Jan Vermant

Since decades, most porous polymeric membranes are prepared via phase inversion. It allows the formation of a variety of porous structures, but in spite of all research, it remains difficult to control the porosity of the membrane. Emulsion templating is an interesting alternative to prepare ordered macroporous membranes. In the mid eighties Barby and Haq introduced this principle in the preparation of porous polymers [1].

In inertialess suspensions of rigid particles, the rotational motion of each particle is governed by the so-called freely rotating condition, whereby the total torque acting on the particle must be zero. In this work, we study the effect of viscoelasticity of the suspending liquid on the rotation period of a sphere by means of three-dimensional finite element simulations, for conditions corresponding to a macroscopic shear flow.

The magnetic interfacial needle stress rheometer is a device capable of sensitive rheological interfacial measurements. Yet even for this device, when measuring interfaces with low elastic and viscous moduli, the system response of the instrument contributes significantly to the measured response. To determine the operation limits of the magnetic rod rheometer, we analyze the relative errors that are introduced by linearly subtracting the instrument contribution from the measured response.

The effect of shear flow on the structure and dynamics of monodisperse spherical polystyrene particles suspended at the interface between decane and water was observed. While undisturbed, the particles arrange themselves on a hexagonal lattice due to strong dipole–dipole repulsion resulting from ionizable sulfate groups on their surfaces. As the interface is subjected to shear flow, however, the lattice adopts a new semi-ordered, anisotropic state for which two distinct regimes are observed.

ABSTRACT: When particles are dispersed in viscoelastic rather than Newtonian media, the hydrodynamics will be changed entailing differences in suspension rheology. The disturbance velocity profiles and stress distributions around the particle will depend on the viscoelastic material functions. Even in inertialess flows, changes in particle rotation and migration will occur.

Bacterial swarming is one of the most efficient methods by which bacteria colonize nutrient-rich environments and host tissues. Several mechanisms have been proposed to explain the phenomenon and the associated intricate macroscopic pattern formation, but so far no conclusive evidence has been presented that identifies the factors that control swarming. Vice versa, little is known about how swarming can be controlled.

The synthesis and selected macroscopic properties of a new model system consisting of poly (N-isopropylacrylamide)(PNIPAM)-coated rod-like fd virus particles are presented. The sticky rod-like colloids can be used to study effect of particle shape on gelation transition, the structure and viscoelasticity of isotropic and nematic gels, and to make both open isotropic as well as ordered nematic particle networks.

Particle shape plays an important role in controlling the optical, magnetic, and mechanical properties of nanoparticle suspensions as well as nanocomposites. However, character-izing the size, shape, and the associated polydispersity of nanoparticles is not straightforward. Electron microscopy provides an accurate measurement of the geometric properties, but sample preparation can be laborious, and to obtain statistically relevant data many particles need to be analyzed separately.

Rheological measurements are used to compare clay nanocomposites prepared through melt mixing using two different polypropylene matrices. Steady state and transient nonlinear rheological experiments are employed to separate the contributions of flow induced orientation of the tactoids and particulate network build-up. The conditions under which the rheological properties are dominated by the aggregate network are subsequently identified.

Synopsis The effect of a delicate balance of forces on the interparticle dynamics and structure of monodisperse spherical polystyrene particles suspended at the interface between decane and water was observed as shear flow was applied to the system. A strong dipole–dipole repulsion, due to ionizable surface sulfate groups, induces the particles to arrange themselves on a hexagonal lattice under quiescent conditions.

The origin of the nonlinearity observed in Bagley plots of thermotropic copolyesters has been investigated experimentally. With the aid of a two-piston slit rheometer the dependence of the viscosity on the hydrostatic pressure has been determined directly. This dependence only accounts for part of the nonlinearity of the Bagley plots. Using slit dies with either a convergent or a divergent entrance, concave as well as convex pressure profiles are obtained.

Flow small-angle light scattering and linear conservative dichroism are used to follow, in situ and time resolved, the flow-induced changes of the microstructure in viscous emulsions such as immiscible polymer blends. A dilute system consisting of poly (butadiene) droplets dispersed in a poly (isobutene) matrix has been used as a model system. Contrary to earlier rheo-optical work on such materials, the structure has been probed in the plane formed by the flow and the velocity gradient directions.

Steric stabilization is a method that is often used to properly disperse small particles. It can be applied in aqueous as well as non-aqueous media. The rheological properties of sterically stabilized dispersions are discussed here. The various controlling parameters and the physical mechanisms involved are reviewed. Brownian hard spheres are used as a reference. Scaling relations are presented that make it possible to reduce data sets and to predict properties.

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.

Abstract We review the sequences of structural states that can be induced in colloidal suspensions by the application of flow. Structure formation during flow is strongly affected by the delicate balance among interparticle forces, Brownian motion and hydrodynamic interactions. The resulting nonequilibrium microstructure is in turn a principal determinant of the suspension rheology. Colloidal suspensions with near hard-sphere interactions develop an anisotropic, amorphous structure at low dimensionless shear rates.

Flow can change the rate at which solutes adsorb on surfaces by changing mass transfer to the surface, but moreover, flow can induce changes in the conformation of macromolecules in solution by providing sufficient stresses to perturb the segmental distribution function. However, there are few studies where the effect of flow on macromolecules has been shown to alter the structure of macromolecules adsorbed on surfaces.

Abstract Swarming motility is suggested to be a social phenomenon that enables groups of bacteria to coordinately and rapidly move atop solid surfaces. This multicellular behavior, during which the apparently organized bacterial populations are embedded in an extracellular slime layer, has previously been linked with biofilm formation and virulence. Many population density-controlled activities involve the activation of complex signaling pathways using small diffusible molecules, also known as autoinducers.

A well-known polyelectrolyte salt, ammonium polymethacrylate (Darvan-C) is used to stabilise ethanol-based Al2O3 and Ce-ZrO2 suspensions with butylamine addition. The sequence in which n-butylamine and Darvan-C are added to the suspension greatly affects the properties of the wet deposit obtained by electrophoretic deposition. To investigate this effect, electrical conductivity of the suspension and the shear rate dependence of its viscosity are investigated.

New practical applications of the magnetic force are reported on a regular basis. Spectacular examples are levitating trains, as operational in Shanghai1 and the magnetic confinement of hot plasma in the ITER project. 2 In the fight against cancer, biocompatible magnetic nanoparticles are injected into the tumor and heated with an external alternating magnetic field to create local hyperthermia. 3 Ferrofluids are applied in many areas such as automotives, aerospace, medicine, electronics, mechanical engineering and optics.

Ethanol-based Al2O3 and Ce–ZrO2 suspensions are stabilized with ammonium poly-methacrylate (Darvan-C) and n-butylamine. The effect of the additives on the suspension stability is studied by zeta potential and viscosity measurements. When n-butylamine is added to the particles dispersed in ethanol, the suspension is electrostatically stabilized, the viscosity decreases and a shear-thinning behavior is observed. The suspensions stabilized with Darvan-C have a low Newtonian viscosity at concentrations above 140μl/g.

Versatile ferrofluids based on polyethylene glycol coated iron oxide nanoparticles were obtained by a facile protocol and thoroughly characterized. Superparamagnetic iron oxide nanoparticles synthesized using a modified forced hydrolysis method were functionalized with polyethylene glycol silane (PEG silane), precipitated and dried. These functionalized particles are dispersable in a range of solvents and concentrations depending on the desired properties.

Abstract Viscosity and density are physical properties of fluids that are critical for manifold processes in industry. Beside their direct importance of determining the flow characteristics, they can also be used to quantify the chemical condition, eg, of lubricants, fuels, electrolytes, or the progress of crystallization or polymerization. Rheometers are commonly used for measuring the material properties to a high accuracy.

Abstract The properties of filled polymers and nanocomposites are strongly linked to the adequate dispersion of the solid phase into a polymeric matrix. However, obtaining the degree of dispersion within a polymer composite system is far from trivial. Typical methods for microstructural analysis such as electron and optical microscopy or scattering methods only investigate the local microstructure. In addition they are either labor intensive or may yield data that are difficult to analyze.

10 Rheology Bulletin, 77 (2) July 2008 suming on the faculty and teaching assistants; in Leuven we have addressed this problem by instating a formal system for spending” quality time 'with the teaching staff, where the students get two vouchers, each valid for half an hour of consulting time from the teaching assistants.

Abstract The intrinsically coupled effects of the curvature of the flow-field and of the viscoelastic nature of suspending medium on the cross-stream lateral migration of a single non-Brownian sphere in wide-gap Couette flow are studied. Quantitative videomicroscopy experiments using a counterrotating device are compared to the results of 3D finite element simulations.

1. Introduction 2. Polarimetry 2.1. Definitions 2.2. Experimental Techniques 2.3. Linear Birefringence Measurements: Case-Studies 2.3.1. The Stress-Optical Relation in Polymers 2.3.2. Separation of the Intrinsic and Form Birefringence 2.4. Linear Conservative Dichroism: Case-studies 2.4.1. Immiscible Polymer Blends 2.4.2. Filled Polymeric Systems 2.5. Conclusion 3. Light scattering 3.1. Theoretical Background 3.2. Experimental Set-ups 3.3. Small Angle Light Scattering: Case-studies 3.3.1. Immiscible Polymer Blends 3.3.2.

The assembly of rod-like particles by weak attractive inter-actions is important for the properties of several soft biological materials as well as in tailoring the properties of colloidal gels in a variety of applications. Under-standing the link between the physicochemical parameters such as size and aspect ratio, volume fraction, interparticle forces with the resulting microstructure and the rheological response remains however challenging.

Abstract: A simple and versatile method for making chemically patterned anisotropic colloidal particles is proposed and demonstrated for two different types of patterning. Using a combination of thermo/mechanical stretching followed by a wet chemical treatment of a sterically stabilized latex, both patchy ellipsoidal particles with sticky interactions near the tips as well as particles with tunable fluorescent patterns could be easily produced.

Synopsis Aggregated suspensions of rodlike particles are commonly encountered in soft biological materials and their solidlike response at extremely low volume fractions is also exploited technologically. Understanding the link between the physicochemical parameters such as size, aspect ratio, volume fraction, and interparticle forces with the resulting microstructure and the subsequent rheological response remains challenging.

Page 1. CHARACTERIZATION OF ANISOTROPIC PARTICLES BY RHEO-OPTICS Naveen Krishna Reddy1, Peter Lang2, Jorge P. Juste3, Isabel P. Santos3, Luis M. Liz-Marzan3, Jan KG Dhont2, Jan Vermant1 1 Katholieke Universiteit Leuven, Chemical Engineering, 3001 Leuven, Belgium 2Forschungszentrum Jülich, IFF, Weiche Materie, 52425 Jülich, Germany 3Universidade de Vigo, Departamento de Quimica Fisica, 36310 Vigo, Spain Page 2.

The forces between colloidal particles at a decane-water interface, in the presence of low concentrations of a monovalent salt (NaCl) and the surfactant sodium dodecyl sulfate (SDS) in the aqueous subphase, have been studied using laser tweezers. In the absence of electrolyte and surfactant, particle interactions exhibit a long-range repulsion, yet the variation of the interaction for different particle pairs is found to be considerable.

The behavior of a colloidal suspension of rodlike fd viruses in the nematic phase, subjected to steady state and transient shear flows, is studied. The monodisperse nature of these rods combined with relatively small textural contribution to the overall stress make this a suitable model system to investigate the effects of flow on the nonequilibrium phase diagram. Transient rheological experiments are used to determine the critical shear rates at which director tumbling, wagging, and flow-aligning occurs.

Abstract: We present a comprehensive model to account for the behavior of suspended nanoparticles under magnetohydrodynamic conditions. The Lorentz force not only drags nanoparticle flocs toward the walls reducing the distance between flocs resulting a more negative total pair interaction potential energy, but also produces extra magnetic-induced stresses inside a floc leading to a change of pair interaction distance thus giving rise to a less negative total potential energy.