Ultradilute Solution Research Articles

Coil-stretch transition and the breakdown of computations for viscoelastic fluid flow around a confined cylinder

The breakdown of finite element (FEM) computations for the steady symmetric two-dimensional flow of dilute and ultradilute Oldroyd-B fluids around a cylinder in a channel, at Weissenberg numbers Wi=O(1), is shown to arise due to a coil-stretch transition experienced by polymer molecules in the wake of the cylinder in the vicinity of the location of the stress maximum on the centerline. In dilute Oldroyd-B fluids, due to the modification of the flow caused by the presence of the polymer, the coil-stretch transition leads to the stress maximum diverging toward infinity at a finite value of Wi≈0.7. On the other hand, in ultradilute solutions, the stress maximum approaches infinity only as Wi→∞. In FENE-P fluids, the coil-stretch transition leads to the mean extension of the molecules saturating to a value close to the fully-extended length, with the maximum stress remaining bounded with increasing Wi. An estimation of the number of finite elements required to achieve convergence for ultradilute Oldroyd-B fluids reveals that obtaining solutions at Wi>1 is not feasible.

Polymer Field Effect Transistors of F8T2 Prepared by Evaporative Spray Deposition using Ultradilute Solution Technique

The evaporative spray deposition using ultradilute solution (ESDUS) technique enables layer-by-layer and pixel-by-pixel deposition of polymer semiconductors. In this study we prepared thin films of dioctylfluorene-co-bithiophene) alternating copolymer, F8T2, by ESDUS and by a conventional spin-coat. These films were made into fabricated field effect transistors to investigate carrier transportation properties. Since the organic films were formed by deposition of aerosol particles in ESDUS, the carrier transportation properties in ESDUS films had been expected lower than those in spin-coat films, which are formed through spread polymer solution. However, the device performance was almost identical, indicating that the polymer deposited as particles undergoes rearrangement process on the substrate.

Self‐Organization of Heteroepitaxial CeO2 Nanodots Grown from Chemical Solutions

The appearance of new functionalities and devices arising from sizeand shape-dependent properties has triggered the interest in creating well-defined structures at the nanometricscale. While conventional lithography is used to fabricate structures in the range of hundreds of nanometers, self-organizing and self-assembling processes following a bottom-up approach can easily decrease this size limit and also cover cost-effectively much larger surfaces. Semiconductors, metals, oxides and molecular materials are different areas where such principles are intensively pursued to generate nanostructures. In particular, self-assembling based on the stress associated to heteroepitaxial growth is an attractive fabrication route in which the generation of semiconductor nanostructures has been extensively investigated and, recently, it has spread to other emerging fields such as oxide-based nanotechnology. Complex oxides attract a great interest for a wealth of different physical and chemical properties and applications such as ferromagnetism, ferroelectricity, colossal magnetoresistance, high dielectric constants, catalysis, optical properties, high temperature superconductivity, solar cells, etc. Based on these properties, many device concepts are under investigation requiring lateral confinement at the nanometric scale, therefore, it constitutes a real scientific challenge to understand the formation mechanisms of nanometric structures. So far the mechanisms that drive self-organized nanodot growth have been studied in certain detail in epitaxial materials prepared from vapor deposition techniques, including oxides, and either Stranski–Krastanov or Volmer–Weber mechanisms were found to apply. On the other hand, vicinal substrates have been widely considered as templates for growing low dimensional nanostructures. The role of lattice steps on the growth mechanism of oxide films has been analyzed by several authors, though the formation of self-organized nanostructures using the terraces as templates has been very scarce. Much less attention has been devoted to investigate the capabilities of Chemical Solution Deposition (CSD), a preparation methodology bearing a high interest for many functionalities and practical applications requiring the use of large areas or long lengths. Particularly, CeO2 is a key material which is being extensively investigated because of its very high intrinsic interest in many areas such as catalysis, ionic conductivity, optical and dielectric properties, buffer layer for oxide superconductors, or nanotemplates to manipulate the vortex properties in superconducting materials. In the last case, the use of self-organized templates in combination with superconducting films could lead to a wealth of new superconducting phenomena. It appears then as highly demanding, scientifically and technologically, the study of the mechanisms generating nanostructured networks of CeO2. Very thin films grown by CSD have been found to be unstable under high temperature annealings and so they can become the source of nanostructure generation, though under these conditions the size and morphology control may become problematic. In this work, a new methodology of general validity has been devised for the formation of quasi one-dimensional arrays of nanodots based on the control of the interfacial energy of heteroepitaxial structures grown from ultradiluted chemical solutions. The terraces of vicinal single crystals of LaAlO3 (LAO) or SrTiO3 (STO) perovskite substrates have been used as templates where CeO2 and Ce1–xGdxO2–y (CGO) nanostructures have been grown by CSD. We have been able to confine within the terraces rows of nanodots with heights ∼ 20 times higher than the unit cell steps of the vicinal substrate. We will show that the underlying mechanism leading to nanodot self-organization is grounded on an enhanced energy barrier at the surface steps due to the structural dissimilarity of the heterostructure. Figure 1a displays a typical image of a heat treated LAO vicinal substrate where the straightness of the lattice steps is clearly appreciated and Figure 1b shows the corresponding line scan where single perovskite unit cell steps are distinguished, thus demonstrating that only one type of crystal termination has been achieved. From the width of the terraces we have determined that the miscut angles of the substrates used in this work were in the range h = 0.2 ± 0.1°. A reproC O M M U N IC A IO N

Layered Polymer Film for Organic Devices Prepared by Evaporative Spray Deposition from Ultra-Dilute Solution

not Available.

A computational study of the effect of viscoelasticity on slot coating flow of dilute polymer solutions

We study the effect of viscoelasticity on the fluid dynamics of slot coating flow of dilute polymer solutions. The fluid is modeled by the Oldroyd-B and FENE-P equations in a conformation tensor formulation. The fully coupled equations of the flow are solved by the DEVSS-TG finite element method together with the elliptic domain mapping method to capture the unknown free surface. We observe that dilute solutions, where the presence of polymer molecules affects the flow field, behave qualitatively differently from ultra-dilute solutions, where the presence of polymer molecules does not alter the flow field. In dilute solutions: (1) the stagnation point on the free surface moves towards the static contact line and the recirculation zone shrinks with increasing Weissenberg number; (2) once the stagnation point reaches the static contact line, the hoop stress on the free surface changes sign from negative to positive, which destabilizes the flow; (3) the region of most severe polymer stretch and distortion also moves to the static contact line. The field variables, such as velocity, velocity gradient and conformation tensor, become singular due to a geometric singularity at the static contact line which leads to the failure of the computational method. In contrast, in ultra-dilute solutions, the computations fail when the mesh can not capture steep stress boundary layers at the free surface. The low-flow limit of inertialess slot coating flow is examined in terms of the Elastocapillary number; the coating window for uniform coating narrows as the liquid grows more elastic.

Layered Polymer Light Emitting Diode Prepared by Evaporative Spray Deposition from Ultradilute Solution (ESDUS) Technique at Various Substrate Temperature

The evaporative spray deposition from ultra-dilute solution (ESDUS) technique enables preparing a polymer layered structure. The bi-layer structure consisted of two π-conjugated polymers, where the top layer was deposited from tetrahydrofuran (THF) solution with the ESDUS technique onto the preceding bottom layer composed of the other THF-soluble polymer. It has been reported that the external quantum efficiency (EQE) of the polymer light emitting diode (PLED) having the bi-layer structure is significantly higher than a corresponding single-layer device. In the present study, the polymer bi-layer films are prepared at substrate temperatures of 65 °C and 80 °C. While the photoluminescence of the bi-layer film prepared at 80 °C indicated the separation of the two components in the film, that of the film at 65 °C suggested partially mixed structure. The PLED composed of the film at 80 °C exhibited significantly better performance than that at 65 °C and the PLED composed of the spin-coated mixed film. The mixture structure of the two component films can be controlled simply by the substrate temperature during ESDUS deposition.

Organic semiconductor device preparation by spray deposition using ultradilute solution

Organic semiconductor device preparation by spray deposition using ultradilute solution

How dilute are dilute solutions in extensional flows?

We investigate the concentration dependence of the characteristic relaxation time of dilute polymer solutions in transient uniaxial elongational flow. A series of monodisperse polystyrene solutions of five different molecular weights (1.8×106⩽M⩽8.3×106g∕mol) with concentrations spanning five orders of magnitude were dissolved in two solvents of differing solvent quality (diethylphthalate and oligomeric styrene). Optical measurements with a capillary breakup extensional rheometer of the rate of filament thinning and the time to breakup in each fluid are used to determine the characteristic relaxation time. A criterion for a lower sensitivity limit is introduced, in the form of a minimum concentration cmin necessary for experimental resolution of the effects of polymeric viscoelasticity. This criterion is validated by experiment and comparison to numerical calculations with a multimode bead-spring dumbbell model. These calculations also rationalize previous paradoxical observations of extensional thinning in fluid threads of ultradilute polymer solutions in which stress relaxation apparently occurred faster than predicted by the Zimm theory. Above this minimum sensitivity limit we show that the effective relaxation time of moderately dilute solutions (0.01⩽c∕c*⩽1) in transient extensional flow rises substantially above the fitted value of the relaxation time extracted from small amplitude oscillatory shear flow and above the Zimm relaxation time computed from kinetic theory and intrinsic viscosity measurements. This effective relaxation time exhibits a power-law scaling with the reduced concentration (c∕c*) and the magnitude of the exponent varies with the thermodynamic quality of the solvent. The scaling of this “self-concentration” effect appears to be roughly consistent to that predicted when the dynamics of the partially elongated and overlapping polymer chains are described within the framework of blob theories for semi-dilute solutions.

Synthesis and luminescent properties of a novel Eu-containing nanoparticle

We synthesized Eu-containing nanoparticles using the ultra-dilute solution method and characterized by FT-IR, NMR, GPC, DSC, and UV–visible absorption/photoluminescence spectra. The size of Eu-containing nanoparticles was 30–150nm and the nanoparticles were soluble in common organic solvents. A study of the dependence of emission intensities of the Eu-containing nanoparticles on the Eu content showed that the emission intensities increased nearly linearly with increasing Eu content. In addition, no significant emission concentration quenching phenomenon was observed at the Eu content of 0–9.5mol%.

Small-Area Deposition of Light Emission Polymer by Evaporative Spray Deposition from Ultradilute Solution Technique

The evaporative spray deposition from ultradilute solution (ESDUS) technique has enabled the preparation of organic devices from highly diluted solutions of polymer semiconductors. Here, we report another feature of this technique, the pixel-size deposition of a π-conjugated polymer. Small-area coating was carried out using a simple shadow mask with pixel-size holes. A blue-emission polymer, PFHB, was deposited through the mask. The resultant film had a pixel pattern of 10 µm separation. Moreover, a green-emission polymer, CN-PDHFV, was subsequently deposited over the first patterned layer. The pixel pattern was maintained clear without getting blurred even using the same solvent, tetrahydrofuran, for each polymer deposition. An electroluminescent (EL) device consisting of a patterned partial double layer showed a green emission from a single layer area.

Electroosmotic flow suppression in capillary electrophoresis: Chemisorption of trimethoxy silane-modified polydimethylacrylamide

Adsorbed polymers are widely used to suppress electroosmotic flow (EOF) in capillary electrophoresis (CE). Polymeric coatings, physisorbed onto the surface of the capillary wall, are often unstable under harsh conditions. This can be attributed to the reversible nature of the coating which becomes apparent when the adsorbed layer competes with a second species in the electrophoresis buffer solution for attachment/interaction with the capillary surface. In an effort to overcome the problem of coating instability, trimethoxysilane-modified polydimethylacrylamide was synthesized. This copolymer rapidly adsorbs on the wall from ultradilute aqueous solutions. After incubation at a temperature of 60 degrees C silyl groups, which extend from the polymer backbone, form condensation bonds with the silanols on the capillary surface. This enables subsequent formation of strong covalent bonds between the copolymer and the capillary wall. In this research, we establish that physisorption of polymer chains to the surface is essential for close alignment of surface and polymer silane groups which facilitates the formation of covalent bonds.

Ultra-Fast High-Resolution Agarose Electrophoresis of DNA and RNA using Low-Molarity Conductive Media

Current DNA electrophoretic solutions employ high ionic concentrations and require long electrophoretic run times. Here we demonstrate that high and low molecular weight double-stranded DNA, single-stranded DNA (ssDNA), and RNA can be separated rapidly in agarose-based low-molarity conductive media. Separation of small DNA fragments was optimized by substituting 1-mM solutions of alkali metals or a nonbiological amine that distributed voltage with a minute current. These ultra-dilute solutions can separate DNA at least 15-fold faster Low-molarity media at 5-10 mM adequately separated RNA and larger DNA fragments as well. These novel media reduce the Joule heating of the electrophoretic system and allow for easy-to-use, ultra-fast separation of DNA fragments.

Formation of Oriented Nanostructures from Single Molecules of Conjugated Polymers in Microdroplets of Solution: The Role of Solvent

We examine the role of solution-phase polymer geometries on the favorability of forming oriented nanostructures from single conjugated polymer molecules formed from microdroplets of ultradilute solution. To our knowledge, this is the first time single molecule fluorescence correlation spectroscopy has been used to extract structural information from solution-phase fluorescence measurements. We find that production of oriented species is strongly favored in “poorer” solvents, where the polymer chains have more compact solution-phase structures. While charging during droplet generation is essential for orientation, we conclude that most of the internal organization associated with these interesting structures takes place in solution, independent of the droplets.

Visualization of individual DNA molecules in a small-scale coating flow

Individual DNA molecules in an ultradilute solution were observed with a fluorescence microscope as they flow between a scaled-down rotating roll and a stationary glass knife. The roll picks up a thin layer of liquid from a pool and drags it to the knife, establishing a bead delineated by two menisci. At low roll speed the flow is premetered and there is a large recirculation. The DNA experiences nearly rectilinear shear flow at the minimum gap position where there is a zero velocity surface. We report the mean and the distribution of fractional extension of DNA molecules and show that the mean fractional extension asymptotes to 0.5, in agreement with the results of Smith et al. [D. E. Smith et al., Science 283, 1724 (1999)]. Interestingly, no polymer distortion is observed at the two menisci. At high roll speed, capillarity is not strong enough to drive backflow; the big recirculation under the coverslip breaks into two smaller recirculations and two separation surfaces arise upstream and downstream of the location of the minimum gap. At the upstream separation surface, most DNA molecules are extended parallel to the knife as they traverse the field of view. We report the distribution of DNA extension and shape in this flow region. Slow, nodular recirculations are present under the upstream and downstream free surfaces. Notably, most DNA molecules stretch axially as they move in these slow recirculating regions.

The Hydrodynamics of DNA Electrophoretic Stretch and Relaxation in a Polymer Solution

Theories of DNA electrophoretic separations generally treat the DNA as a free draining polymer moving in an electric field at a rate that depends on the effective charge density of the molecule. Separations can occur in sieving media ranging from ultradilute polymer solutions to tightly cross-linked gels. It has recently been shown that DNA is not free-draining when both electric and nonelectric forces simultaneously act on the molecule, as occurs when DNA collides with a polymer during electrophoretic separations. Here we show that a semidilute polymer solution screens the hydrodynamic interaction that results from the application of these forces. Fluorescently labeled DNA tethered at one end in a semidilute solution of hydroxyl-ethyl cellulose stretch more in an electric field than they stretch in free solution, and approach free-draining behavior. The steady stretching behavior is predicted without adjustable parameters by a theory developed by Stigter using a hydrodynamic screening length found from effective medium theory. Data on the relaxation of stretched molecules after the electric field is removed agree with the Rouse model prediction, which neglects hydrodynamic interactions. The slowest relaxation time constant, τ R, scales with chain length as τ R ∼ L 1.9±0.17 when analyzed by the data collapse method, and as τ R ∼ L 2.17±0.17 when analyzed by multiexponential fit.

Detection of fine magnetic particles coated on a thread using an HTS–SQUID

Polymer-coated magnetic particles, which contain superparamagnetic ferrite nanoparticles, were attached to a nylon thread of 0.35 mm in diameter and were detected by an HTS–SQUID. The length of the sample attached into the thread was within 3 mm and its interval was 30 mm. The particles were magnetized by a coil applied dc field or by a magnet of 1.4 T. The thread ran 2 mm under the SQUID with 20–100 mm/s of the rate. Signals of magnetic beads were detected and the peak-to-peak amplitude of the signals was directly proportional to the applied field and the weight of the magnetic particles. Obtained peak-to-peak amplitude for 20 ng of magnetite particles was 350 pT at 0.25 mT of applied dc field with noise of 18 pT, and estimated detection limit was 10 ng. S/ N ratio was improved by the remanence measurement using the magnet and 5.8 ng of detection limit was obtained. This measurement has been proved to be promising for the continuous analysis of ultra dilute DNA solution.

Preparation of organic bulk heterojunction photovoltaic cells by evaporative spray deposition from ultradilute solution

An organic bulk heterojunction photovoltaic cell (OPC) having a thin layer consisting of a functionalized fullerene and a poly-p-phenylenevinylene (PPV) derivative was prepared from a highly diluted tetrahydrofuran solution at the concentration below 1 ppm by the evaporative spray deposition from ultradilute solution (ESDUS) technique. The power conversion efficiency of the OPC prepared from 1:1 mixture solution of fullerene/PPV derivative at the concentration of 1 ppm was 0.69%, which was almost comparable to that prepared by the conventional spin coating technique from 0.5 wt % chlorobenzene solution of the mixture. Although the bulk heterojunction OPCs are known to show relatively high power conversion efficiency, they have two major problems for development of higher device performance. One is a solubility problem of used materials and the other is a strong dependence of the device performance on the preparation solvent. The former has been a stumbling block to improve the performance by examining various combinations of organic materials. The latter has compelled to use relatively toxic solvents such as a chlorobenzene to prepare the devices. Those problems could be solved by the ESDUS technique to fabricate an organic film of almost insoluble materials.

Organic Devices Prepared by Evaporative Spray Deposition from Ultra-dilute Solution

Organic Devices Prepared by Evaporative Spray Deposition from Ultra-dilute Solution

Epidural analgesia for labor: safety and success

Safe and successful provision of epidural analgesia for labor requires an understanding of the risks of the technique and attention to detail when performing the technique. The frequency of the commonest complication of epidural analgesia, accidental dural puncture, can probably best be minimized by regular practice of the technique. Some catheters, intended for the epidural space, will find themselves intrathecal, intravascular, or subdural. Although such misplacements probably cannot be prevented, their consequences can be minimized by careful, incremental injection of small doses of local anesthetic. The traditional epidural “test dose” of 45 mg of lidocaine and 15 μg of epinephrine can have undesired effects when injected intrathecally, intravenously, or epidurally. Successful epidural analgesia requires first placing a catheter in the epidural space. The combined spinal epidural technique is a very reliable way to accomplish this task. Subsequently, an appropriate dose of drug must be injected or infused by using an effective delivery system. Continuous infusion of ultra-dilute solutions of local anesthetics and opioids will relieve labor pain in some women, but the frequent need for supplemental medications severely limits their efficacy. Patient-controlled epidural analgesia, using slightly more concentrated local anesthetic solutions produces excellent analgesia with less local anesthetic and less need for rescue medication than continuous infusion techniques.

Novel Approach to Chemically Amplified Resist Materials for Next Generation of Lithography

Our research takes the perspective of how to establish resist structures for nanolithography, we prepared nanoparticles with sizes ranging from 10 to 30 nm by intramolecular self-crosslinking of methylmethacrylate (MMA), 2-hydroxyethyl methacrylate (HEMA) and tert-butyl methacrylate (t-BMA) terpolymer (MMA:HEMA:t-BMA=50%:20%:30%) in an ultradilute solution. As shown in images of pattern side walls by AFM, the chain entanglement of linear polymers made pattern edges and side walls extremely rough.