Jules J. Magda

Molecular dynamics of narrow, liquid‐filled pores

Molecular dynamics studies are reported for a 6‐12 Lennard–Jones liquid in pore channels ranging from about 2–12 molecules wide. The pore walls are modeled as flat surfaces interacting with the fluid molecules via a continuous potential varying only with perpendicular distance from the wall. Liquid density profiles, solvation forces, interfacial tensions, and self‐diffusion coefficients along the pore axis were computed. The density profiles indicate multilayer adsorption in the pore, whereas the locally defined diffusion coefficients do not vary significantly across the pore. The pore‐averaged diffusivity as well as the solvation force oscillate with varying pore width at constant chemical potential. For pore widths greater than ten molecular diameters, the average diffusion coefficient is almost equal to its bulk value, and the solvation force equals the bulk pressure. In the smaller pores the mean square displacement normal to the pore walls never achieves linearity in time, and thus does not reach a d...

Molecular dynamics of flow in micropores

The method of nonequilibrium molecular dynamics is used to study the viscosity and flow properties of strongly inhomogeneous liquids, a particular case of which is a liquid confined in a micropore only a few molecular diameters wide. Fluid inhomogeneity is introduced by imposing an external potential that in one case simulates flat solid walls and in the other case causes density peaks in the middle of a thin liquid film. For comparison a homogeneous fluid is also simulated. In both types of inhomogeneous fluid, the shear stress and effective viscosity are smaller than in the homogeneous fluid. The density profiles and the diffusivities in the micropore were found to be independent of flow, even at the extremely high rates, 1010–1011 s−1 of the simulation. The Green–Kubo relation is found to be valid for the diffusivity under the flow studied. We propose a local average density model (LADM) of viscosity and diffusivity, in which the local transport coefficients are those of homogeneous fluid at a mean den...

Rheological properties of cross-linked hyaluronan-gelatin hydrogels for tissue engineering.

Hydrogels that mimic the natural extracellular matrix (ECM) are used in three-dimensional cell culture, cell therapy, and tissue engineering. A semi-synthetic ECM based on cross-linked hyaluronana offers experimental control of both composition and gel stiffness. The mechanical properties of the ECM in part determine the ultimate cell phenotype. We now describe a rheological study of synthetic ECM hydrogels with storage shear moduli that span three orders of magnitude, from 11 to 3 500 Pa, a range important for engineering of soft tissues. The concentration of the chemically modified HA and the cross-linking density were the main determinants of gel stiffness. Increase in the ratio of thiol-modified gelatin reduced gel stiffness by diluting the effective concentration of the HA component.

Rheological differences among liquid‐crystalline polymers. II. Disappearance of negative N1 in densely packed lyotropes and thermotropes

The steady‐state viscosities η and first normal stress differences N1 as functions of the shear rate γ are measured for highly concentrated and densely packed solutions and melts of hydroxypropylcellulose (HPC) and a polyester [OQO(OEt)10] made from the condensation of 1,10‐decane‐bis‐benzoyl chloride and ethoxy substituted hydroquinone. As the concentration of HPC is increased, systematic departures from the predictions of the Doi theory are found. First, beginning at a concentration C of around 30% by weight, the zero‐shear viscosity begins to increase, rather than decrease, with increasing concentration. Then, at a concentration of 35%, the shear rate γmax at which N1 reaches a local maximum begins to decrease, rather than increase, with concentration, the viscoelastic activation energy of the solution begins to exceed that of the solvent, and the shape of the curve of N1 versus shear rate begins to become temperature sensitive. At a concentration of 55% at room temperature, the region of negative N1...

Deformation‐dependent hydrodynamic interaction in flows of dilute polymer solutions

We have extended the bead–spring model of Zimm to include hydrodynamic interaction that depends upon the conformation of the polymer and hence the deformation rate. Our computational scheme is based on the model of Fixman, and it allows us to investigate numerically chains as large as 500 beads. The major approximation is the replacement of the hydrodynamic interaction tensor with its configurational average. We find that the incorporation of configuration‐dependent hydrodynamic interaction qualitatively changes the Zimm predictions for both the shear and the elongational viscosity. In particular, the elongational viscosity becomes a multivalued function of the rate of elongation. This result is consistent with the coil‐to‐stretch transition predicted by de Gennes and Hinch.

Fabrication of highly uniform nanoparticles from recombinant silk-elastin-like protein polymers for therapeutic agent delivery.

Here we generate silk-elastin-like protein (SELP) polymeric nanoparticles and demonstrate precise control over their dimensions using an electrospray differential mobility analyzer (ES-DMA). Electrospray produces droplets encompassing several polymer strands. Evaporation ensues, leading polymer strands to accumulate at the droplet interface, forming a hollow nanoparticle. The resulting nanoparticle size distributions, which govern particle yield, depend on buffer concentration to the -1/3 power, polymer concentration to the 1/3 power, and ratio of silk-to-elastin blocks. Three recombinantly tuned ratios of 8:16, 4:8, and 4:16, respectively named SELP-815K, SELP-47K, and SELP-415K, are employed, with the latter ratio resulting in a thinner shell and larger diameter for the nanoparticles than the former. The DMA narrows the size distribution by electrostatically classifying the aerosolized nanoparticles. These highly uniform nanoparticles have variations of 1.2 and 1.4 nm for 24.0 and 36.0 nm particles, respectively. Transmission electron microscopy reveals the nanoparticles to be faceted, as a buckling instability releases compression energy arising from evaporation after the shell has formed by bending it. A thermodynamic equilibrium exists between compression and bending energies, where the facet length is half the particle diameter, in agreement with experiments. Rod-like particles also formed from polymer-stabilized filaments when the viscous length exceeds the jet radius at higher solution viscosities. The unusual uniformity in composition and dimension indicates the potential of these nanoparticles to deliver bioactive and imaging agents.

Rheological differences among liquid‐crystalline polymers. I. The first and second normal stress differences of PBG solutions

In this paper and its sequel, we attempt to understand the rheological differences between lyotropic and thermotropic liquid‐crystalline polymers by contrasting the rheology of three different liquid‐crystalline polymers at concentrations ranging from moderate (12%–30%) to highly concentrated (30%–50%) to densely packed (50%–100%). This first paper presents the steady‐state first and second normal stress differences N1 and N2 as functions of the shear rate γ for solutions of poly(γ‐benzyl‐glutamate) with molecular weight 238 000, in the solvent metacresol, at concentrations C ranging from 12.5% to 40% by weight. Predictions of N1 and N2 for this range of concentration are obtained from the Doi molecular theory for rod‐like nematics, using both an approximate, and a nearly exact, method for solving the Doi equation. The predictions of N1, and to a lesser extent N2, agree qualitatively with measured values for all concentrations. In particular, the range of shear rates over which N1 is negative shifts upwa...

Second normal stress difference of a Boger fluid

Abstract Measurement of the second normal stress difference for a highly elastic, constant viscosity ‘Boger fluid’ is reported. Two very different experimental techniques have been used: (1) measurement of the height of the free surface in rod-climbing flow; (2) measurement of the pressure distribution in cone-and-plate shearing flow. The second normal stress difference is at least 30 times smaller in magnitude than the first normal stress difference, and opposite in sign. The results should prove useful in distinguishing between the various constitutive equations proposed for Boger fluids.

The transport properties of rod‐like particles via molecular dynamics. I. Bulk fluid

Molecular dynamics studies of pure bulk fluids of hard, infinitely thin rods are reported. In the semidilute fluid above 70 rods/L3, the rotational diffusion coefficient scales with concentration, with an exponent approaching, but not reaching −2, the value predicted by the Doi–Edwards theory. We estimate the semidilute transition concentration to be at least 70 rods/L3, a concentration twice as large as previously reported from simulations of this rod fluid, but more consistent with recently published theoretical estimates. In the Doi–Edwards theory, a given rod executes a series of step rotations through a sequence of ‘‘cages’’ formed by neighboring rods. Using the simulation results, we estimate a value of τt, the average lifetime of a cage. As one would expect from the Doi–Edwards theory, we find that τt decreases sharply when we turn off the hard core repulsive interactions between the rods which comprise the cages. This result is in disagreement with a recently published Brownian dynamics study. In ...

Monolithic rheometer plate fabricated using silicon micromachining technology and containing miniature pressure sensors for N1 and N2 measurements

An attachment suitable for any standard torsional rheometer is described; it extends the capability of the rheometer, thereby enabling simultaneous measurement of the first (N1) and second (N2) normal stress differences. The attachment is a monolithic rheometer plate 25 mm in diameter that contains eight miniature capacitive pressure sensors, each with a pressure-sensing area less than or equal to 1 mm2. It is found that when the sensor plate is adapted to an ARES rheometer, highly accurate values of N1 and N2 can be measured at room temperature for a standard elastic polymer solution (SRM-1490 from the National Institute of Standards and Technology). Compared to standard measurement methods for N1, use of novel sensor plates of this type should reduce the axial compliance and allow smaller sample sizes to be studied, since no spring-type normal force transducers are required. In addition, a rheometer adapted with the sensor plate, unlike a conventional rheometer, can be used to measure N1 and N2 with a s...