Norbert Willenbacher

Capillary Forces in Suspension Rheology

The addition of a small amount of a nonwetting immiscible fluid to a suspension can drastically alter its rheological properties. The rheology of suspensions (solid particles dispersed in a fluid) is controlled primarily through the volume fraction of solids. We show that the addition of small amounts of a secondary fluid, immiscible with the continuous phase of the suspension, causes agglomeration due to capillary forces and creates particle networks, dramatically altering the bulk rheological behavior from predominantly viscous or weakly elastic to highly elastic or gel-like. This universal phenomenon is observed for a rich variety of particle/liquid systems, independent of whether the second liquid wets the particles better or worse than the primary liquid. These admixtures form stable suspensions where settling would otherwise occur and may serve as a precursor for microporous polymer foams, or lightweight ceramics.

Characterization of the viscoelastic behavior of complex fluids using the piezoelastic axial vibrator

The piezoelectric axial vibrator (PAV) is a squeeze-flow rheometer working at frequencies f between 1 and 4000Hz. It can be used to measure the storage modulus G′ and the loss modulus G″ of complex fluids in this frequency range. Using polymer solutions with known G′ and G″ it is shown that the PAV gives reliable mechanical spectra for frequencies between 10 and 3000Hz. The measurements done with the PAV are combined with a conventional mechanical rheometer (10−3⩽f⩽15Hz) and a set of torsional resonators (f=13, 25, and 77kHz) to obtain G′ and G″ between 10−3Hz and 77kHz. Using this combination we present the first analysis of the viscoelasticity of an aqueous suspension of thermosensitive latex particles in this range of frequency. It is demonstrated that the combination of the three devices gives the entire mechanical spectra without resort to the time-temperature superposition principle.

Linear-to-Branched Micelles Transition: A Rheometry and Diffusing Wave Spectroscopy (DWS) Study

The frequency-dependent shear modulus of aqueous wormlike micellar solutions of cetylpyridinium chloride (CPyCl) and sodium salicylate (NaSal) has been measured over a broad frequency range from 10(-2) to 10(6) rad/s using diffusing wave spectroscopy (DWS) based tracer microrheology as well as mechanical techniques including rotational rheometry and oscillatory squeeze flow. Good agreement between mechanical and optical techniques is found in the frequency range from 10(-1) to 10(5) rad/s (Willenbacher, N.; Oelschlaeger, C.; Schopferer, M.; Fischer, P.; Cardinaux, F.; Scheffold, F. Phys. Rev. Lett. 2007, 99 (6), 068302). At intermediate frequencies between 10 and 10(4) rad/s, squeeze flow provides most accurate data and is used to determine the plateau modulus G(0), which is related to the cross-link density or mesh size of the entanglement network, as well as the scission energy E(sciss), which is deduced from the temperature dependence of the shear moduli in the plateau zone. In the frequency range above 10(4) rad/s, DWS including a new inertia correction is most reliable and is used to determine the persistence length l(p). The system CPyCl/NaSal is known to exhibit two maxima in zero-shear viscosity and terminal relaxation time as the salt/surfactant ratio R is varied (Rehage, H.; Hoffman, H. J. Phys. Chem. 1988, 92 (16), 4712-4719). The first maximum is attributed to a transition from linear to branched micelles (Lequeux, F. Europhys. Lett. 1992, 19 (8), 675-681), and the second one is accompanied by a charge reversal due to strongly binding counterions. Here, we discuss the variation of G(0), E(sciss), and l(p) with salt/surfactant ratio R at constant surfactant concentration of 100 mM CPyCl. G(0) increases at the linear-to-branched micelles transition, and this is attributed to the additional contribution of branching points to the cross-link density. E(sciss) exhibits two maxima analogous to the zero-shear viscosity, which can be understood in terms of the variation of micellar length and variation of the amount of branched micelles and contour length between branching points consistent with the results of a comprehensive cryo-transmission electron microscopy (TEM) study (Abezgauz, L.; Ramon, O.; Danino, D. Department of Biotechnology and Food Engineering, Technion, Haifa, Israel. European Colloid and Interface Society, Geneva, 2007). The persistence length decreases with increasing R. This decrease is stronger than expected from the decrease of Debye length according to the Odijk-Skolnick-Fixman (OSF) theory and is attributed to the penetration of salicylate ions into the micelles; the linear-to-branched transition obviously does not have an effect on l(p).

Desmin and Vimentin Intermediate Filament Networks: Their Viscoelastic Properties Investigated by Mechanical Rheometry

We have investigated the viscoelastic properties of the cytoplasmic intermediate filament (IF) proteins desmin and vimentin. Mechanical measurements were supported by time-dependent electron microscopy studies of the assembly process under similar conditions. Network formation starts within 2 min, but it takes more than 30 min until equilibrium mechanical network strength is reached. Filament bundling is more pronounced for desmin than for vimentin. Desmin filaments (persistence length l(p) approximately 900 nm) are stiffer than vimentin filaments (l(p) approximately 400 nm), but both IFs are much more flexible than microfilaments. The concentration dependence of the plateau modulus G(0) approximately c(alpha) is much weaker than predicted theoretically for networks of semiflexible filaments. This is more pronounced for vimentin (alpha=0.47) than for desmin (alpha=0.70). Both networks exhibit strain stiffening at large shear deformations. At the transition from linear to nonlinear viscoelastic response, only desmin shows characteristics of nonaffine network deformation. Strain stiffening and the maximum modulus occur at strain amplitudes about an order of magnitude larger than those for microfilaments. This is probably attributable to axial slippage within the tetramer building blocks of the IFs. Network deformation beyond a critical strain gamma(max) results in irreversible damage. Strain stiffening sets in at lower concentrations, is more pronounced, and is less sensitive to ionic strength for desmin than for vimentin. Hence, desmin exhibits strain stiffening even at low-salt concentrations, which is not observed for vimentin, and we conclude that the strength of electrostatic repulsion compared to the strength of attractive interactions forming the network junctions is significantly weaker for desmin than for vimentin filaments. These findings indicate that both IFs exhibit distinct mechanical properties that are adapted to their respective cellular surroundings [i.e., myocytes (desmin) and fibroblasts (vimentin)].

Miniemulsion polymerization of acrylated methyl oleate for pressure sensitive adhesives

The focus of this work was to improve the aqueous emulsion polymerization of a highly water-insoluble monomer derived from plant oil, acrylated methyl oleate. Conventional emulsion polymerization requires excessive amounts of surfactant and long reaction time. Miniemulsion polymerization improved the polymerization significantly. Only a fraction of the surfactant and reaction time that was necessary in the conventional emulsion polymerization is required. The resulting polymers have properties comparable to petroleum-based polymers commonly used in pressure sensitive adhesive applications.

The elastic interaction of high-spin and low-spin complex molecules in spin-crossover compounds

Several transition metal compounds show a transition from the low-spin (LS) to the high-spin (HS) electronic state with increasing temperature. The cooperative nature of the transition is usually parametrised by an interaction constant Gamma , the origin of which is still under discussion. In the frame of the lattice expansion mode, the interaction Gamma is attributed to the elastic interaction between the spin-changing ions as a result of the deformation of the crystal accompanying the transition. The authors calculate the complete elastic energy originating from the so-called image pressure in closed form by considering the crystal as an isotropic homogeneous elastic medium with the spin-changing ions as incompressible inclusions described by the full elastic dipole tensors PHS and PLS, respectively. The calculated values of Gamma based on X-ray data and reasonable estimates of the elastic constants of the compounds (Fe(2-pic)3)Cl2.Sol (2-pic=2-aminomethylpyridine, Sol=MeOH,EtOH) and (Fe(2-pic-ND2)3)Cl2.EtOD are compared with the experimental values of Gamma .

Characterizing complex fluids with high frequency rheology using torsional resonators at multiple frequencies

A set of torsional resonators is used to characterize the linear viscoelastic behavior of complex fluids in the kilohertz range. The frequency dependence of the elastic and loss modulus of a hard sphere dispersion, electrostatically and electrosterically stabilized particles, worm-like micelles, polystyrene microgels, and polymer solutions is studied. The results are compared to theoretical predictions for these systems. The utility of the instrument for characterizing the high frequency rheology of complex fluids is demonstrated. This is especially relevant for suspensions or dilute solutions and gels, where time-temperature superposition often fails and the relaxation spectrum is inaccessible from conventional oscillatory shear rotational rheometry.

pH Effects on the Molecular Structure of beta-Lactoglobulin Modified Air-Water Interfaces and Its Impact on Foam Rheology

Macroscopic properties of aqueous β-lactoglobulin (BLG) foams and the molecular properties of BLG modified air-water interfaces as their major structural element were investigated with a unique combination of foam rheology measurements and interfacial sensitive methods such as sum-frequency generation and interfacial dilatational rheology. The molecular structure and protein-protein interactions at the air-water interface can be changed substantially with the solution pH and result in major changes in interfacial dilational and foam rheology. At a pH near the interfacial isoelectric point BLG molecules carry zero net charge and disordered multilayers with the highest interfacial dilatational elasticity are formed at the air-water interface. Increasing or decreasing the pH with respect to the isoelectric point leads to the formation of a BLG monolayer with repulsive electrostatic interactions among the adsorbed molecules which decrease the interfacial dilational elasticity. The latter molecular information does explain the behavior of BLG foams in our rheological studies, where in fact the highest apparent yield stresses and storage moduli are established with foams from electrolyte solutions with a pH close to the isoelectric point of BLG. At this pH the gas bubbles of the foam are stabilized by BLG multilayers with attractive intermolecular interactions at the ubiquitous air-water interfaces, while BLG layers with repulsive interactions decrease the apparent yield stress and storage moduli as stabilization of gas bubbles with a monolayer of BLG is less effective.

Tuning suspension rheology using capillary forces

When a small amount (less than 1%) of a second immiscible liquid is added to the continuous phase of a suspension, the rheological properties of the admixture are dramatically altered and it can change from a fluid-like to a gel-like state. The rheological properties can be modified through changes in the particle size, temperature induced changes to the interfacial tension, and through the addition of surfactants. These changes are experimentally investigated in this current work, concentrating on the capillary (non-wetting) state with additional experiments in the pendular (wetting) state. The yield stress scales with the interfacial tension and reciprocal radius (Γ/r) as expected due to the capillary nature of the attractive force leading to network formation and gelation. The addition of surfactants leads to a drastic reduction in the yield stress and viscosity, which is attributed to a partial trapping of the secondary fluid within surfactant micelles.

Particle configurations and gelation in capillary suspensions

When a small amount (less than 1%) of a second immiscible liquid is added to the continuous phase of a suspension, the rheological properties of the admixture are dramatically altered and can change from a fluid-like to a gel-like state. These so-called capillary suspensions transition to a gel-like state both if the secondary liquid preferentially wets the particles (pendular state) and even if the secondary liquid wets the particles less well than the primary fluid (capillary state). The mechanism of network formation and the distribution of the secondary liquid in the capillary state has not been investigated so far. Here, we discuss the formation of particle clusters—which are assumed to be the basic building blocks of the observed sample-spanning network—as a function of the contact angle and secondary fluid volume. The presence and strength of these clusters is directly related to the experimentally observed rheological features of capillary state suspensions.