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Brownian dynamics simulations are performed to study the binding kinetics in the dilute-sphere limit by considering interactions of two spheres under shear flow across the entire range of Peclet numbers, spanning both perikinetic (diffusion-controlled) and orthokinetic (flow-controlled) coagulation regimes. The dilute regime is attained by carrying out two-sphere simulations in periodic boxes of different sizes and aspect ratios and extrapolating toward the infinite box limit. Effects of particle type (Janus and isotropic particles), shear rate, hydrodynamic interactions, and inter-particle potential are explored. We find that rectangular boxes with appropriate aspect ratios overcome a particle "shadow effect" that cannot be overcome with cubic boxes unless huge boxes are used. With rectangular boxes, we obtain converged binding kinetics for the whole Peclet number range, while cubic boxes of increasing size allow converged results only in the absence of flow. We consider the effect of binding both in a secondary minimum controlled by a combination of electrostatic repulsion and depletion attraction, as well as in a primary minimum governed by induced-dipole attraction. Results are computed using both realistic interaction potentials and by replacing the potential with a simple cutoff gap distance at which binding is deemed to occur. Results agree with several existing reports including Smoluchowski predictions in the zero- and infinite-shear-rate limits, and high-Pe perturbation results of Feke and Schowalter [J. Fluid Mech. 133, 17-35 (1983)] at Peclet numbers (Pe) above 100. Finally, we compute binding times for anisotropic Janus particles which have both repulsive and attractive faces, for a wide range of Pe number. [ABSTRACT FROM AUTHOR]

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. [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 develop an analytical model to predict the collapse conformation for a single semiflexible polymer chain in solution, given its length, diameter, stiffness, and self-attractiveness. We construct conformational phase diagrams containing three collapsed states, namely torus, bundle, and globule over a range of dimensionless ratios of the three energy parameters, namely solvent-water surface energy (γs), energy of bundle end folds (γe), and bending energy per unit length in a torus (γb). Our phase diagram captures the general phase behavior of a single long chain (>10 Kuhn lengths) at moderately high (order unity) dimensionless temperature, which is the ratio of thermal energy to the attractive interaction between neighboring monomers. We find that the phase behavior approaches an asymptotic limit when the dimensionless chain length to diameter ratio (L*) exceeds 300. We successfully validate our analytical results with Brownian Dynamics (BD) simulations, using a mapping of the simulation parameters to those used in the phase diagram. We evaluate the effect of three different bending potentials in the range of moderately high dimensionless temperature, a regime not been previously explored by simulations, and find qualitative agreement between the model and simulation results. We, thus, demonstrate that a rather simplified analytical model can be used to qualitatively predict the final collapsed state of a given polymer chain. [ABSTRACT FROM AUTHOR]

While phenomenological expressions have been given in the past for polymer flux due to gradients in polymer stress or gradients in velocity gradient, we here provide a systematic perturbation theory for the coupled equations for polymer rheology, fluid dynamics, and polymermass transport. Analytical results are obtained for the steady-state concentration of dilute dumbbells in the absence of hydrodynamic interactions, as a function of the Weissenberg number Wi, Peclet number Pe, and the "gradient number" Gd, where the latter is the ratio of polymer equilibrium size to the characteristic distance over which velocity gradients change. At lowest order in all three perturbation variables, the theory yields a Poisson's equation with the driving term given by 2c Dt² vj,k,j vj,j,k 2cDt²vj,k,j vj,j,k, where c cis the average concentration, D is the dumbbell diffusivity, t tis the relaxation time, and v is velocity with Einstein notation for the subscripts. We find using Brownian dynamics (BD) simulations in a simple Taylor vortexflow that this formula becomes accurate for small values of Gd, Wi, and Pe. For small Gd and Wi, but large Pe, we obtain accurate solutions from the Poisson's equation augmented with a simple convection term. For gradients beyond second order in Gd, continuum equations lose utility, but mesoscopic methods such as BD can then be used to determine polymer migration. We give a hierarchy of approximations that shows which equations and methods are required as the Peclet number Pe, the Weissenberg number Wi, and/or the gradient number Gd, exceed unity or as the polymer becomes concentrated or hydrodynamic interactions become important. [ABSTRACT FROM AUTHOR]

We apply our recently developed mesoscopic simulation method for entangled wormlike micelle (WLM) solutions to extract multiple micellar characteristic lengths and time constants: i.e., average micelle length, breakage rate, and entanglement and persistence lengths, from linear rheological measurements on commercial surfactant solutions, one containing sodium lauryl one ether sulfate (SLE1S), and the other containing both SLE1S and cocamidopropyl betaine, as well as a perfume mixture, in both cases with a sample salt (NaCl) added. Measurements include both mechanical rheometry and diffusing wave spectroscopy, the latter providing the high-frequency data needed to determine micelle persistence length accurately. By fitting the experimental data (G' and G") across the entire frequency range through our iteration procedure, the method is of practical use in predicting micellar parameters, which are difficult to obtain from other theoretical or experimental methods. The dependence of micellar parameters on added salt concentration, and the effect of micelle breakage mechanisms on viscoelasticity of WLM solutions, are also discussed. [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 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]

A detailed study of the effects of hydrodynamic interaction (HI) and excluded volume (EV), on isolated bead-rod chains in shear flows, where the 'rods' are mimicked by stiff Fraenkel springs is presented. It is observed that the deformation behavior at weak and intermediate shear rates is qualitatively similar to that observed for polymer chains in the absence of EV and HI, while that at high-shear rates is sensitive to modeling details and chain resolution. Our simulations with varying degrees of resolution reveal universality in chain behavior in the presence of EV (without HI), while the onset of the transition to a compressed chain in the presence of HI (without EV) shifts to higher shear rates with increasing chain resolution. The results also highlight the success of the bead-spring models in predicting the chain behavior in shear flows in the presence of HI, when the HI parameter is appropriately chosen. © 2014 American Institute of Chemical Engineers AIChE J, 60: 1400-1412, 2014 [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]

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]

A correction to an article "Brownian Dynamics Simulations With Stiff Finitely Extensible Nonlinear Elastic-Fraenkel Springs as Approximations to Rods in Bead-Rod Models," by Chih-Chen Hsieh, Seman Jain and Ronald G. Larson published online on January 30, 2006 is presented.

Wecalculate potentials of mean force (PMFs) and mean first passagetimes for a surfactant to escape a micelle, for both ionic sodiumdodecyl sulfate (SDS) and nonionic ethoxylated alcohol (C12E5) micellesusing both atomistic and coarse-grained molecular dynamics (MD) simulations.The PMFs are obtained by umbrella sampling and used in a Smoluchowskifirst-passage-time theory to obtain the times for a surfactant toescape a micelle. The calculated mean first passage time for an SDSmolecule to break away from a micelle (with an aggregation numberof 60) is around 2 μs, which is consistent with previous experimentalmeasurements of the “fast relaxation time” for exchangeof surfactants between the micellar phase and the bulk solvent. Thecorresponding escape time calculated for a nonionic ethoxylated alcoholC12E5, with the same tail length as SDS, is 60 μs, which issignificantly longer than for SDS primarily because the PMF for surfactantdesorption is about 3kT smaller than for C12E5. We also show thattwo coarse-grained (CG) force fields, MARTINI and SDK, give predictionssimilar to the atomistic CHARMM force field for the nonionic C12E5surfactant, but for the ionic SDS surfactant, the CG simulations givea PMF similar to that obtained with CHARMM only if long-range electrostaticinteractions are included in the CG simulations, rather than usinga shifted truncated electrostatic interaction. We also calculate thatthe mean first passage time for an SDS and a C12E5 to escape froma latex binder surface is of the order of milliseconds, which is morethan 100 times longer than the time for escape from the micelle, indicatingthat in latex waterborne coatings, SDS and C12E5 surfactants likelybind preferentially to the latex polymer interface rather than formmicelles, at least at low surfactant concentrations. [ABSTRACT FROM AUTHOR]

We present a scheme to calculate wormlike micelle scission free energies from a potential of mean force (PMF) derived from a weighted histogram analysis method (WHAM) applied to coarse grained dissipative particle dynamics (DPD) simulations. In contrast to previous related work, we use a specially chosen external potential based on a reaction coordinate that reversibly drives surfactants out of the nascent scission location. For the application to a model body wash formulation, we predict how addition of NaCl and small molecules such as perfume raw materials (PRMs) affect scission energies. The results show qualitative agreement and correct trends compared to recently determined scission energies for the same system; however, a more rigorous parametrization of the underlying DPD potential is required for quantitative agreement. [ABSTRACT FROM AUTHOR]

To obtain a molecular level viewof the interface of a latex particlewith water solvent, atomistic simulations using two different forcefields were performed for a slab of methyl methacrylate (MMA)/n-butyl acrylate (BA) random copolymer in water. The carbonylgroups at the polymer/water interface were found to orient significantlytoward the water phase. We calculated the copolymer/water and thecopolymer/vacuum interfacial tensions, and predict a contact anglefor water on copolymer of 77°, consistent with experimental values.We also calculated bulk densities of MMA/BA copolymer, and PMMA andPBA homopolymers, which compare well with experiment, and the structurefactor for PMMA, which agrees with neutron scattering results. Thesimulated Tgof PMMA is not far from itsexperimental value, despite the much shorter chains and faster coolingrates of simulations versus experiments, because these two effectstend to cancel each other. When we correct for these effects, thepredicted Tgis within 5 K of the experimentalvalue. [ABSTRACT FROM AUTHOR]