Linear to Branched Micelles Transition: a Rheometry and Diffusive Wave Spectroscopy (DWS) Study
Claude OelschlaegerMichael SchopfererFrank ScheffoldNorbert WillenbacherAlbert CoGary L. LealRalph H. ColbyA. Jeffrey Giacomin
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Abstract:
The frequency‐dependent shear modulus of aqueous wormlike micellar solutions of cetylpyridinium chloride (CPyCl) and sodium salicylate (NaSal) has been measured in a broad frequency range from 10−2 to 106 rad/s using DWS based tracer microrheology as well as mechanical techniques including rotational rheometry, oscillatory squeeze flow and torsional resonance. Good agreement between mechanical and optical techniques has been found in the frequency range from 10−1 to 105 rad/s [1]. At intermediate frequencies between 10 and 104 rad/s squeeze flow provides most accurate data and is used to determine the plateau modulus G0, which is related to the crosslink density or mesh size of the entanglement network as well as the scission energy Esciss, which is deduced form the temperature dependence of the shear moduli in the plateau zone. In the frequency range above 104 rad/s DWS including a new inertia correction is most reliable and is used to determine the persistence length lp. This quantity is calculated from the loss modulus G″, which exhibits a ω3/4‐scaling in this frequency range as expected for semiflexible objects like wormlike micelles, according to the statistical mechanical theory of Gittes and MacKintosh [2]. The system CPyCl/NaSal is known to exhibit two maxima in zero‐shear viscosity and terminal relaxation time as the salt/surfactant ratio R varies [3]. The first maximum is attributed to a transition from linear to branched micelles [4], the second one is accompanied by a charge reversal due to strongly binding counterions. Recently, the changes in micellar length, branching and branching point density as well as formation of rings has been documented in a comprehensive TEM‐study [5]. Here we discuss the variation of G0, Esciss and lp with salt/surfactant ratio R at constant surfactant concentration of 100 mM CPyCl. G0 increases at the linear‐to‐branched micelles transition, and this is attributed to the additional contribution of branching points to the crosslink density. Esciss exhibits two maxima analogous 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 [5]. The persistence length decreases slightly with increasing R. This is considered to be an ionic strength effect, the linear‐to‐branched transition obviously does not have an effect on lp.Keywords:
Rheometry
Microrheology
Shear modulus
Microrheology
Rheometry
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We observed the diffusive motion of a micron-sized bead in an entangled-DNA solution to investigate the effect of the viscoelasticity on the bead motion. In the absence of external stress (passive microrheology), subdiffusion appears in the timescale of 0.1–10 s, and the normal diffusion recovers in longer timescales. We evaluated the apparent viscosity and elasticity, which yields a simple relaxation time for the viscoelastic medium. We found that the absence of DNA-length dependence for the time-dependent diffusion is explained by the simple relaxation of the viscoelastic media rather than the reptation dynamics, including the disentanglement. On the other hand, in the presence of a small external stress in active microrheology, the bead motion showed clear length dependence owing to the viscoelasticity. These results suggest that the viscoelasticity of the entangled DNA is highly sensitive to the external stress, even in the linear response regime.
Microrheology
Reptation
Stress relaxation
Elasticity
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Magnetic rotational spectroscopy is based on the use of magnetic micron-size wires for viscosity measurements. Submitted to a rotational magnetic field with increasing frequency, the wires undergo a hydrodynamic instability between a synchronous and an asynchronous regime. From a comparison between predictions and experiments, the static shear viscosity and elastic modulus of wormlike micellar solutions are here determined. The values agree with the determination by cone-and-plate rheometry.
Microrheology
Rheometry
Rotational viscosity
Rotating magnetic field
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We review here different methods to measure the bulk viscosity of complex fluids using micron-sized magnetic wires. The wires are characterized by length of a few microns and diameter of a few hundreds of nanometers. We first draw analogies between cone-and-plate rheometry and wire-based microrheology. In particular we highlight that magnetic wires can be operated as stress-controlled rheometers for two types of testing, the creep-recovery and steady shear experiments. In the context of biophysical applications, the cytoplasm of different cell lines including fibroblasts, epithelial and tumor cells is studied. It reveals that the interior of living cells can be described as a viscoelastic liquid with a static viscosity comprised between 10 and 100 Pas. We extend the previous approaches and show that the proposed technique can also provide time resolved viscosity data, which for cells display strong temporal fluctuations. The present work demonstrates the high potential of the magnetic wires for quantitative rheometry in confined espaces.
Rheometry
Microrheology
Nanometre
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Hyaluronic acid (HA) is a polysaccharide widely used in biomedical applications, due to its elevated biocompatibility and the peculiar viscoelastic properties of its solutions. Although the viscoelastic behaviour of HA solutions has been extensively studied in the literature it has been often reported in the range of low frequency (1-100 Hz) and high salt concentration, whereas the main rheological peculiarities of this molecule are expected at high frequency (>100 Hz) and low salt concentration. In this work we studied the viscoelastic properties of low molecular weight HA (155 kDa) in wide range of concentrations (0.01-20 mg/ml) at low ionic strength and over an extended frequency range (0.1-1000 Hz) using both optical tweezers and conventional rheometry. Good agreement between the high frequency dynamic behaviour (optical tweezers) and the viscoelastic properties at low frequency (rheometry) was found. We also found that, in apparent contradiction with polyelectrolyte solution theory, HA solution behaves as liquid-like viscoelastic fluid (G''>G') even at concentrations higher than the entanglement concentration where a weak-gel behavior should be expected.
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Microrheology
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The viscoelastic properties of associative polymers are important not only for their use as rheology modifiers but also to understand their complex structure in aqueous media. In this study, the dynamics of comblike hydrophobically modified alkali swellable associative (HASE) polymers are probed using diffusing wave spectroscopy (DWS) based tracer microrheology. DWS-based tracer microrheology accurately probes the dynamics of HASE polymers, and the extracted microrheological moduli versus frequency profile obtained from this technique closely matches that obtained from rotational rheometry measurements. Quantitatively, however, the moduli extracted from DWS-based tracer microrheology measurements are slightly higher than those obtained using rotational rheometry. The creep compliance, elastic modulus, and relaxation time concentration scaling behavior exhibits a power-law dependence. The length scale associated with the elastic to glassy behavior change is obtained from the time-dependent diffusion coefficient. The Zimm-Rouse type scaling is recovered at high frequencies but shows a concentration effect switching from Zimm to more Rouse-like behavior at higher concentrations.
Microrheology
TRACER
Associative property
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Microrheology
Rheometry
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Mean squared displacement
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The viscoelastic behavior of polymer solutions is commonly measured using oscillating shear rheometry, however, the accuracy of such methods is limited by the oscillating frequency of the equipment and since the relaxation time of the dilute polymer solutions is short, this requires measurement at very high frequencies. Microrheology has been proposed to overcome this technical challenge. Yet the equipment for resolving the statistics of particle displacements in microrheology is expensive. In this work, we measured the viscoelastic behavior of Methocel solutions at various concentrations using a conventional epi-fluorescence microscope coupled to a high-speed intensified camera. Statistical Particle Tracking is used in analyzing the mean-squared displacement of the dispersive particles. Relaxation times ranging from 0.76 - 9.00 ms and viscoelastic moduli, G' between 11.34 and 3.39 are reported for Methocel solutions of concentrations between 0.063 - 0.5%
Microrheology
Rheometry
Particle (ecology)
Mean squared displacement
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Microrheology
Rheometry
Rotational viscosity
Shear modulus
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