Volume 59, Issue 1, January 2015
Index of content:
Nonlinearity from FT-rheology for liquid crystal 8CB under large amplitude oscillatory shear (LAOS) flow59(2015); http://dx.doi.org/10.1122/1.4901288View Description Hide Description
This study systematically investigated the nonlinear stress behavior of liquid crystal (8CB, 4-4′-n-octyl-cyanobiphenyl) in lamellar smectic A phase under large amplitude oscillatory shear (LAOS) flow. To investigate the nonlinear stress response under LAOS flow, the nonlinearity (I 3/1) from Fourier transform-rheology as a function of applied shear time (3600 s) was calculated according to changes in both strain amplitude γ 0 and frequency ω. The storage modulus G′(t) and loss modulus G″(t) from the conventional rheometer program under various LAOS flow conditions decreased and reached equilibrium as a function of time. This could be attributed to shear alignment of the lamellar smectic A structure. On the contrary, with G′(t) and G″(t), the nonlinearity I 3/1(t) showed three different behaviors depending on the magnitude of strain amplitude: (1) Region I: Increased (increased and reached equilibrium), (2) region II: Increased and decreased (showed maximum value; decreased and reached equilibrium), and (3) region III: Decreased (decreased and reached equilibrium) as a function of time. These three different time-dependent behaviors of nonlinearity (I 3/1) were shown to be related with the alignment behavior of the lamellar structure. With stress decomposition method, the viscous and elastic stresses of 3600 s were calculated. Viscous and elastic stresses showed different behavior at region I and region III. With an equilibrium value of 3600 s, the G′, G″, and nonlinearity (I 3/1) were plotted as a function of strain amplitude, γ 0. Interestingly, I 3/1(γ 0) increased and then decreased (maximum) even though G′(γ 0) and G″(γ 0) only decreased with increasing strain amplitude. From these results, it can be concluded that LAOS analysis of nonlinear stress, especially I 3/1 from FT-rheology, is more sensitive to microstructure than storage modulus G′ and loss modulus G″.
59(2015); http://dx.doi.org/10.1122/1.4902000View Description Hide Description
Numerous materials, from biopolymers to filled rubbers, exhibit strain softening at high strain amplitudes during a strain sweep in oscillatory rheology: The modulus decreases with increasing deformation. On the other hand, if the nonlinear elastic response is analyzed within a single oscillation cycle (described by a Lissajous curve), these systems are often reported to exhibit strain hardening . We compare strain sweeps and single cycle LAOS (large amplitude oscillatory shear) analyses of stress vs strain on three very different materials. We conclude that the reported strain hardening is due to the use of a tangent modulus in the LAOS analysis, and that the overall rheology remains strain softening. To show that this conclusion is robust, we demonstrate a rescaling of the modulus that collapses the data from all the oscillatory measurements onto a single master curve that clearly exhibits the correct strain softening behavior.
59(2015); http://dx.doi.org/10.1122/1.4902437View Description Hide Description
Over the past few decades researchers have developed a variety of methods for measuring the mechanical properties of whole cells, including traction force microscopy, atomic force microscopy (AFM), and single-cell tensile testing. Though each of these techniques provides insight into cell mechanics, most also involve some nonideal conditions for acquiring live cell data, such as probing only one portion of a cell at a time, or placing the cell in a nonrepresentative geometry during testing. In the present work, we describe the development of a linear cell monolayer rheometer (LCMR) and its application to measure the mechanics of a live, confluent monolayer of stromal vascular cells. In the LCMR, a monolayer of cells is contacted on both top and bottom by two collagen-coated plates and allowed to adhere. The top plate then shears the monolayer by stepping forward to induce a predetermined step strain, while a force transducer attached to the top plate collects stress information. The stress and strain data are then used to determine the maximum relaxation modulus recorded after step-strain, , referred to as the zero-time relaxation modulus of the cell monolayer. The present study validates the ability of the LCMR to quantify cell mechanics by measuring the change in of a confluent cell monolayer upon the selective inhibition of three major cytoskeletal components (actin microfilaments, vimentin intermediate filaments, and microtubules). The LCMR results indicate that both actin- and vimentin-deficient cells had ∼50% lower values than wild-type, whereas tubulin deficiency resulted in ∼100% higher values. These findings constitute the first use of a cell monolayer rheometer to quantitatively distinguish the roles of different cytoskeletal elements in maintaining cell stiffness and structure. Significantly, they are consistent with results obtained using single-cell mechanical testing methods, suggesting that the rheology-based LCMR technique may be a useful tool for rapid analysis of cell mechanics by shearing an entire cell monolayer.
59(2015); http://dx.doi.org/10.1122/1.4902928View Description Hide Description
The functions and structures of biological mucus are closely linked to rheology. In this article, the skin mucus of loach (Misgurnus anguillicaudatus) was proved to be a weak hydrogel susceptible to shear rate, time, and history, exhibiting: (i) Two-region breakdown of its gel structure during oscillatory strain sweep; (ii) rate-dependent thickening followed by three-region thinning with increased shear rate, and straight thinning with decreased shear rate; and (iii) time-dependent rheopexy at low shear rates, and thixotropy at high shear rates. An interesting correlation between the shear rate- and time-dependent rheological behaviors was also revealed, i.e., the rheopexy-thixotropy transition coincided with the first-second shear thinning region transition. Apart from rheology, a structure of colloidal network was observed in loach skin mucus using transmission electron microscopy. The complex rheology was speculated to result from inter- and intracolloid structural alterations. The unique rheology associated with the colloidal network structure, which has never been previously reported in vertebrate mucus, may play a key role in the functions (e.g., flow, reannealing, lubrication, and barrier) of the mucus.
59(2015); http://dx.doi.org/10.1122/1.4901750View Description Hide Description
We perform experiments on jammed suspensions of microgels with different constituent properties to determine their stress relaxation behavior on flow cessation. We observe that the stress relaxes through a two-step process: A rapid initial relaxation where internal stresses are trapped followed by a much slower decay. Trapped internal stresses are related to the solvent viscosity, particle elasticity, and volume fraction through a universal scaling. The second slower relaxation of the internal stress is characterized by a single exponential decay, which is independent of the preshear stress and relatively insensitive to the material properties of the microgel suspension. Particle-scale simulations are used to understand the microscopic mechanisms which drive the amplitude and the kinetics of the stress relaxation as well as the local particle dynamics in each regime. The rapid initial relaxation occurs through ballistic particle motion, where the number of contacts and average compression return to their static values but the asymmetry of the pair distribution function remains as a signature of the internal stress.
Regular perturbation analysis of small amplitude oscillatory dilatation of an interface in a capillary pressure tensiometer59(2015); http://dx.doi.org/10.1122/1.4902546View Description Hide Description
The dilatational rheology of complex fluid-fluid interfaces is linked to the stability and bulk rheology of emulsions and foams. Dilatational rheology can be measured by pinning a bubble or droplet at the tip of a capillary, subjecting the interface shape to small amplitude oscillations, and recording the resulting pressure jump across the interface. The complex dilatational modulus is obtained by differentiating the interfacial stress with respect to the area change of the interface. In this paper, we perform a regular asymptotic expansion to analyze the interface response in pressure-controlled capillary pressure tensiometers to determine the dilatational modulus as a function of the measured radius of curvature. We show that small amplitude oscillatory dilation of a spherical bubble is neither stress nor strain rate controlled. The resulting dilatational modulus contains contributions from both surface tension effects as well as extra stresses. Depending on the specifics of the interface, each contribution can be a function of the dilation rate and the radius of the bubble. Thus, the radius of curvature can be used as a control parameter with which to separate surface tension and interfacial rheology effects, aiding in validation of interfacial constitutive models. We examine the limits of validity of the small amplitude assumption and provide guidelines for determining the operating limits of a capillary pressure tensiometer. Finally, we compare several existing devices, including a microtensiometer we developed previously that oscillates the pressure inside small (R ∼ 10 μm) droplets.
59(2015); http://dx.doi.org/10.1122/1.4903498View Description Hide Description
Individual molecule dynamics have been shown to influence significantly the bulk rheological and microstructural properties of short-chain, unentangled, linear polyethylene liquids undergoing high strain-rate flows. The objective of this work was to extend this analysis to a linear polyethylene composed of macromolecules of a much greater length and entanglement density; i.e., a liquid consisting of C400H802 molecules, with approximately ten kinks per chain at equilibrium, as calculated by the Z1 code of Kröger [Comput. Phys. Commun. 168, 209–232 (2005)]. To achieve this, we performed nonequilibrium molecular dynamics (NEMD) simulations of a model system using the well-established potential model of Siepmann et al. [Nature 365, 330–332 (1993)] for a wide range of Weissenberg numbers (Wi) under steady shear flow. A recent study by Baig et al. [Macromolecules 43, 6886–6902 (2010)] examined this same system using NEMD simulations, but focused on the bulk rheological and microstructural properties as calculated from ensemble averages of the chains comprising the macromolecular liquids. In so doing, some key features of the system dynamics were not fully elucidated, which this article aims to highlight. Specifically, it was found that this polyethylene liquid displays multiple timescales associated with not only the decorrelation of the end-to-end vector (commonly related to the Rouse time or disengagement time, depending on the entanglement density of the liquid), but also ones associated with the retraction and rotation cycles of the individual molecules. Furthermore, when accounting for these individual chain dynamics, the “longest” relaxation time of the system was higher by a factor of 1.7, independent of shear rate, when calculated self-consistently due to the coupling of relaxation modes. Brownian dynamics (BD) simulations were also performed on an analogous free-draining bead-rod chain model to compare the rotation and retraction dynamics of a single chain in dilute solution with individual molecular motions in the melt. These BD simulations revealed that the dynamics of the free-draining chain are qualitatively and quantitatively similar to those of the individual chains comprising the polyethylene melt at strain rates in excess of Wi ≈ 50, implying a possible breakdown of reptation theory in the high shear limit. An examination of the bulk-average properties revealed the effects of the chain rotation and retraction cycles upon commonly modeled microstructural properties, such as the distribution function of the chain end-to-end vector and the entanglement number density.
59(2015); http://dx.doi.org/10.1122/1.4903312View Description Hide Description
The linear rheology of nanoparticle filled polymer (NPFP) melts has been a quite charming but controversial topic of long standing. This article reviews recent research advances to provide a general understanding of its universal appearance and underlying mechanism. This work summarizes the rheological criteria for determining the so-called liquid-to-solid transition with increasing filler content, the contradictory ideas of four kinds of time-concentration superposition principles proposed for constructing master curves of linear rheology, and a wide range of constitutive and phenomenological models focused on creating rheological contributions of the polymer, filler, and interface region from different perspectives. Controversies about microstructures of NPFPs including filler structure and chain dynamics of the matrix are briefly described. Several open questions are highlighted to outline the most likely general framework for the further investigation of the linear rheology of NPFP melts.
59(2015); http://dx.doi.org/10.1122/1.4902356View Description Hide Description
This work presents a technique and develops an apparatus that allows the application of homogeneous external magnetic fields (parallel or perpendicular to the deformation axis) to a fluid sample undergoing extensional flow kinematics while measuring the filament thinning using the commercial version of the capillary breakup extensional rheometer (Haake™ CaBER™ 1, Thermo Scientific). We also present innovative rheological measurements of several commercial ferrofluids (FFs) and one magnetorheological fluid (MRF) under uniaxial extensional flow. The experimental results demonstrate that FFs exhibit a Newtonian-like behavior in the absence of magnetic fields. When a magnetic field is applied perpendicular to the extensional flow, no significant effects are observed similar to shear experiments. However, when the external magnetic field is aligned with the extensional flow, the filament takes longer to break up but otherwise behaves as a Newtonian fluid. In the case of the MRF, due to the higher concentration of particles and larger particle size, the differences in the extensional behaviors are much more dramatic regardless of the orientation of the magnetic field compared to the case when no magnetic field is applied.
59(2015); http://dx.doi.org/10.1122/1.4903495View Description Hide Description
Very recently, a new convective constraint release (CCR) single-mode (toy) model [Ianniruberto and Marrucci, J. Rheol. 58, 89–102 (2014)] has been proposed to account for the nowadays well-ascertained fact that flow induces some degree of disentanglement, the more so the faster is the flow [Baig et al., Macromolecules 43, 6886–6902 (2010)]. The previous work, successful in explaining some qualitative aspects of the nonlinear step strain response, is here extended to the multimode case by developing a model allowing for a spectrum of relaxation times in both orientational and stretch dynamics, the purpose being one of performing a quantitative comparison with literature data on nearly monodisperse linear polymers. Several data of relaxation after shear step strain and of time-dependent stress response in shear and elongational startup are considered. The overall agreement of the new multimode CCR model with data appears satisfactory.
59(2015); http://dx.doi.org/10.1122/1.4904394View Description Hide Description
Sometimes the creep ringing phenomena are observed as results of the coupling between the instrumental inertia and the viscoelasticity of the materials. Especially, biopolymer solutions are apt to be the victim of creep ringing because creep test is usually conducted to enlarge the frequency range of dynamic data. To overcome the creep ringing problems, we developed a general method which extracts pure material response functions from creep data with ringing by the use of Laplace transform and nonlinear regression method. The method is not based on spring-dashpot models and gives continuous relaxation time spectrum.