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Adhesion of the bacterial strain Staphylococcus epidermidis 3399 to titanium-oxy-nitride (TiNOX (R)) substrata with different specific resistivities was studied in a parallel plate flow chamber, while simultaneously measuring the electric potential of the substrata. During adhesion, bacteria either donated or accepted electrons to the substrata depending on the specific resistivity of the substratum and bacteria that had donated electrons to the substratum adhered more strongly than bacteria that had accepted electrons from the substratum. These results demonstrate that electron transfer plays a role in bacterial adhesion to conducting surfaces, which has hitherto been neglected. (C) 2000 Elsevier Science B.V. All rights reserved.

Adhesion of the bacterial strain Staphylococcus epidermidis 3399 to titanium-oxy-nitride (TiNOX (R)) substrata with different specific resistivities was studied in a parallel plate flow chamber, while simultaneously measuring the electric potential of the substrata. During adhesion, bacteria either donated or accepted electrons to the substrata depending on the specific resistivity of the substratum and bacteria that had donated electrons to the substratum adhered more strongly than bacteria that had accepted electrons from the substratum. These results demonstrate that electron transfer plays a role in bacterial adhesion to conducting surfaces, which has hitherto been neglected. (C) 2000 Elsevier Science B.V. All rights reserved.

The influence of surface characteristics of the microorganisms and the substrate materials, such as electrokinetic potencial and hydrophobicity, in the adhesion process was investigated in a previous work [1]. As neither surface charge nor hydrophobicity can fully explain the process of bacterial adhesion, the effect of divalent cations was experimentally studied. Adhesion assays were performed in sodium phosphate buffer saline (PBS) medium and in a medium with the same ionic strength containing Ca2+, Mg2+ and Fe2+. In the presence of divalent cations, contrary to what happened in PBS, adhesion was more favourable to the more negatively charge polymeric material. This points out a strong contribution of ion bridging in culture medium. In the absence of divalent cations, a higher hydrophobicity of the support is required in the process of adhesion., Instituto de Biotecnologia e Química Fina (IBQF) - Project 01/REGII/6/96, Fundação para a Ciência e a Tecnologia - Praxis XXI, BD/9121/96, BCC11961/97.

The main topic of this thesis is Poisson–Boltzmann theory for suspensions of charged colloids in two of its approximations: cell-type approximations that explicitly take into account non-linear effects near the colloidal surfaces, such as charge renormalization, at the expense of neglecting any explicit multi-body interactions; and (ii) linear approximations that do take into account explicit multi-body interactions but neglect any non-linear effects. These approximations give contradictory results with regards to the existence of spinodal instabilities at low salinity. Firstly, we review Poisson–Boltzmann theory and its cell approximation, and derive a complete description of the linear approximation in a semi-grand canonical framework; we show that this theory gives rise to so-called volume terms, which drive spinodal instabilities at low salinity, and which also give important contributions to the osmotic pressure of such colloidal suspensions. We then construct a novel theory by combining the cell-type and the linear approximations. Taking the strong points of each, the newly constructed theory takes into account both the non-linear behavior near the colloidal surfaces and the explicit multi-body interactions between the colloids. Using this theory, we calculate phase-diagrams as a function of the salt concentration and the colloidal density for many values of the charge Z and the radius over Bjerrum-length ratio λ B / a . We find that spinodal instabilities occur for systems with Z λ B / a ≥ 25 , and that these instabilities for large charges Z are connected to the gas–liquid instability of the primitive model with small (Z=1–10) valencies, suggesting that both instabilities have the same physical origin. Furthermore, we study charge regulation, which describes the chemical equilibrium between ions bound to the colloidal surfaces and free ions. We first study this effect in the Poisson–Boltzmann cell model, and calculate the net charge of the colloids as a function of the particle size, the dissociation constant, the colloid density and the salt concentration. We scanned a large part of parameter space for spinodal instabilities, but find no such instabilities within this model. Finally, we include the charge regulation effects into the newly developed multi-centered non-linear Poisson–Boltzmann theory. For silica particles, we calculate the charge Z as a function of the colloid density and the salt concentration, and we find that, for almost all systems examined, the coupling parameter Z λ B / a ≤ 10 . We thus conclude that, in this model with charge regulation, the coupling parameter is too small for spinodal instabilities to occur. We explicitly calculated phase-diagrams for a large number of colloidal charges and radii, and indeed find no instabilities. Therefore, we conclude that the spinodal instabilities that were found in the model with fixed colloidal charge are probably hard to reach in experimental setups with silica particles.

The main topic of this thesis is Poisson–Boltzmann theory for suspensions of charged colloids in two of its approximations: cell-type approximations that explicitly take into account non-linear effects near the colloidal surfaces, such as charge renormalization, at the expense of neglecting any explicit multi-body interactions; and (ii) linear approximations that do take into account explicit multi-body interactions but neglect any non-linear effects. These approximations give contradictory results with regards to the existence of spinodal instabilities at low salinity. Firstly, we review Poisson–Boltzmann theory and its cell approximation, and derive a complete description of the linear approximation in a semi-grand canonical framework; we show that this theory gives rise to so-called volume terms, which drive spinodal instabilities at low salinity, and which also give important contributions to the osmotic pressure of such colloidal suspensions. We then construct a novel theory by combining the cell-type and the linear approximations. Taking the strong points of each, the newly constructed theory takes into account both the non-linear behavior near the colloidal surfaces and the explicit multi-body interactions between the colloids. Using this theory, we calculate phase-diagrams as a function of the salt concentration and the colloidal density for many values of the charge Z and the radius over Bjerrum-length ratio λ B / a . We find that spinodal instabilities occur for systems with Z λ B / a ≥ 25 , and that these instabilities for large charges Z are connected to the gas–liquid instability of the primitive model with small (Z=1–10) valencies, suggesting that both instabilities have the same physical origin. Furthermore, we study charge regulation, which describes the chemical equilibrium between ions bound to the colloidal surfaces and free ions. We first study this effect in the Poisson–Boltzmann cell model, and calculate the net charge of the colloids as a function of the particle size, the dissociation constant, the colloid density and the salt concentration. We scanned a large part of parameter space for spinodal instabilities, but find no such instabilities within this model. Finally, we include the charge regulation effects into the newly developed multi-centered non-linear Poisson–Boltzmann theory. For silica particles, we calculate the charge Z as a function of the colloid density and the salt concentration, and we find that, for almost all systems examined, the coupling parameter Z λ B / a ≤ 10 . We thus conclude that, in this model with charge regulation, the coupling parameter is too small for spinodal instabilities to occur. We explicitly calculated phase-diagrams for a large number of colloidal charges and radii, and indeed find no instabilities. Therefore, we conclude that the spinodal instabilities that were found in the model with fixed colloidal charge are probably hard to reach in experimental setups with silica particles.

In dieser Arbeit wurden zum ersten Mal alle elastischen Konstanten von Kolloidkristallen experimentell bestimmt. Die untersuchte kolloidale Suspension bestand aus mikroskopisch kleinen, sterisch stabilisierten Plexiglaspartikeln in einer Lösungsmittelmischung aus Decalin und Cycloheptylbromid. Mittels konfokaler Mikroskopie wurden die Trajektorien der Partikel mit hoher Genauigkeit gemessen.In der flüssigen Phase ergab sich aus der Analyse der radialen Paarverteilungsfunktion bestimmt, dass die Wechselwirkung bei kleinen Volumenbrüchen paarweise-additiv und im Einklang mit der DLVO-Theorie ist.In der kristallinen Phase, bei höheren Volumenbrüchen, bildeten sich polykristalline Strukturen mit großen Körnern von kubisch-flächenzentrierter Symmetrie. Die Untersuchung der thermisch angeregten Gitterschwingungen dieser Kristalle führte zu der Messung aller Kraftkonstanten und aller elastischen Konstanten. Es ergab sich, dass die gemessenen Kraftkonstanten der Vorstellung von paarweise-additiven, zentralsymmetrischen Kräften widersprechen und dass Vielteilcheneffekte zu berücksichtigen sind. Für die elastischen Konstanten ergab sich eine Verletzung der Cauchy-Relation. Dies zeigt ebenfalls, dass die Kräfte zwischen den Partikeln nicht zentralsymmetrisch sind. Somit ist es nicht möglich, die Wechselwirkung in Kristallen aus geladenen kolloidalen Teilchen im Rahmen der DLVO-Theorie zu erklären.

Adhesion of the bacterial strain Staphylococcus epidermidis 3399 to titanium-oxy-nitride (TiNOX (R)) substrata with different specific resistivities was studied in a parallel plate flow chamber, while simultaneously measuring the electric potential of the substrata. During adhesion, bacteria either donated or accepted electrons to the substrata depending on the specific resistivity of the substratum and bacteria that had donated electrons to the substratum adhered more strongly than bacteria that had accepted electrons from the substratum. These results demonstrate that electron transfer plays a role in bacterial adhesion to conducting surfaces, which has hitherto been neglected. (C) 2000 Elsevier Science B.V. All rights reserved.

The object of this thesis is to apply scanning probemicroscopy (SPM) to the field of advanced powder processing.Measurement of interparticle surface forces at conditionsrelevant to ceramic processing has been performed together withthorough studies of powder friction. Surface force measurements between silicon nitride andsilica surfaces in 1-bromonaphtalene and diiodomethane resultedin an attractive van der Waals force in both media for thesymmetric systems and a repulsive van der Waals force for theasymmetric systems. This agreed well with theoreticalpredictions from Lifshitz theory. Measurements in electrolytesolutions between silicon nitride surfaces with a varyingdegree of oxidation showed that silanol groups dominated theinteractions at a high degree of oxidation, whereas theinfluence of amine species became stronger after surfaceetching. Surface charge, surface potential and density ofsurface groups have been extracted from DLVO computer modellingof the experimental force curves. Surface force measurementshave been carried out using a nitrided silica sphere as acolloidal probe, representative for commercial silicon nitridepowders. Adsorption of poly(acrylic acid) above the isoelectricpoint generated a thin polymer layer of an essentially flatconformation. Friction force measurements were attainable via novelcalibration procedures of both the lateral photodetectorresponse and the cantilever torsional spring constant.Combining the method for detector calibration with evaluationof static friction slopes simplified the choice of appropriatecontact mechanics theory to evaluate the friction measurements.Applying the method on friction measurements between ironsurfaces coated with commercial lubricants showed a substantialsurface deformation not seen from the friction force alone. Thenanorheological properties of silica surfaces coated with twodifferent stearic acids have been evaluated from friction forcemeasurements. Steady sliding motion was replaced by highlyregular stick-slip motion at a critical load and velocity. Thestick-slip motion was successfully described and fitted to aphenomenological model. The contact area evaluated fromMaugis-Dugdale theory revealed the contact diameter to be veryclose to the stick-slip periodicity. Friction measurements were carried out between individualspray-dried ceramic granules as a function of binderconcentration and relative humidity. The hygroscopic nature ofthe binder resulted in a higher adhesion force but lowerfriction coefficient with increasing humidity. This effect wasascribed to softening of the binder and a lowering of the shearstrength of the binder rich surface layer on the granules.Comparison of the friction force between two granules and agranule and a hard metal surface confirmed that resistance toflow during the initial stages of powder pressing is dominatedby granule-wall friction and adhesion. Keywords: Atomic force microscope, AFM, SPM, van der Waalsinteraction, DLVO-theory, surface forces, colloidal probe,force curve, friction, adhesion, stick-slip, cantilever,calibration, spring constant, silicon nitride, iron,silica. NR 20140805

For the description of general bacterial adhesion phenomena two different physicochemical approaches are available. The first one, based on a surface Gibbs energy balance, assumes intimate contact between the interacting surfaces. According to this approach adhesion is solely related to the Gibbs energies of the surfaces involved. The second approach, based on colloid chemical theories (DLVO theory), allows for two types of adhesion: 1. secondary minimum adhesion, which is often weak and reversible, and 2. irreversible primary minimum adhesion. In the first case a thin water film is present between the interacting surfaces. In the DLVO approach adhesion is determined by long range interactions, i.e., Van der Waals and electrostatic interactions. Van der Waals interactions may be related to the hydrophobicity of the cell wall. For the measurement of bacterial hydrophobicity and electrokinetic potential several macroscopic methods are available. Based on a literature review of the influence of both surface characteristics on adhesion, it is concluded that the surface Gibbs energy balance approach is not adequate to describe the majority of adhesion phenomena. On the other hand the DLVO-theory describes the observations fairly well, especially in the case of reversible (secondary minimum) adhesion. The influence of adsorbing (in)organic compounds, extracellular polymers and cell surface appendages on adhesion can also be predicted by a DLVO-type approach.