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The structural anisotropy and shear rheology of colloidal gels under startup of shear flow are calculated by Brownian dynamics simulations modified to include particle surface-mediated attractions, which is a model for thermoreversible colloidal gels. Shear-induced structural changes are analyzed in both real and reciprocal space through computation of the pair distribution function and structure factor. A distinct structural anisotropy is evident as alignment along the compressional and vorticity axes. A spherical harmonics expansion of pair distribution function is calculated to analyze structural anisotropy. In reciprocal space, structural anisotropy is quantified through an alignment factor, which shows overshoot behavior similar to the stresses. Based on the microstructure analysis, the evolution of the structural anisotropy is explained by the anisotropic rupture of the colloidal gel microstructure. This result provides evidence for how shearing creates structural anisotropy. The structural anisotropy during flow startup is quantitatively related to rheological behavior via a stress-small angle neutron scattering (SANS) rule that is modified to consider the influence of the rupture of the colloidal gel structure. A simple structure kinetic equation accounting for bond creation and rupture provides a unifying view of the results. This new, modified stress-SANS rule quantitatively links the shear-induced structural anisotropy to the nonlinear rheology. [ABSTRACT FROM AUTHOR]

The formation of Taylor cone jet is a key process in electrohydrodynamic printing that is used to produce high resolution patterns. Even though the inks are complex fluids composed of particles, binder, and solvent, most of previous researches have assumed the ink as a Newtonian fluid. In this study, we investigate the effect of viscoelasticity of the ink on Taylor cone jet using two model systems designed to control the elasticity and viscosity of the ink independently. The elasticity and viscosity improve the stability by expanding the operating windows for Taylor cone jet. The results can be summarized in terms of two parameters: elasticity parameter, ξ, and viscosity parameter, χ. The increase in elasticity widens the range of voltage for Taylor cone jet zone, while the range of flow rate remains independent of elasticity. The effect of elasticity is dominant for ξ > 1while it is nearly negligible for ξ < 1. When the viscosity is increased, the Taylor cone jet zone is widened mainly by the flow rate whenχ < 1, while the voltage stabilizes the Taylor cone jet for χ > 1. This study will contribute to the optimal design of EHD printing ink by providing the operating window maps in terms of material properties. [ABSTRACT FROM AUTHOR]

The microchannel flow of complex fluids has drawn wide attention due to possible applications to high end technologies. But its physics has rarely been understood due to the difficulties in getting the microstructural change of the complex fluids during the flow in confined complex geometries. To understand the characteristics of these complex flows, we studied the dynamics of aggregating particle suspensions in three dimensional 4:1 contraction flow using the self-consistent particle simulation method. It combines both Brownian dynamics and finite element methods self-consistently, which enables to consider long-range hydrodynamic interaction effectively. We observed heterogeneous cluster formation and analyzed it in terms of the competition between flow and interparticle forces. In addition, spatio-temporal fluctuation which originates from the microstructural change and its interaction with the flow was observed. The fluctuation was complex with no characteristic periodicity or pattern. Heterogeneous structure formation and spatio-temporal fluctuation were found to be the main factors that characterize the complex flow of the particulate suspensions in the micro-contraction channel flow. [ABSTRACT FROM AUTHOR]

The flow of viscoelastic fluids in a micro-fabricated 4:1 planar contraction channel was studied through flow visualization. As the Weissenberg number (Wi) increased, the flow developed from a Newtonian-like to vortex growth, and the transient start-up flow at high Wi was found to experience all the steady patterns at lower Wi flows. The flow sequence was different depending on the fluids and channel dimensions, however, in all the cases we could reach, the steady patterns at a low Wi flow could be matched 1:1 with the transient patterns at a high Wi flow. The plot of Wi and time when the two sets (transient and steady) were matched showed a clear functional relationship, from which the time-Weissenberg number superposition could be confirmed. [ABSTRACT FROM AUTHOR]

The first normal stress difference (N1) in large amplitude oscillatory shear (LAOS) consists of two contributions: one from non-oscillating nonzero mean value and the other from oscillating even harmonics, while the nonlinear shear stress can be expressed as a sum of odd harmonics. In this study, the two nonlinear contributions of N1 in entangled polymer solutions have been analyzed by systematical comparison with their shear stress counterparts (storage modulus and odd harmonics, respectively) as a function of strain amplitude and of Deborah number (De). The contributions of N1 were greater than those of the shear stress in the LAOS region. The nonzero mean value of N1 decreased with an increase in Deborah number, but conversely, the second harmonic of N1 increased. The relative intensity (the second harmonic over the fourth harmonic of N1) as an indicator of the nonlinearity of N1 decreased with the Deborah number. We also compared experimental results to model predictions from the eight-mode Giesekus constitutive equation. For the model, we obtained two sets of nonlinear parameters from the best fit of model predictions with both steady shear viscosity and the first normal stress difference coefficient. The model could not quantitatively predict the first normal stress difference behavior in LAOS even with these nonlinear parameter sets, which fit the steady data very well. This study shows that the contribution of the first normal stress difference in LAOS is as vital as that of shear stress, and it can be used as a good estimate of the performance of the constitutive equation. [ABSTRACT FROM AUTHOR]

The phase morphology, rheology, mechanical properties, and free volume of poly(trimethylene terephthalate)/polycarbonate (PTT/PC) blends have been investigated as a function of composition. The morphology indicated a two-phase structure, and the blends showed co continuous phase morphology between 30−60 wt % of poly carbonate. Due to the high viscosity of PC, PTT is more finely dispersed in the PC matrix than PC in the PTT matrix. The rheological measurements of the blends revealed that the complex viscosity increased with increase in PC content. Relatively low interfacial tension values of the blends determined using Palierne and Choi−Schowalter methods indicated that there is considerable interaction between the blend components (PTT and PC) due to the transesterification reactions. A random copolyester formed as a result of the transesterification acted as a compatibilizer in the initial stages of reactions which is the main factor for the change in miscibility. The phase morphology and the interfacial adhesion influence the mechanical properties such that addition of the PC phase decreases the tensile strength and Young’s modulus of the system. The free volume data from PALS results showed a slight positive deviation from the known linear additivity rule with increase in PC content, suggesting the blends are partially miscible. Our results show a good correlation among the phase morphology, rheology, mechanical, and free volume parameters. [ABSTRACT FROM AUTHOR]

We suggest a numerical method to describe particle dynamics at the fluid interface. We adopt a coupling strategy by combining the color gradient lattice Boltzmann method (CGLBM) and smoothed profile method (SPM). The proposed scheme correctly resolves the momentum transfer among the solid particles and fluid phases while effectively controlling the wetting condition. To validate the present algorithm (CGLBM-SPM), we perform several simulation tests like wetting a single solid particle and capillary interactions in two solid particles floating at the fluid interface. Simulation results show a good agreement with the analytical solutions available and look qualitatively reasonable. From these analyses, we conclude that the key features of the particle dynamics at the fluid interface are correctly resolved in our simulation method. In addition, we apply the present method for spinodal decomposition of a ternary mixture, which contains two-immiscible fluids with solid particles. By adding solid particles, fluid segregation is much suppressed than in the binary liquid mixture case. Furthermore, it has different morphology, such as with the jamming structure of the particles at the fluid interface, and captured images are similar to bicontinuous interfacially jammed emulsion gels in literature. From these results, we confirm the feasibility of the present method to describe soft matters; in particular, emulsion systems that contain solid particles at the interface. [ABSTRACT FROM AUTHOR]

The local shear stress of non-Brownian suspensions was investigated using the lattice Boltzmann method coupled with the smoothed profile method. Previous studies have only focused on the bulk rheology of complex fluids because the local rheology of complex fluids was not accessible due to technical limitations. In this study, the local shear stress of two-dimensional solid particle suspensions in Couette flow was investigated with the method of planes to correlate non-Newtonian fluid behavior with the structural evolution of concentrated particle suspensions. Shear thickening was successfully captured for highly concentrated suspensions at high particle Reynolds number, and both the local rheology and local structure of the suspensions were analyzed. It was also found that the linear correlation between the local particle stress and local particle volume fraction was dramatically reduced during shear thickening. These results clearly show how the change in local structure of suspensions influences the local and bulk rheology of the suspensions. [ABSTRACT FROM AUTHOR]

For melt-compounded poly(lactic acid) composites containing graphene nanoplatelets (GNPs) at low volume fractions, ΦGNP≤0.34 vol.%, this study focused on correlation between the composite properties and a large-scale structure of GNPs formed by a strong alternating current electric field (with the frequency and intensity of 60Hz and 1.75kV/mm, respectively). The electric field was applied to the composite in a rheometer through the parallel plate fixture, so that the parallel plates served as the rheological measurement fixture as well as the electrodes. Optical microscopy, transmission electron microscopy, and two-dimensional wide-angle X-ray diffraction measurements revealed that almost randomly oriented, thin stacks of GNPs in the as-fabricated composites were aligned by the electric field in the field direction and further organized into an extended chain (or column) of stacks. This chained-stack structure bridged two parallel plate electrodes when ΦGNP was above a threshold value, ΦGNP*≅0.17vol.%. Corresponding to this structural change, the liquidlike rheological response of the as-fabricated composites, characterized by the storage modulus G' almost proportional to ω2 at intermediate-to-low angular frequencies ω, became the solidlike response, G'~ω0 at low ω (for the composites having ΦGNP≥ΦGNP*), after application of the electric field. In contrast, the loss modulus G" was insensitive to the electric field and remained almost proportional to ω at low ω, suggesting that the chained-stack structure formed by the electric field was an elastic structure bridging the parallel plates. This bridge also served as an electrically conductive path between the plates so that an insulator-conductor transition occurred when the composites with ΦGNP≥ΦGNP* were subjected to the electric field. Quantitative analysis of the equilibrium modulus and conductivity of the composites suggested that the chained-stack structure formed by the electric field was not a rigid single slab but included junctions being much softer and less conductive compared to the body of GNP (each graphene nanoplate). Moreover, analysis of the ΦGNP dependence on the equilibrium modulus and static electrical conductance after the transition (ΦGNP≥0.17vol.%) suggested that an elastically inert secondary structure was formed by the GNP stacks remaining out of the chained-stack structure (primary structure). This secondary structure appeared to be in soft contact with the primary structure to provide an extra conducting path. [ABSTRACT FROM AUTHOR]

We report a type of colloidal gel, induced by a minute incremental addition of mutually attractive small particles (size ~12 mil) to a suspension of highly charged large particles (size ~500 nm). The gel's morphological behavior does not follow the typical power-law scaling for fractal clusters. Its unique scaling behavior has two distinct power-law indices, based on particle volume fraction. We show the unique scaling behavior arises when nonfractal networks of large particles are bridged by small-particle clusters, which occurs between a lower and upper critical boundary of small particle volume fraction. [ABSTRACT FROM AUTHOR]

Theeffect of sodium carboxymethyl cellulose (CMC) and styrenebutadiene rubber (SBR) on film formation was evaluated by analyzingboth drying stress and pore size distribution. We discovered thatthe SBR in a graphite/SBR slurry filled the voids among the graphiteparticles with the increase in SBR concentration. In addition, theCMC in the graphite/CMC slurry was adsorbed onto the graphite surface,which caused the graphite particles to coalesce. In the graphite/CMC/SBRslurry, a large amount of SBR was needed to increase the mechanicalstrength of the film in the low concentration region of CMC. However,in high concentration region of CMC, the SBR did not affect the mechanicalstrength of the film any longer. On the basis of the drying stressmeasurements, we could draw a processing window map that clearly showsthe effect of CMC and SBR on the mechanical strength of the film,which will be useful in the design of anode slurries. [ABSTRACT FROM AUTHOR]

Phenomenological studies on mechanical hemolysis in rotary blood pumps have provided empirical relationships that predict hemoglobin release as an exponential function of shear rate and time. However, these relations are not universally valid in all flow circumstances, particularly in small gap clearances. The experiments in this study were conducted at multiple operating points based on flow rate, impeller speed, and tip gap clearance. Fresh bovine red blood cells were resuspended in phosphate-buffered saline at about 30% hematocrit, and circulated for 30 min in a centrifugal blood pump with a variable tip gap, designed specifically for these studies. Blood damage indices were found to increase with increased impeller speed or decreased flow rate. The hemolysis index for 50-µm tip gap was found to be less than 200-µm gap, despite increased shear rate. This is explained by a cell screening effect that prevents cells from entering the smaller gap. It is suggested that these parameters should be reflected in the hemolysis model not only for the design, but for the practical use of rotary blood pumps, and that further investigation is needed to explore other possible factors contributing to hemolysis. [ABSTRACT FROM AUTHOR]

Nonlinear response of linear and branched polymers has been investigated under medium strain amplitude oscillatory shear (strain amplitude range from 10% to 100%) with Fourier-transform rheology. A power law relationship was found between the relative third intensity (I3/I1), which is an indicator of nonlinearity, and the strain amplitude at low and medium strain amplitudes. On a log-log plot, the intercept and slope of I3/I1 were investigated at different excitation frequencies and temperatures. Simulation results with three different constitutive equations [Giesekus, exponential Phan-Thien Tanner (E-PTT), pom-pom model] were also compared. Experimental results show that the intercept was affected by the excitation frequency and temperature, and the slope of I3/I1 for linear polymer remained constant regardless of molecular weight, molecular weight distribution, and excitation frequency in accordance with the predictions of the constitutive equations (Giesekus and E-PTT). It should be noted that the slope of I3/I1 for branched polymer was lower than that of linear polymer, unlike the prediction of the pom-pom model. Among the molecular architecture and processing parameters (e.g., molecular weight, molecular weight distribution, frequency, and temperature), the slope of I3/I1 under medium amplitude oscillatory shear was found to depend only on the long chain branching, which means that it can be used as a measure of the degree of branching. The failure of the pom-pom model in predicting the nonlinear shear behavior was also pointed out. [ABSTRACT FROM AUTHOR]

Silica nanoparticle (SiNP)-poly(vinyl alcohol) (PVOH) coating is an important material system in paper coating applications, where particle distribution critically affects coating performance. In the present study, the authors investigated a role of physicochemical interaction between SiNP surface and PVOH chain in SiNP distribution in the coating layer, with a comparison of the suspension at pH 3 (good interaction) and pH 10 (poor interaction) as PVOH concentration was varied. Rheological properties and sedimentation behavior of the suspensions showed the dispersion stability of SiNP at pH 3 was improved by the addition of PVOH, whereas it was independent of the PVOH concentration at pH 10. Scanning electron microscopy and small angle x-ray scattering intensity of dried coating layer showed the uniform and dense structure with homogeneous distribution of SiNPs at pH 3, where spatial arrangement of SiNPs depended on the addition of PVOH. However, non-uniform and porous structures with SiNP aggregates were observed at pH 10, where the spatial arrangement of SiNPs was independent to the addition of PVOH. The stress development during drying of the coating suggested that the mechanical property was related to the spatial arrangement of individual SiNPs at pH 3, whereas to the distribution of SiNPs aggregates at pH 10. [ABSTRACT FROM AUTHOR]

Current heterogeneous cell assembly techniques in coculture systems rely on irreversible cell layering or a cell separation membrane. However, the techniques possess major drawbacks of inefficiency in direct interactions of the assembled cell layers and cell separation following coculture, which hamper characterization and therapeutic applications of the cells following coculture. Here, we develop a reversible cell layering platform for assembly of heterogeneous cells that allows both active direct cell-cell interactions and facile cell separation. Anionic maleimide-chondroitin-sulfate is grafted onto the surface membrane of myogenic C2C12 cells and human mesenchymal stem cells (hMSCs) to modify the surface charge of the cells without cytotoxicity. A highly porous chitosan thin film is formed in situ interspacing between the heterogeneous cell layers via ionic cross-linking of cationic chitosan and anionic functionalized cells, forming compactly assembled double-layered cell constructs. The chitosan film enables layering of the cells, which allows active direct interactions between the cell layers, and facile delayering of the cells through simple treatment with mild shear stress. The developed platform promotes the myogenic commitment of hMSCs via direct contact with C2C12 cells, mimicking the interactions that trigger stem cell differentiation in vivo. Delivery of the myogenic committed cells to muscle-injured animal models shows evident muscle regeneration. [ABSTRACT FROM AUTHOR]

Dispersions of nanoparticles (NPs) in a polymer matrix are a key element to set various properties of solution-cast polymer nanocomposite films. While the dispersion state of NPs in nanocomposite films has been extensively studied over the past decades, the structural development during drying and the relation of liquid and solid structure still remains poorly understood. In the present work, we study how NP dispersions develop during drying in polymer nanocomposite films, particularly focusing on the role of particle-polymer interaction in the structural development. Aqueous dispersions of nanosilica and poly(vinyl alcohol) are used as model NP/polymer mixtures, where the particle-polymer interaction can systemically be varied via the pH. A novel vertical small-angle X-ray scattering technique enables us to directly observe the development of the NP's dispersion state during drying. At a high pH 10, where silica particles have a poor affinity to PVA, SAXS intensity evolution shows phase separation during drying, resulting in the formation of dense aggregates of bare particles in the solid film. On the other hand, at a low pH 3, where silica particles have a good affinity with PVA, the SAXS data indicate a gradual densification of the NPs while maintaining a constant interparticle distance, which is originating from adsorbed PVA. The resulting solid film after drying exhibits an improved dispersion of NPs. The evaluation of the interparticle interaction suggests that the adsorbed polymer plays generally a key role in the uniform distribution of NPs in solid films, as it sterically stabilizes NPs over short ranges during all drying stages whereas depletion attraction dominates at longer ranges. [ABSTRACT FROM AUTHOR]

The effect of annealing on the miscibility and phase behavior of Sorona {poly(trimethylene terephthalate), PTT} and bisphenol A polycarbonate (PC) blends was examined. These blends exhibited heterogeneous phase-separated morphology and two well-spaced glass transition temperatures (Tgs) indicating immiscibility. The Sorona/PC blends were thermally annealed at 260 °C for different times to induce various extents of transreactions between the two polymers. After annealing at high temperature the original two Tgs of blends were found to merge into one single Tg, exhibiting a homogeneous morphology. It is interesting to note that upon extended annealing the original semicrystalline morphology transformed into an amorphous nature. This is attributed to chemical transreactions between the PTT and PC chain segments as evidenced with FTIR, DSC, DMA, 1H NMR, and WAXS measurements. A new characteristic aryl C−O−C vibration band present at 1070 cm−1in the FTIR spectra of the annealed blends indicated the formation of an aromatic polyester structure due to the transreactions between PTT and PC. The sequence structures of the produced copolyesters were determined by a NMR triad analysis, which showed that the randomness increased with time of heating. WAXS analysis confirmed that the PTT/PC blends completely lost their crystallinity when annealed at 260 °C for a period of 120 min or longer, indicating the formation of fully random copolyesters. A random copolymer formed as a result of the transreactions between PTT and PC serves as a compatibilizer at the beginning, and upon extended annealing this became the main species of the system which is finally transformed to a homogeneous and amorphous phase. [ABSTRACT FROM AUTHOR]

Linear viscoelastic and dielectric measurements were conducted for a blend of polyisoprene (PI, M19.9× 103) and poly(4-tert-butyl styrene) (PtBS, M69.5× 103) with a PI/PtBS composition of 8/2 (w/w). In general, PI and PtBS exhibit the lower-critical-solution-temperature (LCST) type phase behavior. At temperatures examined, T≤ 70 °C, our PI/PtBS blend was in a statically homogeneous state. The PI chain has the so-called type-A dipoles parallel along the backbone, and its large-scale (global) motion activates prominent dielectric relaxation, while the PtBS chain has no type-A dipoles and its global motion is dielectrically inert. In fact, at angular frequencies between 101s-1and 105s-1and at T≤ 70 °C, the dielectric signal of the blend was exclusively attributed to the PI chains therein. The time−temperature superposition failed for the dielectric loss ‘ ‘ of the PI chains, despite the fact that the blend was statically homogeneous. This result suggested that the frictional environment for the global motion was not the same for all PI chains. Namely, the PtBS chains relaxed more slowly than PI (as revealed from comparison of Gand ‘ ‘ data) and their dynamic concentration fluctuation was frozen in the time scale of PI relaxation to give a spatially nonuniform frictional environment for the PI chains. The magnitude of this frictional nonuniformity changed with Tthereby leading to the failure of the time−temperature superposition for PI. In contrast, the superposition worked excellently for the viscoelastic modulus Gof the PtBS chains (obtained by subtracting the PI contribution from the blend modulus). This result suggested that the PI chains relaxing faster than PtBS erased the heterogeneity in the time scale of the PtBS relaxation to provide all PtBS chains with the same frictional environment thereby allowing this relaxation to obey the superposition. [ABSTRACT FROM AUTHOR]

We control the hydrophobicity of submicrometer silica spheres by modifying their surface with −CH3, −CHCH2, −(CH2)2CH3, −CH2(CH2)4CH2−, −C6H5, −(CH2)7CH3, and −(CH2)11CH3groups through a modified one-step process. The scanning electron microscopy (SEM), quasi-elastic light scattering (QELS), UV−visible spectra, nitrogen sorption, and water vapor adsorption methods are used to characterize the particles. The SEM micrographs of the particles demonstrate that the modified particles are uniformly spherical, monodisperse, and well-shaped with the particle size ranging from 130 to 149 nm depending on the modified organic groups. In aqueous solution, the particles modified with phenyl groups have an obvious UV absorption peak at around 210 nm, whereas the other modified particles and unmodified particles do not have any UV−visible absorption peaks. There exist obvious differences in the amount of water vapor adsorbed depending on the type of surface functional groups of the modified particles. Compared with the unmodified particles, the modified particles have a lower water vapor adsorption because of the improved hydrophobicity of the particle surface. As a potential application, we prepared polystyrene/SiO2nanocomposites by blending polystyrene with the synthesized particles. Water contact angle measurements show that the surface of the composite prepared with the modified particles are more hydrophobic. Confocal microcopy demonstrates that the particles are less agglomerated in the nanocomposite as the particles become more hydrophobic. These comprehensive experimental results demonstrate that the hydrophobicity of the particles can be easily controlled by surface modification with different organosilanes through a modified one-step process. [ABSTRACT FROM AUTHOR]