Viscoelastic Liquid Research Articles

A phase transition approach to elucidate the propagation of shear waves in viscoelastic materials

In the field of acoustics, a medium has traditionally been considered a liquid if shear waves cannot propagate. For more complex liquids, such as those containing polymer chains or surfactant aggregates, this definition begins to be unclear. By adopting a rheological model-independent approach, this work investigated by means of dynamic elastography, the liquid–solid phase transitions in viscoelastic liquid media. When the storage shear modulus G′ dominated the loss shear modulus G″, a minimal shear wave attenuation frequency region was defined and the medium was considered solid. When G″ dominated G′, the shear waves were strongly attenuated and the medium was considered liquid. The investigated medium, an aqueous solution of xanthan gum, behaved as a bandpass filter with transition bands, showing liquid–solid–liquid behavior from low to high frequency. During these transitions bands, shear waves still propagated but highly attenuated. The limiting values where shear waves were no longer observed were identified as the low and high cutoff frequencies. Finally, the ability of various rheological models to predict the phase transition frequencies and describe the dispersion curves was tested. A three-element rheological model, the Jeffreys model, was required to accurately fit the experimental response of the medium at different concentrations over the entire frequency range. Shear wave propagation methods can overcome the technical limitations of traditional rheometry and explore higher frequencies, rarely investigated in viscoelastic liquids.

Weakly nonlinear focused ultrasound in viscoelastic media containing multiple bubbles

To facilitate practical medical applications such as cancer treatment utilizing focused ultrasound and bubbles, a mathematical model that can describe the soft viscoelasticity of human body, the nonlinear propagation of focused ultrasound, and the nonlinear oscillations of multiple bubbles is theoretically derived and numerically solved. The Zener viscoelastic model and Keller–Miksis bubble equation, which have been used for analyses of single or few bubbles in viscoelastic liquid, are used to model the liquid containing multiple bubbles. From the theoretical analysis based on the perturbation expansion with the multiple-scales method, the Khokhlov–Zabolotskaya–Kuznetsov (KZK) equation, which has been used as a mathematical model of weakly nonlinear propagation in single phase liquid, is extended to viscoelastic liquid containing multiple bubbles. The results show that liquid elasticity decreases the magnitudes of the nonlinearity, dissipation, and dispersion of ultrasound and increases the phase velocity of the ultrasound and linear natural frequency of the bubble oscillation. From the numerical calculation of resultant KZK equation, the spatial distribution of the liquid pressure fluctuation for the focused ultrasound is obtained for cases in which the liquid is water or liver tissue. In addition, frequency analysis is carried out using the fast Fourier transform, and the generation of higher harmonic components is compared for water and liver tissue. The elasticity suppresses the generation of higher harmonic components and promotes the remnant of the fundamental frequency components. This indicates that the elasticity of liquid suppresses shock wave formation in practical applications.

Role of slip in the stability of viscoelastic liquid flow through a channel

For a horizontal channel flow of a weakly viscoelastic liquid that follows the constitutive model of Walters’ liquid B′′, a linear stability analysis is performed when the channel walls are slippery. In this case, the viscoelastic channel flow is driven by the streamwise pressure gradient. We explore the primary instability for a symmetric slip flow with the same slip length of the channel walls, an asymmetric slip flow with different slip lengths of the channel walls, and an analogy to the Poiseuille–Couette flow of a viscoelastic liquid with a slip effect on the lower wall but no slip effect on the upper wall. We observe that the viscoelastic parameter shows a destabilizing impact, but the slip length shows a stabilizing impact on the most unstable shear mode in all three flow configurations. Moreover, the Poiseuille and Poiseuille–Couette flows for the viscoelastic liquid are linearly more unstable to infinitesimal disturbances than those of the Newtonian liquid. As a result, wall velocity needs to be higher than in the Newtonian liquid to stabilize the viscoelastic Poiseuille–Couette flow. Using the asymptotic analysis, we have determined the critical value of the slip length, or equivalently, the critical value of the lower wall velocity, above which the asymmetric slip flow and the Poiseuille–Couette flow of the viscoelastic liquid are permanently stable to infinitesimal disturbances.

Wave Dynamics of a Gas Bubble Covered with a Hyperelastic Shell in a Viscoelastic Liquid

Wave Dynamics of a Gas Bubble Covered with a Hyperelastic Shell in a Viscoelastic Liquid

Electro-wetting induced dynamic manipulation of symmetrically coalescing viscoelastic liquid bridges

Merging of isolated liquid drops is a common phenomenon that may greatly be influenced by adding polymeric contents to the liquid. Here, we bring out an exclusive control on the dynamics of the intermediate liquid bridge, thus, formed via exploiting the interactions of an exciting electric field with a trace amount of polymeric inclusions present in the intermingling drops. Our results unveil a unique competition of the elastic recovery and time-oscillatory forcing during the drop-unification at early times. However, damped oscillations as a specific signature of the polymer concentration feature eventual stabilization of the bridge at later instants of time. We rationalize these experimental findings in light of a simple unified theory that holds its critical implications in droplet manipulation in a wide variety of applications encompassing digital microfluidics, chemical processing, and biomedical analytics.

Effect of liquid elasticity on nonlinear pressure waves in a viscoelastic bubbly liquid

The importance of viscoelasticity of biological media that are used in medical ultrasounds has been discussed in the literature. Furthermore, the use of microbubbles in biological media drastically improves the efficiency of both diagnostic and therapeutic ultrasounds. Weakly nonlinear wave equations for ultrasound propagation in liquids containing microbubbles have long been studied, although the viscoelasticity of the liquid phase has been ignored for simplicity. In this study, we derived a nonlinear wave equation for ultrasound propagation in a viscoelastic liquid containing microbubbles by considering the effect of the elasticity of the liquid. Additionally, we evaluated how the elasticity of the liquid modifies the nonlinear, dissipation, and dispersion effects of the ultrasound in a few tissue models (i.e., liver, muscle, breast cancer, fat, and skin models and that without shear elasticity). The results revealed that liquid shear elasticity decreases the nonlinear and dissipation effects and increases the dispersion effect, and this tendency is more significantly observed in the breast cancer tissue compared with other tissues. Furthermore, we numerically solved the nonlinear wave equation and investigated the changes in ultrasonic wave evolution with and without shear elasticity.

Steady-state extensional rheology of a dilute suspension of spheres in a dilute polymer solution

It is well-known and intuitive that addition of a dilute volume concentration of spheres increases the viscosity of a Newtonian liquid. We find that a viscoelastic liquid undergoing uniaxial extensional flow at small $D\phantom{\rule{0}{0ex}}e$ (extension rate times the polymer relaxation time) experiences a larger increase in viscosity than the Newtonian case due to wakes of extra polymer stretch produced downstream of the spheres. However, when $D\phantom{\rule{0}{0ex}}e\ensuremath{\gtrsim}0.6$, the highly stretched polymer in the bulk fluid collapses near the particle surface and the suspension deforms more easily than the particle-free viscoelastic fluid.

Versatile Epoxidized Soybean Oil-Based Resin with Excellent Adhesion and Film-Forming Property

Deriving environmentally friendly and renewable polymers from epoxidized plant oils (EPOs) is one of the promising strategies for realizing the sustainable development of materials industry. Direct crosslinking EPOs without pre-modifications is a straightforward, efficient, and cost-effective way for the utilization of EPOs. Herein, a facile and green strategy that crosslinks epoxidized soybean oil with highly branched and flexible polyamine via ring-opening and amidation polymerizations successfully produced a novel resin with exceptional properties and versatile potential applications. After completely cured, the resin appeared to be super strong structural adhesive as it provided a room-temperature bonding strength of 12–24 MPa for metal and non-metal substrates, as well as a steel bonding strength of 11.6 MPa at cryogenic temperature (−196 °C), which is far superior to the previously reported oil-based adhesives. Meanwhile, the resin exhibited excellent film-forming ability. Particularly, by controlling the curing temperature, the films with tunable mechanical properties were obtained with a tensile strength of 3–40 MPa and an elongation at break of 17–230%. The features of uncured resin, including liquid viscoelasticity, good wettability, and high molecular weight, make it a qualified pressure-sensitive adhesive which demonstrated satisfied peel strength for various substrates.

Thermocapillary effects on absolute and convective instability of viscoelastic liquid jets

The effect of heat transmission on the absolute instability (AI) and convective instability (CI) of axisymmetrical disturbances in a viscoelastic liquid jet that falls under gravity is investigated. In general, when heat is included to the interface of a viscoelastic jet, it can be used to process droplet sizes and breakup lengths even more. We describe the jet’s dynamics mathematically using the Upper-Convected Maxwell (UCM) model. On the basis of the jet’s slenderness, an asymptotic approach is used to simplify the problem and obtain solutions of steady basic flow, which are then linearly analyzed for absolute and convective instability. When traveling wave modes are considered, a dispersion relation between the wavenumber and the growth rate of viscoelastic jets is derived, which can then be solved numerically using the Newton–Raphson method. The impact of varying some non-dimensional parameters is shown on absolute and convective instability. In this work, absolute instability is explored by employing a mapping technique called the cusp map method. For a variety of parameter regimes, the convective-to-absolute instability boundary (CAIB) is determined. We have found that absolute-to-convective transitions occur at lower critical Weber numbers when heating is included to the interface of the viscoelastic jets.

Stabilization of the flat Poiseuille-type flow for viscoelastic polymeric liquid

This paper presents a numerical study for the problem of the one-dimensional flow of viscoelastic liquid polymers between two parallel plates. The equations of a rheologically modified Vinogradov–Pokrovskii (mVP) model is used for the formulation of the problem. It is shown that the problem could have multiple steady-state solutions. The evaluation of non-steady solutions was performed to see if the time-dependent solutions got eventually attracted by the steady ones. Also for the case of multiple steady solutions, it was checked which one attracts the non-steady solution if any. The evaluation of time-dependent solutions was used to estimate the stability of equilibrium states. It is revealed that stable steady-state regimes of the problem exist under certain conditions, and also there could be no more than one stable regime for any given set of parameters. The calculations were performed to estimate the values of Reynolds and Weissenberg numbers corresponding to either stable or unstable steady regimes. The result indicates that instability of the steady flow could possibly occur for arbitrary low Reynolds numbers under certain balance of viscous and elastic forces.

Mathematical Modeling of the Wave Dynamics of an Encapsulated Perfluorocarbon Droplet in a Viscoelastic Liquid

A mathematical model has been developed and a numerical study of vapor bubble growth as a result of acoustic evaporation of an encapsulated perfluorocarbon droplet in a viscoelastic liquid is presented. The viscoelasticity of the droplet shell and the carrier liquid is taken into account according to the Kelvin–Voigt rheological model. The problem is reduced to solving a system of ordinary differential equations for the radius and temperature of the bubble, the radius of the droplet and the shell together with the thermal conductivity equation for the internal liquid. Spatial discretization of the thermal conductivity equation is carried out using an implicit finite difference scheme. ODEs are solved by the fifth order Runge–Kutta method with an adaptive computational step. To check the correctness of the numerical calculation in a particular case, the theory has been compared with known experimental data. The influence of the shear modulus of the shell and the carrier liquid, and the shell thickness on the radial dynamics of a vapor bubble inside an encapsulated droplet in an external viscoelastic liquid is demonstrated.

The Elasticity of Polymer Melts and Solutions in Shear and Extension Flows.

This review is devoted to understanding the role of elasticity in the main flow modes of polymeric viscoelastic liquids-shearing and extension. The flow through short capillaries is the central topic for discussing the input of elasticity to the effects, which are especially interesting for shear. An analysis of the experimental data made it possible to show that the energy losses in such flows are determined by the Deborah and Weissenberg numbers. These criteria are responsible for abnormally high entrance effects, as well as for mechanical losses in short capillaries. In addition, the Weissenberg number determines the threshold of the flow instability due to the liquid-to-solid transition. In extension, this criterion shows whether deformation takes place as flow or as elastic strain. However, the stability of a free jet in extension depends not only on the viscoelastic properties of a polymeric substance but also on the driving forces: gravity, surface tension, etc. An analysis of the influence of different force combinations on the shape of the stretched jet is presented. The concept of the role of elasticity in the deformation of polymeric liquids is crucial for any kind of polymer processing.

Unrelaxed tensile stress impacts on the instability of viscoelastic sheets with the gas velocity oscillation

A temporal instability analysis of a viscoelastic liquid sheet with unrelaxed tensile stresses in the oscillation gaseous flow field has been studied with the Floquet method in the present research. Effects of the oscillated amplitude and frequency of incoming gaseous flow on the instability features in the different instability regions are analyzed systematically. Results reveal that the unrelaxed tensile stress has a positive effect on stabilizing the liquid sheet, while the higher oscillation amplitude would promote the sheet breakup. Besides, it is indicated that when the stress tensor would be controlled by the viscoelastic effects, elastic impacts would suppress the breakup, while it shows an opposite impact when the stress tensor is dominated by the tensile stress. The existence of parametric instability regions is influenced by the oscillated and elastic parameters as well.

On the effects of fluid elasticity and gas holdup on Taylor bubble rising dynamics in viscoelastic media

In this work, the rise of Taylor bubbles in a vertical tube filled with viscoelastic media is investigated by means of volume-of-fluid-based direct numerical simulations. The rheological behaviors of the nonlinear viscoelastic liquids are described by the exponential Phan-Thien–Tanner constitutive model. The applicability of our simulations to capture the liquid film around a Taylor bubble has been validated by comparisons with numerical and experimental data in the literature. The effects of fluid elasticity [indicated by the Weissenberg (Wi) number] and gas holdup are mainly discussed in respect of Taylor bubble dynamics (e.g., rising velocity, flow field, stress field, liquid film, and so on). Our results show that the Taylor bubble steady rise velocity is logarithmically correlated with Wi and the slope is nearly constant at low Wi numbers. When the fluid elasticity is moderate, the viscoelastic stress in the liquid film is large, and the rising bubble is stretched to form a thin filament tail with a negative wake. Moreover, the fluid elasticity has negligible effects on the steady bubble rising velocities and the liquid film thickness for large Wi numbers. With increasing fluid elasticity and gas holdup, the long Taylor bubble fluctuates due to the surrounding liquid pulling deformation and gas pressure. The viscoelastic stress profiles exhibit a major non-monotonic dependence on the distance to the walls, which seems to be squeezed as the liquid film gets thinner under high elasticity. The dynamical Taylor bubble is found to be significantly affected by the surrounding fluid viscoelasticity and partially independent of the gas holdup, which will guide the design of microreactors in chemical applications.

Two layered immiscible flow of viscoelastic liquid in a vertical porous channel with Hall current, thermal radiation and chemical reaction

This investigation deals with the combined effects of Hall current, thermal radiation, heat source and chemical reaction on heat and mass characteristics in a vertical porous channel filled with a two layered viscoelastic liquid under the influence of oscillatory pressure gradient. The exact solution of the governing equations for the fluid velocity, temperature, and concentration are obtained by applying the regular perturbation method with appropriate boundary and interface conditions. The numerical values of the shear stress at the plates are shown in tabular form for various values of the relevant parameters, whereas those of the fluid velocity, temperature, concentration, and rate of heat transfer are displayed graphically. Our analysis indicates that with the raise of first-order chemical reaction and ratio of mass diffusion parameters, the temperature and concentration of the liquid raises. With the raise of Hall parameter the skin friction decreases at the walls of the channel. To validate the analytical results, a comparative study of the numerical evaluation of the closed-form results is performed with the results obtained by shooting technique along with the RK-fourth order method. Graphical results demonstrate that the imposed boundary and interface conditions are satisfied by the flow variables.

Transient MHD flow of fractional Burger's fluid model through a parallel microchannel with heat transfer and slip boundary condition

The combined influence of magnetic field, electro-osmotic flow (EOF), and pressure gradient on the unsteady magnetohydrodynamic fluid flow through a parallel microchannel is investigated. The Burger's liquid model is used for the fractional partial differential equation, which allows us to study the behavior of viscoelastic liquid velocity profile in the parallel microchannel. The Laplace transform (LT) in concert with the Fourier cosine transform are used to obtain the analytical solution of the velocity profile. Furthermore, using the method of separation of variables, the energy equation, Joule heating, energy dissipation, and electromagnetic effects are calculated to obtain the temperature within the microchannel. The influence of some relevant parameters like heat transfer, temperature distribution, Hartmann number (Ha), and Brinkman number (Br) on fluid flow velocity are presented graphically and discussed. The results obtained show that temperature decreases with Ha and Br of the transverse electric field for slip flow. Close to the middle of the microchannel, fluid flow velocity increases with decrease in the delay time parameter value and increase in the Burger's parameter value, while the opposite trend is found for the velocity near to the middle of the microchannel. In addition, Burger's liquid is quite general, such that Oldroyd-B, Maxwell, and Newtonian liquids are readily obtained as limiting cases. The study is significant in the application fields of chemical analysis and biological analysis, drug delivery, bacteria detection, and some others.

Extra-Soft Self-Healable Supramolecular Silicone Elastomers from Bis-Amide-PDMS Multiblock Copolymers Prepared by Aza-Michael Reaction

Silicone elastomers involving self-associating bis-amide groups were synthesized by aza-Michael addition between N,N′-methylenebis(acrylamide) and low molecular weight bis(3-aminopropyl)-terminated poly(dimethylsiloxane). In mild conditions that favor monoaddition (reaction of a primary amine) over diaddition (reaction of the resulting secondary amine), a library of non-crosslinked copolymers was prepared by tuning the vinyl-to-primary amine reactive function molar ratio. Almost no diadduct was observed for a reactive function molar ratio lower than 0.8, thus leading to linear polymers with predictable molecular weights. On the other hand, diaddition occurs at higher molar ratios such that polymers with some branches, high molecular weights, and large dispersity were synthesized. Regardless of the polymers considered, the amide groups develop some H-bond interactions as demonstrated by FTIR, leading to phase-separated domains, as revealed by DSC, which alters rheological and viscoelastic properties of the polymers. By increasing the molecular weight of the polymers, it was possible to turn the copolymers from viscoelastic liquids to viscoelastic solids and further to elastomers. Moreover, the latter material presents self-healing propensity at room temperature, which may be related to the diffusion of short chains, while the mechanical properties (strain and stress at break) are endowed by the long entangled linear-like chains, properties that are recovered after healing.

INTERFACIAL INSTABILITY ANALYSIS OF VISCOUS-VISCOELASTIC FLUID INTERFACE IN AN ANNULAR POROUS MEDIUM

The transport of mass and heat from one fluid to another fluid in a porous medium has numerous applications such as film boiling and heat exchangers. This paper reports the results of linear stability analysis of the capillary instability of viscoelastic liquid-viscous gas interface with heat and mass transfer. The viscoelastic liquid and viscous gas lie in an annular region enclosed by two rigid cylinders. The annular region is a saturated porous medium with constant porosity and permeability. The analysis is performed using the potential flow theory for viscoelastic liquid satisfying the Oldroyd-B model. A third-order polynomial in terms of growth rate is achieved and analyzed numerically. It is observed that porous media slows the growth of perturbations, but an increase in porosity makes the interface unstable. The interface goes towards stability when the transfer of heat-mass increases.

Graphene quantum dot doped viscoelastic lyotropic liquid crystal nanocolloids for antibacterial applications.

Graphene quantum dots (GQDs) are prepared and characterized via X-ray diffraction (XRD), UV-Visible spectroscopy, atomic force microscopy (AFM), transmission electron microscopy (TEM) and photoluminescence (PL). GQDs are doped (5 mg and 10 mg) in the lyotropic liquid crystalline (LLC) lamellar and hexagonal phases to prepare GQD/LLC nanocolloids. Polarizing optical microscopy and X-ray diffraction measurement reveals that GQDs do not affect the lamellar and hexagonal LLC structures and may organize on their interface. Pure LLC phases and nanocolloids are studied for steady and dynamic rheological behavior. LLC phases and GQD/LLC nanocolloids possess shear thinning and frequency dependent liquid viscoelastic behavior. A complex moduli study of LLCs and GQD/LLC nanocolloids is carried out which indicates the gel to viscous transition in LLCs and GQD/LLC nanocolloids as a function of frequency. LLC phases and GQD/LLC nanocolloids are tested for antibacterial activity against Listeria ivanovii. The effect of surfactant concentration, LLC phase geometry and GQD concentration has been studied and discussed. A probable mechanism for the strong antimicrobial activity of LLCs and GQD/LLC nanocolloids is presented considering intermolecular interactions. The viscoelastic behavior and strong antibacterial activity (inhibition zone 49.2 mm) of LLCs and GQD/LLC nanocolloids make them valuable candidates for lubrication, cleaning, cosmetics and pharmaceutical applications.

An On-Chip Viscoelasticity Sensor for Biological Fluids.

There are so many non-Newtonian fluids in our daily life, such as milk, blood, cytoplasm, and mucus, most of which are viscoelastic heterogeneous liquid containing cells, inorganic ion, metabolites, and hormones. In microfluidic microparticle-manipulating applications, the target particles are practically distributed within the biological fluids like blood and urine. The viscoelasticity of biological fluid is constantly ignored for simplicity especially when the fluid is substantially diluted and contains rather complex components. However, even the fluid's ultraweak viscoelasticity actually affects the microparticle migration and may bring a completely different behavior compared with the Newtonian fluids. As a result, a robust and easy operated on-chip viscoelasticity sensor is potential and desired in many research and industrial fields, including assay sample preparation, clinical diagnostics, and on-chip sensor. In this work, we employed stable non-Newtonian fluid-polyethylene oxide (PEO) solutions with various concentrations to investigate and calibrate effects of the weak fluidic viscoelasticity on microparticle behaviors in a double-layered microfluidic channel. An analogy-based database of fluidic patterns for viscoelasticity sensing and relaxation time measurement was established. Then, we tested different biological fluids including blood plasma and fetal bovine serum and proved that they exhibited similar viscoelasticity effects to the PEO solutions with the corresponding concentration, which reached a good agreement with available results by references. The detection limitation of relaxation time can reach 1 ms. It promised a robust and integrated on-chip microfluidic viscoelasticity sensor for different biological fluids without complicated calculations.