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To predict the complex rheology and shear-rate inhomogeneities of lamellar gel networks, we propose a simple thixotropic constitutive model with an elastoplastic stress and a smoothly decreasing modulus near a solid boundary, motivated by the analysis of the effect of confinement and shear flow on lamellae orientation near surfaces. We show that the model qualitatively captures the important features of the lamellar gel shear rheology observed in experiments [Datta et al., J. Rheol. 64(4), 851–862 (2020)]. These include thixotropic shear thinning that is intermediate between constant viscosity and constant stress, a power-law slow creep under small constant shear stress and abrupt transition to fast creep at higher stress, as well as partial recovery of strain upon stress removal. In addition, the model correctly predicts a gap-dependent rheology and roughly predicts the amplitude dependence of storage and loss moduli in oscillatory tests despite having only a single thixotropic time constant. Most importantly, the introduction of the modulus gradient enables the model to predict the unique shear-banding phenomenon of lamellar gel networks wherein a thin, fast-shearing band exists near the boundary that widens only slowly with increased apparent shear rate until a very high rate is reached, while the bulk moves as a plug [Datta et al., J. Rheol. 64(4), 851–862 (2020)]. We discuss the influence of a lower modulus near the boundary and its possible origin in the underlying lamellar structure of the material. [ABSTRACT FROM AUTHOR]

Using PYS, TraPPE, OPLS-L, and Flexible-Williams (FW) force field models, atomistic simulations at temperatures ranging from 450 K to 600 K are performed to predict the melt density ρ, the persistence length Np, the nematic coupling constant α, and crystallization dynamics for pentacontane (C50). The coupling constant α arises from packing entropy of rodlike Kuhn segments and increases with increasing ρ and Np. Together with a self-consistent field theory, Np and α are then used to predict the isotropic-to-nematic (IN) transition temperature for polyethylene (PE) oligomers as a function of chain length. The nematic phase is found to be metastable since the IN transition temperature lies below the crystal melting temperatures for C50 in simulations using different force fields. Finally, isothermal simulations of crystallization for PE C50 oligomers and C1000 polymers show that crystal nucleation may be much accelerated by quenching below the IN transition temperature, where chains in the isotropic state first rapidly form nematic ordered domains, within which crystalline order then grows. We also find that the PYS, TraPPE, and FW models overpredict the melting temperature for C50 by around 50 K, while the most flexible OPLS-L model gives a melting temperature within around 10 K of the experimental value. Although giving a more accurate melting temperature, the slow crystallization kinetics of the OPLS-L model may limit its application in direct simulations of PE crystallization. [ABSTRACT FROM AUTHOR]

We develop a general framework for transport of polyions, solvent and salt, with intended application to Layer-by-Layer (LbL) assembly of polyelectrolyte monolayers (PEMs). The formulation for the first time includes electrostatics, chemical potential gradients, and mechanical stress gradients as driving forces for mass transport. The general model allows all species to be mobile throughout the process and avoids the assumptions of stepwise instantaneous equilibrium and/or immobilized structures typical of previous approaches, while reducing to these models in appropriate limits. A simple constitutive equation is derived for a mixture of oppositely charged polyelectrolytes that accounts for network strand dilution and cross-chain ion pairing by appending reactive terms to the Smoluchowski probability diffusion equation for network strand end-to-end vectors. The resulting general framework encompasses the Poisson equation describing the electrostatic potential distribution, an osmotic pressure balance, a stress constitutive equation, and a generalized flux law of polymer transport. The computational domain is split into a PEM phase and an external solution phase with an appropriate boundary condition derived for the interface between the two. The mobile species (water and small salt ions) are taken to be in a state of dynamic equilibrium with their distributions enslaved to the perturbations in the two polyion compositions. The proposed model captures the swelling response of PEM films to external solutions. For the first time, we studied the effects of the temporal evolution of electrostatic and stress distribution on the rate of chain loss and absorption during rinsing and dipping of an idealized and arbitrarily selected and rigid brush layer into external solutions. The temporal evolution provides a kinetic basis for the ability of LbL films to grow under conditions that thermodynamics alone suggests would cause them to be washed away and to account for partial desorption during washing. The proposed transport framework constitutes a solid basis for eventual quantitative modeling of LbL assembly and transport in polyion networks more generally. [ABSTRACT FROM AUTHOR]

A mesoscopic simulation method, the "pointer algorithm," is here shown to capture accurately the rheology of a variety of surfactant solutions, including CTAB/NaNO3, CPyCl/NaSal, and CTAB/NaSal, at different salt and surfactant concentrations presented in the literature. In addition, correlations derived from this method are shown to allow the average micelle length ⟨ L ⟩ to be estimated from G min ′ G min ″ = 0.317 ⟨ L ⟩ l e 0.82 where G min ′ / G min ″ is the ratio of storage to loss modulus at the frequency where G″ exhibits a local minimum, and l e is the entanglement length, which can be estimated from the modulus. We also obtain from the pointer algorithm a formula whereby the micelle breakage time (τbr) can be estimated from the longest relaxation time ( τ R ). The pointer algorithm's predictions are also shown to match those of a more microscopic slip-spring model, which had been previously validated by comparison to polymer rheological data. Thus, the work provides both a method and example estimates of these parameters as functions of surfactant and salt concentration, filling a major gap in characterization of these solutions. Finally, we investigate the determination of the micelle persistence length from high-frequency data. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Using fully atomistic simulations, we find that the structure of the critical urea crystal nucleus (monoclinic, four molecules per unit cell) in an aqueous solution differs from the known crystal structure of bulk urea (orthorhombic, two molecules per unit cell). Following a frequently used "seeding technique" combined with the classical nucleation theory, we also find that at room temperature the ~ critical nucleus is very large (containing 530 molecules) and the nucleation rate is very slow (5 × 10-24 cm-3 s-1), suggesting that the homogeneous nucleation of urea is improbable at room temperature. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

To understand the origin of transmembrane potentials, formation of transient pores, and the movement of anions and cations across lipid membranes, we have performed systematic atomistic molecular dynamics simulations of palmitoyl-oleoyl-phosphatidylcholine (POPC) lipids. A double bilayer setup was employed and different transmembrane potentials were generated by varying the anion (Cl-) and cation (Na+) concentrations in the two water compartments. A transmembrane potential of ∼350 mV was thereby generated per bilayer for a unit charge imbalance. For transmembrane potential differences of up to ∼1.4 V, the bilayers were stable, over the time scale of the simulations (10–50 ns). At larger imposed potential differences, one of the two bilayers breaks down through formation of a water pore, leading to both anion and cation translocations through the pore. The anions typically have a short residence time inside the pore, while the cations show a wider range of residence times depending on whether they bind to a lipid molecule or not. Over the time scale of the simulations, we do not observe the discharge of the entire potential difference, nor do we observe pore closing, although we observe that the size of the pore decreases as more ions translocate. We also observed a rare lipid flip-flop, in which a lipid molecule translocated from one bilayer leaflet to the opposite leaflet, assisted by the water pore. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

To meet the challenge of efficient modeling of film blowing with realistic constitutive equations for commercial thermoplastic melts, we present a multistage optimization modeling framework that integrates polymerization reaction modeling, rheology modeling, and bubble-shape prediction. A direct link is thereby created between the polymer architecture and the bubble shape of low-density polyethylene (LDPE) through a three-stage modeling protocol. Stage 1 aims to get complete polymer structure information from a limited set of linear and nonlinear rheological data and the measured averaged molecular weight. An optimization loop uses the Tobita algorithm for polymer reaction and the BoB model for rheology to minimize the deviation between experimental data and model predictions. Stage 2 is designed to obtain a representative reduced ensemble of LDPE in the Rolie-double-poly (RDP) model to reduce the computational cost of rheology calculations during processing. The parameters of the reduced molecular components are obtained by fitting the RDP model to a wide range of rheology data predicted by the BoB model applied to the full ensemble of polymer architectures obtained in stage 1. In stage 3, the reduced-ensemble RDP model is coupled to measured temperature profiles using time–temperature superposition, and the bubble shape and strain rate history of a fluid particle in the bubble are obtained by minimizing error in the momentum balance equations. We show that each stage of the process yields successful fitting, and at the end, we obtain an a priori prediction of height-dependent bubble radius and velocity in agreement with experiment. With this multistage optimization strategy, we link the polymer compositions to the bubble properties during the film blowing of LDPE. [ABSTRACT FROM AUTHOR]

We employ Brownian dynamics simulations combined with forward flux sampling and theoretical first-passage time analyses to describe the rates of transitions between loops and bridges of telechelic polymers between solid surfaces representing e.g. latex colloids in the limit that the telechelic stickers bind strongly enough to the surfaces to make free chains very rare. It is shown that the bridge formation rate can be expressed by combining times for two processes, namely, the escape of one end sticker from the narrow-but-deep association well near the colloidal surface and the longer-range motion of the chain end to the other surface inhibited by stretching free energy. We find that when using multibead chains to represent the telechelic polymers, the longer-range motion requires use of a multidimensional first passage time analysis that we borrow from the work of Likhtman and co-workers, which was originally developed to describe polymer end fluctuations in a one-dimensional reptation tube. From these ingredients, we develop analytical expressions for the loop-to-bridge and bridge-to-loop transition rates as functions of the number of beads per polymer, the ratio of gap to the equilibrium chain length, and the end sticker association energy to the colloids/surfaces. We also suggest that a 20-to-1 mapping of Kuhn steps to springs may allow the analysis to be applied to real chains, rendering the analysis applicable to a broad range of industrial and biological processes. [ABSTRACT FROM AUTHOR]

We build a Brownian dynamics simulation model to study the linear rheology of latex particles interacting with telechelic polymers (for example, so-called “HEUR” polymers), with the latter modeled as Finitely Extensible Nonlinear Elastic (FENE) dumbbells with ends that stick to Brownian colloidal spheres. We identify from this model four relaxation modes, including (1) a single chain relaxation time, τ′FENE, which is closely related to τFENE, the relaxation time of a free chain predicted using the FENE model, but is around 1.4 times longer because of surface constraints on polymer configuration; (2) a loop translational-rotational relaxation time, τloop, produced by migration of the chain over the particle surface, allowing it to further relax its orientational conformation while still trapped on the particle surface; (3) a bridge formation/breakage relaxation time, τbridge; and (4) one or more colloidal particle cluster relaxation times. We show that τloop scales as the square of particle radius. τbridge is the same as the lifetime of a bridge and increases exponentially with the adsorption energy of a telechelic hydrophobic sticker to the latex particle surface. The one or more long particle cluster relaxation modes are influenced by the diffusion coefficient of the latex particles, the polymer concentration, and the volume fraction of latex particles. These multiple relaxation times help explain the observed rheological behavior of water-borne coatings and differ from the rheology of reversible networks composed of only telechelic HEUR chains, which frequently show single-relaxation-time behavior. [ABSTRACT FROM AUTHOR]

We use all-atom molecular dynamics simulations to extract ΔGeff, the free energy of binding of potassium ions K+ to the partially charged polyelectrolyte poly(acrylic acid), or PAA, in dilute regimes. Upon increasing the charge fraction of PAA, the chains adopt more extended conformations, and simultaneously, potassium ions bind more strongly (i.e., with more negative ΔGeff) to the highly charged chains to relieve electrostatic repulsions between charged monomers along the chains. We compare the simulation results with the predictions of a model that describes potassium binding to PAA chains as a reversible reaction whose binding free energy (ΔGeff) is adjusted from its intrinsic value (ΔG) by electrostatic correlations, captured by a random phase approximation. The bare or intrinsic binding free energy ΔG, which is an input in the model, depends on the binding species and is obtained from the radial distribution function of K+ around the charged monomer of a singly charged, short PAA chain in dilute solutions. We find that the model yields semi-quantitative predictions for ΔGeff and the degree of potassium binding to PAA chains, α, as a function of PAA charge fraction without using fitting parameters. [ABSTRACT FROM AUTHOR]

Into an extended Voorn-Overbeek (EVO) free energy model of polyelectrolyte (PE) complexation and phase behavior, we incorporate three classes of short-ranged electrostatic effects, namely counterion association-dissociation, cross-chain ion pairing (IP), and charge regulation by treating each as a reversible chemical reaction leading to a corresponding law of mass action in a self-consistent fashion. The importance of each reaction is controlled by a corresponding chemistry-dependent standard free energy input parameter. Our model also accounts for Born (or ion solvation) energy using a linear mixing rule for the effective dielectric constant. In monophasic systems, the proposed model can qualitatively explain the observed shifts in acidity and basicity observed in potentiometric titration of weak PEs in the presence of salt and oppositely charged PEs in accordance with Le Châtelier's principle. We demonstrate how a competition between counterion condensation (CC) and IP alone can explain the complex coacervation of strongly charged PEs as well as the existence of a critical salt concentration. Binodal diagrams predicted in our model are also affected by long-ranged electrostatics and are most sensitive to IP strength both for weak and strong PEs. The extent of IP increases in the dense phase at the expense of reduced CC upon coacervation consistent with counter release view of complex coacervation. We compare binodal diagrams predicted by our model against experimental data for both weakly and strongly dissociating polyions pairs and find a plausible parameter set that leads to an acceptable and partial agreement with experiments in the two cases, respectively. [ABSTRACT FROM AUTHOR]

Architectural coatings/paints play a very important role in our world, making our houses and workplaces beautiful, comfortable, and energy-efficient. Polymeric components of coating formulations include latex or binder, dispersants, and thickeners or rheology modifiers [RM]). This review is devoted to the RMs, primarily associative thickeners [AT], specifically highlighting the relationship between the thickener molecular structure, paint formulation, rheology, and application. We briefly describe the chemistries of various associative thickeners ("hydrophobic alkali-swellable emulsion" [HASE], hydrophobically modified hydroxyethylcellulose [HMHEC], and "hydrophobically modified ethoxylated urethane" [HEUR]) polymers. Concentrating on the HEUR thickeners, we then review the structure and rheology of their aqueous solutions and of model paints containing HEURs and latex particles in water. For model paints, we review experimental studies (rheology, NMR, and SANS) and theoretical models from molecular-level classical Molecular Dynamics [MD] to mesoscale coarse-grained MD to large-scale rheology calculations. Lastly, influence of rheological metrics on specific paint application features (flow and leveling, sag resistance, and brush drug) is discussed. We conclude by highlighting opportunities and challenges for future research. [Display omitted] [ABSTRACT FROM AUTHOR]

Weperform coarse-grained molecular dynamics simulations for sodiumdodecyl sulfate (SDS) surfactant using a modification of the Dry Martiniforce field (Arnarez et al. 2014) with implicit water. After inclusionof particle mesh Ewald (PME) electrostatics, an artificially highdielectric constant for water (εr= 150), and reparameterization,we obtain structural and thermodynamic properties of SDS micellesthat are close to those obtained from the standard Martini force fieldwith explicit water, which in turn match those of atomistic simulations.The gains in computational efficiency obtained by removing explicitwater allow direct simulations of the self-assembly of SDS in solution.We observe surfactant exchange among micelles and micelle fissionand fusion and obtain realistic, equilibrated micelle size distributionsat modest computational cost, as well as a transition to cylindricalmicelles at high surfactant concentration or with added salt. We furtherapply this parametrized force field to study the adsorption of SDSonto hydrophobic surfaces and calculate the adsorption kinetics andequilibrium adsorption isotherm. The greatly increased speed of computationof surfactant self-assembly made possible by this Dry Martini methodshould allow future simulation of competitive adsorption of multiplesurfactant species to surfaces, as well as simulation of micellarshape transitions. [ABSTRACT FROM AUTHOR]

We study polymer migration in a periodic pressure-driven sudden contraction-expansion flow with contraction dimension comparable to the polymer radius of gyration, for which several polymer migration mechanisms can be important: (1) sieving by the thin channel of polymers too large to easily enter them; (2) deformation-hydrodynamic coupling, including wall-hydrodynamic interaction, which causes polymers to drift away from the walls towards the center of the channel; (3) streamline-curvature-induced migration, in which polymers traveling along curved streamlines migrate towards the center of curvature; and (4) depletion-convection coupling, in which depletion layers in thin channels are convected across wide side chambers, creating a one-sided diffusion barrier that leads to depletion from the side chamber. We use both Stochastic Rotation Dynamics (SRD), which includes hydrodynamic interaction (HI), and simple Brownian dynamics (BD), with HI omitted and flow field given by finite element analysis. The similarity in results from SRD and BD at Weissenberg number Wi less than 10 (where Wi is based on the shear rate in the narrow region of the contraction channel) shows that HI (Mechanism 2) has only a weak effect on polymer migration in our tight geometry. At Wi > 1, the polymer migrates towards the centerline in the wide region, due mainly to streamline-curvature-induced (SCI) migration (Mechanism 3), but also to depletion-convection-induced migration (Mechanism 4). And we demonstrate these two mechanisms more explicitly in a pressure-driven flow in a grooved channel that is significantly wider than the polymer. SCI migration dominates in the contraction geometry, and produces a migration velocity proportional to Wi². Using the central limit theorem, we accurately predict the position and width of a band of polymer passing through N periodic contractions, thereby demonstrating the potential for SCI migration as a mechanism of size separation in a multi-step planar contraction channel. We find that the best separation is achieved at Wi around 2, where SCI migration has the greatest resolving power between polymers of different size. We also find that sieving (Mechanism 1) is dominant at low Wi less than unity, where the chains with large radius of gyration are delayed in their entry to the thin channel, relative to shorter polymers. This sieving separation mechanism differs from that of size-exclusion chromatography which yields faster migration by the shorter chains. Our strategy of combining simulation methods with the central limit theorem could also be used to predict separation efficiencies of a wide variety of polymers and colloids in microfluidic geometries. [ABSTRACT FROM AUTHOR]

We present a fast "pointer" simulation method that extends the model of Cates and coworkers for the rheology of entangled wormlike micelles. Our method includes not only reptation, breakage/rejoining, contour length fluctuations, and Rouse modes, which were included in Cates' model, but also constraint release, bending modes, and a cross-over to the tight entanglement regime, which had not been previously considered. Our method also contains correlations in micelle length across multiple breakage/rejoining cycles, not included in previous approaches. Our method uses "pointers" that track the ends of unrelaxed regions along each micelle, thereby allowing efficient simulations of relaxation dynamics for ensembles containing thousands of micelles, to obtain accurate results without preaveraging or neglecting correlations. A modified genetic algorithm is applied to transform the simulation data from the time to the frequency domain. The method can span several regimes of behavior depending on the relative rates of reptation, contour length fluctuations, breakage/rejoining, and high frequency modes and is suitable for predicting the rheological behavior of experimental solutions for wormlike micelles. This new simulation method allows extraction of multiple micellar parameters simultaneously by fitting experimental rheology data across the entire available frequency range. Values of average micelle length and breakage rate thereby obtained can be an order of magnitude different from previous estimates based on "local" frequency dependencies predicted by Cates' model. These differences are due to more complete physics included in our method and the fitting of G' and G" data across the entire frequency range. We also provide quantitative relationships between these parameters and rheological behaviors that improve on previous simple scaling results. [ABSTRACT FROM AUTHOR]

We present a simple constitutive model based on Kuhn segment alignment that predicts the observed monotonic extension thinning in steady state viscosity ηE even at extension rates above the inverse Rouse time (έe > τ-1R) for entangled polystyrene melts [Bach et al., Macromolecules 36, 5174-5179 (2003)], while preserving the extension thickening typically seen in entangled solutions for έ > τ-1R [Bhattacharjee et al., Macromolecules 35, 10131-10148 (2002)]. We tested two mechanisms by which Kuhn segment alignment can affect terminal relaxation time sd and rheology within a modified Doi-Edwards-Marrucci-Grizzuti (DEMG) model, which is a simplified tube model. The first mechanism is an increase in the tube diameter, as is inferred from ideas of Doi and Edwards [The Theory of Polymer Dynamics (Clarendon, Oxford, 1986)] and Sussman and Schweizer [Macromolecules 45, 3270-3284 (2012a)], while the second mechanism is a decrease in segmental friction, as described in recent work of Yaoita et al. [Macromolecules 45, 2773-2782 (2012)]. We find that the DEMG model, modified to allow tube diameter enlargement and reduction of entanglements, when done self-consistently, fails to predict extension thinning for entangled melts. The DEMG model modified by the second mechanism, to allow Kuhn segment alignment effects on local friction, correctly predicts extension thinning for entangled melts, while retaining thickening in entangled solutions with high solvent volume fractions. [ABSTRACT FROM AUTHOR]

Using multiscale modeling, includingmolecular dynamics simulations, with both united-atom and coarse-grainedforce fields, as well as Brownian dynamics simulations with stillhigher levels of coarse-graining, we explain the long-mysterious absenceof high frequency modes in the viscoelastic spectrum of isolated polymerchains in good solvents, reported years ago by Schrag and co-workers.The relaxation spectrum we obtain for a chain of 30 monomers at atomisticresolution is, remarkably, a single exponential, while that of a coarse-grainedchain of 100 monomers is well fit by only two modes. These resultsare very surprising in view of the many degrees of freedom possessedby these chains, and in view of the many relaxation modes presentin melts of such chains. However, the result agrees perfectly withexperimental observations of Schrag and co-workers. We also performed Browniandynamics (BD) simulations in which the explicit solvent moleculesare replaced by a viscous continuum, using chain models of varyingdegrees of resolution, both in the presence and absence of hydrodynamicinteractions (HI). Although the local dynamics is suppressed by theaddition of bending, torsion, side groups and excluded volume interactions(as suggested in Jain and Larson(15)), none of the BD simulations predicta single exponential relaxation for a short polymer chain. The comparisonof the relaxation of the bond vectors from different models indicatesan additional slow-down in the presence of explicit solvent molecules,which is critical to obtain a single exponential relaxation for shortchains. Our results indicate that the chain dynamics at small lengthscales (down to a few Kuhn steps) are significantly different fromthe predictions of models based on a continuum solvent, and finallyhelp explain the experimental results of Schrag and co-workers. [ABSTRACT FROM AUTHOR]

The average length of threadlike micelles in a semidilute solution has been inferred from the ratio of loss to storage modulus at a local minimum in loss modulus, which occurs at frequencies well above terminal. This method has yielded average micelle lengths of less than 1 μm or so for cetyltrimethylammonium surfactants at concentrations of around 0.1 to 1 M [Khatory et al., Langmuir 9, 1456-1464 (1993)]. Here, I use established correlations for polymer semidilute solutions to show that this estimate of micelle length is likely too small by around an order of magnitude. More accurate estimates can be obtained from the terminal relaxation time and the viscosity near the cross-over from dilute to semidilute, which show typical micelle average lengths that are well above 1 μm in length, for cetyltrimethylammonium solutions. [ABSTRACT FROM AUTHOR]

The effects of self-entanglements (spring-spring uncrossability) and solvent quality on the static and dynamic properties of a polymer chain in shear and extensional flows are investigated using Brownian dynamics simulations. We model the polymer chain by a sequence of beads connected by finitely extensible non-linear elastic springs, and spring-spring uncrossability is enforced by applying a spring-spring repulsive potential together with an adaptive time-stepping. Our findings suggest that chain uncrossability has an insignificant effect on the dynamics of a polymer chain. Furthermore, we considered four different combinations of intramolecular interactions: (i) no interactions, (ii) repulsive spring-spring and attractive bead-bead interactions, (iii) only repulsive bead-bead interactions, and (iv) only repulsive spring-spring interactions. The first two cases model 'theta' solvents, where the radius-of-gyration of a polymer chain, Rg∼N0.5, where N is the number of beads. For appropriately chosen parameters of interaction potentials, the last two cases model good solvents, where Rg∼N0.59. In the presence of a simple shear flow, the stretching behavior is alike in all the cases except case (ii). In case (ii), the polymer chain forms 'pearl-necklace-like' structures at zero Weissenberg number, Wi. With an increase in Wi, Rg first increases due to stretching of bonds between neighboring 'pearls,' and then decreases when the decrease in the number of pearls becomes limiting. In the presence of an extensional flow, normal stretching behavior is observed even in case (ii); the number of pearls increases with increasing extension rate because larger pearls break into smaller 'pearls.' The results show that even for fixed overall solvent quality for a chain at rest (e.g., a theta solvent), the ability of the chain to stretch in a shear flow is sensitive to the nature of intramolecular interactions. A chain with short-range attraction (between beads) and a long-range repulsion (between springs) tuned to balance each other and so create a theta condition at rest does not stretch much in a shear flow, while a chain for which the theta condition is achieved by applying no interactions at all does stretch in a shear flow. [ABSTRACT FROM AUTHOR]

Abstract: To explore the role of lung surfactant proteins SP-B and SP-C in storing and redelivering lipid from lipid monolayers during the compression and re-expansion occurring in lungs during breathing, we simulate the folding of lipid monolayers with and without these proteins. We utilize the MARTINI coarse-grained force field to simulate monolayers containing pure dipalmitoylphosphatidylcholine (DPPC) and DPPC mixed with palmitoyloleoylphosphatidylglycerol (POPG), palmitic acid (PA), and/or peptides. The peptides considered include the 25-residue N-terminal fragment of SP-B (SP-B1 – 25), SP-C, and several SP-B1 – 25 mutants in which charged and hydrophilic residues are replaced by hydrophobic ones, or vice-versa. We observe two folding mechanisms: folding by the amplification of undulations and folding by nucleation about a defect. The first mechanism is observed in monolayers containing either POPG or peptides, while the second mechanism is observed only with peptides present, and involves the lipid-mediated aggregation of the peptides into a defect, from which the fold can nucleate. Fold nucleation from a defect displays a dependence on the hydrophobic character of the peptides; if the number of hydrophobic residues is decreased significantly, monolayer folding does not occur. The addition of POPG or peptides to the DPPC monolayer has a fluidizing effect, which assists monolayer folding. In contrast, the addition of PA has a charge-dependent condensing affect on DPPC monolayers containing SP-C. The peptides appear to play a significant role in the folding process, and provide a larger driving force for folding than POPG. In addition to promoting fold formation, the peptides also display fusogenic behavior, which can lead to surface refining. [ABSTRACT FROM AUTHOR]

Although design arguments for the existence of God are sometimes dismissed as God of the Gaps apologetics, reasons for rejecting them based on the history of science, philosophy, religion, and pragmatism are not as compelling as is often implied. I argue that using multiple evidences of design in nature, with regular updates to accommodate new findings, can be a sound and convincing approach to apologetics. [ABSTRACT FROM AUTHOR]

Abstract: Using atomic force microscopy (AFM), we find that RecA-single-stranded DNA (RecA-ssDNA) filaments, in the presence of single-stranded DNA-binding (SSB) protein, organize into left-handed bundles, which differ from the previously reported disordered aggregates formed when SSB is excluded from the reaction. In addition, we see both left- and right-handedness on bundles of two filaments. These two-filament supercoils, individual filaments, and other smaller bundles further organize into more complicated bundles, showing overall left-handedness which cannot be explained by earlier arguments that presumed supercoiling is absent in RecA-ssDNA filaments. This novel finding and our previous results regarding supercoiling of RecA-double-stranded DNA (RecA-dsDNA) filaments are, however, consistent with each other and can possibly be explained by the intrinsic tendency of RecA-DNA filaments, in their fully coated form, to order themselves into helical bundles, independent of the DNA inside the filaments (ssDNA or dsDNA). RecA–RecA interactions may dominate the bundling process, while the original conformation of DNA inside filaments and other factors (mechanical properties of filaments, concentration of filaments, and Mg2+ concentration) could contribute to the variation in the appearance and pitch of supercoils. The tendency of RecA-DNA filaments to form ordered supercoils and their presence during strand exchange suggest a possible biological importance of supercoiled filaments. [Copyright &y& Elsevier]

I argue that there are strong reasons for resisting as a principle of science the concept of “anthropic selection.” This concept asserts that the existence of “observers” in a universe can be used as a condition that selects physical laws and constants necessary for intelligent life from different laws or physical constants prevailing in a vast number of other universes, to thereby explain why the properties of our universe are conducive to intelligent life. My reasons for limiting “anthropic selection” to the realm of speculation rather than permitting it to creep into mainstream science include our inability to estimate the probabilities of emergence of “observers” in a universe, the lack of testability through direct observation of the assumed high variability of the constants of nature, the lack of a clear definition of an “observer,” and the arbitrariness in how and to what questions anthropic selection is applied. [ABSTRACT FROM AUTHOR]

Abstract: The effect of salt on the binding of the antimicrobial peptide magainin to POPC lipid bilayers is studied by 40–50 ns molecular dynamics simulations of a POPC bilayer in the presence of different concentrations of Na+ and Cl− ions, corresponding to effective concentrations of 0, 100, 150, 200, 250 and 300 millimolar NaCl, with and without a single molecule of antimicrobial peptide magainin. Simulations without magainin showed that increasing salt concentration leads to the decrease in the area per lipid, a decrease in the head group tilt of the lipids, as well as increased order of lipid tails, in agreement with other recent simulations. Simulations with magainin show that peptide binding to the lipids is stronger at lower concentrations of salt. The peptides disorder the lipids in their immediate vicinity, but this effect is diminished as the salt concentration increases. Our studies indicate that while 50 ns simulations give information on peptide hydrogen bonding and lipid tail ordering that is insensitive to the initial peptide orientation, this run time is not sufficient to equilibrate the peptide position and orientation within the bilayer. [Copyright &y& Elsevier]

We show here both experimentally and theoretically that the formation of "coffee-ring" deposits observed at the edge of drying water droplets requires not only a pinned contact line (Deegan et al. Nature1997, 389, 827) but also suppression of Marangoni flow. For simple organic fluids, deposition actually occurs preferentially at the center of the droplet, due to a recirculatory flow driven by surface-tension gradients produced by the latent heat of evaporation. The manipulation of this Marangoni flow in a drying droplet should allow one in principle to control and redirect evaporation-driven deposition and assembly of colloids and other materials. [ABSTRACT FROM AUTHOR]

The linear viscoelastic properties of binary blends of monodisperse linear polymers of various polymers, including polyisoprene, polybutadiene, and polystyrene, are compared with the predictions of the model of Park and Larson (2004), which is an extension of the Milner-McLeish model [Milner and McLeish (1998)]. For each polymer system the model parameters needed in the predictions of the binary blend are obtained from experimental data for monodisperse polymers. For well entangled blends of nearly monodisperse polymers, the model predictions are in good agreement with experimental data for a wide range of values of the Graessley parameter Gr, which is proportional to the ratio of the reptation time of the long chain in the undilated tube to the constraint-release Rouse time. Our results show that for polymers of multiple chemical types, the terminal behavior of the long chain is controlled by reptation in an undilated tube for small GrGrc, where Grc is around 0.06. For values of Gr near Grc, behavior intermediate between reptation in a dilated and an undilated tube is found. [ABSTRACT FROM AUTHOR]

The existence of coil-stretch hysteresis in extensional flows of long-chain polymers in dilute solutions has recently been demonstrated using enormously long deoxyribonucleic acid (DNA) molecules [Schroeder et al., Science 301, 1515–1519 (2003)]; however, there is no demonstration of hysteresis for synthetic polymers of more modest molecular size. We here use Brownian dynamics simulations of bead-spring chains to predict that the minimum molecular weight of polystyrene in a dilute theta solvent that can show a “coil-stretch hysteresis” in a uniaxial extensional flow is around 0.5 million Daltons. We find that the threshold value of the ratio of effective drag coefficients in the stretched versus coiled states, ζstretch/ζcoil, is about 4.5 for the occurrence of hysteresis, close to the value observed for DNA. [ABSTRACT FROM AUTHOR]

To test the universality of the dependence of zero-shear viscosity on concentration for both flexible and locally semiflexible polymers in good solvents, we collected multiple literature data sets and measured the zero shear viscosity of λ-phage DNA over a range of semidilute concentrations. We found that all experimental data above a critical concentration c/ce>0.5 fall on a single empirical curve given by ηp/ηRouse=(45±2)×(c/ce)2.95±0.07 and this scaling law is in good agreement with the theoretical one, ηp/ηRouse≈(c/ce)2.4/(3ν-1) with ν the excluded volume exponent, ηp=η0-ηs the polymer contribution to the zero shear viscosity of the solution with η0 the zero-shear viscosity and ηs the solvent viscosity, ηRouse the hypothetical Rouse polymer viscosity, and ce the entanglement concentration of the polymer solution (Menezes and Graessley, 1982; Raspaud et al., 1995; Osaki et al., 2001). This scaling law provides a basis for estimating viscosities for arbitrary semidilute entangled polymer solutions from a knowledge of the solvent viscosity, the entanglement molecular weight in the melt, the excluded volume exponent, the second virial coefficient, and the intrinsic viscosity. [ABSTRACT FROM AUTHOR]

We have performed molecular dynamics simulations of the interactions of two α-helical anti-microbial peptides, magainin2 and its synthetic analog of MSI-78, with palmitoyl-oleoyl-phosphatidylcholine (POPC) lipid bilayers. We used various initial positions and orientations of the peptide with respect to the lipid bilayer, including a surface-bound state parallel to the interface, a trans-membrane state, and a partially inserted state. Our 20ns long simulations show that both magainin2 and MSI-78 are most stable in the lipid environment, with the peptide destabilized to different extents in both aqueous and lipid/water interfacial environments. We found that there are strong specific interactions between the lysine residues of the peptides and the lipid head-group regions. MSI-78, owing to its large number of lysines, shows better binding characteristics and overall stability when compared to magainin2. We also find that both peptides destabilize the bilayer environment, as observed by the increase in lipid tail disorder and the induction of local curvature on the lipid head-groups by the peptides. From all the simulations, we conclude that the hydrogen bonding interactions between the lysines of the peptides and the oxygens of the polar lipid head-groups are the strongest and determine the overall peptide binding characteristics to the lipids. [Copyright &y& Elsevier]

After reviewing and organizing the literature on the problem of inertial cross-stream migration of rigid spheres in various geometries including tubes and channels, we use Dissipative Particle Dynamics (DPD) simulations to study the simplest case of migration of a single neutrally or non-neutrally buoyant sphere with diameter 20% of the gap in plane Poiseuille flow and assess the potential and limitations of DPD simulations for this and similar problems. We find that the neutrally buoyant sphere lags by up to 6% behind the surrounding fluid and is focused at a position around 50% of the distance between the channel center and the wall. With Re increasing from around 100 to 500, the sphere migrates closer to the channel center. With flow driven by gravity, a much denser non-neutrally buoyant sphere leads the surrounding fluid and is focused at a position closer to the wall, around 60% the distance from the channel center to the wall, in qualitative agreement with previous work. The lower values of the Schmidt number Sc in DPD simulations relative to real fluids, due to the relatively large diffusivity of DPD beads, are shown to not significantly affect the consistency of our DPD results with literature results although they make results noisy at low Re (i.e., ≲ 50). However, the increase in Ma and Wi with increasing Re leads to compressible flow effects and in some cases viscoelastic effects at high Re depending on the DPD parameters chosen. Even for optimally chosen parameters, we require Re ≲ 500 to avoid strong compressibility effects. Thus, the relative simplicity of the DPD method for complex fluid flows is offset by the need to control the effects of unphysically high values of other parameters, such as Ma and Wi, which seriously limits the range of conditions under which DPD simulations give valid results in fluid transport problems. [ABSTRACT FROM AUTHOR]

Reversible ion binding equilibria in polyelectrolyte solutions are strongly affected by interactions between dissociated ionic species. We examine how the structural correlations between ionic groups on polyelectrolytes impact the counterion binding. Treating the electrostatic correlation free energy using the classical Debye-Hückel expression leads to complete counterion dissociation in the concentrated regime. This unphysical behavior is shown to stem from improper regularization of the self-energy of dissociated ions and polyions and is mitigated by smearing point-like charges across a finite width. The influence of the self-energy on counterion binding is elaborated on by generalizing the Debye-Hückel free energy to polyelectrolytes with variable fractal dimension and stiffness. In the dilute regime, a greater propensity for binding is found for chains with more compact architectures, which in turn reduces the harsh self-repulsions of tightly packed arrangements of charge. In the concentrated regime, the effects of electrostatic correlations weaken due to screening and the extent of binding is governed by a balance of short-ranged interactions and the translational entropy of ions. [ABSTRACT FROM AUTHOR]

A frame-invariant constitutive model is developed to describe the changes in droplet anisotropy and stress in a dilute blend of Newtonian droplets in a Newtonian matrix after imposition of a large step strain. The model, based on the Tomotika theory [Proc. R. Soc. London, Ser. A 150 (1935) 322], describes the evolution of components of the anisotropy tensor from values consistent with a cylindrical shape with undulations to values consistent with a series of fragmented spherical droplets. This model is compared to experimental measurements of droplet shape anisotropy for single droplets that were deformed to various degrees and then allowed to relax. Here, the droplet shape anisotropy is defined using the Doi–Ohta anisotropy tensor [J. Chem. Phys. 95 (1991) 1242], which is obtained by integrating over the droplet interfacial position as determined by microscopic image analysis. For moderately extended droplets, the anisotropy is affected by the ends of the droplet, and thus does not agree with breakup model predictions based on the Tomotika theory, which neglects contributions from the ends of the droplet. When droplet end-effects are subtracted, and only the effects of undulations in the droplet center remain, good agreement between theory and experiment is achieved. Data at various strains also superimpose on a single breakup line when end-effects are neglected. [Copyright &y& Elsevier]

Examines the use of polymer-enhanced media within microfabricated devices in the electrostretching of DNA molecules. Structure and functions of individual molecules; Interaction of DNA molecules with enzymes; Development of dielectrophoresis devices for DNA stretching.

We use a quartz crystal microbalance with dissipation (QCM-D) to investigate the swelling/de-swelling and hysteresis in brushes of weakly ionizable polyanion poly(acrylic acid) (PAA) brushes and bilayers containing a PAA brush and a poly(ethylene imine) (PEI) overlayer. We show that for a long PAA chain (Mw = 39 kDa), at low grafting density (σ < 0.05 chains/nm2) and at a pH value (=4.2) at which it is partially charged, in the low-salt "osmotic brush" regime, the brush height no longer increases with increased grafting density as is seen for shorter brushes and denser grafting, but shows a slight decrease in height, in qualitative agreement with predictions of scaling theory. In a cycle of stepped pH changes, we also show that at a low grafting density of σ = 0.023 chains/nm2 and Mw = 39 kDa, there is hysteresis in swelling over timescales of many minutes. For higher grafting densities σ = 0.87 chains/nm2 and shorter chains (2 kDa), we see little or no measurable hysteresis, and, at intermediate chain length 14 kDa and grafting density σ = 0.06 chains/nm2, hysteresis is observed at short timescales but is greatly reduced at longer timescales. These results are similarly observed when bilayers are made by adsorbing onto the PAA brush a layer of the polycation PEI. In addition, we also note hysteresis in swelling upon changes of salt concentration when pH is fixed. These results show the rich thermodynamics and kinetics of even monolayers and bilayers of polyelectrolyte films. [ABSTRACT FROM AUTHOR]

Presents quantitative predictions of impact forces and deflections resulting from the impact tests of elastomeric materials. Prediction of impact-induced loads; Linear viscoelastic elements in equipment; Rational design of elastomeric components in impact-tolerant portable electronic equipment; High deflections of soft elastomeric materials; High impact forces in elastomeric pads.

Brownian dynamics simulations of single-patch Janus particles under sedimentation equilibrium reveal that the phases found at fixed temperature and volume fraction are extremely sensitive to small changes in lateral box dimension. We trace this sensitivity to an uncontrolled parameter, namely, the pressure component parallel to the hexagonally ordered layers formed through sedimentation. We employ a flexible-cell constant-pressure scheme to achieve explicit control over this usually overlooked parameter, enabling the estimation of phase behavior under given pressure anisotropy. Our results show an increase in the stability range of an orientationally ordered lamellar phase with lateral layer compression and suggest a novel mechanism to control solid-solid phase transitions with negligible change in system volume, thus showing prospect for design of novel structures and switchable crystals from anisotropic building blocks. [ABSTRACT FROM AUTHOR]

We investigate theoretically charge regulation of weakly dissociating polyacids by potentiometric titration of their aqueous solutions. By treating deprotonation and cation binding to the polyacids as reversible reactions in our model, the ionization constant of acid groups along the polyacid chain is adjusted from its intrinsic value by electrostatic correlations. If electrostatic correlations are modeled with a Debye-Hückel electrostatic free energy that ignores the connectivity of acid groups, the theory follows Henderson-Hasselbalch behavior for monoacids at low polyacid concentrations, and does not capture the shift of the ionization constant from its intrinsic value with increasing pH. Using a random phase approximation (RPA) that captures the chain connectivity, the shift of the ionization constant is predicted, which is found to directly originate from the electrostatic repulsions along the polyacid chain. We make predictions for titration of rodlike and Gaussian coil polyacids, and find that the former is followed by hydrophilic poly(acrylic acid), while the latter is followed more closely by hydrophobic poly(acrylamido-2-methyl-1-propanesulfonic acid). The results suggest that development of more advanced free energy models that allow chain configurations to self-adjust with changing pH and chain hydrophobicity could provide accurate a priori modeling of charge regulation of polyacids. Image, graphical abstract [ABSTRACT FROM AUTHOR]

We develop a Brownian dynamics simulation method with full hydrodynamic interactions (HI) to study the recognition kinetics between two patterned colloidal spheres. We use a general resistance matrix (12*12) to describe both the far and near-field hydrodynamics of translation, rotation, and translation-rotation coupling between the two spheres, adopted from Jeffrey and Onishi [J. Fluid Mech. 139, 261 (1984)]. We apply the method to the specific binding of 'patchy' spheres, including effects of depletion attraction and orientation-specific binding, as are present in 'Janus' spheres whose surfaces contain hydrophobic and hydrophilic faces [Q. Chen, S. C. Bae, and S. Granick, Nature (London) 469, 381 (2011)]. The binding times obtained between two non-patterned spheres (of equal or unequal diameter) with or without HI extrapolated to infinite dilution are shown to be in good agreement with theoretical predictions of the Smoluchowski equation. In addition, the binding times for pairs of spheres for three cases of surface patterning of the two spheres (uniform-uniform, uniform-Janus, and Janus-Janus) are compared with or without rotational motion. [ABSTRACT FROM AUTHOR]

DNA stretching in entangling media with oscillating electric fields is useful in DNA sequencing. We propose that stretching occurs around hairpin loops at the chain ends, caused by interactions with the medium. Brownian dynamics simulations show that this mechanism explains several experimental observations, including a resonance with varying field frequency. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 W i , gradient number G d (which defines the level of polymer chain confinement), and Péclet number P e 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 W i. However, in the absence of wall-depletion, there is no first order contribution and, therefore, the lowest-order concentration variation is proportional to W i 2. An upper limit of W i = 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 G d greater than 0.5, the continuum theory fails to be accurate and mesoscopic simulations that track individual polymer molecules are needed. [ABSTRACT FROM AUTHOR]

We develop a method which incorporates spring-spring repulsions into Brownian dynamics simulations of flexible polymers. The distance of closest approach between two springs is computed, and a repulsive force is then applied based on this distance. Repulsive potentials of the exponential and power-law forms are considered. We demonstrate that our method is capable of accounting for excluded-volume effects in start-up of extensional flow. Equilibrium simulations indicate that spring-spring repulsions can be used to prevent the passage of two springs through each other, and thus maintain the topological integrity of polymer molecules. The method developed here is expected to be useful for simulating entanglement phenomena in both single and multichain systems. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

[Display omitted] 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. We compute the escape free energies △G esc of surfactant from micelles and the scission free energies △G sciss 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). For spherical micelles, △G esc 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 △G esc , both at R ~ 0.7, while △G sciss 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. [ABSTRACT FROM AUTHOR]