Colloidal particle-surface interactions in atmospheric and aquatic systems

Colloidal particles suspended in a liquid or gas phase often interact with solid-liquid or solid-gas interfaces. In this study, experimental data through atomic force microscopy (AFM) and neutron reflectometry (NR) and theoretical results of colloidal particle-surface interactions were obtained and compared. Atmospheric and aquatic environments were considered for the interactions of microbial colloidal particles and nano-sized silica particles with planar surfaces. Spores of Bacillus thuringiensis (Bt), members of the Bacillus cereus group, were examined as the microbial particles simulating the pathogens Bacillus cereus and Bacillus anthracis which are potentially dangerous to human health. Model planar surfaces used in this study include gold which is an electrically conductive surface, as well as mica and silica which are charged, nonconductive surfaces.;A mathematical model was developed to calculate the adhesion force between a spherical particle and a planar surface in atmospheric systems as the sum of the capillary force and the van der Waals force. The electrostatic interaction was initially neglected in the model. The two-force mechanisms that were considered are functions of relative humidity. The capillary force increases as the relative humidity value increases, whereas the van der Waals force decreases as the humidity increases. Adhesion force measurements by AFM were conducted using a particle or a Bt-spore modified AFM cantilever probe. The measured adhesion force between a silica particle and a gold surface is comparable to the model calculations, while there is some disagreement between the measured adhesion force for the Bt spore-gold surface system and the calculated values. The discrepancy may be the result of neglecting the electrostatic force, and of the surface roughness of the spore that was observed from the imaging studies.;In continuation of the study, the electrostatic force was investigated as one of the components of the adhesion force between Bt spores and planar surfaces in atmospheric systems. The surface potentials of a Bt spore and a non-conductive surface, such as mica, were experimentally obtained using a combined AFM-scanning surface potential microscopy (SSPM) technique. The surface charge of the mica and the Bt spore decreases with increasing humidity. The Coulombic force was introduced for the spore-mica system and an electrostatic image force was introduced for the spore-gold system. The Coulombic force for spore-mica (both charged, non-conductive surfaces) is repulsive because the components are similarly charged, while the image force for the spore-gold system is attractive. The magnitude of both forces decreases with increasing humidity as a result of the decreasing surface charge density as humidity increases. The electrostatic forces were added to other force components, e.g., van der Waals and capillary forces, to obtain the adhesion force for each system. The repulsive Coulombic force for the spore-mica system decreases the magnitude of the adhesion force, while the attractive image force for the spore-gold system increases the previously obtained adhesion force. It was shown that the electrostatic (Coulombic and image) forces play a significant role in the adhesion force between spores and planar surfaces.;In this part of the study, bacterial spore interactions with planar surfaces in aquatic environments, including adhesion forces and force-distance profiles, were investigated. The characteristics of Bt spores were determined using electron microscopy and electrokinetic measurements. The surface potentials of the spores and mica surface used in the experiments were measured as a function of pH and ionic strength. The Derjaguin-Landau-Verwey-Overbeek (DLVO) theory was employed to predict the interaction force between the spore and planar surfaces as a function of the separation distance, and a force balance was used to explain the adhesion force or "pulling-off" force. Theoretical estimations were compared to experimental measurements obtained from AFM. The DLVO-based calculations are consistent with AFM force measurements, while the calculated adhesion force shows some deviations from the measurements. The deviations can be minimized by considering the roughness of the Bt spore and substrate surfaces. (Abstract shortened by UMI.).