Interfacial rheology of globular proteins

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2011.Cataloged from PDF version of thesis.Includes bibliographical references (p. 107-122).Protein-surfactant mixtures appear in many industrial and biological applications. Indeed, a fluid as vital as blood contains a mixture of serum albumin proteins with various other smaller surface-active components. Proteins and other surface active molecules are often adsorbed at an air-liquid or liquid-liquid interface due to favorable thermodynamics, and these interfaces play a role in such diverse fields as stabilizing foams and emulsions, controlling high speed coatings, the blood clotting process, designing synthetic replacements of biological fluids, etc. It is thus important to understand the interfacial behavior of protein-surfactant mixtures, especially the role it can play in bulk measurements of material properties. A complete description of the dynamics of these multi-component systems is still elusive. In this thesis, a comprehensive study of the rheology of a model globular protein solution is described. In conventional bulk rheological studies of globular protein solutions a yield-like behavior and shear thinning in the viscosity at low shear rates is observed, even for concentrations as low as 0.03% by weight. Typical explanations for this yield stress involve the introduction of long range colloidal forces that are stronger than electrostatic forces, and which stabilize a colloidal structure formed by the protein molecules. However, evidence for this structure from small angle X-ray scattering and small angle neutron scattering is lacking. In the first part of this thesis, it will be shown using interfacial steady and oscillatory shear experiments how the yield-like behavior observed in bulk rheology in fact arises out of a purely interfacial effect. We show that the yield-like behavior and shear-thinning observed in bulk rheology can be modeled by treating the interfacial layer of the adsorbed protein as a pure plastic material that yields at a critical strain. This model also accounts for the geometric dependence seen in bulk rheology experiments. The second part of this thesis investigates the rheological consequences of adding low molecular weight surfactants to globular protein solutions. Because non-ionic surfactants compete for the interface with the protein molecules, the resulting dynamics at the fluid interface can be complex. In addition to steady and oscillatory interfacial shear rheology, a new phenomenon called interfacial creep ringing-a term used to denote the periodic oscillations arising from the coupling between instrument inertia and interface elasticity-arises. It is demonstrated how these damped inertioelastic oscillations can be used to rapidly estimate interfacial viscoelastic moduli and also study the effect of the addition of non-ionic surfactants to globular protein solutions. We show that exploiting the interfacial creep ringing technique is useful as a way to rapidly determine the effects of interfacial viscoelasticity. It is also observed that increasing the concentration of the competing surfactant leads to progressively decreasing ringing frequencies and amplitudes, until visible ringing completely disappears beyond a certain concentration. Finally, we indicate future avenues of work including surface particle tracking to study these complex dynamics of protein surfactant mixtures.by Aditya Jaishankar.S.M

Interfacial viscoelasticity, yielding and creep ringing of globular protein–surfactant mixtures

Protein–surfactant mixtures arise in many industrial and biological systems, and indeed, blood itself is a mixture of serum albumins along with various other surface-active components. Bovine serum albumin (BSA) solutions, and globular proteins in general, exhibit an apparent yield stress in bulk rheological measurements at surprisingly low concentrations. By contrasting interfacial rheological measurements with corresponding interface-free data obtained using a microfluidic rheometer, we show that the apparent yield stress exhibited by these solutions arises from the presence of a viscoelastic layer formed due to the adsorption of protein molecules at the air–water interface. The coupling between instrument inertia and surface elasticity in a controlled stress device also results in a distinctive damped oscillatory strain response during creep experiments known as“creep ringing”. We show that this response can be exploited to extract the interfacial storage and loss moduli of the protein interface. The interfacial creep response at small strains can be described by a simple second order system, such as the linear Jeffreys model, however the interfacial response rapidly becomes nonlinear beyond strains of order 1%. We use the two complementary techniques of interfacial rheometry and microfluidic rheometry to examine the systematic changes in the surface and bulk material functions for mixtures of a common non-ionic surfactant, polysorbate 80, and BSA. It is observed that the nonlinear viscoelastic properties of the interface are significantly suppressed by the presence of even a relatively small amount of surfactant (c[subscript surf] > 10[superscript −3] wt.%). Preferential interfacial adsorption of the mobile surfactant at these surfactant concentrations results in complete elimination of the bulk apparent yield stress exhibited by surfactant-free BSA solutions.Akzo Nobel (Firm

Shear History Extensional Rheology Experiment II (SHERE II) Microgravity Rheology with Non-Newtonian Polymeric Fluids

The primary objective of SHERE II is to study the effect of torsional preshear on the subsequent extensional behavior of filled viscoelastic suspensions. Microgravity environment eliminates gravitational sagging that makes Earth-based experiments of extensional rheology challenging. Experiments may serve as an idealized model system to study the properties of lunar regolith-polymeric binder based construction materials. Filled polymeric suspensions are ubiquitous in foods, cosmetics, detergents, biomedical materials, etc

Probing hydrogen bond interactions in a shear thickening polysaccharide using nonlinear shear and extensional rheology

Mamaku gum is a polysaccharide extracted from the fronds of the black tree fern found in New Zealand. The cooked pith has traditionally been used for various medicinal purposes and as a food source by the Maori people of New Zealand. It has potential applications as a thickener in the food industry and as a palliative for patients with dysphagia. Studies on the shear rheology of Mamaku gum have revealed that the gum exhibits shear thickening at a critical shear rate due to a transition from intra- to inter-molecular chain interactions upon shear-induced chain elongation. In this paper, we demonstrate that these interactions are primarily due to hydrogen bonding. We perform extensional rheology on mixtures of Mamaku gum and urea (a known disruptor of hydrogen bonds) to quantify the nature of these interactions. Capillary Breakup Extensional Rheometry (CaBER) performed on the pure Mamaku gum solutions yield plateau values of the Trouton ratio as high as ∼10[superscript 4], showing that the viscoelasticity of the gum in uniaxial elongation is much higher than in shear. For all Mamaku concentrations tested, the extensional viscosity decreases upon increasing urea concentration. Furthermore, the relaxation time decreases exponentially with increasing urea concentration. This exponential relationship is independent of the Mamaku concentration, and is identical to the relationships between urea concentration and characteristic timescales measured in nonlinear shear rheology. We show using the sticky reptation model for polymers with multiple sticker groups along the backbone how such a relationship is consistent with a linear decrease in the free energy for hydrogen bond dissociation. We then demonstrate that a time-concentration superposition principle can be used to collapse the viscoelastic properties of the Mamaku-gum/urea mixtures.United States. National Aeronautics and Space Administration (Grant NNX09AV99G

Predicting Hemiwicking Dynamics on Textured Substrates

The ability to predict liquid transport rates on textured surfaces is key to the design and optimization of devices and processes such as oil recovery, coatings, reaction-separation, high-throughput screening, and thermal management. In this work we develop a fully analytical model to predict the propagation coefficients for liquids hemiwicking through micropillar arrays. This is carried out by balancing the capillary driving force and a viscous resistive force and solving the Navier–Stokes equation for representative channels. The model is validated against a large data set of experimental hemiwicking coefficients harvested from the literature and measured in-house using high-speed imaging. The theoretical predictions show excellent agreement with the measured values and improved accuracy compared to previously proposed models. Furthermore, using lattice Boltzmann (LB) simulations, we demonstrate that the present model is applicable over a broad range of geometries. The scaling of velocity with texture geometry, implicit in our model, is compared against experimental data, where good agreement is observed for most practical systems. The analytical expression presented here offers a tool for developing design guidelines for surface chemistry and microstructure selection for liquid propagation on textured surfaces

Metal-coordination: using one of nature's tricks to control soft material mechanics

Growing evidence supports a critical role of dynamic metal-coordination crosslinking in soft biological material properties such as self-healing and underwater adhesion. Using bio-inspired metal-coordinating polymers, initial efforts to mimic these properties have shown promise. Here we demonstrate how bio-inspired aqueous polymer network mechanics can be easily controlled via metal-coordination crosslink dynamics; metal ion-based crosslink stability control allows aqueous polymer network relaxation times to be finely tuned over several orders of magnitude. In addition to further biological material insights, our demonstration of this compositional scaling mechanism should provide inspiration for new polymer material property-control designs.National Science Foundation (U.S.). Materials Research Science and Engineering Centers (Program) (DMR-0820054)Danish Council for Independent Research (Natural Sciences for a Post-Doctoral Fellowship 272-08-0087)University of Chicago. Materials Research Science and Engineering Center (DMR 0820054

A fractional K-BKZ constitutive formulation for describing the nonlinear rheology of multiscale complex fluids

The relaxation processes of a wide variety of soft materials frequently contain one or more broad regions of power-law like or stretched exponential relaxation in time and frequency. Fractional constitutive equations have been shown to be excellent models for capturing the linear viscoelastic behavior of such materials, and their relaxation modulus can be quantitatively described very generally in terms of a Mittag–Leffler function. However, these fractional constitutive models cannot describe the nonlinear behavior of such power-law materials. We use the example of Xanthan gum to show how predictions of nonlinear viscometric properties, such as shear-thinning in the viscosity and in the first normal stress coefficient, can be quantitatively described in terms a nonlinear fractional constitutive model. We adopt an integral K-BKZ framework and suitably modify it for power-law materials exhibiting Mittag–Leffler type relaxation dynamics at small strains. Only one additional parameter is needed to predict nonlinear rheology, which is introduced through an experimentally measured damping function. Empirical rules such as the Cox–Merz rule and Gleissle mirror relations are frequently used to estimate the nonlinear response of complex fluids from linear rheological data. We use the fractional model framework to assess the performance of such heuristic rules and quantify the systematic offsets, or shift factors, that can be observed between experimental data and the predicted nonlinear response. We also demonstrate how an appropriate choice of fractional constitutive model and damping function results in a nonlinear viscoelastic constitutive model that predicts a flow curve identical to the elastic Herschel-Bulkley model. This new constitutive equation satisfies the Rutgers-Delaware rule, which is appropriate for yielding materials. This K-BKZ framework can be used to generate canonical three-element mechanical models that provide nonlinear viscoelastic generalizations of other empirical inelastic models such as the Cross model. In addition to describing nonlinear viscometric responses, we are also able to provide accurate expressions for the linear viscoelastic behavior of complex materials that exhibit strongly shear-thinning Cross-type or Carreau-type flow curves. The findings in this work provide a coherent and quantitative way of translating between the linear and nonlinear rheology of multiscale materials, using a constitutive modeling approach that involves only a few material parameters

Cervical Mucus Properties Stratify Risk for Preterm Birth

Background: Ascending infection from the colonized vagina to the normally sterile intrauterine cavity is a well-documented cause of preterm birth. The primary physical barrier to microbial ascension is the cervical canal, which is filled with a dense and protective mucus plug. Despite its central role in separating the vaginal from the intrauterine tract, the barrier properties of cervical mucus have not been studied in preterm birth. Methods and Findings: To study the protective function of the cervical mucus in preterm birth we performed a pilot case-control study to measure the viscoelasticity and permeability properties of mucus obtained from pregnant women at high-risk and low-risk for preterm birth. Using extensional and shear rheology we found that cervical mucus from women at high-risk for preterm birth was more extensible and forms significantly weaker gels compared to cervical mucus from women at low-risk of preterm birth. Moreover, permeability measurements using fluorescent microbeads show that high-risk mucus was more permeable compared with low-risk mucus. Conclusions: Our findings suggest that critical biophysical barrier properties of cervical mucus in women at high-risk for preterm birth are compromised compared to women with healthy pregnancy. We hypothesize that impaired barrier properties of cervical mucus could contribute to increased rates of intrauterine infection seen in women with preterm birth. We furthermore suggest that a robust association of spinnbarkeit and preterm birth could be an effectively exploited biomarker for preterm birth prediction.Massachusetts Institute of Technology. Charles E. Reed Faculty Initiative FundBurroughs Wellcome Fund (Preterm Birth Research Grant)National Science Foundation (U.S.). Graduate Research Fellowship Progra

The linear and nonlinear rheology of multiscale complex fluids

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2014.Cataloged from PDF version of thesis.Includes bibliographical references (pages 311-337).The microstructures of many complex fluids are typically characterized by a broad distribution of internal length scales. Examples of such multiscale materials include physically and chemically cross-linked gels, emulsions, soft colloidal glasses and concentrated suspensions. Due to the complex microstructure, these materials exhibit multiscale power law relaxation under externally imposed deformation. Compact constitutive frameworks that can accurately describe and predict both the linear as well as the nonlinear rheology of such complex fluids have remained elusive. Moreover, the rheological behavior of these materials under extensional deformations, which is important in applications such as spraying and fiber spinning, is relatively poorly understood. The primary contribution of this thesis is the development of a compact constitutive modeling framework to quantitatively describe the rheology of multiscale complex fluids. In the linear limit of small deformations, fractional constitutive equations in conjunction with the concept of quasi-properties have been shown to provide accurate physical descriptions of the broad power law relaxation dynamics exhibited by multiscale materials. In this thesis we very generally show how fractional constitutive equations enable the prediction of the rheological response of multiscale fluids under complex deformation profiles. As a specific example, we analyze the damped inertio-elastic oscillations exhibited at early times by viscoelastic interfacial layers upon the imposition of a constant stress, and the subsequent long time power law creep. We also analyze the small strain lubrication flow regime of a typical tack experiment performed on a crosslinked power law gel, where the extensional deformation of the complex material plays an important role. We extend these models to the large strain nonlinear regime using an integral K-BKZ framework coupled with a strain damping function. We demonstrate in a general manner that nonlinear rheological responses such as shear-thinning and positive first normal stress coefficients can be predicted a priori from linear viscoelastic data and a single additional nonlinear parameter introduced through the damping function. We also demonstrate that well-known empirical rheological models utilized to describe nonlinear behavior such as the Herschel-Bulkley, Cross and Carreau models can be derived using the K-BKZ framework by selecting a suitable fractional relaxation kernel and an appropriate damping function. Additionally, we derive expressions for linear viscometric functions as well as the first normal stress coefficient for materials that exhibit steady shear flow behavior predicted by the above empirical models. Our approach also quantifies the applicability of widely known empirical rheological rules for nonlinear rheology such as the Cox-Merz rule. The second contribution of this thesis is in increasing the understanding of the rheological behavior of multiscale complex fluids in extensional flow fields. For this purpose we utilize a variety of experimental extensional rheology techniques such as Capillary Breakup Extensional Rheometry (CaBER), Filament Stretching Extensional Rheometry (FiSER) and an Optimized Shape Cross-slot Extensional Rheometer (OSCER). Due to their ubiquity in industrial applications as well as in biologically relevant complex fluids, we primarily study aqueous polysaccharide systems (for example Mamaku gum). With the help of these detailed experiments, we investigate and quantify the strength of hydrogen-bonding interactions in this multiscale physically associated gel. We also investigate the extensional rheology of Hyaluronic acid, which has been shown to be an important factor in proper synovial fluid function. The findings of this thesis are widely applicable given the widespread use of multiscale complex fluids in industrial, and biological applications. The fractional constitutive framework derived here overcomes the limitations of current modeling approaches that invoke a large number of empirical constitutive parameters. Our simple models will be useful for quantitative material diagnostics and quality control comparisons as well as for computational simulations. Moreover, the experimental findings on the extensional rheology of multiscale polysaccharide systems will help in the formulation of biologically relevant complex fluids for the treatment of physiological conditions such as osteoarthritis and dysphagia.by Aditya Jaishankar.Ph. D

Describing the firmness, springiness and rubberiness of food gels using fractional calculus. Part II: Measurements on semi-hard cheese

We use the framework of fractional calculus to quantify the linear viscoelastic properties of full-fat, low-fat, and zero-fat, semi-hard cheeses over a range of temperatures and water/protein ratios. These fractional constitutive models correctly predict the time-dependence and interrelation of the firmness, springiness, and rubberiness of these emulsion-filled hydrocolloidal gels. Our equations for the firmness F, springiness S, and rubberiness R, also correctly predict the effect of changing the magnitude or time-scale of the stress loading on the material even in the case of irreversible flow events, when cheese progressively transitions from a solid to a liquid. Finally we show how our FSR-equations can be used in a texture engineering context; they provide rational guidance to product reformulation studies and allow for extrapolation of a firmness measurement to practical situations in which the gel is subjected to prolonged creep loading. Keywords Rational reformulation Food gels Structure-texture engineering Constitutive model Fractional calculus Scott Blai