Zero-shear Viscosity Research Articles

Synergistic enhancement of poly(lactide) melt strength via copolymerization and multi-arm branching strategy

The lower melt strength of poly(lactide) (PLA) limits its broader applications. Here, a strategy combining copolymerization with multi-arm branching was propose to enhance the melt strength of PLA. Initially, stereoisomeric cyclic ester monomers (CEM) synthesized via zeolite catalysis were copolymerized into PLA chains. Subsequently, rheological testing revealed that the zero-shear viscosity (η0) of linear PLA increased by 467 % with only 1 mol% of CEM units. Our study further systematically explored the relationship between the side group structure and chirality of the comonomers and the rheological properties of the copolymers. CEMs with long-chain branched structures and opposite chirality had the best enhancement effect. In order to further enhance the melt strength, we successfully achieved alterations in polymer topology by employing trimethylolpropane as an initiator, corresponding three-arm copolymers achieve up to a 67-fold increase in η0 (1.0 kPa∙s to 68.1 kPa∙s). Tensile tests indicated that the mechanical properties of the copolymers were comparable to those of PLA, with a tensile strength of approximately 65 MPa. Additionally, due to the high melt strength, we successfully produced closed-cell PLA-based foam materials with uniform pore sizes. In summary, this study furnishes a feasible method for designing polymer materials possessing the desired melt strength.

Emulsification in nearly Newtonian and non-Newtonian media of wormlike micelles

Emulsification experiments with four silicone oils, having viscosities ranging from 0.01 to 30 Pa.s, were conducted in two types of media: nearly Newtonian polyvinyl alcohol (PVA) solutions and non-Newtonian mixtures induced by worm-like micelles in solutions of sodium laureth sulfate and cocoamidopropyl betaine (BS) with NaCl. The increased viscosity of BS solutions upon the addition of NaCl did not significantly affect the drop size in the formed emulsions. In contrast, the increased viscosity of solutions with higher PVA concentrations significantly reduced the drop sizes for all silicone oils. A theoretical expression predicting the maximum drop size in both types of media (nearly Newtonian and non-Newtonian) was derived and validated against experimental data. The expression accounts for shear-thinning behavior in both the aqueous and oil phases. Interfacial stress dominates the breakage of less viscous oils, while viscous stress inside the breaking drop plays a leading role for more viscous oils. The formation of emulsions with similar sizes in non-Newtonian solutions of BS with different NaCl concentrations was explained by their strongly shear-thinning behavior, which leads to nearly similar viscosity at high shear rates, despite their zero-shear viscosities differing by more than two orders of magnitude.

Computer-aided design of thermosetting benzoxazoles containing bis-endoalkynyl groups: Low melting points and high thermal stability

High-temperature-resistant polybenzoxazoles (PBOs) have recently become a prominent research area due to their potential applications in aviation and aerospace. However, achieving a balance between thermal stability and processability remains a significant challenge. In this study, a computer-aided method to develop PBOs with high thermal stability and processability is explored. First, thermoset benzoxazoles (CPBOs) are designed using a material genomic approach. Subsequently, their zero shear viscosity, temperature at 50 % thermal weight loss, dielectric constant and dielectric loss are predicted using a computer-aided method. Finally, two screened thermosetting benzoxazoles, CPBO-1 and CPBO-6, are synthesized and experimentally validated. The experiments indicate that their melting points are below 100 °C, with the lowest melt viscosities being 0.5 Pa•s and 1.5 Pa•s, respectively. The corresponding polymers, pCPBO-1 and pCPBO-6, feature high thermal stability. The 5 % weight loss temperature of pCPBO-1 in N2 is 618.9 °C, while the dielectric constant and dielectric loss are 3.1 and 0.0063, respectively. These are excellent values for thermosetting resins. This computer-aided screening method is more efficient and cost-effective compared to conventional trial-and-error methods.

Simultaneous effects of temperature and backbone length on static and dynamic properties of high-density polyethylene-1-butene copolymer melt: Equilibrium molecular dynamics approach

Abstract Temperature and chain length play significant roles in determining the physical properties of polymer melts. In the current computational research, a molecular dynamics (MD) approach was implemented to describe the static and dynamic properties of (1) high-density polyethylene-1-butene with 120 beads in backbone (PE120) and (2) entangled high-density polyethylene-1-butene with 600 beads in the backbone (PE600). The transferable potentials for phase equilibria force fields were used for CH2 beads in a defined initial condition. First, the equilibrium phase of the designed systems was reported with total energy and density convergency at various initial temperatures (T 0 = 450, 470, and 490 K). Also, gyration radius (R g) and end-to-end distance (R) were calculated for the static behavior description of the two PEs. Zero-shear viscosity (η 0), mean square displacement, and diffusion coefficient (D) were estimated to define the dynamic behavior of PE120 and PE600 systems. MD outputs predicted that 10 ns is sufficient for equilibrium phase detection inside polymeric samples. After equilibrium phase detection, R g converged to 14.97 and 17.35 Å in PE120 and PE600, respectively (T 0 = 450 K). Furthermore, MD outputs show that temperature variation can considerably affect the time evolution of the system. Numerically, the η 0 of PE120 and PE600 converged to 49 and 168 cp at 450 K. These results of η 0 parameter as a function of temperature are an important output of MD simulations. The results predicted that η 0 decreases to 24 and 44 cp for PE120 and PE600 samples with an increase in temperature from 450 to 490 K. With the creation of the entanglements network, D reached the highest value of 2 × 10−9 m2·s−1 among the designed polymeric systems. The results are in good consistency with experimental reports. It is expected that the result of this study can be used in designing improved polymeric systems for real applications.

Flow enhancement produced by a pulsatile flow of shear-thinning fluids in circular and concentric annular tubes

Although the analysis of the flow enhancement of non-Newtonian fluids produced by pulsatile flows through tubes is common in the literature, the case of Carreau fluid has not been analyzed, which is the aim of this work. This study determines the flow enhancement caused by the pulsatile fluid flow through (a) a circular tube and (b) a concentric annular tube. We show that the flow rate enhancement of the shear-thinning fluid is controlled by the Carreau number Cu, the Womersley number Wo, the fluid power-law index n, the ratio between the outer and inner radii κ, a parameter β that represents the ratio between the infinite and zero-shear viscosities, and the amplitude of the oscillatory signal ɛ. In both cases (a) and (b), a numerical solution of the start-up of the hydrodynamic is evaluated. With the aid of the velocity solution, the volumetric flow rate is determined under periodic conditions after the initial transient has vanished. Then, the fractional increase in the mean flow rate I due to the pulsatile pressure gradient is calculated. Furthermore, an asymptotic solution for small, intermediate, and very large values of the Carreau number is performed to provide physical insight into flow enhancement.

The possibility of the computer simulation-assisted IDDSI framework for the development of thickened brown rice paste

Increasing requests for thickened fluid food are demanded with population aging, while the limited information provided by the International Dysphagia Diet Standardisation Initiative (IDDSI) is insufficient for food development. Recently, the introduction of computer simulation seems to be able to overcome this dilemma. Here, a thickened fluid system (xanthan gum and konjac glucomannan, XG and KGM) at different ratios was kept at the same IDDSI level 3. An obvious synergy was observed in the ratio of 1:9 (XG: KGM) with high surface tension, zero-shear viscosity, firmness and cohesion, and thus was used to prepare the brown rice paste. From computer simulation, the brown rice pastes (0.3 % and 0.5 % thickener) splashed and that with higher thickener content resulted in more residue. The thickener content of 0.7 % provided enough viscosity and cohesion to avoid splash, and most of the bolus flowed consistently, showing the best sensory quality and swallowing properties.

Impact of polymer molecular weight and dispersity on the coalescence of polypropylene powders suitable for laser powder bed fusion

Understanding and controlling the fundamental processes governing the coalescence of polymers is vital to enable the design of polymeric materials for improved mechanical performance and quality of manufactured structures, including those fabricated via additive manufacturing techniques. Our group has utilized thermally induced phase separation (TIPS) to produce spherical and size controlled polypropylene (PP) powders from 12,000 (12k), 250,000 (250k), and 340,000 (340k) molecular weight (Mw) PP, including 50-50 wt% blends of 12k/250k,12k/340k, and 250k/340k, and a 33-33-33 wt% blend of 12k/250k/340k to investigate the impact of polymer Mw, and thus zero-shear viscosity, on particle coalescence and its implication in laser powder bed fusion. The particles exhibit similar size distributions with an average particle size, Dx(50), in the range of 58 μm–86 μm. The coalescence behavior of the powders, evaluated via hot-stage microscopy, show that adding 12k PP in the blend significantly alters the coalescence dynamics of the 250k and 340k PP, dramatically increasing their coalescence rate. The substantial drop in zero-shear viscosity with addition of 12k PP provides the driving force for the pronounced enhancement in coalescence. Strong agreement between experimental results and the Hopper model of coalescence is observed only if corrected for extensional flow, exemplifying the importance of extensional flow on the coalescence process. The results also indicate that the 12k PP in the blend does not surface segregate in the TIPS process, but is homogeneously distributed in the blend. More broadly, these results provide molecular-level insight into how control of powder molecular weight characteristics and viscosity can offer pathways to optimize the consolidation of particles in manufacturing processes, including laser powder bed fusion.

Nanocomposite formulations of titanium dioxide and algal biomass: A rheological characterization for topical cosmetics and dermal applications

This study investigates rheological properties of novel, algae-based nanocomposites developed for potential use in dermal applications. The purpose of this investigation is to develop a highly tunable, biocompatible cosmetic base formulation and demonstrate how rheology relates to spreadability in its application. It was hypothesized that Spirulina biomass could be used in conjunction with titania (TiO2) nanoparticles and crosslinking and binding agents of calcium and/or citric acid to form nanocomposite materials with highly tunable rheological characteristics in absence of chemical surfactants. In this study, Arthrospira platensis (Spirulina) biomass, titanium (IV) dioxide, calcium chloride, and citric acid are used to develop a formulation that is then tested for a wide range of rheological characteristics and sensory-related properties. The rheological tests include amplitude sweep, frequency sweep, viscosity flow curve, 3-interval strain oscillatory thixotropy, and creep – recovery. Additionally, yield stress, complex viscosity, zero-shear viscosity, and power law indices were determined. The sensory-related properties that are discussed include formulation pH, static load spreadability, and dynamic shearing spreadability. After completing the investigation, it was concluded that the surfactant-free formulations were highly tunable, in terms of viscosity, rigidity, and thixotropy.

Wormlike Micellar Glycerol Solutions Formed from a Double-Tailed Surfactant with Two Quaternary Ammonium Head Groups.

Herein, a quaternary ammonium surfactant with dual heads and tails, N1,N1,N1,N3,N3-pentamethyl-N3-(3-(2-tetradecylhexadecanamido)propyl)propane-1,3-diaminium dibromide, abbreviated as Di-C14-N2, was synthesized. For the first time, clear observation of aggregate structures formed by surfactants in pure glycerol systems was achieved using cryogenic transmission electron microscopy (cryo-TEM). The system's rheological properties were analyzed using both steady-state shear and oscillatory rheological measurements. The lubricating efficiency of the Di-C14-N2 glycerol solution was assessed for its tribological properties using a tribological wear tester, white light interferometer, and scanning electron microscope. In glycerol, Di-C14-N2 formed long wormlike micelles, which resulted in a glycerol solution with the zero-shear viscosity of 1013 Pa·s at 90 mM, which is the most viscous glycerol system up to now. The system displayed distinct rheological properties from the aqueous system, as evidenced by two intersections in the loss and storage moduli. The formed wormlike micelles in glycerol lead to a significant alteration in the viscoelasticity of the system, thus endowing the Di-C14-N2 glycerol solution with potential as an eco-friendly lubricant. The friction coefficient of the system was found to be 23% lower and the wear rate was 83% lower than that of pure glycerol after the addition of Di-C14-N2. This demonstrates that the addition of Di-C14-N2 greatly improves the frictional properties of pure glycerol. This study offers the possibility of directly observing the aggregate structures formed by surfactants in pure glycerol systems. It contributes to the exploration of the self-assembly behavior of surfactants in nonaqueous polar media, thereby aiding in a deeper understanding of the correlation between molecular structure, mesoscale structure, and macroscopic properties.

An alternative approach for characterization of the linear viscoelastic behavior of asphalt binders

This study aimed to develop an alternative approach to characterise the linear viscoelastic behaviour of asphalt binders. A rheogram model was firstly proposed based on the Carreau–Yasuda model; its accuracy was substantiated by high R 2 values. The dynamic shear modulus master curve model was then obtained using the relationship between the magnitude of complex viscosity and dynamic shear modulus and the two definitions of the time-temperature shift factor. Afterwards, the phase angle master curve model was derived according to the approximate Kramers–Kronig relation. The proposed approach was demonstrated to be capable of accurately characterising the linear viscoelastic behaviour of selected asphalt binders. In addition, the predicted zero shear viscosity (ZSV) could be potentially employed to evaluate the asphalt binder rutting resistance. Compared with the widely-used Christensen-Anderson-Marasteanu (CAM) model, the developed master curves models were better at constructing the phase angle master curves of selected asphalt binders.

Melt blending of commercial linear polyethylene with low-entangled ultra-high molecular weight polyethylene: From dispersion compatibility to viscoelastic scaling laws

This study investigates the dispersion and compatibility of low-entangled “dis-entangled” UHMWPE (dis-UH) in a high-density polyethylene (HDPE) matrix using solvent-free melt-blending conditions and compares it with entangled UHMWPE (eUH) in the same matrix. The findings reveal that dis-UH/HDPE exhibits a significantly lower viscosity ratio than eUH/HDPE (1 and 4, respectively), indicating a lower critical capillary number (Cacritical), thus enhanced dispersion and compatibility. Blends with varying dis-UH content up to 20 wt% show homogeneity, evidenced by DSC and SEM analysis, and demonstrate improved mechanical properties by 36 % in the maximum stress (σmax) and 39 % in Young's modulus (E). Linear viscoelasticity assessments reveal that higher dis-UH content slow the dynamics and increase the apparent weight average molecular weight (Mw), consistent with previous reports for linear entangled PE. The zero-shear viscosity (η0) scaling with Mw (η0∝Mn) is adjusted for high polydispersity, yielding a transitional point in the scaling exponent (n) from 3.6 to 3 at a reptation number of entanglement segments (Mr/Me) of ∼287, in line with theoretical predictions. To rationalize the success of the homogenization process, we propose a qualitative molecular picture inspired from the constraint release Rouse mechanism involved in the disorientation process of bi-disperse linear polymers. In the case of dis-UH/HDPE blends, with initially lower density of long-long entanglements within dis-UH, and the highest density of short-short entanglements within HDPE matrix, the formation of long-short entanglements between dis-UH and HDPE is facilitated, which results in successful homogenization process. In the contrary, the establishment of long-short entanglements in eUH/HDPE blends will require unwinding of the long-long entanglements, which holds a higher kinetic barrier compared to dis-UH/HDPE blends, leading to unsuccessful homogenization.

Effect of Dispersion Media Composition on the Structure and Rheology of the Short-Sidechain Perfluorosulfonic Acid Ionomer Dispersions

Perflourosulfonic acid (PFSA) ionomers are used in many applications including polymer-electrolyte membrane fuel cells and electrolysers owing to their many attractive properties such as excellent ion-conductivity, chemical resistance, mechanical properties, and thermal stability. In polymer-electrolyte membrane fuel cells and electrolysers, they are used as the membrane and as a component in the catalyst layers, to provide proton conductivity and serve as a binder for the catalyst particles, which are two key components of these devices. Understanding the structure and rheological properties of the ionomer dispersions is important for solution-processed fabrication of the membranes and the catalyst layers, and will enable better control their final structure/morphology, and improve the device performance. A water-alcohol solvent mixture is a common dispersion media for ionomer dispersions as well as the catalyst inks. While significant efforts exist on the structure of ionomer dispersions in water-alcohol solvent mixtures, their rheological properties, particularly the effect of solvent composition at non-dilute concentrations remain less explored. In this talk, the effect of water-alcohol (isopropanol) composition of the dispersion media on the rheological properties of ionomer dispersions will be presented. The results of small-angle x-ray scattering characterization of the ionomer dispersions will be also discussed. As a model ionomer, a short sidechain perfluorinated ionomer (PFSA), produced by 3M, was used. In dispersions with low ionomer concentrations, the zero-shear viscosity scaling with concentration was found to be similar for all alcohol fractions in the solvent mixture, following Fuoss’s law for polyelectrolytes. Whereas at higher concentrations, beyond the semi-dilute unentangled regime, their scaling was strongly dependent on alcohol fraction, where the scaling exponent increasing with increasing the alcohol fraction. Furthermore, the dispersions showed dramatic shear-thickening and strain-stiffening behaviors at higher alcohol fractions. The rheological observations suggest water-alcohol composition significantly alters the interactions between ionomer, and consequently their structure, and has strong implications in processing as well as on the morphology/structure of the membranes and the catalyst layers.

Extraction of basil seed gum: Optimization and functional properties

Basil seed gum (BSG), a natural vegetable gum, was extracted by ethanol precipitation method using basil seeds as raw material. Based on the single factor experiments, Box-Behnken response surface test was performed to optimize the extraction conditions, and the physicochemical and functional properties of the extracted basil seed gum, as well as its microstructure, were analyzed in detail. The results showed that the optimal extraction conditions of basil seed gum were as follows: liquid-to-feed ratio of 62:1, temperature of 50 °C, extraction time of 29.50 min, and pH = 8.1, with yield of 11.28 %. BSG was mainly composed of total sugars (93.09 %), uronic acid (18.83 %), proteins (2.77 %), and ash (4.35 %), and its monosaccharides included D-(+)-anhydrous glucose (58.26 %), D-galactose (21.40 %), D-mannose (11.96 %), D-(+)-galacturonic acid (7.82 %), D-arabinose (0.28 %), D-(+)-xylose (0.28 %), and L-rhamnose (0.002 %). Comparative analysis of the functional properties of basil seed gum and three commercial vegetable gums (linseed gum, caraway seed gum, guar gum) showed that basil seed gum had excellent water retention properties, and its water absorption and water holding properties were much better than those of three commercial gums. Rheological characterization showed that BSG was a pseudoplastic fluid with high zero-shear viscosity. In addition, the basil seed gum powder showed an irregular flaky fibrous structure under microscope and was a semi-crystalline polymer. In view of the high aqueous absorption and retention properties of basil seed gum, it has promising applications in food, cosmetic and medical devices.

Effect of molecular structure on the dynamics and viscoelasticity of vitrimers

Vitrimers are a class of dynamic polymer networks that perform bond-exchange reactions (BERs) under environmental stimuli while preserving network connectivity. The viscoelastic properties of vitrimers are not only dependent on the kinetics of BERs, but also the architectures of polymer matrix. Here, we investigate the influence of polymer architectures on the dynamics and linear viscoelasticity of vitrimers through varying the sticker (reactive bead) distribution in the precursor chain and the BERs energy barrier (Ub) by performing hybrid molecular dynamics-Monte Carlo simulations. Increasing Ub, vitrimers with block and gradient distributions exhibit considerably faster relaxations of chain ends, Rouse mode, and stress and lower zero-shear viscosity than the analogs with random and uniform distributions, in qualitative agreement with the predictions of the sticky Rouse model. In contrast, the distribution of stickers has a small influence on the relaxation of dynamic bonds regardless of the Ub value. By comparing the molecular structures of vitrimers with different distributions of stickers, we find that the molecular defects, especially the relatively longer dangling end in block and gradient distributions, impair the effect of dynamic bonds on the dynamics and viscoelasticity of vitrimers. This suggests that the dangling defects can be leveraged for further development of recyclable and healable materials with fast stress relaxation and improved flowability.

Effect of Salt Content on Solubilization of Hydrophobic Polymer by Wormlike Micelles of Ionic Surfactant

The effect of concentration of inorganic salt KCl on solubilization of hydrophobic polymer poly(4-vinylpyridine) (P4VP) in aqueous solutions of ionic surfactant potassium oleate was experimentally investigated. As was shown by rheology, in the range of concentrations from 1 to 5 wt.% of salt the polymer-free solutions of surfactant mainly contain linear wormlike micelles (WLMs). The formation of branched WLMs was observed in the solutions containing 5-9 wt.% of KCl. In the presence of higher salt content the phase separation occurred, indicating the formation of a highly branched saturated network of WLMs. Enhanced screening of the charged groups of potassium oleate by oppositely charged ions of KCl caused elongation and branching of micelles that reflected in zero-shear viscosity of the solutions passing through the maximum. In the phase diagram, the saturation concentration with P4VP of both linear and branched WLMs decreased with increasing salt content. In the case of linear WLMs, this could be explained by the constant number of embedded polymer chains per 1 WLM, while simultaneously decreasing number of WLMs in the system. As for the branched WLMs, it was caused by shortening of the linear parts of micelles due to the formation of branching points. For this reason, almost no P4VP was dissolved in the solution containing the highly branched saturated network.

Synthesis and investigation of acrylamide and 2-acrylamido-2-methylpropanesulfonic acid copolymer-modified graphene oxide nanomaterials with intensive viscosification capacity: An experimental and molecular dynamics study

The polymer-modified nanomaterials with spontaneously generated cross-linked network greatly improve the dispersion viscosity, which is crucial for extending the sweep range and enhancing oil recovery. Recently, graphene oxide (GO) nanosheets have attracted extensive attention due to their high chemical reactivity and specific surface. In this paper, the AA molecule, synthesized by acrylamide (AM) and 2-acrylamido-2-methypropanesulfonic acid (AMPS) copolymerization, modified GO nanosheets (GAA) material is firstly synthesized and characterized. With the same content of AMPS segments, GAAn systems consistently exhibit better dispersion stability than AAn systems, along with higher absolute zeta potential values. The comprehensive investigation on the rheological property of GAA system reveals that GAA15 with the highest AMPS content exhibits the largest zero-shear viscosity (409.27 mPa·s) and the longest relaxation time. Based on the molecular dynamic simulation approaches, the aggregation morphology and stability of AA and GAA molecules are evaluated via the configuration analysis and the statistics of hydrogen bond numbers. The abundant binding sites from GO segments of GAA promote additional hydrogen bonds between GAA molecules and water, resulting in a more stable configuration. Molecular surface potential analysis and the independent gradient model based on Hirshfeld partition (IGMH) methods are further used to visualize the non-bonded interaction existed in cross-linked networks, which also predict and validate non-bonding interaction sites among molecular segments. The simulation results indicate higher viscosity in the GAA system should be ascribed to the formation of dispersion-strengthened cross-linked structures (AM-GO plane and GO-GO stacking segments). In summary, the microscopic viscosity-increasing mechanism of novel GAA system is proposed from the perspective of intermolecular interactions, which lays a preliminary theoretical foundation for the application of GAA material in the oil industry.

Development of microstructure-rheological and electrical properties relationship in PS/POE/HNTs blend nanocomposites using machine learning

Halloysite nanotubes (HNTs) and polypropylene-grafted maleic anhydride (PP-g-MA) were studied for their effects in blends of polystyrene (PS) and polyolefin elastomer (POE). The method used to prepare PS/POE blends (90/10 and 80/20 wt/wt) containing 1, 3, and 5 phr HNTs with or without PP-g-MA (a compatibilizer) was melt blending. Structural and morphological studies using X-ray diffraction analysis (XRD), scanning electron microscopy assisted energy dispersive X-ray spectroscopy (SEM-EDS), and transmission electron microscopy (TEM) confirmed a matrix-droplet morphology and the sample containing compatibilizer has better microstructure than the other formulations. The presence of both PP-g-MA and HNTs together has been discovered to improve the viscoelastic properties of the solid, as evidenced by increased storage modulus and complex viscosity. A notable change occurred in the rheological behavior of the PS/POE blends containing 5 phr of PP-g-MA and HNTs. The dependence of zero-shear viscosity on HNTs loading (0–5 phr) was approximated as a polynomial curve by fitting the experimental data with the Carreau-Yasuda model. Computational fluid dynamics (CFD) simulations were also used to study the effects of HNTs loading on changes in fluid flow patterns and shear rates. The calculated effective viscosities at a given shear rate (0.05 1/s) were in qualitative agreement with the experimental results. Moreover, we utilized various machine-learning techniques to predict the complex viscosity of PS/POE blends and their nanocomposites. The results showed that Extreme Gradient Boosting (XGBoost) outperformed other predictive models based on evaluation metrics. Four-point probe measurements found that the samples containing 5 phr HNT had the lowest conductivities due to the presence of aggregated structures. However, the homogeneous distribution of HNT led to a sudden rise in conductivity in the samples containing 5 phr of PP-g-MA. Computer modeling results of samples with uniform and non-uniform HNT distributions showed that the conductivity decreased with HNT loading and decreased considerably compared to the uniform distribution.

Spreading of a viscoelastic drop on a solid substrate

We study the spreading of Newtonian viscous (aqueous glycerin solution) and viscoelastic (aqueous polymer solution) drops on solid substrates with different wettabilities. For drops of the same zero-shear viscosity, we find in the early stages of spreading that viscoelastic drops (i) spread faster and (ii) their contact radius shows a different power law vs time than Newtonian drops. We argue that the effect of viscoelasticity is only observable for experimental time scales of the order of or larger than the internal relaxation time of the viscoelastic polymer solution. We attribute this behaviour to the shear thinning of the viscoelastic polymer solution. When approaching the contact line, the shear rate increases and the steady-state viscosity of the viscoelastic drop is lower than that of the Newtonian drop. We support our experimental findings with a simple (first-order) perturbation model that qualitatively agrees with our findings.

Switchable microscale stress response of actin-vimentin composites emerges from scale-dependent interactions.

The mechanical properties of the mammalian cell regulate many cellular functions and are largely dictated by the cytoskeleton, a composite network of protein filaments, including actin, microtubules, and intermediate filaments. Interactions between these distinct filaments give rise to emergent mechanical properties that are difficult to generate synthetically, and recent studies have made great strides in advancing our understanding of the mechanical interplay between actin and microtubule filaments. While intermediate filaments play critical roles in the stress response of cells, their effect on the rheological properties of the composite cytoskeleton remains poorly understood. Here, we use optical tweezers microrheology to measure the linear viscoelastic properties and nonlinear stress response of composites of actin and vimentin with varying molar ratios of actin to vimentin. We reveal a surprising, nearly opposite effect of actin-vimentin network mechanics compared to single-component networks in the linear versus nonlinear regimes. Namely, the linear elastic plateau modulus and zero-shear viscosity are markedly reduced in composites compared to single-component networks of actin or vimentin, whereas the initial response force and stiffness are maximized in composites versus single-component networks in the nonlinear regime. While these emergent trends are indicative of distinct interactions between actin and vimentin, nonlinear stiffening and longtime stress response appear to both be dictated primarily by actin, at odds with previous bulk rheology studies. We demonstrate that these complex, scale-dependent effects arise from the varied contributions of network density, filament stiffness, non-specific interactions, and poroelasticity to the mechanical response at different spatiotemporal scales. Cells may harness this complex behavior to facilitate distinct stress responses at different scales and in response to different stimuli to allow for their hallmark multifunctionality.

UV aging assessment of asphalt binder: influence of duration, zinc oxide, and aging condition

ABSTRACT Ultraviolet radiation penetrates the atmosphere resulting in photo-oxidative aging of asphalt. The conventional long-term aging method, such as pressure-aging vessel, ages the asphalt considering thermo-oxidative principle. However, asphalt undergoes predominantly photo-oxidative aging (POA) in field. Giving due importance to UV aging method will result in resilient asphalt, which can negate the impact of climate change on asphalt pavements to a certain extent. In this study, asphalt was aged using UV rays, considering different aging durations; 7, 14, and 21 days, aging temperature (room temperature and 60°C), and modification with zinc oxide nanoparticles (ZnO) as experimental factors. Further, thin asphalt layers were also subjected to natural aging by exposing them to atmosphere to develop equivalencies between laboratory and natural aging. The rheological studies indicated significant aging occurred in the laboratory in the first 7 days, followed by a decreased rate. The naturally-aged specimens depicted significant cracks after 1-, 3-, and 6-months of aging, which was relatively less in specimens with ZnO indicating its anti-POA properties. The carbonyl index was reduced by 30–75% with ZnO compared to binders without ZnO. Zero shear viscosity (ZSV) indicated that one-month naturally-aged asphalt with ZnO resulted in similar ZSV as PAV-aged binder with ZnO.