinite. The exact many-body potential is approximated by one- and two-body potentials. The algorithm is implemented and tested for a Lennard–Jones fluid. The advantages of the present formulation over the widely used periodic boundary conditions are discussed. [ABSTRACT FROM AUTHOR]

A protocol for measuring the interaction, deformation and adhesion of soft polymeric substrates with the atomic force microscope (AFM) is described. The technique obtains the photodiode response of the AFM (constant compliance factor) by independent calibration against the rigid substrate adjacent to the deformable particle or patchy film. The zero of separation is taken as the end-point of the jump into contact. A method is given for correcting the velocity dependence of the piezodrive expansion factor, the neglect of which will cause artefacts in dynamic viscoelastic measurements. It is emphasised that conventional force curve analysis, which uses the apparently linear large force region for calibration, will generate erroneous results for deformable substrates. Results are obtained for cellulose particles and for polystyrene films, and their Young's moduli are found to be 22 MPa and 100 MPa, respectively. The latter is about a factor of 30 less than for bulk polystyrene, which indicates that the polystyrene surface is in a less glassy state than the bulk. [ABSTRACT FROM AUTHOR]

A summary is given of the author's recent experimental and theoretical work involving the atomic force microscope, with the focus being upon dynamic measurements. Four topics are covered: calibration techniques for the friction force microscope, quantitative measurements of friction and the effect of adhesion, measurement and theory for the deformation and adhesion of viscoelastic particles, and the interaction and adhesion of hydrophobic surfaces due to bridging nanobubbles. [ABSTRACT FROM AUTHOR]

A generalized Markov superposition approximation is used for the higher order correlation functions in a systematic expansion for the entropy. The results are applicable to certain onedimensional problems, such as the information content of time series or signals, or possibly also to polymer adsorption at a wall. The case of digitization of the signal into many levels is treated in detail, and the dependence of the information content on sampling and digitization is analysed. [ABSTRACT FROM AUTHOR]

The exclusion of solvent from solute clusters provides an additional contribution to aggregation. A derivation of the well known equations for equilibrium aggregation is given, and an explicit expression for the solvent-exclusion effect is obtained for linear aggregates. The analytical result is shown to be in qualitative agreement with recent Monte Carlo simulations of model discotic solutions, which found a concentration dependence of the equilibrium constant. The latter behaviour is argued to stem from a peculiarity in the simulation protocol, namely that the total pressure was fixed, which means that the solvent pressure must increase due to the decreased osmotic pressure at lower solute concentrations. Finite size effects are shown to have a small effect in the simulations. Two further contributions to the observed increased aggregation at low temperatures and concentrations are analysed. For intracolumn aggregation it is argued that it is more appropriate to use the length of the column rather than the volume of the system in the definition of the aggregation constant. The possibility is raised that the increased aggregation seen at low solute concentrations could manifest a type of phase transition, either to one-dimensional columns, driven by feedback between the solvent pressure and the number of aggregates, or to a solvent-depleted globule. [ABSTRACT FROM AUTHOR]