Session L08: Non Newtonian Fluid : Rheology

New methodology for rheological properties calculation of discotic nematic Liquid Crystals

In this study, the main objective is to propose a methodology to calculate various rheological properties and viscosity coefficients for a discotic nematic LiquidCcrystal (DNLC). Liquid Crystals (LCs) are anisotropic viscoelastic materials with fluid-like and crystal-like properties. The anisotropy of the viscosity coefficients, with respect to different flow directions, is a unique property of the liquid crystalline phase. Using the presented method, the required viscosity coefficients for different concentrations of Graphene Oxide (GO) dispersion, as a DNLC, were obtained. GO, the most processable derivative of graphene, is oxygen-functionalized graphene and has attracted enormous attention due to the unique liquid crystal and rheological properties. Shear-thinning rheological behavior of the nematic GO dispersion has opened an easy way to fabricate graphene-based devices in micro and macro scales. Our results showed that the alignment viscosity from the analytical method was consistent with the experimental one. Using the calculated viscosity coefficients, the numerical simulation results of the nematic GO under flow were consistent with the experimental results.   

Lubrication theory applied to the Landau deGennes theory

Liquid crystals (LCs) are anisotropic, viscoelastic materials with properties intermediate of solids and liquids. They are useful structural and functional materials; due to their ability to form ordered layers close to the bounding surfaces they are used as lubricants. The material properties and the behavior of LCs are dependent on the microstructure of the LCs which is affected by the strength of the applied hydrodynamic field and elastic effects of the bounding surfaces. Working towards the goal of studying thin films of LCs, the tensorial Landau de-Gennes theory is simplified using the Reynolds scaling approach. The solution of the fully coupled system of Navier-Stokes equations with a modified stress tensor which accounts for the viscoelastic contribution and the equations of Landau de-Gennes (LdG) theory was obtained on COMSOL Multiphysics\texttrademark . The simplified equations from the derived lubrication theory were solved on MATLAB and the results were validated using a Couette flow by comparison with the simulations of the fully coupled system. As the LdG theory is a multiscale theory, the solution of the coupled system requires important computational resources. The advantage of the simplified equations of Lubrication theory is that it allows the reduction of study time.   

Electrostatics and Rheology in Semidilute Polyelectrolyte Solutions

Polyelectrolyte (PE) solutions, which are charged polymers in aqueous solvents, have been studied from many perspectives in the past century. However, the effect of added salt on semidilute PE solutions remains unclear. To understand the electrostatic interactions among the polyions and electrolyte ions, we use a mean-field approach to determine the electrostatic energy of a PE solution at various polymer concentration $n_p$ and salt concentration $n_s$. We derive asymptotic approximations for the potential and ion distributions in nano-confined charged systems by probing the Poisson-Boltzmann equation within and beyond the Debye-H\"uckel linearization, and obtain distinct scaling laws for the electrostatic energy and viscosity for PE solutions at different regimes of $n_p/n_s$. One of our predictions coincides with the empirical Fuoss law for viscosity $\eta \sim n_p^{0.5}$ under the same assumption $n_s \propto n_p$ as in de Gennes’ scaling theory. Our theory also captures an unexplained empirical observation $\eta \propto n_p^{0.68}$ for salt-free PE solutions [1], and provides more physical insights on the effects of salt and charge fraction on the properties of PE solutions. [1] C. G. Lopez, R. H. Colby, P. Graham and J. T. Cabral, Macromolecules, 50, 332 (2017)   

Influence of pH value on gel reaction for fluid flow pattern in a circular flow pipe.

Influence of pH value for flow pattern and pressure variation with PVA--borax gel reaction in a circular flow pipe are experimentally investigated. The working fluids are 10 mass{\%} polyvinyl alcohol (PVA) solution and 3 mass{\%} borax solution. The pH value of borax solution is varied from 6.9 to 11.7 using sodium hydroxide (NaOH) or hydrogen chloride (HCl) solution. Three distinct flow patterns with gel reaction in a flow pipe are described at each pH value as, 1. Parallel flow, 2. Capsule flow and 3. Clogging. In the parallel flow, the injected borax is stretched by main flow velocity and it smoothly flows to downstream direction. The pressure is not varied since the gel reaction does not effect for flow pattern. In the capsule flow, the injected borax forms capsule shape and it flows to downstream region. The capsule is produced by gel sheet which is generated the interface between the injected borax and PVA solution. As a result, the injected borax is separated from PVA region. In clogging, injected borax forms fine finger shape product. Finally, injected material stays on the main flow pipe. The pressure cannot recover to the initial pressure because formed gel is adhered to the wall of main flow pipe. Thus, the pressure monotonically increases with time.   

Macromolecular Architecture and Complex Viscosity

General rigid bead-rod theory [Hassager, \textit{J Chem Phys,} \textbf{60}, 4001 (1974)] explains polymer viscoelasticity from macromolecular orientation. By means of general rigid bead-rod theory, we relate the complex viscosity of polymeric liquids to the architecture of axisymmetric macromolecules. In this work, we explore the zero-shear and complex viscosities of 24 different axisymmetric polymer configurations. When non-dimensionalized with the zero-shear viscosity, the complex viscosity depends on the dimensionless frequency and the sole dimensionless architectural parameter, the \textit{macromolecular}\textit{ lopsidedness}. In this work, in this way, we compare and contrast the elastic and viscous components of the complex viscosities of macromolecular chains that are straight, branched, ringed, or star-branched. We explore the effects of branch position along a straight chain, branched-chain backbone length, branched-chain branch-functionality, branch spacing along a straight chain (including pom-poms), the number of branches along a straight chain, ringed polymer perimeter, branch-functionality in planar stars, and branch dimensionality.   

Rheology of Pluronics: morphological transitions induced by temperature and concentration

Pluronics are a class of water-soluble triblock copolymers made by a sequence polyethylene oxide (PEO)-polypropylene oxide (PPO)-polyethylene oxide (PEO) segments. Due to the presence of hydrophilic and hydrophobic parts on the same molecule, they have the ability to self-assembly in water. By varying the molecular weights of EO and PO sections, it is possible to obtain different types of Pluronics, recognized by a different code. In this work, we study the structures detected in aqueous solutions of Pluronic F68 at different polymer concentration and temperature by means of rheology and Small Angle X-ray Scattering. Various concentrations ranging between 10{\%} and 80{\%} by weight of Pluronic in water were tested. We performed dynamic temperature ramp tests in linear regime at different ramp rates and we were able to evaluate, via a rheological extraction, the temperatures at which transitions occur. The temperature at which the transitions appear were measured as a function of the triblock-copolymer concentration. Molecular structures were analyzed via SAXS measurements, which showed a liquid to body-centered cubic phase transition. The results allowed for the determination of a complete rheological phase diagram water/F68.   

Dynamics of DNA-bridged particle dimers in well-entangled polymer solutions under large amplitude oscillatory shear (LAOS)

Although evidence for shear-banding flows in highly entangled polymer solutions has accumulated over the last two decades, the shear-induced microscopic conformational changes of individual chains that trigger shear banding remains unknown. Here, using a custom-built high-resolution rheo-confocal shear cell, we experimentally study the dynamics of DNA-bridged particle dimers in the shear-banding flow of well-entangled double-stranded DNA (dsDNA) solutions under LAOS to reveal the microscopic dynamics of entangled DNA chains in shear-banding flows. In our experiments, we first confirm that the velocity profiles of the entangled DNA solutions are inhomogeneous at high Weissenberg number (Wi) and develop into strong shear-banding flows with two distinct shear bands. Second, we investigate the dynamics of the particle dimers linked by long linear dsDNA chains and measure the distribution of dimer orientations in the high and low shear-rate bands. Quantitative analyses of the spatially distinct dynamics of such dsDNA-bridged dimers in the two co-existing bands provide important insights into the microscopic origin of the shear-banding flows in entangled polymer solutions.