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Brownian dynamics simulations of shear flow of a colloidal suspension bridged by telechelic polymers with “sticky” end groups.

A thorough study is made of the dependences on salt concentration and polymer chain lengths of the low-frequency plateau of coacervates of poly (diallyl dimethyl ammonium chloride), PDADMAC, and poly (sodium 4-styrenesulfonate), PSS. The reliability and reproducibility of these measurements are carefully checked by determining the frequency-dependent stress limits of the rheometer through the use of reference fluids and by repeat experiments with coacervates. Long-time frequency sweeps show that coacervates with less salt are more repeatable than those with higher salt. A low-frequency plateau reliably appears only below a critical salt concentration, and the magnitude of the plateau depends strongly on salt concentration and chain lengths of both polycation and polyanion. It is only present for the molecular weight of the polycation, PDADMAC, higher than 100 kDa, but the magnitude of the plateau is more strongly influenced by the chain length of the polyanion, PSS. Possible causes of the low-frequency plateau are discussed.

We examine linear and nonlinear shear and extensional rheological properties using a “micelle-slip-spring model” [T. Sato et al., J. Rheol. 64, 1045–1061 (2020)] that incorporates breakage and rejoining events into the slip-spring model originally developed by Likhtman [Macromolecules 38, 6128–6139 (2005)] for unbreakable polymers. We here employ the Fraenkel potential for main chain springs and slip-springs to address the effect of finite extensibility. Moreover, to improve extensional properties under a strong extensional flow, stress-induced micelle breakage (SIMB) is incorporated into the micelle-slip-spring model. Thus, this model is the first model that includes the entanglement constraint, Rouse modes, finite extensibility, breakage and rejoining events, and stress-induced micelle breakage. Computational expense currently limits the model to micellar solutions with moderate numbers of entanglements (≲7), but for such solutions, nearly quantitative agreement is attained for the start-up of the shearing flow. The model in the extensional flow cannot yet be tested owing to the lack of data for this entanglement level. The transient and steady shear properties predicted by the micelle-slip-spring model for a moderate shear rate region without significant chain stretch are fit well by the Giesekus model but not by the Phan–Thien/Tanner (PTT) model, which is consistent with the ability of the Giesekus model to match experimental shear data. The extensional viscosities obtained by the micelle-slip-spring model with SIMB show thickening followed by thinning, which is in qualitative agreement with experimental trends. Additionally, the extensional rheological properties of the micelle-slip-spring model with or without SIMB are poorly predicted by both the Giesekus and the PTT models using a single nonlinear parameter. Thus, future work should seek a constitutive model able to capture the behavior of the slip-spring model in shear and extensional flows and so provide an accurate, efficient model of micellar solution rheology.

We investigate the transition between the overdamped and underdamped regimes in Langevin dynamics simulations with significant conservative forces by comparing direct simulations with theories of Kramers, Mel'nikov and Meshkov (MM), and Larson and Lightfoot (LL). The need for clarification is made evident by noting that the most commonly cited theories of Kramers and MM do not apply in the overdamped limit to escape times from a Lennard-Jones (LJ) potential, because Kramers and MM do not account for the flatness of the LJ potential at the escape position, which allows for a region of nearly free Brownian diffusion near the escape position. While the little-known LL approach does consider an LJ potential, it does not properly consider the underdamped regime, and so a complete description is only achieved by combining the LL and MM results into a single general equation, which we validate for the first time by an explicit comparison with Langevin simulations.

Our recent continuum theory for stress-gradient-induced migration of polymers in confined solutions, including the depletion from the solid boundaries [Hajizadeh, E., and R. G. Larson, Soft Matter, 13, 5942–5949 (2017)], is applied to a two-dimensional rotational shearing flow in the gap between eccentric cylinders. Analytical results for the steady-state distribution of polymer dumbbells in the limit of dilute polymer solution c/c∗≪1 (c∗ is the chain overlap concentration) and in the absence of hydrodynamic interactions are obtained. The effects of eccentricity e and of three perturbation variables, namely, Weissenberg number Wi, gradient number Gd(which defines the level of polymer chain confinement), and Peclet number Pe on the polymer concentration pattern, are investigated. The stress-gradient-induced migration results in polymer migration toward the inner cylinder, while wall-depletion-induced migration results in near-zero polymer concentration close to flow boundaries, which couples to a stress-gradient-induced migration effect. In the presence of wall-depletion, we obtain first order concentration variation proportional to Wi. However, in the absence of wall-depletion, there is no first order contribution and, therefore, the lowest-order concentration variation is proportional to Wi2. An upper limit of Wi=1.6 exists, beyond which the numerical solution demands an excessive under-relaxation to converge. In addition, for a high degree of polymer chain confinement, i.e., for Gd greater than 0.5, the continuum theory fails to be accurate and mesoscopic simulations that track individual polymer molecules are needed.

Chociaż argumenty z projektu, mające przemawiać za istnieniem Boga, są czasem odrzucane, gdyż uznaje się je za apologetykę wykorzystującą argument z Boga w lukach wiedzy, to jednak powody ich kwestionowania, mające podstawę w historii nauki, filozofii, religii i pragmatyzmie, nie są aż tak silne, jak się to często insynuuje. Twierdzę, że korzystanie z wielorakich dostrzegalnych w świecie przyrody świadectw projektu oraz ich regularne uaktualnianie w celu uzgodnienia z nowymi ustaleniami, może stanowić rozsądne i przekonujące podejście do apologetyki.

The linear viscoelasticity of coacervates formed by oppositely charged polyelectrolytes in the salt solution is reviewed, with a focus on time-temperature, time-salt, time-pH, and time-hydration superpositions, and on fundamental relaxation mechanisms. A variety of polyelectrolyte pairs are covered, showing the frequent, but not universal, success of the time-salt superposition. Master curves in many cases are similar to those for neutral polymers, including Rouse and reptation theories. However, in some cases, solidlike, as opposed to fluidlike, response is observed at low frequencies, especially at low salt concentrations. Some coacervates seem to fit “sticky diffusion” theory reasonably well, wherein relaxation is controlled by the breakage rate of ion pairs; the dependence of the “sticker” lifetime on salt concentration has been explored but is not well understood as yet. It is also possible that local relaxation is not controlled by breakage of ion pairs but by cooperative, “glassy,” relaxation of monomers, salt ions, and water molecules. Compilation and comparison of different datasets and suggested formulas for rheological time constants are presented, and some suggestions are given for future directions.

We show that the average length ⟨ L ⟩ of threadlike micelles in surfactant solutions predicted by fitting results of a mesoscopic simulation, the “pointer algorithm,” to experimental G′(ω), G″(ω) data, is longer than, and more accurate than, that from a scaling law that equates ⟨ L ⟩ / l e to the modulus ratio G 0 / G m i n ′ ′. Here, G0 is the plateau modulus, G m i n ′ ′ is obtained at the local minimum in G″, and l e is the entanglement length. The accuracy of the pointer algorithm is supported by the agreement of its predictions with results from a recent application of the slip-spring simulation method to threadlike micelles. Improved fits of the pointer algorithm to the slip-spring results are obtained for weakly entangled micelles (with an average number of entanglements of Z

We develop a method for efficient prediction of linear and nonlinear rheology of polydisperse polymers by judicious selection of a small number of representative polymer molecules from the large ensemble of chains comprising the molecular weight and branching distribution. Specifically, we use a numerical inversion of the double reptation model to select five or six representative molecular species to optimally fit the linear rheology of commercial polymers and then use the regressed parameters to compute the nonlinear rheology via the Rolie-double-poly (RDP) model. The method is tested for several model systems, namely, two polydisperse linear polystyrene (PS) samples described in Shivokhin et al. [Polym. Eng. Sci. 56, 1012-1020 (2016)] and Münstedt [J. Rheol. 24, 847-867 (1980)], a commercial linear low density polyethylene (LLDPE), and a low density polyethylene (LDPE) whose rheological properties are measured in the current study. For the linear polymers including the PS samples and the LLDPE, the predictions of the "representative"RDP model of the start-up extensional rheology are comparable to those of the "full"RDP model based on all the species drawn from the gas permeation chromatography characterization. The method then successfully predicts the linear rheology of blends of LLDPE and LDPE using the same representative molecules found by fitting each of the pure polymers. Further fitting the extensional rheology of the LDPE requires using the "priorities"q i and stretch relaxation times τ s, i of the representative molecules as adjustable parameters, whose values are then held fixed when predicting the extensional rheology of blends of the LDPE with the LLDPE roughly as successfully as does branch-on-branch model. The reduction in the number of representative polymer species offers new opportunities for faster simulations of flowing polymers, as well as for the prediction of segmental orientation to be used in the modeling of flow-induced crystallization.

We perform united atom molecular dynamics (MD) simulations to quantify the role of flow-induced nematic order in polyethylene (PE) crystal nucleation. Blends of periodic stretched and nonperiodic u...

We extend the single-chain slip-spring model developed by Likhtman [Macromolecules 38, 6128 (2005)] to describe the dynamics and rheology of entangled polymers to wormlike micellar solutions by incorporating chain breakage and rejoining, which are the key additional dynamics present in wormlike micellar solutions. We show that the linear rheological properties obtained from this micelle slip-spring model are in good agreement with mesoscopic simulations using the “pointer algorithm” [W. Zou and R. G. Larson, J. Rheol. 58, 681 (2014)] and can be fit to experimental results after an adjustment to correct for the too-high flexibility of the micelles assumed in the slip-spring model. Finally, we use this model to predict the nonlinear rheological properties of entangled wormlike micelles, which are the first predictions that include the effects of entanglements, breakage and rejoining, Rouse modes, and stretch of bead-spring micellar chains with Hookean springs.

The time-dependent rates of growth of thickness of wax deposits, as well as the spatial distributions of wax components within the deposits, formed on a cold finger from stirred baths of n-dodecane...

The authors report the rheology and shear flow velocity profiles of pseudo-ternary “lamellar gel networks” commonly found in cosmetic emulsions such as creams, lotions, and hair conditioners, consisting of the cationic surfactant behentrimonium methosulfate, a fatty alcohol mixture of cetyl and stearyl alcohol, and water at surfactant/fatty alcohol mole fractions ranging from 0.1 to 0.6 and water mass fractions of 85%–97%. The high shear-thinning viscosity shows hysteresis in up- and down-ramps of the shear rate, with the gel “remembering” its highest shear for around 12 h before it heals and recovers its original viscosity. These materials exhibit rheological signatures characteristic of a soft glassy material (as described by the soft glassy rheology model) such as shear hysteresis, inhomogeneous velocity gradient, a solidlike creep response, power-law stress decay in step strain experiments, and high storage modulus relative to loss modulus over a wide range of frequencies, confirming its nearly elastic response for small deformations. Using particle image velocimetry (PIV) in steady shear experiments, a transition from a linear velocity profile to a pluglike profile was observed at shear rates above around 0.01 s−1. PIV also revealed macroscopic inhomogeneities including the development of fracture planes and shear bands, with rheology becoming gap-dependent and displaying some characteristics of solidlike friction.

A new type of model branched polymer, asymmetric H-shaped polybutadienes, consisting of central crossbars having various combinations of short and long arms attached to the ends of the crossbars, was synthesized using living anionic polymerization and chlorosilane linking chemistry. The linking agent 4-(dichloromethylsilyl)diphenylethylene provides selective reactivity to attach short or long arms on one side or both sides as desired. The samples were characterized thoroughly by size exclusion chromatography with light scattering detection (SEC-LS) and found to exhibit controlled molecular weights, as well as narrow polydispersity indices (PDIs of 1.01–1.06). Temperature gradient interaction chromatography, a method with far superior resolution as compared to SEC, also shows that these materials are well-defined, with minimal and identifiable impurities.

Direct ink writing is used to print multiple polydimethylsiloxane (PDMS) mixtures with fumed silica or as a two-part commercial liquid silicone rubber (LSR) mixed with polyethylene glycol (PEG) or as a two-part commercial vulcanizing (RTV) silicone. We correlate their printability into a hollow slump cone with rheological measurements, including (1) a shear rate up-ramp followed by (2) a down-ramp in the shear rate, (3) creep tests, and (4) large-amplitude oscillatory shear (LAOS) with increasing amplitude. The PDMS-fumed silica mixtures fail to print even at the highest fumed silica loading used (9 wt. %), while LSR-PEG with 4 or 6 wt. % PEG prints well, and one of the two RTV silicone components is printable, as is the mixture due in part to its rapid chemical curing. The large differences in printability of these materials do not correlate well with any single rheological test. They do correlate with a combination of a measure of material strength, given by either the yield stress σycr from creep tests or the “flow stress” σf at which G′ and G″ cross-over in LAOS, and of material recoverability given by the dynamic yield stress σy− in test 2. The latter is measured during a down-ramp in the shear rate after reaching a maximum shear rate of 1000 s−1, the highest shear rate in the print nozzle.

Using both coarse-grained (CG) and fine-grained (FG) simulations we show how strain hardening in polymeric glasses under uniaxial extension arises from highly stretched strands that form as the pol...

We blend newly synthesized nearly monodisperse four-arm star 1,4-polybutadienes with various well-entangled linear polymers, confirming the conclusions in Desai et al. [ Macromolecules201649 (13)49...

We combine the self-consistent field theory and the Derjaguin approximation to predict the polymer-induced colloidal interactions and the nonuniform distributions of the loops and bridges of telech...

We review thixotropy, its attributes, and accompanying rheological phenomena, such as yielding, hysteresis in shear-rate ramps, the influence of rest time and viscosity bifurcation, and the prevalence and importance of thixotropy in common fluids. While older work is reviewed in brief, the major emphasis is on recent developments, including nonmonotonic responses of stress to changes in strain rate, viscosity bifurcation, shear banding, and kinematic hardening. The major categories of phenomenological constitutive models are reviewed; these can include viscoelasticity and aging, plasticity, kinematic hardening, and thixotropy; and distinctions between these phenomena and thixotropy are discussed. A few available microstructural models are also reviewed, including population balance models and mesoscopic simulations. We end by highlighting important future work that is needed, including further development of microscopic models and their connection to phenomenological constitutive equations, detailed measurements of microstructures and flow fields with bands, and the investigation of flows other than simple shear.

Inertial migration of single neutrally buoyant prolate and oblate rigid spheroids in a plane Poiseuille flow at Reynolds number is studied using dissipative particle dynamics (DPD) simulations. The particles have aspect ratios ( AR p , ratio of the major axis L to the minor axis B) from unity to three for prolate and from unity to eight for oblate spheroids, and the ratio of the diameter of the major axis to the gap ( L / H ) ranges from 0.2 to around 0.4 for both types of the spheroids. The equilibrium positions move closer to the channel center as Re , AR p , and L / H increase. The previously observed effect of the maximum blockage ratio L / H on the equilibrium position is re-confirmed, while differences for oblate vs. prolate shape at fixed L / H , deemed negligible by previous studies, are found in our study to be significant for both the equilibrium position and the rotational behavior, at L / H ⩾ 0.3 and Re ⩾ 200 . The major axis of a prolate spheroid generally tumbles in the flow-gradient plane, and the dependence of its focusing position on L / H likely arises from interactions with the wall during tumbling. We observed that when Re is above around 300, with increasing AR p , an oblate spheroid changes from log-rolling with the minor axis in the vorticity direction ( AR p = 1.5 ), to inclined-rolling with the minor axis tilted at an angle with respect to the vorticity axis ( AR p = 3 ), to a mixed state of inclined-rolling and inclined-tumbling ( AR p = 5 ), and finally to an approximate steady state, where the minor axis remains in the flow-gradient plane and slowly fluctuates between 60 ° and 90 ° from the flow direction ( AR p = 8 ). The utility of the DPD methods for exploring inertial migration phenomena of particles of various shapes is thus demonstrated.

Hypothesis The free energies associated with adsorption/desorption of individual surfactants from micelles and the fusion/scission of long micelles can be used to estimate the rate constants for micellar kinetics as functions of surfactant and salt concentration. Experiments We compute the escape free energies △Gesc of surfactant from micelles and the scission free energies △Gsciss of long micelles from coarse-grained molecular dynamics simulations coupled with umbrella sampling, for micelles of both sodium dodecylsulfate (SDS) in sodium chloride (NaCl) and cetyltrimethylammonium chloride (CTAC) in sodium salicylate (NaSal). Findings For spherical micelles, △Gesc values have maxima at certain aggregation numbers, and at salt-to-surfactant molar concentration ratios R near unity, consistent with experiments. For cylindrical micelles, SDS/NaCl shows a minimum, and CTAC/NaSal a maximum in △Gesc, both at R ~ 0.7, while △Gsciss of CTAC micelles also peaks at around R ~ 0.7 and that of SDS micelles increases monotonically with R. We explain the non-monotonic dependence of escape and scission free energies on R by a combination of electrostatic screening and the decrease of micelle radius with increasing R. Transitions from predominantly spherical to cylindrical micelles, and between adsorption/desorption and fusion/scission kinetics with changing salt concentration can be inferred from the free energies for CTAC/NaSal.

Cationic surfactant mixed with fatty alcohol as cosurfactant in excess water can form stable emulsions, known as "lamellar gel networks," that contain extended and interconnected networks of swollen bilayers, including ones with in-plane liquidlike disorder (L

A new transport model is proposed for paraffin wax deposition onto a cold finger from flowing wax-containing oils. The model solves transient energy and mass balances simultaneously for a reversible first-order kinetic rate for precipitation of pseudo-single-component wax, and the effects of yield stress using a critical solid wax concentration to withstand flow-induced stress at the deposit-fluid interface, Cpi. The model can predict the time evolution of the deposit thickness, and the spatial and temporal evolution of temperature and wax concentration and was validated using experiments involving a cylindrical cold finger. We found that for oils with Cpi close to zero, the deposit thickness growth is dominated by heat transfer. However, mass transfer cannot be neglected as diffusion of wax into the deposit continues to take place even after the deposit has stopped growing. For oils with non-zero Cpi, the deposit growth is slow and accompanied by occasional sloughing.

Overcharging in complex coacervation, in which a polyelectrolyte complex coacervate (PEC) initially containing equal moles of the cationic and anionic monomers absorbs a large excess of one type of polyelectrolyte species, is predicted using a recently developed thermodynamic model describing complexation through a combination of reversible ion binding on the chains and long-range electrostatic correlations. We show that overcharging is favored roughly equally by the translational entropy of released counterions and the binding entropy of polyelectrolytes in the polyelectrolyte complex, thus helping resolve competing explanations for overcharging in the literature. We find that the extent of overcharging is non-monotonic in the concentration of added salt and increases with both strength of ion-pairing between polyions and chain hydrophobicity. The predicted extent of overcharging of the PEC is directly compared with that of multilayers made of poly(diallyldimethylammonium), PDADMA, and poly(styrene-sulfonate), PSS, overcompensated by the polycation in two different salts: KBr and NaCl. Accounting for the specificity of salt ion interactions with the polyelectrolytes, we find good qualitative agreement between theory and experiment.

We compute desorption rates for isolated polymers adsorbed to a solid wall with a rare event sampling technique called multilevel splitting, also known as forward flux sampling. We interpret computed rates with theories based on the conjecture that the product tdesDRg2 of the desorption time tdes and diffusivity D divided by squared radius of gyration Rg scales with exp(h/Rg) where h is the equilibrium ratio of adsorbed surface concentration of polymer &Gamma, to bulk concentration of polymer c. As the polymer&ndash, wall interaction energy is increased, the slope of lntdesDRg2 vs. NVMFkBT nearly approaches unity, as expected for strongly-adsorbing chains, where N is the degree of polymerization and VMF is the height-averaged monomer&ndash, wall interaction energy for a strongly adsorbed chain. However, we also find that this scaling law is only accurate when adsorption strength per monomer exceeds a threshold value on the order of 0.3&ndash, 0.5 kBT for a freely jointed chain without or with excluded volume effects. Below the critical value, we observe that tdesDRg2 becomes nearly constant with N, so that tdes&prop, N&alpha, with &alpha, &asymp, 2. This suggests a crossover from &ldquo, strong&rdquo, detachment-controlled to a &ldquo, weak&rdquo, diffusion-controlled desorption rate as VMF/kBT drops below some threshold. These results may partially explain experimental data, that in some cases show &ldquo, exponential dependence of desorption time on chain length, while in others a &ldquo, power-law dependence is found. However, in the &ldquo, adsorption case, our results suggest much longer desorption times than those measured, while the reverse is true in the weak adsorption limit. We discuss possible reasons for these discrepancies.

We determine experimentally the “dilution exponent” α for entangled polymers from the scaling of terminal crossover frequency with entanglement density from the linear rheology of three 1,4-polybutadiene star polymers that are blended with low-molecular-weight, unentangled linear 1,4-polybutadiene at various star volume fractions, ϕs. Assuming that the rheology of monodisperse stars depends solely on the plateau modulus GN(ϕs) ∝ ϕs1+α, the number of entanglements per chain Me(ϕs) ∝ ϕs–α, and the tube-segment frictional Rouse time τe(ϕs) ∝ ϕs–2α, we show that only an α = 1 scaling superposes the Me(ϕs) dependence of the terminal crossover frequency ωx,t of the blends with those of pure stars, not α = 4/3. This is the first determination of α for star polymers that does not rely on any particular tube model implementation. We also show that a generalized tube model, the “Hierarchical model”, using the “Das” parameter set with α = 1 reasonably predicts the rheological data of the melts and blends featured in...

The influence of a poly(vinyl alcohol) (PVA) molecule on the air/water interfacial structure of adsorbed sodium lauryl ether sulfate with one polyethylene oxide group (SLE1S) was investigated using molecular dynamics simulations. Increasing SLE1S surface density increases the interface thickness and forces both SLE1S and PVA molecules to migrate towards the air (actually vacuum) to different degrees. While most surfactants, when dilute, align parallel with the interface, increasing surfactant surface density forces tail groups gradually to align more perpendicularly to the interface. The three types of oxygen atoms in the SLE1S head group, i.e. the ionic, ester and ether oxygen atoms, form three distinct hydration shells, with PVA monomers appearing both within these shells and between the second and third shell due to its own hydrogen bonding with water and excluded volume of surfactant and water. The ionic oxygen atoms form more hydrogen bonds with water molecules and PVA than do the ester and ether oxygen atoms. The addition of PVA decreases slightly the number of hydrogen bonds between the surfactant oxygen atoms and the water molecules, as PVA chains form new hydrogen bonds with both of them. With increasing surfactant concentration, the PVA is pushed away from the bulk water, decreasing the number of the hydrogen bonds between it and water. These results are relevant to the use of polymers to aid surfactant in aqueous foaming applications.

The traditional Doi–Edwards tube model, applied to extensional flows at strain rates above the inverse Rouse time, predicts that the tube deforms affinely, which implies that the extensional stress reaches its plateau as soon as the chain has become locally fully stretched, even if the chain is still folded, and far from being completely unraveled. By starting from a state in which the chain is in a locally fully stretched, but folded, state, we develop an “entangled kink dynamics algorithm” that predicts the final unraveling of an ensemble of mutually entangled, folded chains, driven by a combination of drag forces and chain tension, with negligible Brownian motion. Equations for motions of both entangled folds and unentangled folds in which two chains hook together at a single fold point are derived and solved, including the effects of constraint release that occurs when the end of one chain passes through the fold at which that chain is entangled. This model predicts that the stress approaches its fina...

We study the rheological responses and shear-rate inhomogeneities and shear banding behaviors of a thixotropic fumed silica suspension in shear startup tests and flow reversal tests. We find that this suspension under transient shear exhibits not only viscoelasticity, yielding, kinematic hardening, and thixotropy, but also time-dependent shear inhomogeneities including bands when the apparent shear rate is below a critical value between 0.1 and 0.25 s-1. Through multiple shear startup tests and flow reversal tests, we find that thixotropy promotes flow heterogeneity while kinematic hardening suppresses it. We propose a simple thixo-plastic constitutive equation that can qualitatively predict the important features of the rheological response and banding dynamics in shear startup tests and flow reversal tests.

We combine mechanical rheometry, diffusing wave spectroscopy (DWS), and small angle neutron scattering (SANS) with a simulation model, the "pointer algorithm", to obtain characteristic lengths and time constants for wormlike micelle (WLM) solutions over a range of salt concentrations encompassing the transition from unentangled to entangled solutions. The solutions contain sodium lauryl ethylene glycol sulfate (SLE1S), cocamidopropyl betaine (CAPB), and NaCl. The pointer algorithm is extended to include relaxation of unentangled micelles, allowing micelle parameters to be extracted from the rheology of partially entangled solutions. DWS provides the data at high frequency needed to determine micelle persistence length accurately. From pointer algorithm fits to rheology, we observe a salt-induced rapid change in micellar length as the solution enters the well-entangled regime and a weaker growth with surfactant concentration consistent with mean-field theory. At a lower surfactant concentration, micelle length and persistence length from SANS are roughly consistent with values from rheology once the lower surfactant concentration used in SANS is accounted for. This is, to our knowledge, the first time that quantitative comparisons of structural features including micelle length are made between rheology and SANS. Finally, scaling laws for micelle diffusion and recombination times indicate that micelle kinetics are reaction controlled leading to mean-field recombination with surrounding micelles over the entire range of concentration of interest except at very low and very high surfactant concentrations where either short micelles or branched micelle clusters are dominant.

We infer the swelling/de-swelling behavior of weakly ionizable poly(acrylic acid) (PAA) brushes of 2-39 kDa molar mass in the presence of KCl concentrations from 0.1-1000 mM, pH = 3, 7, and 9, and grafting densities σ = 0.12-2.15 chains per nm2 using a Quartz Crystal Microbalance with Dissipation (QCM-D), confirming and extending the work of Wu et al. to multiple chain lengths. At pH 7 and 9 (above the pKa ∼ 5), the brush initially swells at low KCl ionic strength (10 mM) in the "osmotic brush" regime, and de-swells at higher salt concentrations, in the "salted brush" regime, and is relatively unaffected at pH 3, below the pKa, as expected. At pH 7, at low and moderate grafting densities, our results in the high-salt "salted brush" regime (Cs10 mM salt) agree with the predicted scaling H ∼ Nσ+1/3Cs-1/3 of brush height H, while in the low-salt "osmotic brush" regime (Cs10 mM salt), we find H ∼ Nσ+1/3Cs+0.28-0.38, whose dependence on Cs agrees with scaling theory for this regime, but the dependence on σ strongly disagrees with it. The predicted linearity in the degree of polymerization N is confirmed. The new results partially confirm scaling theory and clarify where improved theories and additional data are needed.

Motivated by our experimental observations of nanofibre formation via the centrifugal spinning process, we develop a string model to study the behaviours of a Newtonian, viscous curved jet, in a non-orthogonal curvilinear coordinate system including both air-drag effects and solvent evaporation for the first time. In centrifugal spinning a polymeric solution emerges from the nozzle of a spinneret rotating at high speeds around its axis of symmetry and thins as it moves away from the nozzle exit until it finally lands on the collector. Except for the Newtonian fluid assumption, our model includes the key parameters of the curved jet flow, e.g. viscous, inertial, rotational, surface tension, gravitational, mass diffusion within the jet, mass diffusion into air and aerodynamic effects, via Rossby ( ), Reynolds ( ), Weber ( ), Froude ( ), Peclet ( ), air Reynolds ( ) and air Peclet ( ) numbers, and the collector radial position ( ). Our results, including comparison to experiments, reveal that the aerodynamic effects must be considered to enable a correct prediction of the jet trajectory and radius. Decreasing not only renders the jet thinning much faster, but also forces the jet to wrap tighter around the rotation axis. Increasing , and leads to a longer jet. Decreasing causes the jet to wrap tighter around the spinneret but it shows trivial effects on the solvent evaporation. Changes in and do not significantly affect the jet trajectory. Finally, we propose simple relations to estimate the jet radius and the jet breakup length.

We use an analytical mean-field theory and all-atom molecular dynamics (MD) simulations to predict that external tension, together with the nematic coupling interactions, can drive phase separation of long chains from short ones in bidisperse homopolymer melts. The nematic coupling parameter α for polyethylene (PE) oligomers under applied tension is extracted from the MD simulations and used in the mean-field free energy to predict the phase boundary for bidisperse melts in which the longer chains are stretched by uniaxial tension. The predicted phase diagram is validated by direct MD simulations. We also show that extensional flow, and possibly even shear flow, may lead to nematic phase separation in molten PE oligomers, because the flow can impose a stronger tension on the longer chains than the short ones., Using theories and simulations, we show nematic interactions and unequal tension imposed by flow or other fields on chains of different lengths can drive phase separation in homopolymer melts.

We compare the predictions of the Vasquez–Cook–McKinley (VCM) [P. a. Vasquez et al., J. Non-Newton. Fluid. Mech. 144, 122–139 (2007)] model, which treats wormlike micelles as Hookean dumbbells that break at half-length to form two shorter dumbbells, to an analogous Brownian dynamics (BD) simulation of the same physical model. We find a discrepancy between VCM model predictions and trace it to the absence in the VCM model of the spatial position of the nascent breakage point in the long micelle, which is needed to satisfy microscopic reversibility of breakage and fusion. We incorporate microscopic reversibility in the VCM model by extending an ensemble-averaged bead-spring phase space model of Wiest et al. [J. Chem. Phys. 90, 587 (1989)] to include reversible scission of two-spring chains. The revision tracks the conformations of the two halves of the long micelle and transmits this information to the short micelles upon breakage and thereby recovers complete agreement with the BD results. A general method is offered for retaining this information in populations of micelles of many different lengths, represented by an ensemble of many-spring chains. The new model also offers the prospect of including additional physics, such as flow-enhanced micelle growth or breakage, and the ability to check the accuracy of closure approximations using parallel BD simulations.We compare the predictions of the Vasquez–Cook–McKinley (VCM) [P. a. Vasquez et al., J. Non-Newton. Fluid. Mech. 144, 122–139 (2007)] model, which treats wormlike micelles as Hookean dumbbells that break at half-length to form two shorter dumbbells, to an analogous Brownian dynamics (BD) simulation of the same physical model. We find a discrepancy between VCM model predictions and trace it to the absence in the VCM model of the spatial position of the nascent breakage point in the long micelle, which is needed to satisfy microscopic reversibility of breakage and fusion. We incorporate microscopic reversibility in the VCM model by extending an ensemble-averaged bead-spring phase space model of Wiest et al. [J. Chem. Phys. 90, 587 (1989)] to include reversible scission of two-spring chains. The revision tracks the conformations of the two halves of the long micelle and transmits this information to the short micelles upon breakage and thereby recovers complete agreement with the BD results. A general method...

We use coarse-grained (CG) molecular dynamics simulations to determine the effect of uniaxial strain on the stress, scission stress, and scission energy of solutions of wormlike micelles of cetyltrimethylammonium chloride/sodium salicylate (NaSal). We find that the breaking stress, stretch modulus, and scission energy of the charged micelles are nonmonotonic functions of oppositely charged hydrotrope (NaSal) concentration. While the stretch modulus shows a peak at a value of surfactant-to-hydrotrope concentration ratio ( R) close to unity as expected due to neutralization of head-group charge at R = 1, the breaking stress and scission energy produce a peak at R < 1.0 because of thinning of the micelle diameter with increased R. The breaking stress from the simulations depends on the rate of deformation and roughly agrees with the experimental values of Rothstein ( J. Rheol. 2003 , 47 , 1227 ) after extrapolation to the much lower experimental rates. The method and results can be used to predict the effects of flow and mechanical stress on rates of micellar breakage, which is important in the rheology of wormlike micellar solutions.