Soft Glass Rheology Research Articles

Fumed alumina-in-nematic liquid crystal suspensions under shear and electric field

We study rheology, microstructure, and response to an applied electric field (E) in suspensions of fumed alumina (Al2O3) nanoparticles in a nematic liquid crystal (NLC) made of N-(4-methoxybenzylidene)-4-butylaniline (MBBA). Fumed Al2O3/MBBA suspensions exhibit flowability with nanoparticle volume fraction (ϕ) = 0.001 and 0.007, and become solid-like gels at a nanoparticle ϕ = 0.014 and beyond. The dynamic rheology of gel-like suspensions follows the soft glass rheology (SGR) model. The effective noise temperature remains close to 1 for these Al2O3/MBBA suspensions, which serves as an indication of the presence of glassy dynamics. Further, the optical microscopy and the differential scanning calorimetry (DSC) reveal that the incorporation of fumed Al2O3 nanoparticles causes a significant depression in the nematic-isotropic phase transition temperature (TNI). The gel-like suspensions are less sensitive to pre-shear and show a large structural recovery after shear when compared to Al2O3/silicone oil suspensions. At and above a critical nanoparticle ϕ = 0.005, the suspensions exhibit a reversible nematic-isotropic phase transition under the application of E.

Cell membrane biophysics with optical tweezers.

Membrane elastic properties play important roles in regulating cell shape, motility, division and differentiation. Here I review optical tweezer (OT) investigations of membrane surface tension and bending modulus, emphasizing didactic aspects and insights provided for cell biology. OT measurements employ membrane-attached microspheres to extract long cylindrical nanotubes named tethers. The Helfrich-Canham theory yields elastic parameters in terms of tether radius and equilibrium extraction force. It assumes initial point-like microsphere attachment and no cytoskeleton content within tethers. Experimental force-displacement curves reveal violations of those assumptions, and I discuss proposed explanations of such discrepancies, as well as recommended OT protocols. Measurements of elastic parameters for predominant cell types in the central nervous system yield correlations between their values and cell function. Micro-rheology OT experiments extend these correlations to viscoelastic parameters. The results agree with a quasi-universal phenomenological scaling law and are interpreted in terms of the soft glass rheology model. Spontaneously-generated cell nanotube protrusions are also briefly reviewed, emphasizing common features with tethers. Filopodia as well as tunneling nanotubes (TNT), which connect distant cells and allow transfers between their cytoplasms, are discussed, including OT tether pulling from TNTs which mediate communication among bacteria, even of different species. Pathogens, including bacteria, viruses and prions, opportunistically exploit TNTs for cell-to-cell transmission of infection, indicating that TNTs have an ancient evolutionary origin.

Rheological properties of cells measured by optical tweezers.

BackgroundThe viscoelastic properties of cells have been investigated by a variety of techniques. However, the experimental data reported in literature for viscoelastic moduli differ by up to three orders of magnitude. This has been attributed to differences in techniques and models for cell response as well as to the natural variability of cells.ResultsIn this work we develop and apply a new methodology based on optical tweezers to investigate the rheological behavior of fibroblasts, neurons and astrocytes in the frequency range from 1Hz to 35Hz, determining the storage and loss moduli of their membrane-cortex complex. To avoid distortions associated with cell probing techniques, we use a previously developed method that takes into account the influence of under bead cell thickness and bead immersion. These two parameters were carefully measured for the three cell types used. Employing the soft glass rheology model, we obtain the scaling exponent and the Young’s modulus for each cell type. The obtained viscoelastic moduli are in the order of Pa. Among the three cell types, astrocytes have the lowest elastic modulus, while neurons and fibroblasts exhibit a more solid-like behavior.ConclusionsAlthough some discrepancies with previous results remain and may be inevitable in view of natural variability, the methodology developed in this work allows us to explore the viscoelastic behavior of the membrane-cortex complex of different cell types as well as to compare their viscous and elastic moduli, obtained under identical and well-defined experimental conditions, relating them to the cell functions.Electronic supplementary materialThe online version of this article (doi:10.1186/s13628-016-0031-4) contains supplementary material, which is available to authorized users.

Asphaltene-laden interfaces form soft glassy layers in contraction experiments: a mechanism for coalescence blocking.

In previous studies, the adsorption kinetics of asphaltenes at the water-oil interface were interpreted utilizing a Langmuir equation of state (EOS) based on droplet expansion experiments.1-3 Long-term adsorption kinetics followed random sequential adsorption (RSA) theory predictions, asymptotically reaching ∼85% limiting surface coverage, which is similar to limiting random 2D close packing of disks. To extend this work beyond this slow adsorption process, we performed rapid contractions and contraction-expansions of asphaltene-laden interfaces using the pendant drop experiment to emulate a Langmuir trough. This simulates the rapid increase in interfacial asphaltene concentration that occurs during coalescence events. For the contraction of droplets aged in asphaltene solutions, deviation from the EOS consistently occurs at a surface pressure value ∼21 mN/m corresponding to a surface coverage ∼80%. At this point droplets lose the shape required for validity of the Laplace-Young equation, indicating solidlike surface behavior. On further contraction wrinkles appear, which disappear when the droplet is held at constant volume. Surface pressure also decreases down to an equilibrium value near that measured for slow adsorption experiments. This behavior appears to be due to a transition to a glassy interface on contraction past the packing limit, followed by relaxation toward equilibrium by desorption at constant volume. This hypothesis is supported by cycling experiments around the close-packed limit where the transition to and from a solidlike state appears to be both fast and reversible, with little hysteresis. Also, the soft glass rheology model of Sollich is shown to capture previously reported shear behavior during adsorption. The results suggest that the mechanism by which asphaltenes stabilize water-in-oil emulsions is by blocking coalescence due to rapid formation of a glassy interface, in turn caused by interfacial asphaltenes rapidly increasing in concentration beyond the glass transition point.

Listening to Cells: A Non-Contact Optoacoustic Nanoprobe

Cells have a complex composition that yields an intricate rheological behavior, appealing for measurements over a wide frequency range. However the existing techniques cannot exceed the kHz range, and rely for most on injected or contacting functionalized microprobes. Here, we report on an innovative non-contact optoacoustic probing of the mechanical properties of single cells at GHz acoustic frequencies under physiological conditions. We culture cells on a biocompatible Ti6Al4V metal alloy. Low-energy femtosecond laser pulses are focused at the cell-Ti6Al4V interface to a sub-µm spot. The ensuing ultrafast thermal dilatation of the metal launches a high-frequency sound pulse in the cell. Acoustic propagation is measured remotely with an ultrafast laser probe through Brillouin light scattering. This all-optical non-invasive technique offers a broad frequency range, extending up to 1 THz, a sub-µm lateral resolution and a nanometer in-depth resolution. For illustration, we here concentrate on the cell nucleus. Using the laser-generated GHz acoustic waves, we probe the stiffness and viscosity of the nuclei of various cell types. We demonstrate for the first time that, at GHz frequencies, the nucleus stiffness tends to a unique value, much larger than that observed at kHz frequencies. We demonstrate that this universal stiffness reflects the compressional dynamics of the nucleus components. We also show that the evolution of the mechanical properties of the nucleus during cell differentiation is correlated with a specific gene expression pattern. In the frame of soft glass rheology, we project the GHz mechanical properties of the differentiated nuclei to the kHz range. Comparison with the kHz data obtained by alternative techniques suggests the existence of a new absorption process appearing at GHz frequencies. This innovative technique defines a new class of experiments to enlighten cell mechanics in physiological conditions at a subcell scale.

On the relation between hierarchical morphology and mechanical properties of a colloidal 2D gel system

We study a bidimensional gel system formed by a Langmuir film of gold nanoparticles. Its morphology is thoroughly characterized by AFM, SEM, and ellipsometric imaging techniques and shows a complex structure with features on a hierarchy of different sizes following a Levy distribution. The mechanical response arising after the gel point is investigated by Interfacial Shear Rheometry. The film is found to be mainly elastic, with the mechanical moduli scaling as a power law of the reduced concentration, in the same way as the fluctuation time which was measured in a recent X-ray Photon Correlation Spectroscopy experiment. The frequency dependence of the moduli is well described in the framework of the Soft Glass Rheology model (SGR) [Sollich et al. Phys Rev Lett 78, 2020 (1997)]. In this theory a power law distribution of relaxation times is postulated, whose exponent is experimentally determined for the present case. Such a distribution may reflect – in the dynamics – the hierarchical nature of the morphology of the film evidenced by microscopy. A mastercurve can be built, extending time–temperature to a time–concentration superposition principle as in [Cicuta et al. Phys Rev Lett 90, 116103(2003)]. This allows to describe the mechanical response over a frequency range slightly larger than that experimentally accessible. Besides the SGR component, a viscous term is always present, whose origin is investigated also taking into account samples with different preparation histories.

Phase Behavior and Rheological Analysis of Reverse Liquid Crystals and W/I2 and W/H2 Gel Emulsions Using an Amphiphilic Block Copolymer

This article reports the phase behavior determi-nation of a system forming reverse liquid crystals and the formation of novel disperse systems in the two-phase region. The studied system is formed by water, cyclohexane, and Pluronic L-121, an amphiphilic block copolymer considered of special interest due to its aggregation and structural properties. This system forms reverse cubic (I2) and reverse hexagonal (H2) phases at high polymer concentrations. These reverse phases are of particular interest since in the two-phase region, stable high internal phase reverse emulsions can be formed. The characterization of the I2 and H2 phases and of the derived gel emulsions was performed with small-angle X-ray scattering (SAXS) and rheometry, and the influence of temperature and water content was studied. The H2 phase experimented a thermal transition to an I2 phase when temperature was increased, which presented an Fd3m structure. All samples showed a strong shear thinning behavior from low shear rates. The elastic modulus (G') in the I2 phase was around 1 order of magnitude higher than in the H2 phase. G' was predominantly higher than the viscous modulus (G''). In the gel emulsions, G' was nearly frequency-independent, indicating their gel type nature. Contrarily to water-in-oil (W/O) normal emulsions, in W/I2 and W/H2 gel emulsions, G', the complex viscosity (|η*|), and the yield stress (τ0) decreased with increasing water content, since the highly viscous microstructure of the continuous phase was responsible for the high viscosity and elastic behavior of the emulsions, instead of the volume fraction of dispersed phase and droplet size. A rheological analysis, in which the cooperative flow theory, the soft glass rheology model, and the slip plane model were analyzed and compared, was performed to obtain one single model that could describe the non-Maxwellian behavior of both reverse phases and highly concentrated emulsions and to characterize their microstructure with the rheological properties.

Effects of Mechanical Forces on the Cellular Dynamics

The links between biomechanics and human diseases have been the subject of considerable scientific research effort for a number of decades. Recently, the application of soft glass rheology and polymers dynamics concepts from physics and engineering to the study of biological and physiological problems has provided valuable insights into the mechanics of cells function, their mechanical response to external stimuli and their physical and mechanical processes.Here we report the characteristic self-similar cell stress-stiffening response of the two subsequent cytoskeleton dynamics: the early entropic and the later soft glass-like dynamics. We propose that the master relation obtained by a single parameter normalization reflects, in context with the hierarchical nature of cytoskeleton architecture, the intrinsic tight dependence of the two mechanical behaviours.

Soft Glass Rheology in Liquid Crystalline Gels Formed by a Monodisperse Dipeptide

Thermal and extensive rheological characterization of a nematic liquid crystal gelated with a novel monodisperse dipeptide, also a liquid crystal, has been carried out. For certain concentrations, the calorimetric scans display a two-peak profile across the chiral nematic-isotropic (N*-I) transition, a feature reminiscent of the random-dilution to random-field crossover observed in liquid crystal gels formed with aerosil particles. All samples show shear thinning behavior without a Newtonian plateau region at lower shear rates. Small deformation oscillatory data at lower frequencies exhibit a frequency dependence of the storage (G') and loss (G'') moduli that can be described by a weak power-law, characteristic of soft glassy rheological systems. At higher frequencies, while lower concentration composites have a strong frequency dependence with a trend for possible crossover from viscoelastic solid to viscoelastic liquid behavior, the higher-concentration gels show frequency-independent rheograms of entirely elastic nature G' > G''. The plateau modulus of G' is described by a power-law with an exponent again common to soft materials, such as foams, slurries, etc. Other features which are a hallmark of such materials observed in the present study are: (i) above a critical strain, a strain softening of the moduli with a peak in the loss modulus, (ii) power-law variation of the storage modulus in the nonlinear viscoelastic regime, and (iii) absence of Cox-Merz superposition for the complex viscosity. An attractive feature of these gels is the fast recovery upon removal of large strain and qualitatively different temporal behavior of the recovery between the low and high concentration composites, with the latter indicating the presence of two characteristic time scales.

Rheological behavior of mammalian cells.

Rheological properties of living cells determine how cells interact with their mechanical microenvironment and influence their physiological functions. Numerous experimental studies have show that mechanical contractile stress borne by the cytoskeleton and weak power-law viscoelasticity are governing principles of cell rheology, and that the controlling physics is at the level of integrative cytoskeletal lattice properties. Based on these observations, two concepts have emerged as leading models of cytoskeletal mechanics. One is the tensegrity model, which explains the role of the contractile stress in cytoskeletal mechanics, and the other is the soft glass rheology model, which explains the weak power-law viscoelasticity of cells. While these two models are conceptually disparate, the phenomena that they describe are often closely associated in living cells for reasons that are largely unknown. In this review, we discuss current understanding of cell rheology by emphasizing the underlying biophysical mechanism and critically evaluating the existing rheological models.

Rheological Behavior of Living Cells Is Timescale-Dependent

The dynamic mechanical behavior of living cells has been proposed to result from timescale-invariant processes governed by the soft glass rheology theory derived from soft matter physics. But this theory is based on experimental measurements over timescales that are shorter than those most relevant for cell growth and function. Here we report results measured over a wider range of timescales which demonstrate that rheological behaviors of living cells are not timescale-invariant. These findings demonstrate that although soft glass rheology appears to accurately predict certain cell mechanical behaviors, it is not a unified model of cell rheology under biologically relevant conditions and thus, alternative mechanisms need to be considered.

Two regimes, maybe three?

We have begun to understand the physical determinants of cell rheology and their relevance to biological functions. Experiments performed on freshly excised cells offer a new perspective in which soft-glass rheology and prestress seem to play central roles.