Effect of polyethylene pretreatments on the biomimetic deposition and adhesion of calcium phosphate films.

The effect of ultraviolet irradiation and glow discharge (GD) processing of the polyethylene (PE) substrates on deposition of calcium phosphate (CaP) films from supersaturated aqueous calcium phosphate solutions was investigated in this study. CaP coatings deposited on the PE substrates were comprised of elongated clusters of spherical particles and 100% of the free surface area of nearly all of the substrates was covered with a porous CaP film after a 3 day immersion. Nano-scratch tests determined that PE-CaP adhesion was most improved when PE substrates were subjected to 50W GD treatments. As determined by contact angle measurements, the GD-treated PE samples had the highest electron donor parameter of surface energy, suggesting that enhancing the electron donor parameter of PE leads to improved adhesion with the biomimetic CaP coating.

Probing colloidal forces between a Si3N4 AFM tip and single nanoparticles of silica and alumina.

The atomic force microscope (AFM) has been used to measure surface forces between silicon nitride AFM tips and individual nanoparticles deposited on substrates in 10(-4) and 10(-2) M KCl solutions. Silica nanoparticles (10 nm diameter) were deposited on an alumina substrate and alumina particles (5 to 80 nm diameter) were deposited on a mica substrate using aqueous suspensions. Ionic concentrations and pH were used to manage attractive substrate-particle electrostatic forces. The AFM tip was located on deposited nanoparticles using an operator controlled offset to achieve stepwise tip movements. Nanoparticles were found to have a negligible effect on long-range tip-substrate interactions, however, the forces between the tip and nanoparticle were detectable at small separations. Exponentially increasing short-range repulsive forces, attributed to the hydration forces, were observed for silica nanoparticles. The effective range of hydration forces was found to be 2-3 nm with the decay length of 0.8-1.3 nm. These parameters are in a good agreement with the results reported for macroscopic surfaces of silica obtained using the surface force apparatus suggesting that hydration forces for the silica nanoparticles are similar to those for flat silica surfaces. Hydration forces were not observed for either alumina substrates or alumina nanoparticles in both 10(-4) M KCl solution at pH 6.5 and 10(-2) M KCl at pH 10.2. Instead, strong attractive forces between the silicon nitride tip and the alumina (nanoparticles and substrate) were observed.

AFM colloidal forces measured between microscopic probes and flat substrates in nanoparticle suspensions.

Colloidal forces between atomic force microscopy probes of 0.12 and 0.58 N/m spring constant and flat substrates in nanoparticle suspensions were measured. Silicon nitride tips and glass spheres with a diameter of 5 and 15 mum were used as the probes whereas mica and silicon wafer were used as substrates. Aqueous suspensions were made of 5-80 nm alumina and 10 nm silica particles. Oscillatory force profiles were obtained using atomic force microscope. This finding indicates that the nanoparticles remain to be stratified in the intervening liquid films between the probe and substrate during the force measurements. Such structural effects were manifested for systems featuring attractive and weak repulsive interactions of nanoparticles with the probe and substrate. Oscillation of the structural forces shows a periodicity close to the size of nanoparticles in the suspension. When the nanoparticles are oppositely charged to the probes, they tend to coat the probes and hinder probe-substrate contact.

Pull-off force measurements between rough surfaces by atomic force microscopy.

Direct measurements of the pull-off (adhesion) forces between pharmaceutical particles (beclomethasone dipropionate, a peptide-type material, and lactose) with irregular geometry and rough polymeric surfaces (series of polypropylene coatings, polycarbonate, and acrylonitrile-butadiene-styrene) were carried out using the atomic force microscope. These measurements showed that roughness of the interacting surfaces is the significant factor affecting experimentally measured pull-off forces. A broad distribution of pull-off force values was noted in the measurements, caused by a varying adhesive contact area for a particle located on rough substrate. The possibility of multiple points of contact between irregularly shaped pharmaceutical particles and substrate surfaces is demonstrated with nanoindentations of the particle in a fluoro-polymer film. Force-distance curves showing the "sawtooth" pattern are additional evidence that particles make contact with substrates at more than one point. Reduced adhesion of 10- to 14-microm-diameter lactose and peptide material particles to the polypropylene coatings with a roughness of 194 nm was found in this study. Similar pull-off force versus roughness relationships are also reported for the model spherical particles, silanized glass particle with a size of 10 microm and polystyrene particle with a diameter of 9 microm, in contact with polypropylene coatings of varying roughness characteristics. It was found that the model recently proposed by Rabinovich et al. (J. Colloid Interface Sci. 232, 1-16 (2000)) closely predicts the pull-off forces for glass and lactose particles. On the other hand, the adhesion of the peptide material and polystyrene particle to polypropylene is underestimated by about an order of magnitude with the theoretical model, in which the interacting substrates are treated as rigid materials. The underestimate is attributed to the deformation of the peptide material and polystyrene particles.

Mineral matter distribution on coal surface and its effect on coal wettability.

Coal is an organic sedimentary rock composed of organic macerals and mineral matter. As it is demonstrated in this paper the discrete mineralogical nature of coal largely influences the wetting of the coal surface by water. Both advancing and receding contact angles were measured using the captive-bubble technique with an automatic bubble shape analysis software. The distribution and amount of mineral inclusions on the coal surface were determined by scanning electron microscopy and examined using the image analysis system. To determine the amount and size distribution of mineral grains, the coal surface layer, on which the contact angles were measured, was separated from the larger piece used in the measurements by microslicing. The separated surface layer was subjected to a low-temperature ashing followed by particle size analysis. As expected, a significant scatter of contact angle values was obtained for the same coal samples. Increasing the amount of mineral matter on the coal surface reduced the value of both advancing and receding contact angles. Also, the scatter of contact angle values increased with the increasing mineral matter content from about 1 to 50 wt%. The results reveal that an important factor in analysis of contact angle variation on coal surfaces is the size of the hydrophilic mineral inclusions. Both the advancing and the receding contact angles decrease with increasing size of the mineral grains. Additionally, the scatter of contact angle values increase with increasing size of the mineral matter grains. Finally, the results of fractal dimension analysis of mineral matter grains distributed over the coal surface indicate that there is no significant effect from the shape of hydrophilic mineral inclusions on both advancing and receding contact angles.