Session K11: Non-Newtonian Flows: Rheology (8:45am - 9:30am CST)

The first open channel for a yield stress fluid in complex porous media

The prediction of the first fluidized path of a yield stress fluid in complex porous media is a challenging yet an important task to understand several industrial and biological processes. In most cases, the conditions that open this first path are known either through experiments or expensive computations. Here, we present a simple network model to predict the first open channel for a yield stress fluid in a complex porous medium. For porous media made of non-overlapping spheres, we find that the pressure drop required to open the first channel depends on both the sphere relative size to the macroscopic length of the system and on the packing fraction. We also report the statistics on the arc-length of the first open path. Finally, we discuss the implication of our results on the design of porous media for enhanced transport properties.   

Data-Driven Physics-Informed Constitutive Meta-Modeling of Complex Fluids, Comprehensive Framework and Applications

We present a Multi-Fidelity Neural Network (MFNN) architecture for data-driven constitutive meta-modelling of complex fluids and compare its rheological predictions with those of a simple Deep Neural Network (DNN) and experimental measurements. The framework allows for inclusion of underlying physics in form of synthetic data used in training of the ``low fidelity'' network. Generation of these low-fidelity data points here are done using the underlying rheological constitutive models. The high-fidelity network in contrast is trained on limited experimentally measured data. The MFNN is found to be capable of successfully predicting the steady state shear viscosity of a multi-component complex fluid consisting of several different colloidal particle, worm-like micelles and other aromatic particles over a wide range of applied shear rates based on fluid's primary constituting components. We discuss the role of constitutive model used in data generation on overall performance of the MFNN algorithm. Furthermore, rheological predictions were made for the same system with respect to experiment temperature, salinity of the mixture, and sample aging. We show that by incorporating the appropriate physical intuition into the neural network, the MFNN algorithm captures the role of temperature, the salt level added to the mixture, as well as aging within and outside the range of training data parameters. In contrast, a purely data-driven DNN is consistently found to predict erroneous rheological behavior.   

Shear thinning rheology in concentrated suspension of fibers modeled using a load depended friction coefficient

Study of rheology of suspensions of fibers is essential in many industrial applications such as paper and pulp production, biofuel production and material reinforcement. Rheological properties of dense suspensions such as viscosity and normal stress coefficients depend on a large number of parameters such as fiber aspect ratio, volume fraction, flexibility of the fiber, roughness, and friction between fibers. As the solid volume fraction increases, the contribution from the contact force to viscosity becomes more important compared to the hydrodynamic force. We perform direct numerical simulations modeling the fibers as continuous flexible slender bodies governed by the Euler-Bernoulli beam theory. An immersed boundary method is used to solve for the motion of fibers and couple it with the Navier-Stokes equations for the fluid. To model the contact between fibers a normal load dependent coefficient of friction is used which successfully recovers the shear thinning behavior in suspensions observed in experiments. After validating our computational model, we perform a parametric study varying fiber flexibility, volume fraction and examine suspension viscosity and yield stress.   

Signatures of elastoviscous buckling in the dilute rheology of stiff polymers

As an elastic polymer tumbles in shear flow, it experiences compressive viscous forces that can cause it to buckle and undergo a sequence of morphological transitions with increasing flow strength. We use numerical simulations to uncover the effects of these transitions on the steady shear rheology of a dilute suspension of stiff polymers. Our results agree with classic scalings for Brownian rods in relatively weak flows but depart from them above the buckling threshold. These changes in scaling laws are further highlighted in the gyration tensor of the polymer that allows us to quantify the role of filament morphologies in rheology. Signatures of elastoviscous buckling include enhanced shear thinning and an increase in the magnitude of normal stress differences that. We discuss our findings in the light of past work on rigid rods and non-Brownian filaments and highlight the subtle role of thermal fluctuations in triggering instabilities   

Active-passive granular mixtures: shear-thinning flow induced by wiggling larvae

Just as heating a viscous fluid causes its viscosity to drop, we observe that the introduction of active particles into a passive granular material can increase its flowability. In our experiments, we examine this effect by introducing flour beetle larvae into grains of various sizes. We use three different measurement techniques to characterize the behavior of the active-passive mixtures: the macroscopic flow rate via the changing angle of “repose”, the timescale of grain-scale rearrangements via diffusing wave spectroscopy, and the complex viscosity via a rheometer. We find that increasing the percentage of larvae decreases the timescale of microstructure rearrangements, and that the flow rate depends nontrivially on both the percentage of larvae and the grain size. We also find that the mixture is shear-thinning for shear rates faster than the wiggling motion of the larvae. The Cross equation, typically used to quantify the temperature-dependence of polymer solutions or melting alloys, is able to capture the viscosity of the mixtures among all percentages of larvae and grains.   

Rheology and Creep Dynamics of Wormlike Micellar Gels

Long chained surfactants in solution show behavior akin to elastic gels. We work with one such model system and elucidate its rheology and dynamics under an imposed shear. Wormlike micellar gels show a complex creep response marked by long time power law creep at low stresses and delayed flow phenomenon at higher stresses. We quantify these dynamics and show that the fluidization process is characterized by a high degree of variability indicative of heterogeneous yielding processes. These results separate wormlike micellar gels from conventional micellar solutions where such interesting phenomena aren't typically observed.   

Understanding the relationship between plasticity and material microstructure in disordered systems

How soft, disordered materials yield is a question of fundamental interest to material engineers and physicists alike. In this talk, we explore~the relationship between the plastic flow-induced dynamics and microscopic structure of disordered colloidal solids. Our experimental setup~consists of~a~custom-built interfacial stress rheometer, in which a dense monolayer of repulsive colloidal particles~is placed and~undergoes cyclic shear. This~apparatus permits~simultaneous measurement of the material bulk rheology ($G'$,~$G'')$~and dynamic structure factor from particle trajectories, as well as~characterization of the suspension microstructure. We~quantify system-wide~structure using the concept of (structural) excess entropy, the difference between~system~entropy and that of an ideal gas.~The experiments reveal~that structural relaxation induced by plastic flow depends on and scales~with the strain-rate and microscopic order measured at earlier and later times, respectively. Thus, measurement of sample~\textit{static}~structure (excess entropy) provides insight about both strain-rate and constituent rearrangement~\textit{dynamics}~in the sample at earlier times. Moreover,~the relaxation times scale~with shear rate according to a classic shear thinning relation.   

High shear rate capillary rheology of rod-like viruses

Complex fluids composed of long filaments, such as many polymeric and worm-like micellar fluids, undergo significant microstructural changes as they are sheared, resulting in strong shear thinning behavior, shear banding, and other flow instabilities. In order to simultaneously measure structural changes and flow properties, we use capillary \textmu rheo-SANS to study solutions of Fd bacteriophage, a model rod-like fluid system. As we push towards very high shear rates, we measure the dependence of microstructural quantities, such as pair correlations and rotational diffusion coefficients, on shear rate, flexibility, and other factors, and aim to connect them to the fluid's rheological properties.   

Non-affine velocity fields to explain weakly-nonlinear rheology

We derive and interpret the weakly nonlinear response of the Johnson-Segalman/Gordon-Schowalter constitutive model for non-Newtonian fluids. Non-affine "slip" velocity fields cause the nonlinearity in this family of models. From our theoretical derivation, we show how quantitative medium-amplitude oscillatory shear (MAOS) nonlinearities can be associated with these material-level flow physics. Other constitutive models are a subset of the generalized results presented here, including the generalized Corotational Maxwell model. We derive results for a generalized relaxation kernel allowing for complex relaxation spectra, enabling us to reinterpret previously published MAOS experimental data in terms of non-affine flow and deformation. Reference: Ramlawi, N., N. A. Bharadwaj, and R. H. Ewoldt, ``The weakly nonlinear response and non-affine interpretation of the Johnson-Segalman/Gordon-Schowalter model,'' arXiv:2007.08089 [cond-mat.soft], https://arxiv.org/abs/2007.08089   

Pinching dynamics and rheology of polymer-surfactant complexes

The rheological properties of polymer-surfactant mixtures play a significant role in applications ranging from enhanced oil recovery, pharmaceutical and biological fluids, cosmetics, food, soft adhesives and coating. Addition of an ionic surfactant to an aqueous solution of neutral polymer like polyethylene oxide is known to result in a shear rheological response with non-monotonic concentration dependent variation, attributed to association complexes formed by hydrophobic interactions between surfactant monomers and polymers chains, as well as charge effects. Furthermore, the formation of association complexes changes both dynamic and equilibrium surface tension. However, due to a lack of suitable techniques, extensional rheology response of polymer-surfactant mixtures has not been characterized in adequate detail, even though drop formation or liquid transfer applications are influenced by extensional rheology and pinching dynamics. In this study, we examine how pinch-off dynamics and rheological response of polymer solutions are modified by the addition of ionic surfactants, and we discuss the implications for dispensing of multicomponent complex fluids.   

Using Dissipative Particle Dynamics to Investigate the Behaviour of Surfactant Solutions Under Shear Flow

Surfactants are present in many everyday products, including detergents and shampoos. Amphiphilic surfactant molecules will self-assemble into lyotropic liquid crystal structures when in solution. The different structures that form alter the rheology of the solution. There exist a wide range of possible solution phases, each possessing different properties e.g. viscosity. Small scale modelling of the ‘clustering’ behaviour of surfactant molecules in solution helps us to understand the effects of the small scale on the rheology of the material. Multiple simulation methods are possible for this type of investigation, but this poster will focus on the use of Dissipative Particle Dynamics (DPD). DPD is an off-lattice, mesoscopic simulation technique which involves a set of particles moving in continuous space. DPD has benefits over Molecular Dynamics (MD) techniques, and has the potential for reaching longer length and time scales. Most existing research focuses on understanding equilibrium behaviour, however the complex behaviour of surfactant solutions under shear flow is not well understood. This poster will present the different methods that can be used to calculate the shear viscosity of a fluid. The viscosities calculated can be compared with those found experimentally.   

Pinching dynamics, rheology and elastic instabilities of Boger fluids

Boger fluids refer to viscoelastic that exhibit rate-independent shear viscosity. The absence of rate-dependent viscosity allows identification of purely elastic instabilities and comparison with Oldroyd-B model, which is a constitutive model that includes elasticity without allowing for shear thinning. In this study, we investigate the shear and extensional rheology response of Boger fluids to identify the role played by enhancement in solvent viscosity at a fixed polymer concentration. The increase in solvent viscosity and relative contribution of viscous and elastic stresses is contrasted in experiments carried out using~dripping-onto-substrate rheometry, as well as viscoelastic fingering.