Claude Oelschlaeger

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).

Microrheology with fluorescence correlation spectroscopy.

We show that fluorescence correlation spectroscopy (FCS) using a commercial spectrometer can be applied to passive microrheological (MR) experiments. The method probes the local rheological properties of materials on length scales of the focus dimension of the confocal microscope. For a feasibility study, we performed measurements on a high molecular weight poly(ethylene oxide)-water solution to allow direct comparison of the results to previous studies using diffusing wave spectroscopy, quasielastic light scattering, and particle tracking methods. We were able to detect mean-square center-of-mass displacements ranging from somewhat better than [SYMBOL: SEE TEXT]2(t) approximately 100 nm2 up to above Deltar2(t) approximately 10(6) nm2. Thus, we were able to derive the bulk rheological shear moduli covering more than five decades in frequency (from omega<or=10(-1) rad/s to omega approximately 10(4) rad/s). The MR results are compared to results obtained from conventional rheological experiments on the same samples using a rotational rheometer as well as a piezo-driven squeeze flow apparatus. Good agreement between MR results probing the local rheological properties and those obtained by the conventional methods measuring the macroscopic mechanical response is found in the whole frequency range. Spatial resolution in combination with the possibility of using small tracer beads open the opportunity to probe the local, length scale-dependent rheological properties in heterogeneous samples. Small tracer concentrations and small sample sizes make FCS spectroscopy a powerful tool in particular for biological and medical applications.

Elongational deformation of wormlike micellar solutions

We have investigated the uniaxial elongation behavior of six different wormlike micelle systems covering a broad range of surfactant concentrations cs and salt/surfactant ratios R using the capillary breakup elongational rheometry (CaBER). In the fast-breaking limit (high cs and R), filament lifetime tfil is controlled by the equilibrium shear modulus G0 and the breakage time λbr obtained from small oscillatory shear according to tfil/G0∝λbr2/3 and relaxation time ratios λe/λs≈1 are found. When reptation dominates (high cs, low R) λe/λs<1 is observed similar as for solutions of covalently bound polymers. In this concentration regime, the micellar structure seems not to be affected by the strong elongational flow. In contrast, high filament lifetimes up to 1000 s and λe/λs values up to 10 are observed at low cs irrespective of R. This indicates the formation of elongation-induced structures (EISs). A minimum viscosity and a minimum initial diameter are required for creating EIS. Additional filament stretch...

Chain Flexibility and Dynamics of Polysaccharide Hyaluronan in Entangled Solutions: A High Frequency Rheology and Diffusing Wave Spectroscopy Study

We have investigated the linear viscoelastic properties of high molecular weight hyaluronan in aqueous solution using an experimental approach combining mechanical rheometry and optical microrheology. The complex shear modulus has been measured over a broad frequency range from 10(-1) to 10(7) rad/s. Chain flexibility is characterized by the persistence length lpand this parameter has been determined for the first time in the entangled regime here from high frequency modulus data. At NaHA concentrations above the entanglement concentration ce, lp is essentially independent of polymer concentration, temperature, and ionic strength. The latter is consistent with the Odijk-Skolnick-Fixman theory. The scaling exponent describing the concentration dependence of the plateau modulus G0 agrees well with predictions for polymers in good solvents. The scaling exponents for the specific viscosity ηsp and relaxation time TR are slightly higher than theoretically predicted for polyelectrolytes in the high salt limit, indicating, that molecular aggregation occurs at higher polymer concentrations.

Synthesis, Structural and Micromechanical Properties of 3D Hyaluronic Acid-Based Cryogel Scaffolds.

In this study, macroporous, elastic, three-dimensional scaffolds formed of hyaluronic acid mixed with ethylene glycol diglycidyl ether as a chemical cross-linker have been prepared by cryogelation for application in tissue engineering. These cryogels are characterized by large interconnected pores of size ∼50-300 μm and pore wall thickness of ∼5-30 μm as determined from confocal microscopy images. Variation of pH, freezing temperature, and polymerization time allows for control of pore size and shape as well as matrix thickness. These structural properties then determine mechanical strength as well as swelling capacity. Furthermore, increasing hyaluronic acid concentration decreases cryogel pore size, reduces swelling properties, and reinforces mechanical properties. On the other hand, decreasing cross-linker concentration, at a constant hyaluronic acid concentration, increases pore size and swelling capacity but provides less rigidity. Additionally, for the first time, local elastic properties of the polymer matrix and viscous properties of the pores have been characterized using multiple particle tracking microrheology. Local matrix elasticity, relaxation time of hyaluronic acid chains, and the degree of heterogeneity are discussed in detail. These latter properties are crucial for the development of new tissue engineering constructs and will help to understand how local matrix viscoelasticity affects cell cultivation. Finally, elastic moduli obtained in bulk rheology are much higher than corresponding values deduced from microrheology. This discrepancy might be explained by the formation of very highly cross-linked cores in the network where no tracer particle can penetrate.

Tracking errors in 2D multiple particle tracking microrheology

Tracking errors due to particles moving in and out of the focal plane are a fundamental problem of multiple particle tracking microrheology. Here, we present a new approach to treat these errors so that a statistically significant number of particle trajectories with reasonable length are received, which is important for an unbiased analysis of multiple particle tracking data from inhomogeneous fluids. Starting from Crocker and Grier’s tracking algorithm, we identify particle displacements between subsequent images as artificial jumps; if this displacement deviates more than four standard deviations from the mean value, trajectories are terminated at such positions. In a further processing step, trajectories separated by a time gap ��max are merged based on an adaptive search radius criterion accounting for individual particle mobility. For a series of Newtonian fluids covering the viscosity range 6–1300 mPa s, this approach yields the correct viscosity but also results in a viscosity-independent number of trajectories equal to the average number of particles in an image with a minimum length covering at least two orders of magnitude in time. This allows for an unbiased characterization of heterogeneous fluids. For a Carbopol ETD 2050 solution we recover the expected broad variation of particle mobility. Consistent with the widely accepted structural model of highly swollen microgel particles suspended in a polymer solution, we find about 2/3 of the tracers are elastically trapped.

Squeeze flow rheometry as a novel tool for the characterization of highly concentrated protein solutions.

This study aims at defining rheological parameters for the characterization of highly concentrated protein solutions. As a basis for comparing rheological behavior with protein solution characteristics the protein phase behavior of Lysozyme from chicken egg white with concentrations up to 225 mg/mL, changing pH values and additive concentrations was studied in a microbatch scale format. The prepared phase diagrams, scored after 40 days ( t40 ) give insights into the kind and kinetics of the phase transitions that occur. Oscillatory frequency sweep measurements of samples with exactly the same conditions were conducted immediately after preparation ( t0 ). The protein solutions behave viscoelastic and show a characteristic curve shape of the storage modulus (G′) and the loss modulus (G″). The graphs provide information about the cross‐linking degree of the respective sample. The measured rheological parameters were sensitive concerning solution composition, protein concentration and solution inner structure. The rheological moduli G′ and G″ and especially the ratio of these parameters over a frequency range from 100 to 40000 rad/sec give information about the aggregation tendency of the protein under tested conditions. We succeeded to correlate protein phase behavior with the defined rheological key parameter ωCO . This point represents the frequency value of the intersection point from G′ and G″. In our study Lysozyme expressed a ωCO threshold value of 20000 rad/sec as a lower limit for stable protein solutions. The predictability of lysozyme aggregation tendency and crystallization by means of squeeze flow rheometry is shown. Biotechnol. Bioeng. 2016;113: 576–587.

Multiple-Particle Tracking (MPT) Measurements of Heterogeneities in Acrylic Thickener Solutions

S. Neser Faculty of Mathematics and Science, University of Applied Sciences Darmstadt, Schöfferstrasse 3, 64295 Darmstadt, Germany Abstract In this study the method of multiple-particle tracking (MPT) is used to quantify the degree of structural and mechanical microheterogeneity of two polymeric thickener solutions finally aiming at a better understanding of the contribution of microheterogeneities, which commonly occur in solutions of many synthetic as well as biopolymers, on bulk rheology. We have chosen the commercial polyacrylate ester Sterocoll FD and Sterocoll D (BASF Aktiengesellshaft) as model systems. For the Sterocoll FD solution the ensemble-averaged mean square displacement (MSD) is almost linear in time, as expected for such a weakly elastic fluid and relatively similar to that observed for a homogeneous aqueous glycerol solution, used as a reference system. However, the MSD distribution is broader than for the glycerol solution and their statistical analysis clearly reveals a heterogeneous structure on the μm length scale. For the Sterocoll D solution, the average MSD exhibits a subdiffusive behavior, typical for highly elastic solutions. Moreover, the displacements of microspheres at different locations within the solution display a wide range of amplitudes and time dependences. The MSDdistribution is very broad/bimodal and the statistical analysis indicates a degree of inhomogeneity slightly higher than for the Sterocoll FD solution.

Microstructure, local dynamics, and flow behavior of colloidal suspensions with weak attractive interactions

We present a comprehensive micro- and macrorheological study of the effect of weak depletion attraction (Ψdep ≈ 1–10 kBT) on dense colloidal suspensions stabilized by short-range repulsive interactions. We used aqueous polymer dispersions as model system and demonstrated the unique capabilities of multiple particle tracking (MPT) to disclose structural changes in such technically important systems exhibiting many characteristic features of hard sphere systems. Below the hard sphere freezing point ϕc, viscosity increases monotonically with increasing Ψdep due to the transition from a fluid to a fluid/crystalline and finally to a gel state. Above ϕc, increasing attraction strength first results in a viscosity reduction corresponding to the formation of large, permeable crystals and then in a viscosity increase when a network of dense, small crystals forms. The fraction of the fluid and crystal phase, particle concentration in each phase as well as the modulus of the micro-crystals are obtained, the latter decreases with Ψdep. Above the colloidal glass transition strong heterogeneities and different local particle mobility in the repulsive and attractive arrested states are found. Particles are trapped in the cage of neighboring particles rather than in an attractive potential well. The intermediate ergodic state exhibits uniform tracer diffusivity.

Self-assembling all-enzyme hydrogels for biocatalytic flow processes

We describe the construction of binary self-assembling all-enzyme hydrogels that are comprised entirely of two tetrameric globular enzymes, the stereoselective dehydrogenase LbADH and the cofactor-regenerating glucose 1-dehydrogenase GDH. The enzymes were genetically fused with a SpyTag or SpyCatcher domain, respectively, to generate two complementary homo-tetrameric building blocks that polymerise under physiological conditions into porous hydrogels. The biocatalytic gels were used for the highly stereoselective reduction of a prochiral diketone substrate where they showed the typical behaviour of the coupled kinetics of coenzyme regenerating reactions in the substrate channelling regime. They effectively sequestrate the NADPH cofactor even under continuous flow conditions. Owing to their sticky nature, the gels can be readily mounted in simple microfluidic reactors without the need for supportive membranes. The reactors revealed extraordinary high space-time yields with nearly quantitative conversion (>95%), excellent stereoselectivity (d.r. > 99:1), and total turnover numbers of the expensive cofactor NADP(H) that are amongst the highest values ever reported.