Separation of screw-sensed particles in a homogeneous shear field

The Stokes motions of three-dimensional screw-sensed slender particles in a homogeneous shear field are investigated, including the effects of buoyancy. Conclusions are drawn about the possibility of achieving a separation of mixtures of right- and left-handed particles. The linearity of the Stokes equations allows complex flows to be solved by adding the effects of the several terms which describe the flow in which the particle is immersed. The homogeneous shear flow considered here consists of three such terms; solutions for a series of 12 unit motions are sufficient to determine the hydrodynamic resistance tensors. The forces and torques experienced by screw-sensed particles are calculated from these 51 resistance tensors, using slender-filament theory. The results allow an estimate of the range of buoyancy parameters for which gravitational sedimentation can be neglected. The fundamental component of the particle motion is a rotation, at approximately the same angular velocity as that of the fluid. Superimposed on this are variations, of large period, in the particle orientation. A phase plane analysis is used to find the terminal orientations. Very long calculation times are required for the phase portrait. An approximate method based on azimuthally-averaged equations is developed to avoid the requirements for long time integration.     

Measurements and simulations of particle dispersion in a turbulent flow

A particle imaging technique has been used to collect droplet displacement statistics in a round turbulent jet of air. Droplets are injected on the jet axis, and a laser sheet and position-sensitive photomultiplier tube are used to track their radial displacement and time-of-flight. Dispersion statistics can be computed which are Lagrangian or Eulerian in nature. The experiments have been simulated numerically using a second-order closure scheme for the jet and a stochastic simulation for the particle trajectories. Results are presented for non-vaporizing droplets of sizes from 35 to 160 μm. The simulations have underscored the importance of initial conditions and early droplet displacement history on the droplet trajectory for droplets with large inertia relative to the turbulence. Estimates of initial conditions have been made and their effect on dispersion is quantified.     

Synthesis of biodegradable copolymers with low‐toxicity zirconium compounds. V. Multiblock and random copolymers of L‐lactide with trimethylene carbonate obtained in copolymerizations initiated with zirconium(IV) acetylacetonate

Using zirconium(IV) acetylacetonate as an initiator of lactide/trimethylene carbonate copolymerization allowed us to obtain high‐molecular‐weight copolymers with high efficiency. The reactivity ratios of the comonomers were 13.0 for lactide and 0.53 for trimethylene carbonate. Despite the large differences between the values of the reactivity ratios, copolymers with randomized chain structures were obtained. This phenomenon occurred as a result of an intensive intermolecular transesterification process proceeding along with the reaction of copolymer chain growth and modifying its final structure. Conducting the copolymerization at the relatively low temperature of about 110 °C, which minimized the influence of intermolecular transesterification, made it possible to obtain semicrystalline copolymers with multiblock structures. Increasing the temperature of copolymerization up to 180 °C was associated with strong intensification of the transesterification reactions. At this temperature, amorphous copolymers were obtained with identical compositions but highly randomized chain structures. An analysis of the chain microstructures of the obtained copolymers, determining the average length of the blocks, the intermolecular transesterification ratio, and the degree of chain randomization, was conducted by means of NMR spectroscopy. For this purpose, very specific signal assignment in the carbonyl and methylene carbon regions of the ¹³C NMR spectra to appropriate comonomer sequences of polymeric chains was performed. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 3184–3201, 2006     

Synthesis of biodegradable copolymers with low‐toxicity zirconium compounds. IV. Copolymerization of glycolide with trimethylene carbonate and 2,2‐dimethyltrimethylene carbonate: Microstructure analysis of copolymer chains by high‐resolution nuclear magnetic resonance spectroscopy

The results of the copolymerization of glycolide with cyclic trimethylene carbonate and 2,2‐dimethyltrimethylene carbonate are described. The copolymerization was conducted in the presence of low‐toxicity zirconium(IV) acetylacetonate as an initiator. With this kind of initiator, the composition of the comonomer units in the copolymer chains was assumed to be obtained with high efficiency. Despite significant differences in the comonomer reactivity, in copolymers containing comparable amounts of glycolidyl and carbonate sequences, highly randomized chain structures were observed. This effect resulted from strong intermolecular transesterification that proceeded during the studied copolymerization and caused glycolidyl microblock randomization. The assignment of the spectral NMR lines to appropriate comonomer sequences of polymeric chains was performed in the region of methylene protons of glycolidyl units in ¹H NMR spectra of the copolymers and in the carbonyl region of carbon spectra. The equations were formulated for a detailed characterization of the obtained copolymer chains, the average lengths of the blocks, and the transesterification and randomization coefficients. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 98–114, 2006     

The response of microstructure to processing in a series of poly(siloxaneimide) copolymers

Poly(siloxaneimide) (PSI) segmented copolymers exhibit organized microdomains if the blocks are sufficiently incompatible. As with neat diblock and triblock copolymers, the processing route employed to prepare films of PSI materials is expected to influence the dimensions and/or morphology of the resultant microstructure. In this work, small-angle neutron scattering (SANS) is utilized to characterize the disordered microstructure found in films of a series of PSI copolymers which are subjected to solvent casting and various thermal treatments. Microstructural dimensions such as the periodicity and correlation length are deduced from the Teubner-Strey (TS) model for disordered microemulsions. The scattering intensity of each copolymer up to q = 5.0 nm^?1, where q is the scattering vector, is found to scale as q^?2.8+?0.1. Results indicate that processing the materials as cast films or as melt-pressed films allowed to cool slowly has a small, but discernible, effect on microstructural characteristics. SANS profiles of films quenched from elevated temperatures reveal a clear transition in microdomain periodicity, which correlates well with the glass transition temperature of the imide microphase in these and other materials of similar chemical structure. © 1993 John Wiley & Sons, Inc.     

Potential distribution around a charged spherical colloidal particle in a medium containing its counterions and a small amount of added salts

A theory is developed for the potential distribution around a charged spherical colloidal particle carrying ionized groups on the particle surface in a medium containing its counterions (i.e., counterions produced from dissociation of the particle surface groups) and a small amount of added salts on the basis of the theory of Imai and Oosawa. Numerical solutions to the Poisson–Boltzmann equation for the potential distribution are obtained for the case of dilute (but not infinitely dilute) particle suspensions of volume fraction 1 for a1 (where is the Debye–Hückel parameter and a is the particle radius). Here we have taken into account the effects of (i) counterions from the particle surface groups, and (ii) the finite particle volume fraction. These effects, which are usually neglected in the conventional Poisson–Boltzmann equation, are found to be important. It is found that, as in the case of completely salt-free media, there is a certain critical value of the particle charge (which is the same as that for the completely salt-free case). When the particle charge is lower than the critical value, the potential is given by a Coulomb potential. If the particle charge is higher than the critical value, then counterions are accumulated in the vicinity of the particle surface (counterion condensation) and the potential becomes less dependent on the particle charge. The above behaviors can be observed even for the case where the electrolyte concentration is higher than the concentration of counterions from the particle surface groups, if the conditions 1 and a1are both satisfied.     

Cadmium colloids from non-aqueous solvents

Cadmium colloids have been prepared by Chemical Liquid Deposition (CLD). The metal is evaporated to yield atoms which are solvated at liquid nitrogen temperature, and upon warming, stable liquid colloids are formed with particle size ranging between 25–100 Å. Zeta potentials were calculated according to the conversion of Hunter and the Hückel equation, for ethanol and dimethyl sulphoxide. UV/VIS measurement of most of the black colloids showed absorption band around 280 nm. For comparison, we prepared CdS colloid with size 400–625 Å. The colloids are stable to oxidation in air and/or oxygen bubbling. The synthesis of colloids and films from Cd with acetone, 2-butanone, ethanol, 2-propanol, 2-methoxyethanol, DMF and DMSO is reported. Transmission Electron Microscopy (TEM) allows us to determine particle size.     

ToF‐SIMS observation of PTFE surfaces modified by α‐particle irradiation

The surface structure of polytetrafluoroethylene (PTFE) upon α‐particle irradiation has been investigated at doses in the range of 1 × 10⁷ to 1 × 10¹¹ Rad and compared with the surface structure of the unirradiated polymer. Both neat and 25% fiberglass content PTFE were studied. The samples, maintained at nominal room temperature, were irradiated in vacuum by 5.5 MeV ⁴He²⁺ ions generated in a tandem accelerator beam line. Static time‐of‐flight SIMS (ToF‐SIMS) was employed to probe chemical changes at the surface as a function of the irradiation level. In general, the data are indicative of increased cross‐linking at α‐doses less than 1 × 10⁹ Rad, followed by increased fragmentation and unsaturation at α‐doses greater than 1 × 10⁹ Rad. Throughout the irradiation regime, scission is a constant factor promoting cross‐linking, branching, and unsaturation. However, at α‐doses greater than 1 × 10¹⁰ Rad, extreme structural degradation of the polymer becomes evident and is accompanied by conversion to oxygen‐functionalized and aliphatic compounds. Thus, for PTFE in an α‐particle field, an upper exposure limit of ～10¹⁰ Rad is essential for nominal retention of molecular structure. Finally, a quantitative relationship between α‐dose and characteristic fragment ion intensity is developed. Copyright © 2005 John Wiley & Sons, Ltd.     

用于下一代传感和光源的新型材料、光纤及器件

制造技术与复杂模型、设计工具的进步使微纳结构光学器件的实现成为可能。微纳结构光学器件可用于导光与光的相互作用,液态或气态新型光源和传感器件。IPAS致力于新型光学材料研究与开发,将玻璃工艺和光纤开发有机结合,重点研究微纳结构光纤,光纤表面功能处理和器件开发。介绍了IPAS的研究实力和近年的发展概况,其中包括中红外光学材料、纳米粒子嵌入玻璃材料、新型化学和生物传感器(适用于超低量样本及/或体内样本)、激光器件,以及用于光数据处理的新型高非线性光纤。     

Membrane tube pearling induced by a coupling of osmotic pressure and nanoparticle adhesion

ABSTRACT

In this work, a coarse-grained molecular dynamics simulation method that belongs to the class of dissipative particle dynamics scheme with implicit solvent was used to indicate that adsorption of nanoparticles (NPs) inside a lipid membrane tube and pressure difference across the membrane, e.g. osmotic pressure, cooperatively induce membrane tube pearling. We demonstrate that NP adsorption and aggregation initiate the shape transformation of the lipid tube, and pressure difference provides a driving force for pearling transition. Depending on the dynamic coupling of tube shape transition and NP aggregation in the interior of the tube, different shape transitions via four kinds of pearling pathways are recognised, including pearls on a string (i.e. vesicles are interconnected via either a chain or double-chain of NPs) and tube-to-vesicle transition that is dominated kinetically either by NP-membrane attraction or by pressure difference. Considering the fact that biological membranes are semipermeable and many proteins interact with the membranes, these findings not only provide a mechanism of membrane tube pearling but also demonstrate the importance of osmotic pressure and protein–membrane interaction for many cell activities related to shape transitions of biomembrane.