Lutz Heymann

Investigation of the solid–liquid transition of highly concentrated suspensions in oscillatory amplitude sweeps

Suspensions of quasimonodisperse polymethylmethacrylate spheres with mean diameters of 4.7 and 3.1 μm, dispersed in a low-molecular-weight polydimethylsiloxane, were characterized using oscillatory shear amplitude sweeps. Thereby, the solid volume concentration was varied. The influence of the sample preparation, the mode of the experiment (controlled shear rate, controlled shear stress), and the parameters of the amplitude sweep such as logarithmic ramp time, measurement time, and frequency were investigated. The logarithmic ramp and the measurement time were found to be the substantial factors which especially influence the experimental results at low shear stress amplitudes. The frequency has an effect only at higher shear stress amplitudes where the material behaves linearly again. All suspensions showed a Hookean solid behavior at low shear stress amplitudes and a Newtonian fluid behavior at high shear stress amplitudes. In the transition range between the Hookean and the Newtonian behavior the analy...

Shear rheology of a cell monolayer

We report a systematic investigation of the mechanical properties of fibroblast cells using a novel cell monolayer rheology (CMR) technique. The new technique provides quantitative rheological parameters averaged over ~106 cells making the experiments highly reproducible. Using this method, we are able to explore a broad range of cell responses not accessible using other present day techniques. We perform harmonic oscillation experiments and step shear or step stress experiments to reveal different viscoelastic regimes. The evolution of the live cells under externally imposed cyclic loading and unloading is also studied. Remarkably, the initially nonlinear response becomes linear at long timescales as well as at large amplitudes. Within the explored rates, nonlinear behaviour is only revealed by the effect of a nonzero average stress on the response to small, fast deformations. When the cell cytoskeletal crosslinks are made permanent using a fixing agent, the large amplitude linear response disappears and the cells exhibit a stress stiffening response instead. This result shows that the dynamic nature of the cross-links and/or filaments is responsible for the linear stress-strain response seen under large deformations. We rule out the involvement of myosin motors in this using the inhibitor drug blebbistatin. These experiments provide a broad framework for understanding the mechanical responses of the cortical actin cytoskeleton of fibroblasts to different imposed mechanical stimuli.

Thixotropy in macroscopic suspensions of spheres

An experimental study of the viscosity of a macroscopic suspension, i.e. a suspension for which Brownian motion can be neglected, under steady shear is presented. The suspension is prepared with a high packing fraction and is density-matched in a Newtonian carrier fluid. The viscosity of the suspension depends on the shear rate and the time of shearing. It is shown for the first time that a macroscopic suspension shows thixotropic viscosity, i.e. shear-thinning with a long relaxation time as a unique function of shear. The relaxation times show a systematic decrease with increasing shear rate. These relaxation times are larger when decreasing the shear rates, compared to those observed after increasing the shear. The time scales involved are about 10000 times larger than the viscous time scale and about 1000 times smaller than the thermodynamic time scale. The structure of the suspension at the outer cylinder of a viscometer is monitored with a camera, showing the formation of a hexagonal structure. The temporal decrease of the viscosity under shear coincides with the formation of this hexagonal pattern.

Selective cross-linking of oligosilazanes to tailored meltable polysilazanes for the processing of ceramic SiCN fibres

An interesting alternative for the processing of non-oxide ceramic fibres at lower costs than that for the current commercially available fibre types was developed by modifying different commercially available liquid oligosilazanes (ML33 and HTT1800) into polysilazanes by selective cross-linking via the N–H and Si–H groups with tetra-n-butylammoniumfluoride (TBAF) as a catalyst. Termination of the reaction with calcium borohydride allows the processing of meltable solid polysilazanes (ML33S and HTTS) with tailored chemical and thermal properties, to fulfil the requirements for the melt spinning of mechanically stable and homogeneous polymeric fibres. The chemical and thermal stability of the polysilazanes ML33S and HTTS were investigated by using GPC, DSC and rheological measurements. These techniques indicate the dependency of the molecular weight and glass temperature on the catalytical cross-linking conditions. Polymers with up to ∼10 000 g mol−1 show glass–liquid transition (Tg) between 65 and 81 °C and viscoelasticity, which are essential properties for the melt-spinning process. The thermal stability of ML33S is ensured up to 220 °C. In contrast the thermal stability of HTTS is limited to 170 °C due to the presence of vinyl-groups. The viscoelastic behaviour of the polymer melts, measured by oscillatory rheometry, and the sufficient thermal stability allowed the continuous processing of stable green fibres by melt spinning in the temperature range of 110 to 130 °C. After fast electron beam irradiation curing of the green fibres and pyrolysis of continuous amorphous ceramic SiCN fibres from both ML33S and HTTS polysilazanes were successfully synthesized, while a defined Tg point influences the shape and the smoothness positively.

Viscoelasticity of mono- and polydisperse inverse ferrofluids

We report on measurements of a magnetorheological model fluid created by dispersing nonmagnetic microparticles of polystyrene in a commercial ferrofluid. The linear viscoelastic properties as a function of magnetic field strength, particle size, and particle size distribution are studied by oscillatory measurements. We compare the results with a magnetostatic theory proposed by De Gans et al. [Phys. Rev. E 60, 4518 (1999)] for the case of gap spanning chains of particles. We observe these chain structures via a long distance microscope. For monodisperse particles we find good agreement of the measured storage modulus with theory, even for an extended range, where the linear magnetization law is no longer strictly valid. Moreover we compare for the first time results for mono- and polydisperse particles. For the latter, we observe an enhanced storage modulus in the linear regime of the magnetization.

Indirect detection of structural changes in micellar solutions by rheological measurements

Abstract At higher concentration levels the inner structure of micellar solutions cannot be detected directly by optical means. Nevertheless the flow behavior of the micellar solutions reflect their micellar structures. Hence, in this study the material behavior of micellar surfactant solutions was investigated by rheometrical means in steady and oscillatory shear flows with and without thermal preload. The rheological behavior of the solutions was found to be strongly dependent on the concentration. The complex material structure is modeled according to the flow behavior by discrete and continuous relaxation time spectra depending on the concentration. It is found that the material possesses besides the memory for mechanical preload also a strong memory for thermal preload which depends on the strength of preload.

Influence of surface modified particles on the rheological properties of suspensions

The rheological properties of suspensions are influenced by a combination of several physico-chemical parameters. In contrast to the particle shape and solid volume concentration the influence of the physical and chemical properties of particle surfaces on the flow behavior of suspensions is less known. The aim of the present work was to investigate experimentally the influence of the particle porosity and surface modification on the plasto-viscoelastic properties of suspensions.

Transition Pathways between Solid and Liquid States of Dense Suspensions in Transient and Oscillatory Shear Flows

Suspensions containing rigid monodisperse spherical particles in a Newtonian carrier liquid are investigated experimentally, providing evidence for solid and liquid states in transient controlled shear rate (TCSR) experiments starting from the state of rest and experiments in controlled shear deformation (CSD) oscillatory flows. In TCSR experiments it has been found that between solid and liquid states a transition takes place. The dynamics of the transition is shown, with the material in this regime reacting as a highly nonlinear system. A key feature of the transition regime is an intrinsic material instability with a negative slope of the flow and deformation curves in a certain range of shear rates and shear strains, respectively. For the same suspension CSD oscillatory shear experiments have been carried out with varying shear strain amplitudes and angular frequencies. The raw input strain and stress response signals were analyzed by the method of Fourier transform rheology. At low amplitudes only th...