Direct numerical simulation of homogeneous nucleation and growth in a phase-field model using cell dynamics method

The homogeneous nucleation and growth in a simplest two-dimensional phase field model is numerically studied using the cell dynamics method. The whole process from nucleation to growth is simulated and is shown to follow closely the Kolmogorov-Johnson-Mehl-Avrami (KJMA) scenario of phase transformation. Specifically the time evolution of the volume fraction of new stable phase is found to follow closely the KJMA formula. By fitting the KJMA formula directly to the simulation data, not only the Avrami exponent but the magnitude of nucleation rate and, in particular, of incubation time are quantitatively studied. The modified Avrami plot is also used to verify the derived KJMA parameters. It is found that the Avrami exponent is close to the ideal theoretical value m=3. The temperature dependence of nucleation rate follows the activation-type behavior expected from the classical nucleation theory. On the other hand, the temperature dependence of incubation time does not follow the exponential activation-type behavior. Rather the incubation time is inversely proportional to the temperature predicted from the theory of Shneidman and Weinberg [J. Non-Cryst. Solids 160, 89 (1993)]. A need to restrict thermal noise in simulation to deduce correct Avrami exponent is also discussed.     

Free-energy landscape of nucleation with an intermediate metastable phase studied using capillarity approximation

Capillarity approximation is used to study the free-energy landscape of nucleation when an intermediate metastable phase exists. The critical nucleus that corresponds to the saddle point of the free-energy landscape as well as the whole free-energy landscape can be studied using this capillarity approximation, and various scenarios of nucleation and growth can be elucidated. In this study, we consider a model in which a stable solid phase nucleates within a metastable vapor phase when an intermediate metastable liquid phase exists. We predict that a composite critical nucleus that consists of a solid core and a liquid wetting layer as well as pure liquid and pure solid critical nuclei can exist depending not only on the supersaturation of the liquid phase relative to that of the vapor phase but also on the wetting behavior of the liquid surrounding the solid. The existence of liquid critical nucleus indicates that the phase transformation from metastable vapor to stable solid occurs via the intermediate metastable liquid phase, which is quite similar to the scenario of nucleation observed in proteins and colloidal systems. By studying the minimum-free-energy path on the free-energy landscape, we can study the evolution of the composition of solid and liquid within nuclei which is not limited to the critical nucleus.     

Contact angle hysteresis of cylindrical drops on chemically heterogeneous striped surfaces

Contact angle hysteresis of a macroscopic droplet on a heterogeneous but flat substrate is studied using the interface displacement model. First, the apparent contact angle of a droplet on a heterogeneous surface under the condition of constant volume is considered. By assuming a cylindrical liquid-vapor surface (meniscus) and minimizing the total free energy, we derive an equation for the apparent contact angle, which is similar but different from the well-known Cassie's law. Next, using this modified Cassie's law as a guide to predict the behavior of a droplet on a heterogeneous striped surface, we examine several scenarios of contact angle hysteresis using a periodically striped surface model. By changing the volume of the droplet, we predict a sudden jump of the droplet edge, and a continuous change of the apparent contact angle at the edge of two stripes. Our results suggest that as drop volume is increased (advancing contact lines), the predominant drop configuration observed is the one whose contact angle is large; whereas, decreasing drop volume from a large value (receding contact lines) yields drop configuration that predominantly exhibit the smaller contact angle.     

A note on the nucleation with multiple steps: parallel and series nucleation

Parallel and series nucleation are the basic elements of the complex nucleation process when two saddle points exist on the free-energy landscape. It is pointed out that the nucleation rates follow formulas similar to those of parallel and series connection of resistors or conductors in an electric circuit. Necessary formulas to calculate individual nucleation rates at the saddle points and the total nucleation rate are summarized, and the extension to the more complex nucleation process is suggested.     

Synthetic cephalosporins. VI. Synthesis and antibacterial activity of 7-[(Z)-2-(2-aminothiazol-4-yl)-2-(1-carboxy-1-methyl) ethoxyiminoacetamido]- 3-(3-hydroxy-4-pyridon-1-yl)methyl-3-cephem-4-carboxylic acid and related compounds

Synthesis and antibacterial activity of 7-[(Z)-2-(2-aminothiazol-4-yl)-2-(1-carboxy-1-methyl)ethoxyimin oacetamido]- 3-(3-hydroxy-4-pyridon-1-yl)-3-cephem-4-carboxylic acid (12) and its related compounds are described. Compound 12 exhibited excellent antibacterial activity against gram-negative bacteria, and its anti-pseudomonal activity was ten to fifteen times greater than that of ceftazidime.     

Anisotropy in optical transmittance and molecular chain orientation of silver- dispersed uniaxially drawn polyimide films

The anisotropy in optical transmittance in the visible and near-infrared region observed for uniaxially drawn and silver-dispersed polyimide (PI) films was investigated. The films were prepared in a one-step operation that consists of thermal curing and simultaneous uniaxial drawing of poly(amic acid) (PAA) films dissolving 5.7 ∼ 20 mol% of silver nitrate. The PAA was converted to PI by heating, and the PI chains were orientated along the drawing direction during curing. Silver nanoparticles were precipitated in the films when they were cured in air and under nitrogen. In particular, silver nanoparticles aggregated along drawing direction and spheroidal nanoparticles (size of longer axis: 10–25 nm, aspect ratio: ca. 1.5) were observed in the films cured in air, and distinct anisotropy in optical transmittance was observed. The maximum optical anisotropy was obtained with a specific holding time at the final curing temperature (320 °C). In addition, the anisotropy can be controlled by polymer chain orientation when films are cured with the optimal holding time. In optimized preparing conditions, anisotropies in transmittance larger than 500 : 1 were obtained at the wavelengths between 700 and 900 nm, and its optical properties were retained after annealing at 150 °C for 1 hr. The PI films thus obtained can be used as thermally stable thin-film polarizers. Copyright © 2003 John Wiley & Sons, Ltd.     

Nepenthesin, a unique member of a novel subfamily of aspartic proteinases: enzymatic and structural characteristics

Carnivorous plants are known to secrete acid proteinases to digest prey, mainly insects, for nitrogen uptake. In our recent study, we have purified, for the first time, to homogeneity two acid proteinases, nepenthesin I (Nep I) and nepenthesin II (Nep II) from the pitcher fluid of Nepenthes distillatoria and investigated their enzymatic and structural characteristics. Both enzymes were optimally active at pH approx. 2.6 toward acid-denatured hemoglobin; the specificity of Nep I toward oxidized insulin B chain appears to be similar, but slightly wider than those of other aspartic proteinases (APs). At or below 50 degrees C both enzymes were remarkably stable; especially Nep I was extremely stable over a wide range of pH from 3 to 10 for over 30 days. This suggests an evolutionary adaptation of the enzymes to their specific habitat. We have also cloned the cDNAs and deduced the complete amino acid sequences of the precursors of Nep I and Nep II from the pitcher tissue of Nepenthes gracilis. Although the corresponding mature enzymes are homologous with ordinary pepsin-type APs, both enzymes had a high content of cysteine residues (12 residues per molecule), which are assumed to form six unique disulfide bonds as suggested by computer modeling and are thought to contribute toward the remarkable stability of Neps. Moreover, the amino acid sequence identity of Neps with ordinary APs, including plant vacuolar APs, are remarkably low (approx. 20%), and phylogenetic comparison shows that Neps are distantly related to them to form a novel subfamily of APs with a high content of cysteine residues and a characteristic insertion, named 'the Nep-type AP (NAP)-specific insertion', including a large number of novel, orthologous plant APs emerging in the gene/protein databases.