Poly Ethylene Oxide Solutions Research Articles

Multiscale Microrheology Using Fluctuating Filaments as Stealth Probes.

The mechanical properties of soft materials can be probed on small length scales by microrheology. A common approach tracks fluctuations of micrometer-sized beads embedded in the medium to be characterized. This approach yields results that depend on probe size when the medium has structure on comparable length scales. Here, we introduce filament-based microrheology using high-aspect-ratio semiflexible filaments as probes. Such quasi-1D probes are much less invasive than beads due to their small cross sections. Moreover, by imaging transverse bending modes, we simultaneously determine the micromechanical response of the medium on multiple length scales corresponding to the mode wavelengths. We use semiflexible single-walled carbon nanotubes as probes that can be accurately and rapidly imaged based on their stable near-IR fluorescence. We find that the viscoelastic properties of sucrose, polyethylene oxide, and hyaluronic acid solutions measured in this way are in good agreement with those measured by conventional micro- and macrorheology.

Viscoelastic fluid flow in microporous media

This paper describes an experimental investigation of the flow of flexible polymer solutions through a distinctive micro-porous structure (typical pore sizes of 500 microns) with the primary focus on the interaction of the viscoelasticity of the working fluid with the micro-porous structure. In particular, we relate the bulk flow properties to measurable rheological parameters, demonstrating that a key parameter in estimating the pressure drop through the porous media (for a variety of polymer types, concentrations, solvents, molecular weights, and states of degradation) is the extensional relaxation time (or, more correctly, a characteristic time for extensional stress growth). A Weissenberg number (Wi) is calculated as a product of this extensional relaxation time and the nominal shear rate in the flow. Results, suitably normalised with Newtonian pressure-drop data, show a critical Wi of roughly 0.01 where all working fluids in two different pore structures reveal the onset of elastic dominance over viscous forces as the flowrate increases. Such a low critical value of Wi is due to the estimate of a nominal shear rate based on pore size, which severely underestimates the maximum shear rates within the complex pore structure. Significant deviation from the universal behaviour is observed for high concentrations of the polyacrylamide, which is thought to be a result of shear-thinning for these systems. Systematic degradation of the polyethylene oxide solutions caused an exponential decay in elasticity, which is reflected in the pressure-drop measurements. These imply that, although the flow through porous media is known to be a complex combination of both shear and extensional flow, the extensional effects are of primary importance in this context (or that transient stretching in the media is well represented by this relaxation time) of viscoelasticity. The findings highlight the significance of the transient extensional flow through the neck regions between the pores of the media, which are regions of contraction/expansion.

Effects of crosslinking on the physical solid-state and dissolution properties of 3D-printed theophylline tablets

Crosslinking is an established treatment to alter the physicochemical and functional properties of polymers through the creation of bonds between the polymer chains. Polymers can be crosslinked via radiation in the presence of photo-initiator. The aim of the present study was to formulate 3D-printed theophylline (THEO) tablets using polyethylene oxide (PEO) as a carrier polymer, and to study the influence of crosslinking on the drug release behavior. The tablets were 3D-printed using the aqueous solution of PEO and THEO (80:20 w/w) with a micro-extrusion-based printing setup. A photo-initiator (4-hydroxybenzophenone) was added into the printing solutions, and the injectability of the solutions was investigated prior to printing. The 3D-printed tablets were crosslinked after printing using ultraviolet (UV) or gamma-radiation, and crosslinking was verified by means of Fourier-transform infrared (FTIR) spectroscopy. The maximum injection force of aqueous printing solutions of PEO and THEO was close to that observed with the pure PEO solution. Increasing the number of printing layers in the 3D-printed tablets resulted in a slower drug release in vitro. Gamma-radiation in a nitrogen environment and UV-crosslinking made the carrier polymer (PEO) less water-soluble, but such crosslinking did not affect the release rate of the tablets. Surprisingly, even faster drug release behavior was found with the crosslinked 3D-printed tablets compared to that of non-crosslinked tablets. More research work is needed on the impact of 3D-printed tablet layering thickness and crosslinking for tailoring drug release behavior.

Exploiting the drag reduction characteristics of dense pastes transporting in pipe with boundary liquids injection

AbstractWhen dense paste transports in pipe, the pressure loss is enormous. To make use of the wall slip phenomenon, different liquids were injected into the boundary layer in horizontal pipe and pressure loss per unit reduces. Through the pipe transportation tests, the effect of different factors to the resistance was analysed respectively, such as the thickness of boundary layer, the concentration of material, the type of drag reducer, the transport velocity and the flow rate. Results show that the drag reduction effect is much better with a higher thickness and lower transport velocity, and also obtains that PEO (Polyethylene oxide) solution as boundary layer is much more effective than water. With analysis of variance and significance test, the influence degree of the three main factors on the transport resistance is sorted as the thickness of the boundary layer, paste concentration, pipe diameter. The thickness of boundary layer has the greatest influence on the transport resistance. All conclusions provide a basis for how to use the boundary layer rationally in engineering paste pipeline transportation system.

Experimental Resonances in Viscoelastic Microfluidics

Pulsatile flows of viscoelastic fluids are very important for lab-on-a-chip devices, because most biofluids have viscoelastic character and respond distinctively to different periodic forcing. They are also very important for organ-on-a-chip devices, where the natural mechanical conditions of cells are emulated. The resonance frequency of a fluid refers to a particular pulsatile periodicity of the pressure gradient that maximizes the amplitude of flow velocity. For viscoelastic fluids, this one has been measured experimentally only at macroscales, since fine tuning of rheological properties and system size is needed to observe it at microscales. We study the dynamics of a pulsatile (zero-mean flow) fluid slug formed by a viscoelastic fluid bounded by two air-fluid interfaces, in a microchannel of polymethyl methacrylate. We drive the fluid slug by a single-mode periodic pressure drop, imposed by a piezoactuator. We use three biocompatible polymer solutions of polyethylene oxide as model viscoelastic fluids, and find resonances. We propose a model accounting for surface tension and fluid viscoelasticity that has an excellent agreement with our experimental findings. It also provides an alternative way of measuring relaxation times. We validate the method with parameters reported in the literature for two of the solutions, and estimate the relaxation time for the third one.

A weak shear stress microfluidic device based on Viscoelastic Stagnant Region (VSR) for biosensitive particle capture

Particle manipulation in microfluidic devices is of great significance in biological research. However, currently available inertial capture methods require relatively high flow rates, which will cause damage to biological particles, especially for single-celled organisms that are sensitive to environments. Herein, we demonstrate a label-free, size-based, low shear stress manipulation method using the Viscoelastic Stagnant Region (VSR) to capture sensitive bioparticles. This method uses the deformation of molecular chains in the polymer solution which can generate elastic stresses to form vortices which is called VSR because of the extremely low velocity in the contraction-expansion array (CEA) microchannel. Formation and evolution of VSR was observed experimentally using the Micro-PIV system in polyethylene oxide (PEO) solutions with different concentrations. On this basis, 20 μm and 5 μm polystyrene (PS) particles were confined to a certain area in the microchamber and the trajectory of particles motion in VSR was observed. Both the inertial lift force and the viscoelastic force are affected by the particle size, so the method also presented size selectivity. By quantitatively studying the velocity distribution of the particles on the orbit of motion, it was found that the maximum velocity of the particles in the VSR was about 0.02 m s−1, which is only about 1/100 of that in inertial vortices. Moreover, the spiral motion of particles captured by VSR with variable trajectories was first observed, which is different from the equilibrium orbit in inertial vortices and can provide more motion paths for the particles. This method was further applied to the research of Crypthecodinium cohnii (C. cohnii), which is vulnerable but has high nutritious value. We found that the activity of the C. cohnii decreased slightly after being processed by VSR, but was lost after being processed by the inertial vortices. This research, as a simple and low shear stress particle manipulation method, will provide useful guidance for the manipulation, capture and separation of sensitive biological cells under higher biocompatibility, which is of great significance for the study of cellular and molecular biology.

Insulator-based dielectrophoretic focusing and trapping of particles in non-Newtonian fluids.

Insulator-based dielectrophoretic (iDEP) microdevices have been limited to work with Newtonian fluids. We report an experimental study of the fluid rheological effects on iDEP focusing and trapping of polystyrene particles in polyethylene oxide, xanthan gum, and polyacrylamide solutions through a constricted microchannel. Particle focusing and trapping in the mildly viscoelastic polyethylene oxide solution are slightly weaker than in the Newtonian buffer. They are, however, significantly improved in the strongly viscoelastic and shear thinning polyacrylamide solution. These observed particle focusing behaviors exhibit a similar trend with respect to electric field, consistent with a revised theoretical analysis for iDEP focusing in non-Newtonian fluids. No apparent focusing of particles is achieved in the xanthan gum solution, though the iDEP trapping can take place under a much larger electric field than the other fluids. This is attributed to the strong shear thinning-induced influences on both the electroosmotic flow and electrokinetic/dielectrophoretic motions.

Electrical Conductivity of a Stretching Viscoelastic Filament.

Long polymeric chains highly stretched and aligned with the flow confer a strong mechanical anisotropy on a viscoelastic solution. The electrically-driven transport of free ions under such conditions is far from being understood. In this paper, we determine experimentally whether the above-mentioned deviation from isotropy affects the electric charge transport across the liquid. To this end, we measure the electrical conductivity in the flow (stretching) direction of the cylindrical liquid filament formed in the elasto-capillary thinning that arises during the breakup of a viscoelastic liquid bridge. First, we examine the behavior of monodisperse solutions of polyethylene oxide (PEO) in a mixture of glycerine and water. For all the concentrations and molecular weights considered, the filament conductivity remains practically the same as the isotropic conductivity measured under hydrostatic conditions. However, we observe a decrease in the electric current at the end of elasto-capillary regime which may partially be attributed to the reduction of the liquid conductivity. Then, we measure the conductivity of bidisperse solutions of PEO with very different molecular weights. In this case, a significant decrease in conductivity is observed as the filament radius decreases. This constitutes the first experimental evidence of ion mobility reduction in stretching viscoelastic filaments, a relevant effect in applications such as electrospinning.

Polymer effects on viscoelastic fluid flows in a planar constriction microchannel

A comprehensive understanding of the flow of viscoelastic polymer solutions in a contraction and/or an expansion geometry concerns numerous applications. The effects of polymer type, molecular weight, and concentration are investigated through controlled experiments with polyethylene oxide (PEO), polyvinylpyrrolidone (PVP) and hyaluronic acid (HA) solutions in a planar constriction microchannel in a wide range of Reynolds (Re) and Weissenberg (Wi) numbers. The polymer structure renders drastic differences amongst the contraction flows of 3000 ppm PEO, PVP, and HA solutions despite having similar molecular weights. The expansion flow vortices of these solutions remain analogous to those in the inertial flow of water. Increasing the molecular weight or concentration of PEO polymer promotes the elastic instabilities in both the contraction and expansion flows. It, however, suppresses (and even blocks) the inertial vortices in the expansion flow and makes them start from the salient corners, in contrast to the lips of the expansion walls in water. Interestingly, a sudden decrease in the size of inertial vortices is observed in the expansion flow of 500 ppm, 1 megadalton PEO solution when the elastic disturbances start appearing in the contraction flow. The observed flow regimes and vortex development in the polymer solutions are summarized in the same dimensionless Re−Wi and Re−χL parameter spaces, where χL is the normalized vortex length. The elasticity number, El=Wi/Re, is found to determine and distinguish the contraction and expansion flows.

Constriction length dependent instabilities in the microfluidic entry flow of polymer solutions.

Transport phenomena of fluids and particles through contraction and/or expansion geometries have relevance in many applications. Polymer solutions are often the transporter in these processes, giving rise to flow complexities. The separation distance between a contraction and a following expansion in microfluidic entry flow can affect the interplay between the shear and extension force dominated flow regimes, but the process is still little understood. We investigate the rheological responses of such constriction length dependent instabilities with three different polymer solutions and water in planar contraction-expansion microchannels differing only in the constriction length. The viscoelastic polyethylene oxide (PEO) solution is found to exhibit strong constriction length-dependent instabilities in both the contraction and expansion flows. Such a dependence is, however, completely absent from the flow of shear-thinning xanthan gum (XG) solution and Newtonian water. Interestingly, it is only present in the expansion flow of the both shear thinning and viscoelastic polyacrylamide (PAA) solution.

Refining rheological model for description of linear and nonlinear viscoelasticity of polymer systems

The article reviews the current mesoscopic modeling of flows for polymer solutions and melts of various structures. It also demonstrates the unified structure of the rheological constitutive relation, up to a certain dissipative function. Linear and nonlinear effects are considered for the simple shear and uniaxial tension of polymer melts based on the modified Vinogradov and Pokrovskii rheological model. The model can describe with adequate accuracy the linear viscoelasticity of fluid polymer systems, as well as transient processes at the shear flow and uniaxial elongation. It was compared with other experimental data for the melt of an industrial polyethylene sample. The nonlinear viscoelastic response of a polymer material was modeled at large periodic deformations. The calculation results were obtained via the Runge-Kutta approach with in-line subprograms of MATLAB IT environment. They were later compared with the experimental data for the solution of polyethylene oxide in dimethyl sulfoxide. The nonlinear behavior of the polymer sample manifested itself in the distortion of the viscoelastic response of the material at sinusoidal oscillations. In this case the shear stresses are no longer the correct harmonics and a "step" appears at the leading edge of the response. Besides, their amplitude is not proportional to the amplitude of the shift. The paper also considers the superposition of the oscillating shear flow on the simple shear. The distortion of the viscoelastic response is observed here though, unlike the high-amplitude periodic deformation, the curvatures of the upper and lower half-waves of the response differ. Since there is still no experimental data for this case, this work expects to encourage the researchers working in the field of nonlinear viscoelastic properties of polymer solutions and melts for further endeavors.

Formation of Oriented Fibers Using Injection of PEO Solutions inside Electric Fields Defined by Two Parallel Suspended Electrodes

Formation of oriented fibers using injection of polyethylene oxide (PEO) solutions inside electric fields defined by two parallel suspended electrodes is investigated. Images of streams formed with the injection of a large amount of polymeric solution reveal good agreement with electric field distributions obtained with numerical simulation (COMSOL Multiphysics) when appropriate boundary conditions are defined. Oriented fibers with diameters in the range of hundreds of nanometers to micrometers result connected between electrodes (separated by several centimeters) and can be easily collected/transferred keeping their orientation. Fibers with this characteristic find applications in topics such as tissue and sensors engineering. Also, the fibers are flexible and can be shaped with the stylus of a profilometer.

Self-similarity in the breakup of very dilute viscoelastic solutions

Abstract

Assessment of the stability of water in water emulsions using analytical centrifugation

Water in water (W/W) emulsions were prepared by mixing aqueous dextran and polyethylene oxide (PEO) solutions. Addition of different amounts of the polysaccharide chitosan inhibited coalescence of dispersed droplets of the dextran phase in the continuous PEO phase to different extents depending on the concentration and pH. Transmission profiles were measured during centrifugation and analysed. It is shown that the evolution of the transmission profile is determined by the relative rates of coalescence and sedimentation of the droplets. The rate of coalescence of the sedimented droplets into a continuous dextran phase depends on the resistance of the protective chitosan layer to compression by centrifugation. It is concluded that the efficacy of stabilization of W/W emulsions can rapidly be quantitatively assessed, but it is difficult to deduce the behaviour under gravity from that observed during centrifugation.

Effect of polymer solutions on the propagation and structure of freely translating vortex rings

We experimentally study the propagation of vortex rings in aqueous solutions of polyethylene oxide (PEO400) and hydrolysed polyacrylamide (PAMH25). We study the canonical vortex ring as a caricature problem to the phenomenon of turbulent polymer drag reduction. PAMH25 has a similar infinite shear viscosity, ten times the zero shear viscosity and a thousand times the relaxation time of PEO400. We study the vortex rings generated by the trapezoidal motion of a piston-cylinder mechanism using PIV and PLIF at constant impulse. We see that PAMH25 drastically affects the propagation properties and the structure of the vortex ring, while PEO400 shows a behaviour similar to water. We conclude that PAMH25 retards the roll-up process of the ring, due to both the large range of plateau viscosities and large relaxation time. We surmise that the behaviour of the vortex ring observed might be used to unravel the mechanism of polymer drag reduction.

Molecular engineering of thixotropic, sprayable fluids with yield stress using associating polysaccharides

HypothesisMolecular engineering facilitates the development of a complex fluid with contradictory requirements of yield stress and sprayability, while minimizing the amount of structuring material (<0.05 wt%). This unique system can be achieved by a biopolymer hydrogel with tunable inter- and intra-molecular interactions for microstructural robustness and molecular extensibility by the variation of chemical conformations that microstructure breaks up under shear followed by a low molecularly extensible response. ExperimentsBlends of xanthan and konjac glucomannan containing 99.95 wt% water are demonstrated to satisfy these contradictory requirements and formulated as a function of KCl concentrations. A systematic study was performed using shear and extensional rheology and compared to a reference solution of polyethylene oxide (PEO), a well-known, Boger fluid, highlights the role of molecular elasticity in controlling critical rheological properties. Static light scattering (SLS) and simultaneous rheology and small-angle neutron scattering (RheoSANS) are also used to elucidate the equilibrium structure and flow dynamics. FindingsThe blends exhibit a lower yield stress and extensional resistance with added KCl, which leads to good spray characteristics in contrast to strain-hardening PEO. The results suggest that the inter-molecular attractions between the two gums leading to network formation with appropriate stiffness, that break up readily under shear, and low molecular elasticity are critical molecular design parameters necessary to achieve sprayable, yields stress fluids.

Electrospinning of polymer solutions: An analysis of instability in a thinning jet with solvent evaporation

The present study examines the stability of a thinning straight jet in electrospinning process. The linear stability analysis is carried out to obtain the growth/decay rate of axisymmetric disturbance imposed on the thinning jet. The analysis captures the role of solvent evaporation in the stability behavior of the linear path of the electrospinning jet. In contrast to many prior stability analyses which consider the jet as a cylindrical filament with uniform radius, the present study analyzes the stability of a jet which exhibits thinning under realistic electrospinning conditions. Two different polymeric solutions are considered: the poly-isobutene (PIB) Boger fluid with low electrical conductivity and the highly conductive poly-ethylene oxide (PEO) solution in ethanol/water. At low extensional deformation, the rheology of the unentangled PIB-based Boger fluid is described by the Oldroyd-B model. The PEO solution, on the other hand, experiences a very high elongation rate due to the development of strong axial electric stress. The nonlinear rheology of the entangled PEO solution is appropriately described by the eXtended Pom–Pom (XPP) model, a variant of the classical tube model. The analysis reveals that the instability in low conductivity fluid is driven by the capillary forces, whereas the high conductivity fluid exhibits oscillatory conducting mode of instability driven by the coupled effect of axial electric field and surface charges. For both the reference fluids, the increase in solvent evaporation tends to decrease the disturbance growth rate to the extent that the axisymmetric instability is completely suppressed for a strong enough evaporation. The stabilizing role of solvent evaporation is attributed to the enhancement in fluid viscosity and elasticity due to increased polymer concentration upon evaporation. Thus, by suppressing the onset of axisymmetric instability during the straight jet path, solvent evaporation potentially helps produce smooth fibers without bead formation.

Streamwise evolution of drag reduced turbulent boundary layer with polymer solutions

The effect of local polymer solution injection on the evolution of a flat-plate turbulent boundary layer has been investigated experimentally. Polyethylene oxide (PEO) solutions were injected through an inclined slot. The influence of polymer injection on boundary layer development downstream of the slot is assessed at three different concentrations (100 ppm, 500 ppm, and 1000 ppm) using planar velocity field and concentration measurements. A local drag reduction (DR) of up to 60% was obtained in the vicinity of the slot. A systematic change observed in the inverse of the von Kármán constant (k = 1/κ) with an increase in DR is used to define the sub-regimes of the high DR regime, and a linear relation between k and DR is shown to persist over a wide range of Reynolds numbers. The analysis of combined velocity and concentration measurements provides added insight into the associated changes in the boundary layer characteristics and the underlying flow physics.

Drag Reduction Performance of Mechanically Degraded Dilute Polyethylene Oxide Solutions

Abstract Mechanical degradation of dilute solutions of polyethylene oxide (PEO) via chain scission was investigated within a turbulent pipe flow. Comparisons of the drag reduction performance with and without degradation were made by matching the onset of drag reduction conditions, which has been shown for PEO to be related to the mean molecular weight. The bulk flow behavior of both the degraded and nondegraded samples were generally consistent with trends observed in the literature, but a subset of conditions showed significant deviation in the slope increment (drag reduction performance) between the degraded and nondegraded samples. When they deviated, the degraded samples were consistently more efficient than the nondegraded samples even though they had the same mean molecular weight. The deviations were shown to scale with the normalized difference between the initial and final molecular weights. The current data and analysis as well as the literature suggest that the deviations in the polymer performance (slope increment) are related to changes in the molecular weight distribution. More specifically, the improved performance of the degraded samples relative to the nondegraded ones at the mean molecular weight of the degraded sample indicates an excess of longer polymer chains since the higher chain fractions in a degraded solution more effectively control the flow properties when within a certain degree of degradation and Reynolds number.

Conformation and Diffusivity of Ring and Linear Polyethylene Oxide in Aqueous Solution: Molecular Topology Dependent Concentration Effects and Comparison with Experimental Data

AbstractAtomistic molecular dynamics simulations are performed on aqueous solutions of ring and linear polyethylene oxide (PEO) under good solvent conditions for the investigation of their conformation and diffusion dynamics. The systems simulated span a wide range of molecular weights (from 2 to 10 k) and polymer concentrations. In the limit of infinite dilution, both conformational and dynamic properties appear to be practically molecular topology independent as far as their dependence on chain length and concentration is concerned, and a very favorable agreement is observed with respect to relevant well‐established theoretical predictions, experimental data, and earlier simulation studies. Crossing over to the semidilute regime, molecular topology starts to bear a distinct effect on the observed conformations of the two different molecular geometries. On the other hand, the diffusion dynamics of PEO in the semidilute regime appears to be practically molecular topology independent judging from the dependence of the chain center‐of‐mass diffusivity on chain length and concentration. Indeed, and despite the fact that ring molecules exhibit consistently a faster dynamics than their linear homologs over the entire concentration range simulated, the estimated concentration scalings for both types of architecture are strikingly similar. The findings of this work are compared with scaling arguments and available experimental data.