An empirical formulation relating boundary lengths to resolution in specimens showing ‘non-ideally fractal’ dimensions

The concept of fractal dimension offers an elegant basis for the understanding of changes in boundary lengths due to different resolutions. However, in many cases, the usual log-log ‘fractal plot’ does not quite fit the observed data, with most biological structures for instance. An automated boundary following algorithm has been used with an IBAS image analyser on various objects, to evaluate boundary lengths at different magnifications and with a wide range of discrete divider stride lengths on each digitized image. A formula which relates the boundary length to the divider stride length and fits very closely the experimental data has been empirically obtained. Examples are shown with various object boundaries observed by TV-camera, macroscopically or with a light microscope. At different magnifications, the same constants are obtained for a given object, notably a theoretical maximum boundary length (B_m) when the divider stride tends towards zero. The formula allows to calculate a variation of the log-log plot slope which fits very closely the upper convex plots observed when the boundaries do not display an ideally fractal behaviour. Plots derived from the Lineweaver-Burk and Hill ones (used in enzymology) allow a relatively easy graphic determination of the parameters of the proposed formula, which may offer useful criteria for object shape classification, as demonstrated by a biological example. Instead of a true (constant) fractal dimension, it seems that, in many situations, exists a ‘continuous fractal dimensional transition’, inside which an apparently linear segment in a given range of resolution of the usual log-log plot could be explained by asymptotic limits at higher and lower resolutions. The ideal situation with a constant fractal dimension over all resolution ranges could be considered as a limit, achieved when the maximum boundary length (B_m) tends towards infinity.     

A fully automated, ‘thimble-size’ scanning tunnelling microscope

A novel, fully automated high-stability, high-eigenfrequency scanning tunnelling microscope (STM) has been developed. Its key design feature is the application of two piezoelectric ceramic tubes, one for the x-y-z motion of the tip and one for a linear motor (‘nano-worm’) used for the coarse positioning of the tip relative to the specimen. By means of the nano-worm, the tip can be advanced in steps between 16 and 0·2 nm. The walking distance is >2 mm, with a maximum speed of 2000 steps/s. The nano-worm positioning implies that this STM is fully controlled by electronic means, and that no mechanical coupling is needed, which makes operation of the STM extremely convenient. The axial-symmetry construction is rigid, small and temperature-compensated, yielding reduced sensitivity to mechanical and acoustic vibrations and temperature variations. The sample is simply placed on a piece of invar which surrounds the scanner tube and the nano-worm and is held by gravity alone. This allows for easy sample mounting. The performance of the microscope has been tested in air by imaging a variety of surfaces, including graphite and biological samples.     

The precise measurement of the thickness of ultrathin sections by a ‘re-sectioned’ section technique

The thickness of ultrathin tissue sections embedded in Epon-Araldite and cut with a diamond knife was measured by re-sectioning and electron microscopic examination of the section profiles. A secondary section mounted on a Formvar-coated slot grid provided enough normally cut segments (seven to seventeen) for measurements giving a precise estimate of mean thickness, comparable to that obtainable by interference microscopy (±2.3% or less for grey to dark gold sections). The standard deviation of section thickness within sections was never more than 5 nm, corresponding to a coefficient of variation of 6.5% or less for sections more than 48 nm thick. This suggests that variation in section thickness, within sections, may be less than has been supposed, so that quantitative work may be based on thickness measurements made over a limited representative area. A silver interference colour was associated with sections 49–60 nm thick.     

A simple and inexpensive liquid helium cooled ‘slam freezing’ device

A liquid helium cooled copper block device has been designed for the rapid freezing of biological material. The apparatus differs from previous designs in being easily constructed from readily available components. It incorporates a novel and simplified specimen carrying system. Construction of this apparatus is possible for any laboratory and makes the advantages of freeze substitution reliably available for many cell types at minimal cost.     

Surfaces of cryosections: is cryosectioning ‘cutting’ or ‘fracturing’?

In order to determine if cryosectioning involves ‘fracturing’ or ‘cutting’ we examined the surfaces obtained in cryosectioning by a metal-replicating procedure commonly used in freeze-fracture microscopy. Platinum-carbon replicas were made of the surfaces of both the sections and the complementary surfaces of the sample stubs from which the sections were cut. When samples of frozen red cells were sectioned at ?120°C with large knife advancements (1 μm), the chips produced did not resemble sections. Membrane fracture faces, produced by splitting of the lipid bilayer, were found in electron micrographs of replicas of the sample stubs. This demonstrates that a cryomicrotome can be used to produce large intact replicas. When dull knives were used with small knife advancements, both smooth and fractured regions were found. The sections produced with dull knives had a snowflake appearance in the light microscope. When sharp knives were used with small advancements (0·1 μm), replicas of the surfaces were free of fracture faces and the sections had a cellophane-like appearance in the light microscope. Therefore, in cryosectioning a different process other than ‘fracturing’ is responsible. This ‘cutting’ process may be micromelting of a superficial layer by the mechanism of melting-point depression from the pressure exerted by the sharp edge of the knife.     

Automated cell counting in tissue sections: a new approach by ‘multiple grey-level analysis‘

A system is described for the automated quantitation in tissue sections of the follicular and stromal cell populations of the thyroid gland, using a computer-linked TV image analyser. The paper illustrates how the problems of variable staining intensity and clustering of images (nuclei) may be overcome by a novel approach, in which multiple analyses of each field are made at increasing grey-level thresholds, and then ‘synthesized’ by computer to give a composite image. Discrimination between cell (nuclear) types was by a single minimum width criterion. The results were highly reproducible and correlated well with counts obtained by a comparable manual method.     

‘ASKFOR’ — a design analysis aid for bearing applications

This paper describes a time-sharing system which was developed to assist in the provision of a design analysis service for customers. Both the background to the development and details of the present system are given. In particular it is shown how greater flexibility enables the user to solve a very large range of problems. The interactive features of the system are also covered, showing how it has been attempted to make the input as natural as possible by offering a wide range of facilities.     

A variable cell culture chamber for ‘open’ and ‘closed’ cultivation,perfusion and high microscopic resolution of living cells

A multipurpose chamber is described for growing and testing cultured cells. The chamber can be converted from a perfusion chamber to an ‘Open’ or ‘closed’ culture system. The chamber provides optimum conditions for microscopy using all common objectives and condensers for different microscopic methods.     

Cells on biomaterials – some aspects of elemental analysis by means of electron probes

Electron probe X‐ray microanalysis enables concomitant observation of specimens and analysis of their elemental composition. The method is attractive for engineers developing tissue‐compatible biomaterials. Either changes in element composition of cells or biomaterial can be defined according to well‐established preparation and quantification procedures. However, the qualitative and quantitative elemental analysis appears more complicated when cells or thin tissue sections are deposited on biomaterials. X‐ray spectra generated at the cell/tissue–biomaterial interface are modelled using a Monte Carlo simulation of a cell deposited on borosilicate glass. Enhanced electron backscattering from borosilicate glass was noted until the thickness of the biological layer deposited on the substrate reached 1.25 μm. It resulted in significant increase in X‐ray intensities typical for the elements present in the cellular part. In this case, the mean atomic number value of the biomaterial determines the strength of this effect. When elements are present in the cells only, the positive linear relationship appears between X‐ray intensities and cell thickness. Then, spatial dimensions of X‐ray emission for the particular elements are exclusively in the range of the biological part and the intensities of X‐rays become constant. When the elements are present in both the cell and the biomaterial, X‐ray intensities are registered for the biological part and the substrate simultaneously leading to a negative linear relationship of X‐ray intensities in the function of cell thickness. In the case of the analysis of an element typical for the biomaterial, strong decrease in X‐ray emission is observed in the function of cell thickness as the effect of X‐ray absorption and the limited excitation range to biological part rather than to the substrate. Correction procedures for calculations of element concentrations in thin films and coatings deposited on substrates are well established in materials science, but little is known about factors that have to be taken into account to accurately quantify bioelements in thin and semi‐thick biological samples. Thus thorough tests of currently available quantification procedures are required to verify their applicability to cells or tissues deposited on the biomaterials.     

MoS2‐based alloys and nanocomposites for solid lubrication

The development of MoS₂ coatings has involved the modification of substrate surfaces, the addition of metals or compounds to the MoS₂, and variation in the deposition process parameters affecting the properties of deposited films. More recently, multilayer and periodic nanolayer coating structures have also been investigated. At present, work is concentrated on alloys of MoS₂, mainly with various metals, and targeted at terrestrial (ambient air) applications. The addition of metals or compounds to physical‐vapour‐deposited MoS₂ has led to improvements in coating performance, for example, greater stability of friction coefficient, greater film endurance, and increased temperature/oxidation resistance. The metal or compound can be either in the form of nanoscale multilayers or mixed with the MoS₂, sometimes leading to nanoclusters within a MoS₂ matrix. Microstructural analysis seems to show that the primary function of these additives is to suppress the formation of low‐density, columnar structures. At certain concentrations an added metal can also enhance the formation of the tribologically favourable (002) orientation of the MoS₂ crystallites. Other changes in the properties of MoS₂—metal composites may be due to their oxidation resistance, as indicated by the stability of these films against storage in air and their increased endurance when in sliding contacts at elevated temperatures.     

How effective is ‘high-speed’ autoradiography?

A number of methods have recently been described for enhancing silver grain production in light microscope autoradiography by impregnating the emulsion with a 2,5-diphenyloxazole-based fluor system. After extensive testing of these methods it was found that none gave a significant increase in silver grain count. In the course of this work a number of observations were made which seem to account at least partially for these failures, and which question the feasibility of this type of fluorography.

• 1 Tetrahymena nuclei, pulse labelled with ³H-thymidine, contain sufficient radioactivity to produce useful autoradiographs after 2 h exposure without fluorographic enhancement.
• 2 Although fluorographic autoradiography of this type requires that the fluor penetrate the photographic emulsion, direct evidence was obtained that such penetration does not occur. Thus it is unlikely that true fluorographic enhancement occurs with methods based on 2,5-diphenyloxazole impregnation of the emulsion. (This is in contrast to the widely used gel fluorography techniques in which it is the gel rather than the photographic emulsion which is impregnated with fluor.)
• 3 In DNA fibre autoradiography, it is possible to obtain artefactual arrays of silver grains which closely resemble those expected of replicating DNA. This phenomenon alone, however, does not explain a recent report of successful DNA fibre autoradiography after an exposure of only 3 days. Calculations based on the maximum dpm per length of labelled DNA raise further questions concerning this result.

     

Mixed lubrication — an overview

The regime known as ‘mixed’ lubrication lies between those of hydrodynamic and elastohydrodynamic. This paper reviews work on this subject, in order to recognise the key areas of progress, and of ignorance. Film thickness and surface roughness are discussed, to conclude that below certain values of λ, there are limitations to earlier models. Computational solution of the micro-EHD problem has largely succeeded, as well as more analytical approaches. The mixed lubrication regime has been studied by electrical contact resistance, friction measurement, and, more recently, analysis has been aided by optical interferometry — the implications of this work, and future prospects, are addressed.     

Smooth, low scatter metal optics by repeated machining on a ‘no growth’ machine

Specifications mastered well today for metal optical components up to some 10 cm diameter include figure and slope. Diamond-machined optical function surfaces also require a minimum incident light scatter and maximum specular reflectance equalling that of the bulk material. This relates to microfinish, which is less well mastered in micromachining.

We report recent results in the state of the art of diamond machining metal surfaces. We expect that today's physical and economic limit can be pushed further by a special machining process reducing roughness, based upon repeated machining relying on a ‘no growth’ machine. In this method the tool is exposed to the full range of cutting depths between the preset maximum down to ‘zero’, or rather to a limit about which we intend to learn much more. In situ tool inspection is essential to predict results.

The method was presented at Laser 83 and we now present recent results. The process of repeated machining concentrates on flats and spheres.     

A ‘pocket guide’ to total internal reflection fluorescence

The phenomenon of total internal reflection fluorescence (TIRF) was placed in the context of optical microscopy by Daniel Axelrod over three decades ago. TIRF microscopy exploits the properties of an evanescent electromagnetic field to optically section sample regions in the close vicinity of the substrate where the field is induced. The first applications in cell biology targeted investigation of phenomena at the basolateral plasma membrane. The most notable application of TIRF is single‐molecule experiments, which can provide information on fluctuation distributions and rare events, yielding novel insights on the mechanisms governing the molecular interactions that underpin many fundamental processes within the cell. This short review intends to provide a ‘one stop shop’ explanation of the electromagnetic theory behind the remarkable properties of the evanescent field, guide the reader through the principles behind building or choosing your own TIRF system and consider how the most popular applications of the method exploit the evanescent field properties.     

The importance of current density ‘focusing’ in STM image resolution

In a previous paper we incorporated Tersoff and Hamann's model of an STM tip into Lang's transfer Hamiltonian result for the tunnelling current density and applied the resulting simple expression to Tersoff's six-plane-wave model of a monolayer of graphite. In this paper the results for a more realistic model of a graphite surface and a more complete selection of tip positions are presented. They support the previous conclusions that the normal component of the current density takes on both positive and negative values in a complex flow pattern and that its lateral falloff away from the (projected) tip position is much slower than expected from the extraordinary lateral resolution (～1 Å) evident in the best STM images of graphite. This shows that sharp ‘focusing’ of the current density directly under the tip is not a necessary condition for high lateral resolution.     

A method of measuring ‘zero-wear’ of the cylinder liner

In this paper, new methods of linear wear estimation of the cylinder liner are proposed. They can be used if the deepest honed valleys remain unaltered during the wear process. In the first part of the paper, the possibility of using various amplitude parameters as measures of wear is studied. The second part presents a new procedure for cylinder liner wear analysis based on the ‘fax-film’ method that allows us to estimate the wear of the cylinder liner without disassembly of the cylinder-piston-piston ring group.     

Applications of the concept of tribopolymerisation in fuels,lubricants, metalworking,and ‘minimalist’ lubrication

Tribopolymerisation, a novel concept of molecular design developed by Furey and Kajdas, involves the continuous formation of thin polymeric films on rubbing surfaces; the protective films formed are self‐replenishing. The antiwear compounds developed are effective with metals and ceramics and in the liquid and vapour phases. Furthermore, they are ashless and contain no harmful phosphorus or sulphur, and many are biodegradable. The potential applications are diverse and have cost, performance, energy, and environmental advantages. The present paper describes some applications of the concept of tribopolymerisation as a mechanism of boundary lubrication, including recent laboratory research and in‐plant industrial demonstrations. Applications include: (a) ashless antiwear or ‘lubricity’ additives for fuels; (b) ashless lubricants for automotive engines to reduce exhaust catalyst poisoning and environmental emissions; (c) lubrication of ceramic engines or ceramic components; (d) machining and cutting using thin films to reduce friction and ceramic tool wear; (e) vapour‐phase applications in high‐temperature gaseous systems or to counter fuel injector wear problems in natural gas engines; (f) special ‘minimalist’ pre‐treatment compositions for engine assembly and running‐in; and (g) enabling technology in the development of new engines and propulsion systems.     

The effect of mechanical stress on four viscosity improver polymers

The effect of a Bosch injector on the mass distribution function of four viscosity improver (VI) polymers has been studied. Two of these, hydrogenated styrene—isoprene (HSI) and hydrogenated styrene—butadiene (HSB), are di‐block polymers, whereas the others are either comb‐like, polyalkyl methacrylate (PMA), or linear, olefin copolymer (OCP). Measurements were performed by light scattering and viscometric techniques before and after application of mechanical stress. Only slight modifications in the physical properties were detected for HSB and HSI, which can be explained by their micellar behaviour. Conversely, a large variation in mean size and viscometric properties was observed for PMA and OCP. Finally, a model is proposed which links the variation of the mass distribution function to the intrinsic viscosity loss.