Capillary force required to detach micron-sized particles from solid surfaces—Validation with bubbles circulating in water and 2 μm-diameter latex spheres

The adhesion forces holding micron-sized particles to solid surfaces can be studied through the detachment forces developed by the transit of an air–liquid interface in a capillary. Two key variables affect the direction and magnitude of the capillary detachment force: (i) the thickness of the liquid film between the bubble and the capillary walls, and (ii) the effective angle of the triple phase contact between the particles and the interface. Variations in film thickness were calculated using a two-phase flow model. Film thickness was used to determine the time-variation of the capillary force during transit of the bubble. The curve for particle detachment was predicted from the calculated force. This curve proved to be non-linear and gave in situ information on the effective contact angle developing at the particle–bubble interface during detachment. This approach allowed an accurate determination of the detachment force. This theoretical approach was validated using latex particles 2 μm in diameter.     

Synthesis of HAP coating on galvanostatically treated stainless steel substrates by sol–gel method

Producing bioactive hydroxyapatite coatings on metallic implant materials combines the mechanical advantages of implant materials and biological affinity of the hydroxyapatite surface to the natural tissue. In this work, hydroxyapatite was synthesized on 316L stainless steel substrates via sol–gel method by using Ca(NO₃)₂·4H₂O and C₆H₁₅O₃P. In order to improve adherence of the coatings produced, the surface of the substrate was initially modified by electrodepositing nucleus of calcium phosphate compounds. Effect of aging time for preparation of the sol solution and coating characteristics were investigated. The phase compositions and structures of the coatings were characterized by X-ray diffractometry, and scanning electron microscopy was used to determine morphological characteristics of the coatings. Adhesion between the hydroxyapatite coating and the substrate was investigated by using scanning scratch tester. The coating produced on the modified surface by the sol solution aged for 24 h was found to prove better morphological and adhesion properties.     

A simple flow cell system to evaluate in vitro bacterial adhesion on solids

A simple flow cell system was developed for studies of bacterial adhesion on hydrophobic or hydrophilic surfaces in the presence or absence of saliva. The system consists of flow cells made of borosilicate glass tubes (flow cell volume, 0.45 ml) that are connected to syringes mounted on an infusion pump. Hydrophilic or hydrophobic surfaces were obtained by treating the flow cells with acid or dichlorodimethylsilane, respectively. Liquids and bacterial suspensions were withdrawn directly into the flow cells and the flow rate was monitored through the infusion pump. The adhesion of ³H-labeled Streptococcus mutans cells to bare or saliva-coated surfaces was studied in this system by determining the radioactivity retained in the flow cells after rinsing with buffer at different flow rates. The results indicate that factors like the bacterial cell density, the surface properties, the presence of saliva pellicle and the rinsing flow rate significantly affect the adhesion of S. mutans. These results are in agreement with previous findings obtained with other flow cell systems. The present system has the advantages of using small volumes of liquids and allowing defined laminar flow conditions. Moreover, the entire cell, not only selected area spots, are used for the evaluation of bacterial adhesion. No part of the flow cell is re-used, thus avoiding the risk of contamination with other components. The system may therefore be suitable for assays testing the effect of different compounds on bacterial adhesion.     

Simulation of a soil loosening process by means of the modified distinct element method

We apply the Distinct Element Method (DEM) to analyze the dynamic behavior of soil. However, the conventional DEM model for calculation of contact forces between elements has some problems; for example, the movement of elements is too discrete to simulate real soil particle movement. Therefore, we modify the model to solve the difficulties. To investigate the validity of the modified model, we conduct an experiment in which soil is cut with a pendulum-typeblade, and simulate the soil loosening process with the modified DEM model. This paper presents details of the experimental apparatus and the comparison of soil behavior and energy absorption between the simulation and the experiment. Some characteristic phenomena of the experiment are reproduced in the simulation giving us confidence that the modified model is better than the conventional model for the simulation of soil behavior.     

A novel technique for the interfacial characterisation of glass fibre–polypropylene systems

In the present work, a new technique was developed to determine the interfacial properties of two opaque glass fibre/polypropylene (GF/PP) systems via fragmentation tests on single filament model composites. Fragmentation tests usually require the fibre inside the composites to be completely aligned in the loading direction. Since PP matrices are non-transparent, it is not possible to guarantee a priori this condition. Hence, a novel technique was developed to determine the inclination of the filaments embedded in the composites. The fibre–polymer systems were also evaluated by comparing their interfacial properties with the overall mechanical properties determined on pultruded GF/PP composites. The present work shows that the knowledge of the interfacial properties is important, not only to compare alternative fibre/matrix systems, but also to assess whether the level of adhesion in these systems is adequate to fabricate composites with good mechanical properties.     

The adhesion coefficient of fiber-reinforced polymers evaluated by dynamic measurements

A theoretical model, consisting of a series of infinite concentric cylinders surrounding a fiber in a composite material, was introduced in this paper to give a quantitative account of interface phenomena, already experimentally observed. A series of specimens, conveniently designed to represent the theoretical model, were subjected to dynamic modes of loading to measure the amount of adhesion between fibers and matrices by means of an adhesion coefficient developed in the theory. It was found that theoretical results for the adhesion between matrix and filler were compatible with the structural characteristics of the specimens tested.     

Characterisation of the interface of sputter-deposited copper coatings on nitrogen plasma-treated carbon substrates

The adhesion of copper coatings to carbon substrates is very poor, because of lack of diffusion or reaction between the constituents. Because there is technological interest in enhancing the adhesion and improving the interface between copper and carbon, plasma treatment of the carbon substrate was employed in this study. For modification of the carbon surface a nitrogen plasma was used. It was confirmed by pull-off tests that the strength of adhesion of the copper coating can be improved by a factor of more than 10 by plasma pretreatment, even after treatment for a very short time (1 min). To obtain more information about the mechanisms of the processes occurring at the interface SIMS investigations were performed on samples which had been treated for different times (between 1 and 60 min). These measurements confirmed that nitrogen is located on the interface. With increasing pretreatment time the amount of nitrogen detected on the interface increased. Besides characterisation of as-deposited samples, another focus was to study the mechanisms of diffusion of nitrogen if the samples are heat treated at 500°C.     

Cavity growth in soft adhesives

The growth process of cavities nucleated at the interface between a rigid surface and a soft adhesive layer has been investigated with a probe method. A tensile stress was applied to the highly confined layer resulting in a negative hydrostatic pressure in the layer. The statistics of appearance and rate of growth of cavities as a function of applied negative stress were monitored with a CCD camera. If large germs of cavities were initially present, most of the cavities became optically visible above a critical level of stress independent of layer thickness. Cavities grew simultaneously and at the same expansion rate as a function of applied stress. In the absence of large germs, cavities became optically visible one after another, reaching a limiting size controlled by the thickness of the layer independently and very rapidly. Although, for each sample, we observed a statistical distribution of critical stress levels where a cavity expanded, the mean cavitation stress depended both on surface topography and more surprisingly on layer thickness. We believe that this new and somewhat surprising result can be interpreted with a model for the growth of small germs in finite size layers (J. Dollhofer, A. Chiche, V. Muralidharan et al., Int. J. Solids Struct. 41, 6111 (2004)). This model is mainly based on the dual notion of an energy activated transition from an unexpanded metastable state to an expanded stable state and to the proportionality of the activation energy with the elastic energy stored in the adhesive layer.     

Lifting the singular nature of a model for peeling of an adhesive tape

We investigate the dynamics of peeling of an adhesive tape subjected to a constant pull speed. Due to the constraint between the pull force, peel angle and the peel force, the equations of motion derived earlier fall into the category of differential-algebraic equations (DAE) requiring an appropriate algorithm for its numerical solution. By including the kinetic energy arising from the stretched part of the tape in the Lagrangian, we derive equations of motion that support stick-slip jumps as a natural consequence of the inherent dynamics itself, thus circumventing the need to use any special algorithm. In the low mass limit, these equations reproduce solutions obtained using a differential-algebraic algorithm introduced for the earlier singular equations. We find that mass has a strong influence on the dynamics of the model rendering periodic solutions to chaotic and vice versa. Apart from the rich dynamics, the model reproduces several qualitative features of the different waveforms of the peel force function as also the decreasing nature of force drop magnitudes.     

Stability of mechanical properties of molecular layer–deposited alucone

We report on the effect of aging on the mechanical properties of molecular layer–deposited (MLD) thin films. We studied the mechanical failure of the films during uniaxial tensile testing and observed a sixfold difference in the crack-onset strain (COS) and related flexibility within the first two days after the samples were exposed to ambient air. The MLD films made using trimethylaluminum and ethylene glycol are notorious for exhibiting structural changes after the fabrication; we show that these changes are detrimental for mechanical robustness of the films. This information aids to plan the handling or the protection of these films to achieve better performance with these materials. The interfacial shear strains and COSs of the shortly air-exposed 300-nm-thick films were observed to be roughly 0.3% and 1.8%, respectively. These values are the highest reported so far for hybrid organic–inorganic MLD thin films and would extrapolate to about 14% COS for 5-nm-thick film, indicating potential applications as interfacial adhesion layer for films on polymer substrates and as a protective coating in battery applications.