W.R. Bowen

Steps of membrane blocking in flux decline during protein microfiltration

The flow decline is studied in typical experiments with dead-end microfiltration of BSA solutions (1 and 0.1 kg/m3) through Cyclopore® track-etched polycarbonate membranes (pore sizes 0.1, 0.2, 0.4 and 1.0 μm) at pH 5 and with a saline content of NaCl 0.01 M. Results are examined, within the frame of the common blocking mechanisms, interpreted here as successive or simultaneous steps of flow decline rather than as alternative possibilities of theoretical modelling for experimental data. An average deposited volume per unit of permeate volume is defined for the central steps of the flux decline, whose behaviour against the shear stress is studied. Under the conditions studied, an increase in shear stress results in a decrease in deposition, probably due to a reduction in protein-surface interaction times. The initial and final steps are also analyzed in terms of an early blockage of the smallest pores and a formation of a cake up to a limiting height, subsequently followed by the establishment of a final non zero flow.

Comparative surface studies on wet and dry sacrificial thermal oxidation on silicon carbide

Abstract A comparative study on the effect of wet and dry thermal oxidation on 4H-silicon carbide (SiC) and on sacrificial silicon (Si) thermal oxidation on 4H-SiC surface has been conducted using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The AFM images show the formation of ‘nano-islands’ of varying density on the SiC surface after the removal of thermal oxide using hydrofluoric (HF) acid etch. These nano-islands are resistant to HF acid and have been previously linked to residual carbon [1] , [2] , [3] resulting from the oxidation process. This paper presents the use of a sacrificial silicon oxidation (SSO) step as a form of surface preparation that gives a reproducible clean SiC surface. XPS results show a slight electrical shift in binding energy between the wet and dry thermal oxidation on the standard SiC surface, while the surface produced by the SSO technique shows a minimal shift.

Improved thin films of pentacene via pulsed laser deposition at elevated substrate temperatures

Thin films of pentacene were deposited by thermal evaporation and by pulsed laser deposition (PLD). Surface characterization using atomic force microscopy shows that PLD leads to a reduction in surface roughness, which can be reduced further using heated substrates. Observation of enhanced field‐effect carrier mobilities in the PLD films, together with increased electrical conductivity and reduced activation energy for electrical conduction, is consistent with increased molecular ordering in the pentacene films and suggests a direct correlation between molecular ordering in the bulk and surface roughness.

A study of the tensile properties of liquids in confined spaces using an atomic force microscope

We report work in which an atomic force microscope (AFM) is used to stretch (and ultimately, to rupture) a thin film of liquid between a moving colloid sphere and a static plane surface. Under some circumstances, when the sphere and the surface are pulled apart sufficiently rapidly, an unexpected transient decrease in the sphere‐surface separation is recorded. The results of numerical simulations of cavitation bubble dynamics suggest that the growth of a cavitation bubble within a liquid may result in the development of sufficiently large negative pressures to account for this phenomenon. The results of separate experiments, which involve acoustic pulse propagation within metre‐long columns of liquid and high‐speed microphotography (using a novel optical system designed for this work), are used to show that the peak tensile forces recorded in the AFM experiments correspond to the development of tensile stresses that are commensurate with the fluids effective tensile strength (or ‘cavitation threshold’). The results of this study, which, to the best of our knowledge, is the first to apply the AFM in cavitation bubble dynamics work, provide evidence that, in the cavitation of liquids within confined spaces, the growth of a cavity may be more damaging than its subsequent collapse.

Atomic force microscope studies of stainless steel: Surface morphology and colloidal particle adhesion

An atomic force microscope has been used to image four commercial stainless steel samples of widely differing surface finishes. Analysis of the images allowed quantification of surface roughness over different area scales, 50 × 50 μm, 10 × 10 μm and 1 × 1 μm. The atomic force microscope was also used to measure directly the adhesion of a single polymer latex particle (radius ∼5 μm) to the surface in solution using the colloid probe technique. It was found that the adhesion increased with decreasing roughness, except for the smoothest surface which exhibited very regular surface features on the area scale most relevant to adhesion of the particle (1 × 1 μm). There was a good correlation between the variability of adhesion over each surface and the corresponding variability in surface roughness. Measurements of this type should prove useful in the technical/economic choice of surface finish for a particular purpose. As the colloid probe has dimensions comparable to those of bacteria and yeast cells, such measurements should especially be of value in the selection of surface finish likely to minimise bioadhesion.

A Study of Cavitation Phenomena Using an Atomic Force Microscope

We report work in which an Atomic Force Microscope (AFM) is used to deform a liquid film between a colloid sphere and a plane surface. Under some circumstances, when the sphere and the surface are pulled apart rapidly, an unexpected transient decrease of the sphere-surface separation is recorded. Numerical simulations of cavitation bubble dynamics are used to explain how the growth of a cavitation bubble may result in the development of sufficiently large negative pressures to account for this phenomenon. The results of this study provide evidence in support of a mechanism mooted by Israelachvilli and co-workers that in the cavitation of liquids within confined spaces, the growth of a cavity may be more damaging than its subsequent collapse.