Rheologica Acta最新文献

In this work, we have studied the viscoelastic behavior of chemically and physically crosslinked Poly(vinyl alcohol) (PVA) hydrogels near the critical gel point (GP) as well as further away from it, by means of small amplitude (SAOS) and large amplitude (LAOS) oscillatory shear experiments. Chemical crosslinking involved covalent bonding by means of glutaraldehyde as a crosslinker, while physical crosslinking was induced by freeze–thaw cycles. SAOS data analysis allowed evaluation of critical parameters such as the critical relaxation exponent n, gel strength S, and equilibrium modulus Ge, based on the dynamic self-similarity and fractal network structures at the GP. LAOS rheological data analysis showed that the chemically crosslinked system exhibited moderate strain-dependance due to the permanent covalent bonds, whereas the physically crosslinked system displayed significant strain-dependent nonlinearity due to strain dependent interactions at the crosslink entities. LAOS experiments, supported by Chebyshev coefficients and Lissajous-Bowditch plots, highlighted pronounced differences in nonlinear responses, underscoring the influence of crosslinking mechanisms on the network rheological behavior. The findings establish LAOS as a powerful tool for differentiating polymeric network structures, providing insights beyond those attained by conventional linear rheology.

The chemical gel point that occurs in the resin matrix of thermoset composites is crucial to the design of manufacturing process parameters. However, the formation of a physical network of filler material can affect the viscoelastic response of the composite so strongly that conventional rheological indicators of the chemical gel point (like power-law stress relaxation) are no longer observed. Additionally, many industrially relevant thermoset composites have a small linear viscoelastic region, limiting the utility of the high-strain multiwave measurement approach that was developed to monitor the frequency-dependent behavior of rapidly evolving materials. Here, we pair frequency-dependent properties obtained by low-strain Optimally Windowed Chirp (OWCh) measurements with existing rheology-conversion relationships to apply time-cure superposition to the loss tangent of epoxy-amine resins filled with unreactive particles. We show that this advanced rheological approach allows us to track the relative change in the relaxation time, providing another way to identify the elusive chemical gel point of these thermoset composites. The results allow us to assess the applicability of several gel point criteria to crosslinking composites, including the G'-G'' crossover, frequency-independent tan δ, peak in tan δ, and divergence of the relaxation time. Investigation of a resin with a weak filler network reveals all of these gel point criteria, with the frequency-independent tan δ providing the best agreement with the easily identifiable gel point of the neat resin. For resins with a strong physical filler network, frequency independence of tan δ does not occur, but the divergence of the relaxation time matches the gel point of the neat resin and the peak in tan δ.

Particle-laden interfaces have been extensively used due to their excellent capabilities of imparting stability in multiphase materials in what is called Pickering-Ramsden stability. While particles are usually added in amounts that create maximally packed or multilayer coverages on a bubble or droplet interface, it has been reported that even sub-monolayer coverages can impart a finite interfacial yield stress—already strong enough to arrest bubble dissolution. In the present work, we use a model elastoviscoplastic interface and custom-built interfacial rheometry set-ups to interrogate the yielding behavior in both shear and compressional/dilatational deformation modes while simultaneously looking at the 2D microstructure. Depending on surface coverage, either flocculated networks or densely packed particle-laden interfaces are obtained. We specifically investigate the transition from linear to nonlinear behavior in different rheometric experiments and relate the transitions, from elastic to plastic to viscous deformations, to microstructural observations. With full microstructural resolution in two-dimensional systems being easily accessible, the results inform both the deliberate tuning of interfacial mechanics and provide insights into the fundamental mechanisms governing yield in bulk materials.

We develop and test a rheo-optical platform based on a two-axes, parallel plates shear cell coupled to an optical microscope and a photon correlation imaging setup for simultaneous investigation of the rheological response and the microscopic structure and dynamics of soft materials under shear. Each plate of the shear cell is driven by an air bearing linear stage, which is actuated by a voice coil motor. A servo control loop reading the plate displacement through a contactless linear encoder enables both strain-controlled and stress-controlled rheology. Simultaneous actuation of both linear stages enables both parallel and orthogonal superposition rheology. We validate the performance of our device in both oscillatory and transient rheological tests on a microgel soft glass, and we demonstrate its potential through orthogonal superposition rheology experiments. During steady-state flow, we reconstruct the strain field across the gap by tracking the motion of tracer particles to check for slip or shear banding instabilities. At the same time, we measure the microscopic dynamics, both affine and non-affine, resolving them in space and time using photon correlation imaging.

In the search for faster rheometrical measurements techniques for fast time-evolving systems, optimally windowed chirps (OWCh) have recently been proposed for the determination of the complex modulus. However, such chirps are prone to artefacts at high frequencies due to fact that the input power is distributed over a range of frequencies leading to reduced signal-to-noise ratios in noisy conditions. The Tukey window which modulates the amplitude of the excitation disturbance and which is required to avoid spectral leakage directly reduces the signal-to-noise ratio at the edges of the signal leading to a divergence of the measured moduli at high frequencies. A new double exponential chirp (DEC) signal is proposed to overcome these limitations. Its capabilities are demonstrated with orthogonal superposition rheometry as an example of a demanding high-noise environment. The S-shaped time-frequency history of the new chirp signal redistributes the input power over the frequency spectrum. Numerical simulations using the Maxwell and Giesekus models, along with orthogonal superposition measurements on wormlike micellar fluids, demonstrate the effectiveness of the DEC waveform. Parameter optimization with the Giesekus model identifies the ideal input configurations for achieving a maximum signal-to-noise ratio during rheological measurements.

Although the second normal stress difference ({N}_{2}) is connected to several critical flow phenomena, it has received relatively little attention in literature, mainly due to the difficulty of accurately measuring this material function. Even for highly viscous polymeric solutions or melts, a trustworthy ({N}_{2}) measurement is still a challenge, and only a few rheologists tackled this problem for lower viscous materials, mainly due to the sensitivity limits of current experimental setups. In this paper, we re-evaluate a technique that received less attention due to the relatively large sample volumes needed: the normal stress-induced deformation of the free surface of a fluid flowing through a tilted trough. We introduce a new design that derives ({N}_{2}) from the deformation of an interface between a viscoelastic and a Newtonian fluid flowing through a closed pipe.

Viscosity affects lubricant film thickness and the separation of machine parts. It is thus a major parameter to ensure adequate lubrication and machine operation. Viscosity is dependent on operating conditions, especially pressure, which is known to vary up to several GPa in tribological contacts. Few viscometers are capable of performing in-situ measurements, and replicating the combined extreme operating conditions outside the contact zone is difficult. This work employs ultrasound to enable in-situ viscosity measurements under high-pressure and high-shear. The low-shear viscosity behaviour under pressure of distilled water, octane, squalane (SQL), squalane + polyisoprene (SQL+PIP), diisodecylphthalate (DidP), and polyalphaolefin 100 (PAO100) was derived from the literature using the Williams-Landel-Ferry-Yasutomi (WLF-Yasutomi) model. Combined with shear-thinning models from the literature, the viscosity under high-pressure and high-shear ((4.5 times 10^6 ,{text {s}^{-1}})) was determined. An ultrasonic viscometer was instrumented onto a high-pressure test cell. Several fluids were used to calibrate the ultrasonic viscometer under pressure. The ultrasonic viscosities of SQL+PIP and PAO100 were computed at (40 ,^{circ }text {C}), from ambient pressure up to 600 MPa, and compared with literature data. This work contributes to a better understanding of the ultrasonic in-situ viscometry technique. Such insight is crucial to apply this technique to challenging environments. The ultrasonic viscometer also holds significant potential to advance the understanding of complex fluids under high-pressure and high-shear conditions, where conventional measurement methods often fall short. Moreover, the ultrasonic viscometry technique has strong potential for industrial application, where there is a growing need for real-time, in-situ monitoring of fluid properties under varying operating conditions. This can lead to improved process control, safety, and efficiency across a range of industries.