Jan Fransaer

Living on a surface: swarming and biofilm formation

Swarming is the fastest known bacterial mode of surface translocation and enables the rapid colonization of a nutrient-rich environment and host tissues. This complex multicellular behavior requires the integration of chemical and physical signals, which leads to the physiological and morphological differentiation of the bacteria into swarmer cells. Here, we provide a review of recent advances in the study of the regulatory pathways that lead to swarming behavior of different model bacteria. It has now become clear that many of these pathways also affect the formation of biofilms, surface-attached bacterial colonies. Decision-making between rapidly colonizing a surface and biofilm formation is central to bacterial survival among competitors. In the second part of this article, we review recent developments in the understanding of the transition between motile and sessile lifestyles of bacteria.

Electrodeposition and sliding wear resistance of nickel composite coatings containing micron and submicron SiC particles

Abstract SiC particles of three different sizes, namely 5, 0.7 and 0.3 μm, were codeposited with nickel from Watts’ solutions. It was found that for a given number density of particles in the plating solution, the number density of particles in the coating increases with decreasing particle size. The friction and wear behavior of these composite coatings was evaluated in uni- and bi-directional sliding tests against corundum balls. The best sliding wear resistance was obtained with Ni–SiC composite coatings containing 4–5 vol.% submicron SiC particles.

Exploiting particle shape in solid stabilized emulsions

Pickering–Ramsden emulsions and other forms of particle stabilized soft materials have received quite some attention recently because of the relative ease of formulation and the possibility to create novel materials. There is, however, a clear need for approaches that are versatile and efficient. In the present work the effect of aspect ratio of particles on the stability of both water-in-oil and oil-in-water emulsions is investigated experimentally. Two types of non-spherical particles are used. Hydrophobic prolate ellipsoids with aspect ratios ranging from 1 to 9 are obtained by stretching polystyrene latex particles. Hydrophilic spindle type hematite particles have been synthesized with aspect ratios ranging from 1 to 6. A strong dependence of emulsion stability on the aspect ratio of the particles is observed. Optical as well as cryogenic scanning electron microscopy are used to visualize the droplet morphology and particulate structure and reveal fairly densely packed monolayers of ellipsoids, consistent with the mechanism of limited coalescence. Yet stable emulsions are only obtained for particles with a sufficient aspect ratio. Surface rheology on planar monolayers demonstrates the pronounced effect of aspect ratio on the surface moduli. The magnitude of the interfacial viscoelastic properties is shown to strongly depend on the aspect ratio at a given surface coverage. This is most probably due to an increased effective coverage and the occurrence of strong attractive shape induced capillary interactions. The dependence of the surface rheological properties on the aspect ratio of the particles rationalizes the observed emulsion stability as the surface rheological properties play a role in the coalescence process. The results demonstrate that interfaces with controlled surface rheology, as obtained by using shape induced capillary forces and packing effects, can be used for the rational design of Pickering emulsions and other types of high interface materials.

Analysis of the Electrolytic Codeposition of Non‐Brownian Particles with Metals

A model for the electrolytic codeposition of spherical particles with metals on a rotating disk electrode is presented, based on a trajectory analysis of the particle deposition, including convective mass transport, geometrical interception, and migration under specific forces, coupled to a surface immobilization reaction. A number of relevant forces were included and their effects determined. Theoretical predictions of this model are compared with experimental results for the codeposition of spherical polystyrene particles with copper during electrolysis from an acid copper sulfate solution. The influence of fluid flow velocities, particle concentration, and current density on the rate of particle deposition is illustrated. Experiments done on a rotating disk electrode allow the adhesion forces to be determined from the distribution of particles on the surface. It is shown that codeposition is governed by colloidal interactions that can, in first order, be approximated by the Derjaguin‐Landau‐Verwey‐Overbeek interactions plus an additional short range repulsion that was associated with the hydration force.

Self-Assembly and Rheology of Ellipsoidal Particles at Interfaces

Colloidal particles confined at liquid interfaces have important applications, for example in the stabilization of emulsions and foams. Also the self-assembly of particles at interfaces offers potential for novel applications and structured particle films. As the colloidal interactions of colloidal particles at interfaces differ from those in bulk, colloidal microstructures can be achieved at an interface which cannot be produced in bulk. In the present work the particle shape, surface charge, and wetting properties are varied, and the resulting self-assembly of particles at a fluid interface is studied. Model monodisperse micrometer-sized ellipsoidal particles were prepared by a mechanical stretching method. These particles were chosen to be well-suited for investigation by optical microscopy. When deposited at an interface between two fluids, shape-induced capillary interactions compete with the electrostatic repulsion. Changing the surface charge and the position at the interface can be used to manipulate the experimentally observed self-assembly process. The initial microstructure of charged ellipsoids at a decane-water interface consists of individual ellipsoids coexisting with linear chains of ellipsoids, connected at their tips. The aggregation behavior in these monolayers was investigated by optical microscopy combined with quantitative image analysis and a dominant tip-tip aggregation was observed. Microstructural information was quantified by calculating the pair-distribution and orientation-distribution functions, as a function of time. Compared to particles at an oil-water interface, particles of the same surface chemistry and charge at an air-water interface seem to have weaker electrostatic interactions, and they also have a different equilibrium position at the interface. The latter leads to differences in the capillary forces. The subsequent change in the balance between electrostatic and capillary forces gave rise to very dense networks having as a typical building block ellipsoids connected at their tips in triangular or flower-like configuration. These networks were very stable and did not evolve in time. The resulting monolayers responded elastically and buckled under compression. Furthermore, the mechanical properties of these monolayers, as measured by surface shear rheology, showed that the monolayer of ellipsoids exhibit a substantial surface modulus even at low surface coverage and can be used to create more elastic monolayers compared to aggregate networks of spheres of the same size and surface properties.

Improved corrosion resistance through microstructural modifications induced by codepositing SiC-particles with electrolytic nickel

Abstract Micron and submicron-sized SiC-particles (5 and 0.3 μm respectively) were codeposited with nickel from a Watts electrolyte. The Ni–SiC composite coatings showed a better corrosion resistance in a 0.6 M NaCl solution than nickel electrodeposited under the same conditions. The corrosion rate of Ni–SiC decreases by two orders of magnitude with respect to pure Ni coatings. This improved corrosion resistance is quite independent of the size and amount of embedded particles, except for the smallest SiC-particles investigated. In that case, the pitting corrosion potential shifts to more noble values indicating a notable reduction of the localized corrosion susceptibility. This improved corrosion resistance of Ni–SiC coatings containing submicrometric SiC-particles is linked to a change in grain morphology and texture of the coatings. That morphology evolves from columnar grains to small and equiaxed grains.

Sol-gel preparation of high-Tc Bi-Ca-Sr-Cu-O and Y-Ba-Ca-O superconductors

High‐Tc superconducting oxides have been prepared by a liquid‐mix technique using etylene‐diamine‐tetra‐acetic acid (EDTA) as a complexing agent. Bi‐Sr‐Ca‐Cu oxides and Y‐Ba‐Cu oxides were made using this technique giving a better compositional homogeneity, a more precise control of the cation stoichiometry and a decreased firing temperature as compared to conventional material produced by a solid‐state reaction. This technique extends the amorphous citrate process to those systems where no citrate complex exists. EDTA binds with most metallic elements of the periodic table, making this technique a versatile tool in the production and study of these new ceramic materials. Therefore, the method is easily adapted to the preparation of new superconducting oxides.

Copper(I)‐Containing Ionic Liquids for High‐Rate Electrodeposition

New metal-containing ionic liquids [Cu(CH(3)CN)(n)][Tf(2)N] (n=2, 4; Tf(2)N=bis(trifluoromethylsulfonyl)- amide) have been synthesised and used as a non-aqueous electrolyte for the electrodeposition of copper at current densities greater than 25 A dm(-2). The tetrahedral copper(I)-containing cation in [Cu(CH(3)CN)(4)][Tf(2)N] is structurally analogous to quaternary ammonium and phosphonium ionic liquids and overcomes problems of metal solubility and mass transport. Two CH(3)CN ligands are removed at elevated temperatures to give [Cu(CH(3)CN)(2)][Tf(2)N], which can be used as a concentrated non-aqueous electrolyte. The structural and electrochemical characterisation of these compounds is described herein.

Electrochemical investigation of the codeposition of SiC and SiO2 particles with nickel

The use of electrochemical impedance spectroscopy (EIS) for the in situ control of the electrolytic codeposition of Ni/SiO2 and Ni/SiC was investigated. An attempt was made to clarify why silica particles hardly codeposit in comparison to silicon carbide particles. It was found that the presence of SiO2 and SiC particles influences the metal deposition process in different ways. SiC particles that are being embedded in the growing metal layer cause an apparent decrease in the electrode surface area, probably due to blocking off a part of the surface by partly engulfed particles. In the case of SiO2 particles, which embed in the metal matrix to a very limited extent, no blocking was observed. It was found that the presence of particles in the solution causes an apparent increase in the electrode surface area, probably due to increased surface roughness.

Mechanism of electrolytic composite plating: survey and trends

SummaryWith the development of composite plating during the past two decades, new opportunities for electroplating in the field of advanced materials have been created. Although the electroplating of some composite coatings like Ni-SiC, Ni-PTFE, and Co-Cr2O3 are nowadays common industrial practice, the modelling of the codeposition process is still far from being well- developed. The present paper intends to give a survey on the understanding of the mechanism of electrolytic codeposition and on the mathematical models developed over the years to describe the incorporation of solid particles in electrodeposited metals. Finally some trends and expected future developments of composite plating are presented.