Charles Manke - Academia.edu (original) (raw)
Papers by Charles Manke
Journal of Non-newtonian Fluid Mechanics, 1985
Capillary viscometry was performed on dilute non-Newtonian solutions of monodisperse polystyrene ... more Capillary viscometry was performed on dilute non-Newtonian solutions of monodisperse polystyrene in theta solvents. The solvents, blends of lowmolecular-weight polystyrene with styrene, had viscosities (qs) that were varied from 0.22-27 Pa s. Data reduction of the dilute limit, [ q]/[qo] vs. j3 = [q,,]q,Mjl/RT (where i, is shear rate) revealed a parametric dependence on 77, that has not before been reported and is not predicted by most molecular theories of polymer dynamics. It is suggested that an internal viscosity model can explain such a phenomenon.
Macromolecules, Oct 1, 1985
International Journal of Modern Physics B, Jan 20, 2003
A segmental repulsion force is developed to reduce the frequency of artificial chain segment cros... more A segmental repulsion force is developed to reduce the frequency of artificial chain segment crossing events in DPD simulations of the dynamics of polymer melts. The scaling of viscosity and center of mass diffusion coefficients with chain length show transitions in scaling exponents at a critical entanglement chain length. Above the entanglement chain length, the scaling exponent for the diffusion coefficient is close to the experimental value of -2, but the scaling exponent for viscosity is close to +2, which is smaller than the experimental value of +3.4. Without segmental repulsion forces, chain segment crossings occur freely, and the scaling of both diffusion coefficient and viscosity follow the Rouse model for all chain lengths.
Macromolecules, Sep 1, 1994
We report gas solubilities in molten polymers for two systems: the solubility of carbon dioxide i... more We report gas solubilities in molten polymers for two systems: the solubility of carbon dioxide in poly(dimethylsi1oxane) and the solubility of 1,l-difluoroethane in polystyrene are measured in the range of temperatures and pressures where the gas is supercritical. The solubility data are correlated by two lattice-theory-based equations of state, namely, the Sanchez-Lacombe and Panayiotou-Vera equations of state, which employ a single adjustable binary interaction parameter. Both equations of state provide satisfactory descriptions of the solubility data when the binary interaction parameter is allowed to depend on temperature. The utility of the mixture equations of state is illustrated by predictions of swollen volume, isothermal compressibility, and thermal expansion coefficient for the mixtures over the range of data.
A numerical method employing a marching integration, finite difference method is used to determin... more A numerical method employing a marching integration, finite difference method is used to determine the momentum, temperature, and component molar concentration profiles in the tapered annulus of a vertical cylinder epitaxy reactor for silicon deposition from SiCl4 in H2' Results of the study contribute to the understanding of momentum, heat, and mass ,transfer in the vertical cylinder reactor. The numerical results indicate that boundary layers control the deposition profile in the entrance length of the reactor, while downstream rates are governed by the inlet flow rate and susceptor tilt angle.
Oxford University Press eBooks, Jan 8, 2004
Use of supercritical fluids (SCFs), particularly supercritical carbon dioxide, as alternative sol... more Use of supercritical fluids (SCFs), particularly supercritical carbon dioxide, as alternative solvents in polymer synthesis and processing is a rapidly growing research area with successful industrial applications (McCoy, 1999). In some cases, the need for alternative solvents is based on environmental concerns, with regulations mandating replacement solvents. An environmentally mandated example is the 1995 ban of the use of chlorofluorocarbons (CFCs) as physical blowing agents in the manufacture of polymeric foams after CFCs were classified as class-I-ozone-depleting substances (ODPs). Among the alternative blowing agents are gases like CO2 and N2 and refrigerants such as 1,1-difluoroethane (R152a) and 1,1,1,2-tetrafluoroethane (R134a). Under the foaming conditions, at temperatures above the glass transition temperature of a polymer, and at pressures required for flow of highly viscous polymer melts, these alternative blowing agents are frequently supercritical. When polymers are formed into final products by various melt-processing techniques, such as extrusion, injection molding, blow molding, foaming, and spin-coating, extremely high melt viscosity presents a major difficulty. A common method to moderate the processing conditions is to add a liquid solvent or plasticizer to the melt. Solvents and plasticizers lower the glass transition temperature, Tg, of the polymer so that the polymer can be made to flow at lower pressures and temperatures. Replacing liquid solvents with SCFs presents unique processing advantages. Higher diffusivity and lower viscosity of SCFs, compared with liquid solvents, increase rates of dissolution and mixing. The properties of polymer–SCF solutions are tunable via pressure or temperature changes, thus allowing efficient downstream separations. Most importantly, dissolution of an SCF produces very large reductions in melt viscosity compared with a liquid solvent dissolved in the melt. Whether the interest in using SCFs in polymer synthesis and processing is driven by environmental concerns or processing advantages, it is important to understand the rheological behavior of polymer–SCF mixtures. In this chapter, we describe rheological measurements of polymer melts containing dissolved gases for two polymers, polydimethylsiloxane (PDMS) swollen with CO2 at 50 °C and 80 °C and polystyrene (PS) swollen with CO2, R152a, and R134a at 150 °C and 175 °C.
Membranes, Jul 29, 2021
This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY
Journal of Rheology, Sep 1, 1994
Linear viscoelastic fluid theory is used to demonstrate that the infinite frequency dynamic visco... more Linear viscoelastic fluid theory is used to demonstrate that the infinite frequency dynamic viscosity η’∞, the viscosity jump upon start‐up of shear flow η+(0), and the viscosity jump upon cessation of Newtonian shear flow η0−η−(0) are equal. Therefore, these quantities are fundamentally the same material property for fluids that exhibit linear viscoelastic behavior in the limit of small deformations. The η’∞ predictions of several continuum and molecular theories are compared, and implications for theoretical and experimental investigations of these phenomena are discussed.
Computer Physics Communications, Jul 1, 2000
Dissipative Particle Dynamics (DPD), a recently developed mesoscopic simulation technique, is use... more Dissipative Particle Dynamics (DPD), a recently developed mesoscopic simulation technique, is used to model the morphology and dynamical behavior of short-chain diblock copolymer melts. The two blocks, A and B, of the copolymer chain are represented by particles of different DPD "phases", and composition is varied by varying the number of beads in each block. As composition is varied from 0-50% A, the predicted morphology changes, progressively, from a disordered system to BCC spherical domains of A, and then to cylindrical domains and lamelae. These predicted phase structures are in agreement with experimental and theoretical results from the literature. Steady shear flow is simulated by means of Lees-Edwards boundary conditions to investigate the effect of shear flow on morphology, and to evaluate viscosity and normal stresses under shearing. The predicted effects of shear flow include flow-induced microstructural transitions, and enhanced rheological properties for systems having the BCC equilibrium microstructure.
Journal of Rheology, Sep 1, 2002
ABSTRACT The rheology and configurational properties of dilute polymer solutions in steady shear ... more ABSTRACT The rheology and configurational properties of dilute polymer solutions in steady shear flow are modeled by dissipative particle dynamics (DPD), a new mesoscopic simulation method. The simulations represent the polymer as a 10-bead chain of FENE connector springs and the solvent as a sea of free DPD particles. Thermodynamic interactions between the polymer and solvent are modeled by varying the strength of the repulsive forces acting between unlike particle species. Since DPD simulations model the full hydrodynamics of the polymer-solvent system, instantaneous hydrodynamic interactions among beads of the polymer chain emerge naturally from the simulations. The predicted rheological material functions include realistic features such as shear thinning of the viscosity and first normal stress coefficient, and negative values for the ratio Ψ2/Ψ1. The enhancement of η, Ψ1, and Ψ2 by means of chain expansion in good solvents is also realistically represented by the model predictions. The DPD simulations accurately describe the variation of the orientation angle of polymer chains with the shear rate, and predict macromolecular expansion ratios that are in good agreement with the recent flow light scattering measurements of Lee and Muller [Polymer 40, 2501-2510 (1999)].
Journal of Cellular and Molecular Medicine, 2011
Since the discovery and implication of N-ethylmaleimide-sensitive factor (NSF)-attachment protein... more Since the discovery and implication of N-ethylmaleimide-sensitive factor (NSF)-attachment protein receptor (SNARE) proteins in membrane fusion almost two decades ago, there have been significant efforts to understand their involvement at the molecular level. In the current study, we report for the first time the molecular interaction between full-length recombinant t-SNAREs and v-SNARE present in opposing liposomes, leading to the assembly of a t-/v-SNARE ring complex. Using high-resolution electron microscopy, the electron density maps and 3D topography of the membrane-directed SNARE ring complex was determined at nanometre resolution. Similar to the t-/v-SNARE ring complex formed when 50 nm v-SNARE liposomes meet a t-SNARE-reconstituted planer membrane, SNARE rings are also formed when 50 nm diameter isolated synaptic vesicles (SVs) meet a t-SNARE-reconstituted planer lipid membrane. Furthermore, the mathematical prediction of the SNARE ring complex size with reasonable accuracy, and the possible mechanism of membrane-directed t-/v-SNARE ring complex assembly, was determined from the study. Therefore in the present study, using both lipososome-reconstituted recombinant t-/v-SNARE proteins, and native v-SNARE present in isolated SV membrane, the membranedirected molecular assembly of the neuronal SNARE complex was determined for the first time and its size mathematically predicted. These results provide a new molecular understanding of the universal machinery and mechanism of membrane fusion in cells, having fundamental implications in human health and disease.
Journal of Chemical Physics, Feb 22, 2002
Langmuir, Sep 14, 2015
Forces between colloidal particles determine the performances of many industrial processes and pr... more Forces between colloidal particles determine the performances of many industrial processes and products. Colloidal force measurements conducted between a colloidal particle AFM probe and particles immobilized on a flat substrate are valuable in selecting appropriate surfactants for colloidal stabilization. One of the features of inorganic fillers and extenders is the prevalence of rough surfaces-even the polymer latex particles, often used as model colloidal systems including the current study, have rough surfaces albeit at a much smaller scale. Surface roughness is frequently cited as the reason for disparity between experimental observations and theoretical treatment but seldom verified by direct evidence. This work reports the effect of nanoscale surface roughness on colloidal force measurements carried out in the presence of surfactants. We applied a heating method to reduce the mean surface roughness of commercial latex particles from 30 nm to 1 nm. We conducted force measurements using the two types of particles at various salt and surfactant concentrations. The surfactants used were pentaethylene glycol monododecyl ether, Pluronic ® F108, and a styrene/acrylic copolymer Joncryl ® 60. In the absence of the surfactant, nanometer surface roughness affects colloidal forces only in high salt conditions when the Debye length becomes smaller than the surface roughness. The adhesion is stronger between colloids with higher surface roughness and requires a higher surfactant concentration to be eliminated. The effect of surface roughness on colloidal forces was also investigated as a function of the adsorbed surfactant layer structure characterized by AFM indentation and dynamic light scattering. We found that when the layer thickness exceeds the surface roughness the colloidal adhesion is less influenced by surfactant concentration variation. This study demonstrates that surface roughness at the nanoscale can influence colloidal forces significantly and should be taken into account in colloidal dispersion formulations.
Molecular Simulation, Aug 1, 2000
ABSTRACT Dissipative Particle Dynamics (DPD) is employed to model the dynamics and rheology of po... more ABSTRACT Dissipative Particle Dynamics (DPD) is employed to model the dynamics and rheology of polymer solutions, and suspensions of spherical particles with adsorbed polymers. Static and dynamic scaling relationships for the variation of radius of gyration and relaxation time with polymer chain length are reviewed, demonstrating that the DPD polymer solution model correctly represents the effects of hydrodynamic interaction and excluded volume. Rheological simulations for both polymer solutions and polymer-sphere suspensions predict Newtonian viscosities at low shear rate followed by shear-thinning behavior as a reduced shear rate of unity is approached. Both the Newtonian viscosity and the extent of shear-thinning are greatly enhanced in the case of good solvents, compared to the viscosity curves for polymers and polymer-spheres structures dissolved in theta solvents and poor solvents.
Fluid Phase Equilibria, 2006
Phase behavior of -d galactose pentaacetate-carbon dioxide binary system is investigated by dew-... more Phase behavior of -d galactose pentaacetate-carbon dioxide binary system is investigated by dew-point and bubble-point measurements conducted in a high pressure variable volume sapphire cell. The phase envelope for solutions of -d galactose pentaacetate in supercritical CO 2 is obtained for -d galactose pentaacetate concentrations between 2 and 26 wt%, and for the temperature range of 308-323 K. The system exhibits lower critical solution temperature (LCST) behavior and high solubility of -d galactose pentaacetate is observed. The densities of the system are also measured, and liquid-like densities (near 1 g/cm 3) are observed for single-phase solutions of -d galactose pentaacetate in supercritical CO 2 at concentrations of 18 wt% and higher. Viscosity is measured for solutions of 18 and 25 wt% -d galactose pentaacetate in the single-phase region at 313 K and 17 MPa and the viscosity values, 0.095 and 0.103 cp, respectively, are similar in magnitude to the viscosity of pure carbon dioxide.
Chemical Engineering Science, 2010
Thermoset nanocomposites (TSNCs) may offer significantly improved performance over conventional t... more Thermoset nanocomposites (TSNCs) may offer significantly improved performance over conventional thermoset materials, and thus are attractive for wide industrial applications, especially in the coating industry. Design of TSNCs via experiment, however, faces various technical challenges due to design complexity. Computational design can provide deep insights and identify superior design solutions through exploring opportunities in a usually huge design space. This paper introduces a generic computational methodology for the design, characterization, and testing of TSNC-based coatings. A distinct feature of the methodology is its capability of generating quantitative correlations among material formulation, processing condition, coating microstructure and property, coating performance, and processing efficiency. The correlations can enable a comprehensive analysis for optimal TSNC coating design. Case studies will demonstrate the methodological efficacy and attractiveness.
Cell Biology International, Apr 1, 2008
Earlier studies using x-ray diffraction, light scattering, photon correlation spectroscopy, and a... more Earlier studies using x-ray diffraction, light scattering, photon correlation spectroscopy, and atomic force microscopy, strongly suggest that SNARE-induced membrane fusion in cells proceed as a result of calcium bridging opposing bilayers. The bridging of phospholipid heads groups in the opposing bilayers by calcium leads to the release of water from hydrated Ca 2+ ions as well as the loosely coordinated water at PO-lipid head groups. Local dehydration of phospholipid headgroups and the calcium, bridging opposing bilayers, then leads to destabilization of the lipid bilayers and membrane fusion. This hypothesis was tested in the current study by atomistic molecular dynamic simulations in the isobaric-isothermal ensemble using hydrated dimethylphosphate anions (DMP−) and calcium cations. Results from the study demonstrate, formation of DMP-Ca 2+ complexes and the consequent removal of water, supporting the hypothesis. Our study further demonstrates that as a result of Ca2+-DMP self-assembly, the distance between anionic oxygens between the two DMP molecules is reduced to 2.92 Å, which is in close agreement with the 2.8 Å SNARE-induced apposition established between opposing bilayers, reported earlier from x-ray diffraction measurements.
Since the discovery and implication of N-ethylmaleimide-sensitive factor (NSF)-attachment protein... more Since the discovery and implication of N-ethylmaleimide-sensitive factor (NSF)-attachment protein receptor (SNARE) proteins in membrane fusion almost two decades ago, there have been significant efforts to understand their involvement at the molecular level. In the current study, we report for the first time the molecular interaction between full-length recombinant t-SNAREs and v-SNARE present in opposing liposomes, leading to the assembly of a t-/v-SNARE ring complex. Using high-resolution electron microscopy, the electron density maps and 3D topography of the membrane-directed SNARE ring complex was determined at nanometre resolution. Similar to the t-/v-SNARE ring complex formed when 50 nm v-SNARE liposomes meet a t-SNARE-reconstituted planer membrane, SNARE rings are also formed when 50 nm diameter isolated synaptic vesicles (SVs) meet a t-SNARE-reconstituted planer lipid membrane. Furthermore, the mathematical prediction of the SNARE ring complex size with reasonable accuracy, and the possible mechanism of membrane-directed t-/v-SNARE ring complex assembly, was determined from the study. Therefore in the present study, using both lipososome-reconstituted recombinant t-/v-SNARE proteins, and native v-SNARE present in isolated SV membrane, the membranedirected molecular assembly of the neuronal SNARE complex was determined for the first time and its size mathematically predicted. These results provide a new molecular understanding of the universal machinery and mechanism of membrane fusion in cells, having fundamental implications in human health and disease.
Journal of Supercritical Fluids, 2008
1,3,5-tri-tert-butylbenzene (TTBB) is solid at ambient conditions, and has substantial solubility... more 1,3,5-tri-tert-butylbenzene (TTBB) is solid at ambient conditions, and has substantial solubility in liquid and supercritical carbon dioxide. We present the phase behavior of TTBB-CO 2 binary system at temperatures between 298 and 328 K and at pressures up to 20 MPa. Phase diagrams showing the liquid-vapor, solid-liquid and solid-vapor equilibrium envelopes are constructed by pressure-volume-temperature measurements in a variable-volume sapphire cell. TTBB is highly soluble in CO 2 over a wide range of compositions. Single-phase states are achieved at moderate pressures, even with very high TTBB concentrations. For example, at 328 K, a binary system containing TTBB at a concentration of 95% by weight forms a single-phase above 2.04 MPa. TTBB exhibits a significant melting-point depression in the presence of CO 2 , 45 K at 3.11 MPa, where the normal melting point of 343 K is reduced to 298 K. With its high solubility in carbon dioxide, TTBB has potential uses as a binder or template in materials forming processes using dense carbon dioxide.
Journal of Physical Chemistry B, Sep 13, 2010
In an effort to provide insight into the mechanism of Ca(2+)-induced fusion of lipid vesicles, mo... more In an effort to provide insight into the mechanism of Ca(2+)-induced fusion of lipid vesicles, molecular dynamics simulations in the isobaric-isothermal ensemble are used to investigate interactions of Ca(2+) with apposed lipid bilayers in close proximity. Simulations reveal the formation of a Ca(2+)-phospholipid "anhydrous complex" between apposed bilayers, whereas similar calculations performed with Na(+) display only complexation between neighboring lipids within the same bilayer. The binding of Ca(2+) to apposed phospholipids brings large regions of the bilayers into close contact (<4 Å), displacing water from phospholipid head groups in the process and creating regions of local dehydration. Dehydration of the apposed bilayers leads to ordering of the phospholipid tails, which is partially disrupted by the presence of Ca(2+)-phospholipid bridges.
Journal of Non-newtonian Fluid Mechanics, 1985
Capillary viscometry was performed on dilute non-Newtonian solutions of monodisperse polystyrene ... more Capillary viscometry was performed on dilute non-Newtonian solutions of monodisperse polystyrene in theta solvents. The solvents, blends of lowmolecular-weight polystyrene with styrene, had viscosities (qs) that were varied from 0.22-27 Pa s. Data reduction of the dilute limit, [ q]/[qo] vs. j3 = [q,,]q,Mjl/RT (where i, is shear rate) revealed a parametric dependence on 77, that has not before been reported and is not predicted by most molecular theories of polymer dynamics. It is suggested that an internal viscosity model can explain such a phenomenon.
Macromolecules, Oct 1, 1985
International Journal of Modern Physics B, Jan 20, 2003
A segmental repulsion force is developed to reduce the frequency of artificial chain segment cros... more A segmental repulsion force is developed to reduce the frequency of artificial chain segment crossing events in DPD simulations of the dynamics of polymer melts. The scaling of viscosity and center of mass diffusion coefficients with chain length show transitions in scaling exponents at a critical entanglement chain length. Above the entanglement chain length, the scaling exponent for the diffusion coefficient is close to the experimental value of -2, but the scaling exponent for viscosity is close to +2, which is smaller than the experimental value of +3.4. Without segmental repulsion forces, chain segment crossings occur freely, and the scaling of both diffusion coefficient and viscosity follow the Rouse model for all chain lengths.
Macromolecules, Sep 1, 1994
We report gas solubilities in molten polymers for two systems: the solubility of carbon dioxide i... more We report gas solubilities in molten polymers for two systems: the solubility of carbon dioxide in poly(dimethylsi1oxane) and the solubility of 1,l-difluoroethane in polystyrene are measured in the range of temperatures and pressures where the gas is supercritical. The solubility data are correlated by two lattice-theory-based equations of state, namely, the Sanchez-Lacombe and Panayiotou-Vera equations of state, which employ a single adjustable binary interaction parameter. Both equations of state provide satisfactory descriptions of the solubility data when the binary interaction parameter is allowed to depend on temperature. The utility of the mixture equations of state is illustrated by predictions of swollen volume, isothermal compressibility, and thermal expansion coefficient for the mixtures over the range of data.
A numerical method employing a marching integration, finite difference method is used to determin... more A numerical method employing a marching integration, finite difference method is used to determine the momentum, temperature, and component molar concentration profiles in the tapered annulus of a vertical cylinder epitaxy reactor for silicon deposition from SiCl4 in H2' Results of the study contribute to the understanding of momentum, heat, and mass ,transfer in the vertical cylinder reactor. The numerical results indicate that boundary layers control the deposition profile in the entrance length of the reactor, while downstream rates are governed by the inlet flow rate and susceptor tilt angle.
Oxford University Press eBooks, Jan 8, 2004
Use of supercritical fluids (SCFs), particularly supercritical carbon dioxide, as alternative sol... more Use of supercritical fluids (SCFs), particularly supercritical carbon dioxide, as alternative solvents in polymer synthesis and processing is a rapidly growing research area with successful industrial applications (McCoy, 1999). In some cases, the need for alternative solvents is based on environmental concerns, with regulations mandating replacement solvents. An environmentally mandated example is the 1995 ban of the use of chlorofluorocarbons (CFCs) as physical blowing agents in the manufacture of polymeric foams after CFCs were classified as class-I-ozone-depleting substances (ODPs). Among the alternative blowing agents are gases like CO2 and N2 and refrigerants such as 1,1-difluoroethane (R152a) and 1,1,1,2-tetrafluoroethane (R134a). Under the foaming conditions, at temperatures above the glass transition temperature of a polymer, and at pressures required for flow of highly viscous polymer melts, these alternative blowing agents are frequently supercritical. When polymers are formed into final products by various melt-processing techniques, such as extrusion, injection molding, blow molding, foaming, and spin-coating, extremely high melt viscosity presents a major difficulty. A common method to moderate the processing conditions is to add a liquid solvent or plasticizer to the melt. Solvents and plasticizers lower the glass transition temperature, Tg, of the polymer so that the polymer can be made to flow at lower pressures and temperatures. Replacing liquid solvents with SCFs presents unique processing advantages. Higher diffusivity and lower viscosity of SCFs, compared with liquid solvents, increase rates of dissolution and mixing. The properties of polymer–SCF solutions are tunable via pressure or temperature changes, thus allowing efficient downstream separations. Most importantly, dissolution of an SCF produces very large reductions in melt viscosity compared with a liquid solvent dissolved in the melt. Whether the interest in using SCFs in polymer synthesis and processing is driven by environmental concerns or processing advantages, it is important to understand the rheological behavior of polymer–SCF mixtures. In this chapter, we describe rheological measurements of polymer melts containing dissolved gases for two polymers, polydimethylsiloxane (PDMS) swollen with CO2 at 50 °C and 80 °C and polystyrene (PS) swollen with CO2, R152a, and R134a at 150 °C and 175 °C.
Membranes, Jul 29, 2021
This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY
Journal of Rheology, Sep 1, 1994
Linear viscoelastic fluid theory is used to demonstrate that the infinite frequency dynamic visco... more Linear viscoelastic fluid theory is used to demonstrate that the infinite frequency dynamic viscosity η’∞, the viscosity jump upon start‐up of shear flow η+(0), and the viscosity jump upon cessation of Newtonian shear flow η0−η−(0) are equal. Therefore, these quantities are fundamentally the same material property for fluids that exhibit linear viscoelastic behavior in the limit of small deformations. The η’∞ predictions of several continuum and molecular theories are compared, and implications for theoretical and experimental investigations of these phenomena are discussed.
Computer Physics Communications, Jul 1, 2000
Dissipative Particle Dynamics (DPD), a recently developed mesoscopic simulation technique, is use... more Dissipative Particle Dynamics (DPD), a recently developed mesoscopic simulation technique, is used to model the morphology and dynamical behavior of short-chain diblock copolymer melts. The two blocks, A and B, of the copolymer chain are represented by particles of different DPD "phases", and composition is varied by varying the number of beads in each block. As composition is varied from 0-50% A, the predicted morphology changes, progressively, from a disordered system to BCC spherical domains of A, and then to cylindrical domains and lamelae. These predicted phase structures are in agreement with experimental and theoretical results from the literature. Steady shear flow is simulated by means of Lees-Edwards boundary conditions to investigate the effect of shear flow on morphology, and to evaluate viscosity and normal stresses under shearing. The predicted effects of shear flow include flow-induced microstructural transitions, and enhanced rheological properties for systems having the BCC equilibrium microstructure.
Journal of Rheology, Sep 1, 2002
ABSTRACT The rheology and configurational properties of dilute polymer solutions in steady shear ... more ABSTRACT The rheology and configurational properties of dilute polymer solutions in steady shear flow are modeled by dissipative particle dynamics (DPD), a new mesoscopic simulation method. The simulations represent the polymer as a 10-bead chain of FENE connector springs and the solvent as a sea of free DPD particles. Thermodynamic interactions between the polymer and solvent are modeled by varying the strength of the repulsive forces acting between unlike particle species. Since DPD simulations model the full hydrodynamics of the polymer-solvent system, instantaneous hydrodynamic interactions among beads of the polymer chain emerge naturally from the simulations. The predicted rheological material functions include realistic features such as shear thinning of the viscosity and first normal stress coefficient, and negative values for the ratio Ψ2/Ψ1. The enhancement of η, Ψ1, and Ψ2 by means of chain expansion in good solvents is also realistically represented by the model predictions. The DPD simulations accurately describe the variation of the orientation angle of polymer chains with the shear rate, and predict macromolecular expansion ratios that are in good agreement with the recent flow light scattering measurements of Lee and Muller [Polymer 40, 2501-2510 (1999)].
Journal of Cellular and Molecular Medicine, 2011
Since the discovery and implication of N-ethylmaleimide-sensitive factor (NSF)-attachment protein... more Since the discovery and implication of N-ethylmaleimide-sensitive factor (NSF)-attachment protein receptor (SNARE) proteins in membrane fusion almost two decades ago, there have been significant efforts to understand their involvement at the molecular level. In the current study, we report for the first time the molecular interaction between full-length recombinant t-SNAREs and v-SNARE present in opposing liposomes, leading to the assembly of a t-/v-SNARE ring complex. Using high-resolution electron microscopy, the electron density maps and 3D topography of the membrane-directed SNARE ring complex was determined at nanometre resolution. Similar to the t-/v-SNARE ring complex formed when 50 nm v-SNARE liposomes meet a t-SNARE-reconstituted planer membrane, SNARE rings are also formed when 50 nm diameter isolated synaptic vesicles (SVs) meet a t-SNARE-reconstituted planer lipid membrane. Furthermore, the mathematical prediction of the SNARE ring complex size with reasonable accuracy, and the possible mechanism of membrane-directed t-/v-SNARE ring complex assembly, was determined from the study. Therefore in the present study, using both lipososome-reconstituted recombinant t-/v-SNARE proteins, and native v-SNARE present in isolated SV membrane, the membranedirected molecular assembly of the neuronal SNARE complex was determined for the first time and its size mathematically predicted. These results provide a new molecular understanding of the universal machinery and mechanism of membrane fusion in cells, having fundamental implications in human health and disease.
Journal of Chemical Physics, Feb 22, 2002
Langmuir, Sep 14, 2015
Forces between colloidal particles determine the performances of many industrial processes and pr... more Forces between colloidal particles determine the performances of many industrial processes and products. Colloidal force measurements conducted between a colloidal particle AFM probe and particles immobilized on a flat substrate are valuable in selecting appropriate surfactants for colloidal stabilization. One of the features of inorganic fillers and extenders is the prevalence of rough surfaces-even the polymer latex particles, often used as model colloidal systems including the current study, have rough surfaces albeit at a much smaller scale. Surface roughness is frequently cited as the reason for disparity between experimental observations and theoretical treatment but seldom verified by direct evidence. This work reports the effect of nanoscale surface roughness on colloidal force measurements carried out in the presence of surfactants. We applied a heating method to reduce the mean surface roughness of commercial latex particles from 30 nm to 1 nm. We conducted force measurements using the two types of particles at various salt and surfactant concentrations. The surfactants used were pentaethylene glycol monododecyl ether, Pluronic ® F108, and a styrene/acrylic copolymer Joncryl ® 60. In the absence of the surfactant, nanometer surface roughness affects colloidal forces only in high salt conditions when the Debye length becomes smaller than the surface roughness. The adhesion is stronger between colloids with higher surface roughness and requires a higher surfactant concentration to be eliminated. The effect of surface roughness on colloidal forces was also investigated as a function of the adsorbed surfactant layer structure characterized by AFM indentation and dynamic light scattering. We found that when the layer thickness exceeds the surface roughness the colloidal adhesion is less influenced by surfactant concentration variation. This study demonstrates that surface roughness at the nanoscale can influence colloidal forces significantly and should be taken into account in colloidal dispersion formulations.
Molecular Simulation, Aug 1, 2000
ABSTRACT Dissipative Particle Dynamics (DPD) is employed to model the dynamics and rheology of po... more ABSTRACT Dissipative Particle Dynamics (DPD) is employed to model the dynamics and rheology of polymer solutions, and suspensions of spherical particles with adsorbed polymers. Static and dynamic scaling relationships for the variation of radius of gyration and relaxation time with polymer chain length are reviewed, demonstrating that the DPD polymer solution model correctly represents the effects of hydrodynamic interaction and excluded volume. Rheological simulations for both polymer solutions and polymer-sphere suspensions predict Newtonian viscosities at low shear rate followed by shear-thinning behavior as a reduced shear rate of unity is approached. Both the Newtonian viscosity and the extent of shear-thinning are greatly enhanced in the case of good solvents, compared to the viscosity curves for polymers and polymer-spheres structures dissolved in theta solvents and poor solvents.
Fluid Phase Equilibria, 2006
Phase behavior of -d galactose pentaacetate-carbon dioxide binary system is investigated by dew-... more Phase behavior of -d galactose pentaacetate-carbon dioxide binary system is investigated by dew-point and bubble-point measurements conducted in a high pressure variable volume sapphire cell. The phase envelope for solutions of -d galactose pentaacetate in supercritical CO 2 is obtained for -d galactose pentaacetate concentrations between 2 and 26 wt%, and for the temperature range of 308-323 K. The system exhibits lower critical solution temperature (LCST) behavior and high solubility of -d galactose pentaacetate is observed. The densities of the system are also measured, and liquid-like densities (near 1 g/cm 3) are observed for single-phase solutions of -d galactose pentaacetate in supercritical CO 2 at concentrations of 18 wt% and higher. Viscosity is measured for solutions of 18 and 25 wt% -d galactose pentaacetate in the single-phase region at 313 K and 17 MPa and the viscosity values, 0.095 and 0.103 cp, respectively, are similar in magnitude to the viscosity of pure carbon dioxide.
Chemical Engineering Science, 2010
Thermoset nanocomposites (TSNCs) may offer significantly improved performance over conventional t... more Thermoset nanocomposites (TSNCs) may offer significantly improved performance over conventional thermoset materials, and thus are attractive for wide industrial applications, especially in the coating industry. Design of TSNCs via experiment, however, faces various technical challenges due to design complexity. Computational design can provide deep insights and identify superior design solutions through exploring opportunities in a usually huge design space. This paper introduces a generic computational methodology for the design, characterization, and testing of TSNC-based coatings. A distinct feature of the methodology is its capability of generating quantitative correlations among material formulation, processing condition, coating microstructure and property, coating performance, and processing efficiency. The correlations can enable a comprehensive analysis for optimal TSNC coating design. Case studies will demonstrate the methodological efficacy and attractiveness.
Cell Biology International, Apr 1, 2008
Earlier studies using x-ray diffraction, light scattering, photon correlation spectroscopy, and a... more Earlier studies using x-ray diffraction, light scattering, photon correlation spectroscopy, and atomic force microscopy, strongly suggest that SNARE-induced membrane fusion in cells proceed as a result of calcium bridging opposing bilayers. The bridging of phospholipid heads groups in the opposing bilayers by calcium leads to the release of water from hydrated Ca 2+ ions as well as the loosely coordinated water at PO-lipid head groups. Local dehydration of phospholipid headgroups and the calcium, bridging opposing bilayers, then leads to destabilization of the lipid bilayers and membrane fusion. This hypothesis was tested in the current study by atomistic molecular dynamic simulations in the isobaric-isothermal ensemble using hydrated dimethylphosphate anions (DMP−) and calcium cations. Results from the study demonstrate, formation of DMP-Ca 2+ complexes and the consequent removal of water, supporting the hypothesis. Our study further demonstrates that as a result of Ca2+-DMP self-assembly, the distance between anionic oxygens between the two DMP molecules is reduced to 2.92 Å, which is in close agreement with the 2.8 Å SNARE-induced apposition established between opposing bilayers, reported earlier from x-ray diffraction measurements.
Since the discovery and implication of N-ethylmaleimide-sensitive factor (NSF)-attachment protein... more Since the discovery and implication of N-ethylmaleimide-sensitive factor (NSF)-attachment protein receptor (SNARE) proteins in membrane fusion almost two decades ago, there have been significant efforts to understand their involvement at the molecular level. In the current study, we report for the first time the molecular interaction between full-length recombinant t-SNAREs and v-SNARE present in opposing liposomes, leading to the assembly of a t-/v-SNARE ring complex. Using high-resolution electron microscopy, the electron density maps and 3D topography of the membrane-directed SNARE ring complex was determined at nanometre resolution. Similar to the t-/v-SNARE ring complex formed when 50 nm v-SNARE liposomes meet a t-SNARE-reconstituted planer membrane, SNARE rings are also formed when 50 nm diameter isolated synaptic vesicles (SVs) meet a t-SNARE-reconstituted planer lipid membrane. Furthermore, the mathematical prediction of the SNARE ring complex size with reasonable accuracy, and the possible mechanism of membrane-directed t-/v-SNARE ring complex assembly, was determined from the study. Therefore in the present study, using both lipososome-reconstituted recombinant t-/v-SNARE proteins, and native v-SNARE present in isolated SV membrane, the membranedirected molecular assembly of the neuronal SNARE complex was determined for the first time and its size mathematically predicted. These results provide a new molecular understanding of the universal machinery and mechanism of membrane fusion in cells, having fundamental implications in human health and disease.
Journal of Supercritical Fluids, 2008
1,3,5-tri-tert-butylbenzene (TTBB) is solid at ambient conditions, and has substantial solubility... more 1,3,5-tri-tert-butylbenzene (TTBB) is solid at ambient conditions, and has substantial solubility in liquid and supercritical carbon dioxide. We present the phase behavior of TTBB-CO 2 binary system at temperatures between 298 and 328 K and at pressures up to 20 MPa. Phase diagrams showing the liquid-vapor, solid-liquid and solid-vapor equilibrium envelopes are constructed by pressure-volume-temperature measurements in a variable-volume sapphire cell. TTBB is highly soluble in CO 2 over a wide range of compositions. Single-phase states are achieved at moderate pressures, even with very high TTBB concentrations. For example, at 328 K, a binary system containing TTBB at a concentration of 95% by weight forms a single-phase above 2.04 MPa. TTBB exhibits a significant melting-point depression in the presence of CO 2 , 45 K at 3.11 MPa, where the normal melting point of 343 K is reduced to 298 K. With its high solubility in carbon dioxide, TTBB has potential uses as a binder or template in materials forming processes using dense carbon dioxide.
Journal of Physical Chemistry B, Sep 13, 2010
In an effort to provide insight into the mechanism of Ca(2+)-induced fusion of lipid vesicles, mo... more In an effort to provide insight into the mechanism of Ca(2+)-induced fusion of lipid vesicles, molecular dynamics simulations in the isobaric-isothermal ensemble are used to investigate interactions of Ca(2+) with apposed lipid bilayers in close proximity. Simulations reveal the formation of a Ca(2+)-phospholipid "anhydrous complex" between apposed bilayers, whereas similar calculations performed with Na(+) display only complexation between neighboring lipids within the same bilayer. The binding of Ca(2+) to apposed phospholipids brings large regions of the bilayers into close contact (<4 Å), displacing water from phospholipid head groups in the process and creating regions of local dehydration. Dehydration of the apposed bilayers leads to ordering of the phospholipid tails, which is partially disrupted by the presence of Ca(2+)-phospholipid bridges.