Rheo-optical studies of concentrated polystyrene solutions subjected to transient simple shear flow

Concentrated polystyrene solutions were investigated on the inception and cessation of simple shear flow by means of the technique of two-color flow birefringence. Both monodisperse solutions of various molecular weights and bimodal mixtures were studied. The molecular weight affected both the amount of overshoot in the birefringence and the response time on the inception of shear flow. Large overshoots in birefringence, up to 250%, and undershoot in the orientation angle were observed. The shear stress and the first normal stress difference were calculated by using the stress–optical rule. The amount of strain at the peaks in the stress growth curves are presented along with the steady-state viscosity and primary normal stress coefficient. The experimental results are compared qualitatively with theoretical predictions of various molecular models.     

A near-infrared dye for multichannel imaging

A large Stokes shift dye, composed of water-solubility and near-infrared feature, was developed for multichannel imaging applications.     

A microfluidic system with optical laser tweezers to study mechanotransduction and focal adhesion recruitment

We present a new method to locally apply mechanical tensile and compressive force on single cells based on integration of a microfluidic device with an optical laser tweezers. This system can locate a single cell within customized wells exposing a square-like membrane segment to a functionalized bead. Beads are coated with extracellular matrix (ECM) proteins of interest (e.g. fibronectin) to activate specific membrane receptors (e.g. integrins). The functionalized beads are trapped and manipulated by optical tweezers to apply mechanical load on the ECM-integrin-cytoskeleton linkage. Activation of the receptor is visualized by accumulation of expressed fluorescent proteins. This platform facilitates isolation of single cells and excitation by tensile/compressive forces applied directly to the focal adhesion via specific membrane receptors. Protein assembly or recruitment in a focal adhesion can then be monitored and identified using fluorescent imaging. This platform is used to study the recruitment of vinculin upon the application of external tensile force to single endothelial cells. Vinculin appears to be recruited above the forced bead as an elliptical cloud, centered 2.1 ± 0.5 μm from the 2 μm bead center. The mechanical stiffness of the membrane patch inferred from this measurement is 42.9 ± 6.4 pN μm(-1) for a 5 μm × 5 μm membrane segment. This method provides a foundation for further studies of mechanotransduction and tensile stiffness of single cells.     

On-line multichannel Raman spectroscopic detection system for capillary zone electrophoresis

An on-line multichannel Raman spectroscopic detection system for capillary zone electrophoresis using a charge-coupled device as the detector is described. Resonant, near-resonant and non-resonant excitation Raman spectroscopies are employed. The 400 cm-1 spectral window provides adequate information to identify resolved and unresolved compounds. The use of analyte velocity reduction to allow increased data acquisition times is described. With near-resonant enhancement, the technique is shown to allow detection of 500 attomoles of methyl orange from a 5-nl injection volume, corresponding to 1 x 10(-7) M.     

A multichannel native fluorescence detection system for capillary electrophoretic analysis of neurotransmitters in single neurons

A laser-induced native fluorescence detection system optimized for analysis of indolamines and catecholamines by capillary electrophoresis is described. A hollow-cathode metal vapor laser emitting at 224 nm is used for fluorescence excitation, and the emitted fluorescence is spectrally distributed by a series of dichroic beam-splitters into three wavelength channels: 250–310 nm, 310–400 nm, and >400 nm. A separate photomultiplier tube is used for detection of the fluorescence in each of the three wavelength ranges. The instrument provides more information than a single-channel system, without the complexity associfated with a spectrograph/charge-coupled device-based detector. With this instrument, analytes can be separated and identified not only on the basis of their electrophoretic migration time but also on the basis of their multichannel signature, which consists of the ratios of relative fluorescence intensities detected in each wavelength channel. The 224-nm excitation channel resulted in a detection limit of 40 nmol L⁻¹ for dopamine. The utility of this instrument for single-cell analysis was demonstrated by the detection and identification of the neurotransmitters in serotonergic LPeD1 and dopaminergic RPeD1 neurons, isolated from the central nervous system of the well-established neurobiological model Lymnaea stagnalis. Not only can this system detect neurotransmitters in these individual neurons with S/N>50, but analyte identity is confirmed on the basis of spectral characteristics. Lapainis and Scanlan contributed equally to this work.     

Rheo‐optical studies of the effect of shear flow on the structure of elastomer blends

The effect of shear flow on the structure of a phase‐separated, near‐critical blend of 50/50 (w/w) poly(styrene‐ran‐butadiene) and polybutadiene was studied with two different custom‐built rheo‐optical instruments that combined polymer melt flow and small‐angle light scattering (SALS). The deformation of the phase domains during shear flow was nonaffine, and the SALS patterns evolved from a spinodal ring (SR) pattern to a squashed SR with two high‐intensity lobes, to an H‐pattern, to a butterfly pattern with a dark streak along the equator, and finally to a steady‐state, elliptical pattern. The SALS patterns were explained in terms of a network model, in which the strands of the network first orient in the flow direction, then extend in this direction, and finally break up into droplets aligned in the flow direction. According to this picture, the strands in the vorticity direction do not deform until relatively high strains, after which the periodicity of the network begins to disappear. Supporting this model was the observation that the transitions between the different SALS patterns corresponded to inflections and/or maxima in the shear stress or first normal stress difference. Increasing the shear rate changed the kinetics of the structure evolution and reduced the size of the phase‐separated droplets in the steady state. No evidence was obtained for flow‐induced miscibility. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1725–1738, 2004     

A general-purpose minicomputer system for electrochemical studies

A software system (SYS) is described for a 16-bit minicomputer interfaced to a potentiostat and electrochemical cells, as well as various display and signal devices. The software controls the functions common to all electrochemical experiments, such as applying cell voltage, timing, sampling signals, displaying these on graphic devices, and smoothing data; it also loads specific user programs into core, for experiments requiring these functions. In this way, a new experiment can quickly be programmed and running; the software also contains some debugging aids. While the system described is specific to the minicomputer used, its general structure should be capable of implementation on any mini- or micro-computer.     

A visualization system for observing plastic foaming processes under shear stress

Previous studies offered theories to explain shear-induced bubble nucleation and growth phenomena in plastic foaming processes, but empirical verification was limited due to difficulty in observing these processes in situ under an easily adjustable and uniform shear flow. This study presents a novel visualization system that successfully achieved this goal. The system allows easy control of the critical experimental parameters: applied shear strain, shear strain rate, temperature, pressure, pressure drop rate, plastic material and blowing agent. From a foaming visualization study of polystyrene, it was observed that cell nucleation rate and maximum cell density increased with the applied shear strain, which was due to the decreased local system pressure, detachment and growth of microvoids, and elongation of bubbles. This foaming visualization system provides a direct and effective way to investigate the mechanisms of bubble nucleation and growth under dynamic conditions that simulates industrial plastic foaming processes.     

WAXS studies of global molecular orientation induced in nematic liquid crystals by simple shear flow

Global molecular orientation function coefficients for the nematic liquid crystal 4-cyano 4- n -pentylbiphenyl (5CB) in shear flow are presented, being extracted from 2-dimensional Wide-Angle X-ray Scattering data. A linear increase in orientation parameter P2 is observed with a logarithmic increase in shear rate. It is proposed that this arises from an increased number of LC directors aligning to the shear axis. Upon cessation of shear flow, the anisotropy is seen to relax away completely, over a time scale which is inversely proportional to the previously applied shear rate.     

A submersible flow analysis system

A submersible chemical analyzer (SCANNER) has been developed which can perform analyses in situ in the ocean. The SCANNER is based on a modified flow-injection system and can be used to automate virtually any spectrophotometric determination that can be done by flow injection analysis. The SCANNER consists of a multichannel peristaltic pump, solid-state colorimeters, manifold tubing, valves, and an electronic module. All of the components are pressure-tolerant, except the electronics module, which is placed in a pressure housing. Typical detection limits are of the order of 0.1 μM. Sample introduction is continuous. The SCANNER has been tested successfully to pressures of 3300 dbar in the laboratory and to depths of 2500 m in the ocean. Examples of silicate and sulfide determinations around animal communities in a deep-sea hydrothermal vent field are presented.     

A study on the relaxation phenomena of nematic polymers after cessation of shear flow

A viscoelastic model, composed of the Ericksen and Landau-de Gennes nematic continuum theories, is used to study numerically the relaxation phenomena after cessation of simple shear flow for a model rigid rod uniaxial nematic polymer. This model predicts that under certain conditions the relaxation of stored molecular and coupling elastic free energies due to periodic fluctuations in the scalar order parameter results in a transient periodic distortion of the director field. These conditions are that: (1) the ratio of the wavelength scales of the initial periodic spatial variation in the scalar order parameter k_s to the initial periodic planar director orientation fluctuation kφ (i.e. k_S/kφ) and the amplitude of the initial S spatial variation exceed certain minimum values, and (2) kφ is not zero. It is shown that the wavelength selection mechanism is controlled by the director reorientation-induced backflows. The digitized optical patterns of the transient periodic director field show transient periodic optical patterns similar to the transient banded texture nematic polymers exhibit after cessation of shear flow when observed between crossed polars. The numerical results and digitized optical patterns replicate frequently reported experimental observations.     

A flow system for calibration of dissolved oxygen sensors

Well-defined oxygen standard solutions were obtained by the electrolysis of water in a coulometric oxygen generator. The generator was integrated into a flow system that includes the degassing of the carrier electrolyte, the generation of dissolved oxygen and the temperature control of the carrier electrolyte. The current efficiency of oxygen generation was found to be 100% by the Winkler titration method. Calibrations of a home made laboratory sensor and a WTW CellOx dissolved oxygen sensor have been made in a concentration range of 0.02 to 8 mg/L at temperatures from 5°C to 30°C. The calibration of the WTW sensor on water vapour saturated air was compared with the electrochemical calibration method. Both methods gave reliable results provided that the temperature equilibration between the sensor and the ambient air was successful. Received: 24 March 1997 / Revised: 15 May 1997 / Accepted: 15 May 1997     

A chemiluminescence flow injection system for nitrite ion determination

A chemiluminescence system is described for the determination of nitrite ion based on new designs for an ozone generator, liquid-gas separator and chemiluminescence reaction cell. The method is based on the gas-phase chemiluminescence reaction between ozone and nitric oxide, which is generated from the reduction of nitrite with iodide in sulfuric acid solution. The efficiency of the system was evaluated by investigation of the analytical performance characteristics of the system for nitrite determination in batch and flow injection procedures. Under optimal conditions, the chemiluminescence response of the system was linear against the nitrite concentration over the range 1 to 1 × 10⁴ ng ml^?1 in the batch procedure and 10 to 5 × 10³ ng ml^?1 in the flow injection procedure, with detection limits of 1 and 10 ng ml^?1, respectively. The method is highly selective and allows for the determination of nitrite in the presence of high concentrations of several cationic, anionic and nitrogen containing species. It has been successfully applied to the analysis of nitrite in natural water and soil extracts.     

Effect of shear history on the steady shear flow behavior of a thermotropic liquid-crystalline polymer

The steady shear stress (σ) and first normal stress difference (N₁) of a thermotropic liquid-crystalline polyester, poly[(phenylsulfonyl)-p-phenylene 1,10-decamethylene-bis(4-oxybenzoate)] (PSHQ10), in both the isotropic and nematic regions were measured as a function of shear rate (γ), using a cone-and-plate rheometer. For the study, PSHQ10 was synthesized via solution polymerization in our laboratory. The PSHQ10 was found to have (a) the weight-average molecular weight of 45,000 relative to polystyrene standards and a polydispersity index of 2, (b) a glass transition temperature of 88°C, (c) a melting point of 115°C, and (d) a nematic-to-isotropic transition temperature of 175°C. For the measurements of σ and N₁ in the nematic region of PSHQ10, its initial conditions for the startup of shear flow was controlled by (a) first heating an as-cast specimen to 190°C, (b) shearing there at γ = 0.085 s^?1 for about 5 min, and then (c) cooling slowly down to a predetermined temperature (130, 140, 150, 160, or 171°C) in the nematic region. For each γ chosen, after start-up of shear flow, we waited for a sufficiently long time until both the shear stress and first normal stress difference leveled off, giving rise to steady-state values of σ and N₁. Emphasis was placed on investigating the effect of shear history on σ and N₁ of PSHQ10 in the nematic region. For this, the following experiments were conducted: (a) a fresh specimen was sheared continuously by increasing the γ stepwise, and (b) a presheared specimen was further sheared continuously by increasing the γ stepwise. We have found that fresh specimens exhibited ‘shear-thinning’ behavior over the entire range of γ (0.008?0.27 s^?1) tested, whereas the presheared specimens exhibited both zero-shear viscosities and shear-thinning behavior. When using fresh specimens, we found that N₁ was positive over the entire range of γ (0.008–0.27 s^?1) tested. However, when using presheared specimens we found that (a) at very low γ, N₁ initially was negative and then became positive as shearing continued, and (b) at higher γ, N₁ was positive over the entire duration of shearing. © 1994 John Wiley & Sons, Inc.     

A microfluidic generator of dynamic shear stress and biochemical signals based on autonomously oscillatory flow

Biological cells in vivo typically reside in a dynamic flowing microenvironment with extensive biomechanical and biochemical cues varying in time and space. These dynamic biomechanical and biochemical signals together act to regulate cellular behaviors and functions. Microfluidic technology is an important experimental platform for mimicking extracellular flowing microenvironment in vitro. However, most existing microfluidic chips for generating dynamic shear stress and biochemical signals require expensive, large peripheral pumps and external control systems, unsuitable for being placed inside cell incubators to conduct cell biology experiments. This study has developed a microfluidic generator of dynamic shear stress and biochemical signals based on autonomously oscillatory flow. Further, based on the lumped-parameter and distributed-parameter models of multiscale fluid dynamics, the oscillatory flow field and the concentration field of biochemical factors has been simulated at the cell culture region within the designed microfluidic chip. Using the constructed experimental system, the feasibility of the designed microfluidic chip has been validated by simulating biochemical factors with red dye. The simulation results demonstrate that dynamic shear stress and biochemical signals with adjustable period and amplitude can be generated at the cell culture chamber within the microfluidic chip. The amplitudes of dynamic shear stress and biochemical signals is proportional to the pressure difference and inversely proportional to the flow resistance, while their periods are correlated positively with the flow capacity and the flow resistance. The experimental results reveal the feasibility of the designed microfluidic chip. Conclusively, the proposed microfluidic generator based on autonomously oscillatory flow can generate dynamic shear stress and biochemical signals without peripheral pumps and external control systems. In addition to reducing the experimental cost, due to the tiny volume, it is beneficial to be integrated into cell incubators for cell biology experiments. Thus, the proposed microfluidic chip provides a novel experimental platform for cell biology investigations.     

A multichannel electrophoresis microchip platform for rapid chiral selector screening

This study presents a four-channel electrophoresis chip platform, featuring double-cross hydrostatic sample injection, for rapid chiral selector screening. This platform needs only five electrodes to drive microchip electrophoresis in four separate channels for screening four chiral selectors at a time. To demonstrate the performance of this screening platform, eight neutral CDs and their derivatives as chiral selectors were screened towards two FITC-labeled chiral compounds. The screening could be accomplished in less than 2 min. Dimethyl-beta-CD and hydroxypropyl-alpha-CD was demonstrated to be the appropriate selectors for FITC-norfenefrine and FITC-baclofen, respectively. The established platform is easy to operate and suitable for rapid screening process, which is expected to be a potential platform for high-throughput screening of chiral selectors.     

A multichannel electron energy loss spectrometer for low-temperature condensed films

We describe a wide-gap multichannel cylindrical deflection electron energy analyzer suitable for measuring the weak signals characteristic of electronically inelastic electron energy loss spectra. The analyzer has nearly ideal fringing field termination, and its resolution and energy dispersion were characterized as a function of energy by solving numerically the equation of motion of electrons in an ideal cylindrical electric field. The numerical results for the radial location of the electrons at the detector as a function of the entrance location, angle, and energy are closely approximated by a second order polynomial, and match closely with those observed. The detection efficiency of the analyzer is 100-150 times better than that of an equivalent single-channel instrument, but limited energy transmission of the zoom lens system used in our case reduced it by a factor of about 2. The performance of the new instrument was demonstrated by measuring the (3)E(1u) electronic spectrum of benzene in only 2 min and the spectrum of endo-benzotricyclo[4.2.1.0(2.5)]nonane.