Local Optical Probes Research Articles

Experimental investigation and modeling of void drift in modern BWR fuel designs

Abstract Modern fuel designs for Boiling Water Reactor (BWR) include complex features, such as part-length rods (PLRs) and spacer grids with mixing vanes that make the flow very heterogeneous and challenging to simulate accurately (e.g. using sub-channel analysis codes or CFD tools). Due to the lack of reliable local experimental data, the codes have thus far not been validated for the simulation of two-phase flow parameters under BWR operating conditions across the resulting heterogeneous fuel lattice. In order to improve the thermal-hydraulic code predictions, local annular two-phase flow parameters (void and velocity) have been recently measured in a test fuel bundle (including PLRs and spacer grids with mixing vanes) at the Westinghouse FRIGG facility under prototypical BWR core operating conditions. The instruments (local optical probes and Pitot tubes), have been positioned both in an open region downstream PLRs and in a neighboring region surrounded by full length rods (FLRs) at the outlet of the test section. The measurements show higher void fraction and velocity in the region downstream PLRs, thus clearly illustrating a steam crossflow phenomena towards open regions, also known as “void drift”. The measurements are used to support the development and validation of a new void drift crossflow model in the Westinghouse sub-channel analysis code VIPRE-W/MEFISTO-T, applicable to modern fuel rod bundles with PLRs. The results show significant improvements of the code predictions.

Bubbles and microbubbles for hydrodynamic test facilities

Bubbles are known to have important effects in hydrodynamic facilities such as cavitation tunnels, hydraulic flow loops, pumps, valves and turbines test facilities, towing tanks etc. They are also used to simulate real two-phase flows with various void fractions or bubble size distributions under different pressure and velocity conditions. In order to manage safely these problems or to simulate at best the prototype conditions, an arsenal of theoretical and experimental techniques is necessary, including: – The control of dissolved air, which is under the form of independent molecules moving in the water. – The control of nuclei which are generally microbubbles of some tens of micrometers. – The injection of emulsions. Emulsions are a mixture of water and bubbles of millimetric size. – The separation of free air. Free air concerns bubbles of any size. – The resorption and the control of bubbles growing or shrinking by diffusion of air in water. The production and control of bubbles also implies the use of dedicated measuring techniques. Among these are devices to measure the void fraction and size of bubbles in emulsions as well as dissolved gas: – Local optical and electrical probes [14], – Optical techniques, – Venturi measurements (susceptibility meters). Susceptibility is the pressure at which a nucleus must be submitted to explode and create a vapor bubble. – Optical and polarographic gages for dissolved O2 analysis. Finally, microbubbles can be utilized as non-polluting tracers for optical velocity measurements in water and transparent fluids, such as LDV or PIV. Many of these techniques have been developed by YLEC Consultants. Some of these can be utilized in fluids other than water, such as liquid metals.

Ultrafast water dynamics at the interface of the polymerase-DNA binding complex.

DNA polymerases slide on DNA during replication, and the interface must be mobile for various conformational changes. The role of lubricant interfacial water is not understood. In this report, we systematically characterize the water dynamics at the interface and in the active site of a tight binding polymerase (pol β) in its binary complex and ternary state using tryptophan as a local optical probe. Using femtosecond spectroscopy, we observed that upon DNA recognition the surface hydration water is significantly confined and becomes bound water at the interface, but the dynamics are still ultrafast and occur on the picosecond time scale. These interfacial water molecules are not trapped but are mobile in the heterogeneous binding nanospace. Combining our findings with our previous observation of ultrafast water motions at the interface of a loose binding polymerase (Dpo4), we conclude that the binding interface is dynamic and the water molecules in various binding clefts, channels, and caves are mobile and even fluid with different levels of mobility for loose or tight binding polymerases. Such a dynamic interface should be general to all DNA polymerase complexes to ensure the biological function of DNA synthesis.

Atomic-Monolayer Thermoelectric Conversion in Two-Dimensional Chalcogenides

Electronic structures of bismuth telluride, the highest thermoelectric figure of merit at room temperature, are substantially modified by electronic confinements and strong spin-orbit coupling in the few-monolayers thickness regime. Here, by local optical probes, we show direct evidence that efficient thermoelectric current generation is available at the few-monolayers thick bismuth telluride by hot electrons motions through two-dimensional subbands and topologically protected surface states. Our study offers a challenging opportunity to investigate the fundamental photoresponse of topological quantum states, and provides a new design rule for the highly efficient thermoelectric circuitry.

A local optical probe for measuring motion and stress in a nanoelectromechanical system

Nanoelectromechanical systems can be operated as ultrasensitive mass sensors and ultrahigh-frequency resonators, and can also be used to explore fundamental physical phenomena such as nonlinear damping and quantum effects in macroscopic objects. Various dissipation mechanisms are known to limit the mechanical quality factors of nanoelectromechanical systems and to induce aging due to material degradation, so there is a need for methods that can probe the motion of these systems, and the stresses within them, at the nanoscale. Here, we report a non-invasive local optical probe for the quantitative measurement of motion and stress within a nanoelectromechanical system, based on Fizeau interferometry and Raman spectroscopy. The system consists of a multilayer graphene resonator that is clamped to a gold film on an oxidized silicon surface. The resonator and the surface both act as mirrors and therefore define an optical cavity. Fizeau interferometry provides a calibrated measurement of the motion of the resonator, while Raman spectroscopy can probe the strain within the system and allows a purely spectral detection of mechanical resonance at the nanoscale.

Mapping solvation dynamics at the function site of flavodoxin in three redox states.

Flavoproteins are unique redox coenzymes, and the dynamic solvation at their function sites is critical to the understanding of their electron-transfer properties. Here, we report our complete characterization of the function-site solvation of holoflavodoxin in three redox states and of the binding-site solvation of apoflavodoxin. Using intrinsic flavin cofactor and tryptophan residue as the local optical probes with two site-specific mutations, we observed distinct ultrafast solvation dynamics at the function site in the three states and at the related recognition site of the cofactor, ranging from a few to hundreds of picoseconds. The initial ultrafast motion in 1-2.6 ps reflects the local water-network relaxation around the shallow, solvent-exposed function site. The second relaxation in 20-40 ps results from the coupled local water-protein fluctuation. The third dynamics in hundreds of picoseconds is from the intrinsic fluctuation of the loose loops flanking the cofactor at the function site. These solvation dynamics with different amplitudes well correlate with the redox states from the oxidized form, to the more rigid semiquinone and to the much looser hydroquinone. This observation of the redox control of local protein conformation plasticity and water network flexibility is significant, and such an intimate relationship is essential to the biological function of interprotein electron transfer.

Protein Hydration Dynamics and Molecular Mechanism of Coupled Water−Protein Fluctuations

Protein surface hydration is fundamental to its structural stability and flexibility, and water-protein fluctuations are essential to biological function. Here, we report a systematic global mapping of water motions in the hydration layer around a model protein of apomyoglobin in both native and molten globule states. With site-directed mutagenesis, we use intrinsic tryptophan as a local optical probe to scan the protein surface one at a time with single-site specificity. With femtosecond resolution, we examined 16 mutants in two states and observed two types of water-network relaxation with distinct energy and time distributions. The first water motion results from the local collective hydrogen-bond network relaxation and occurs in a few picoseconds. The initial hindered motions, observed in bulk water in femtoseconds, are highly suppressed and drastically slow down due to structured water-network collectivity in the layer. The second water-network relaxation unambiguously results from the lateral cooperative rearrangements in the inner hydration shell and occurs in tens to hundreds of picoseconds. Significantly, this longtime dynamics is the coupled interfacial water-protein motions and is the direct measurement of such cooperative fluctuations. These local protein motions, although highly constrained, are necessary to assist the longtime water-network relaxation. A series of correlations of hydrating water dynamics and coupled fluctuations with local protein's chemical and structural properties were observed. These results are significant and reveal various water behaviors in the hydration layer with wide heterogeneity. We defined a solvation speed and an angular speed to quantify the water-network rigidity and local protein flexibility, respectively. We also observed that the dynamic hydration layer extends to more than 10 A. Finally, from native to molten globule states, the hydration water networks loosen up, and the protein locally becomes more flexible with larger global plasticity and partial unfolding.

Silver cluster induced absorption enhancement and conformation control of peptides

We present a theoretical study of the structural and optical properties of isolated silver cluster-tripeptide hybrid systems. We show that small silver clusters induce enhancement of the optical absorption of peptides. Moreover, silver clusters bound to peptides can reduce the conformational flexibility and invoke changes of the secondary structure. Thus, they might serve as local optical probes in biolabelling applications.

Charge carrier dynamics in organic semiconductors by position dependent optical probing

We investigate charge carrier transport in poly-(3-hexylthiophene) films where sizable lateral photovoltaic effects are observed. Spatiotemporal measurements of the lateral photovoltage (LPV) using a local optical probe are carried out on device structures consisting of the semiconducting polymer film with a Schottky-type backcontact and a front pair of Ohmic contacts. We employ a spreading impedance approach in the context of a discrete circuit element model to obtain a quantitative understanding of the spatial dependence and the frequency response of the lateral photovoltage. An excellent agreement between theoretical model and experiment is found, leading to an understanding of the origin of LPV in organic systems.

Protein surface hydration mapped by site-specific mutations

Water motion at protein surfaces is fundamental to protein structure, stability, dynamics, and function. By using intrinsic tryptophans as local optical probes, and with femtosecond resolution, it is possible to probe surface-water motions in the hydration layer. Here, we report our studies of local hydration dynamics at the surface of the enzyme Staphylococcus nuclease using site-specific mutations. From these studies of the WT and four related mutants, which change local charge distribution and structure, we are able to ascertain the contribution to solvation by protein side chains as relatively insignificant. We determined the time scales of hydration to be 3-5 ps and 100-150 ps. The former is the result of local librational/rotational motions of water near the surface; the latter is a direct measure of surface hydration assisted by fluctuations of the protein. Experimentally, these hydration dynamics of the WT and the four mutants are also consistent with results of the total dynamic Stokes shifts and fluorescence emission maxima and are correlated with their local charge distribution and structure. We discuss the role of protein fluctuation on the time scale of labile hydration and suggest reexamination of recent molecular dynamics simulations.

Femtosecond Studies of Tryptophan Fluorescence Dynamics in Proteins: Local Solvation and Electronic Quenching

We report our systematic examination of tryptophan fluorescence dynamics in proteins with femtosecond resolution. Distinct patterns of femtosecond-resolved fluorescence transients from the blue to the red side of emission have been characterized to distinguish local ultrafast solvation and electronic quenching. It is shown that tryptophan is an ideal local optical probe for hydration dynamics and protein-water interactions as well as an excellent local molecular reporter for ultrafast electron transfer in proteins, as demonstrated by a series of biological systems, here in melittin, human serum albumin, and human thioredoxin, and at lipid interfaces. These studies clarify the assignments in the literature about the ultrafast solvation or quenching dynamics of tryptophan in proteins. We also report a new observation of solvation dynamics at far red-side emission when the relaxation of the local environment is slower than 1 ps. These results provide a molecular basis for using tryptophan as a local molecular probe for ultrafast protein dynamics in general.

Characterization of the fast dynamics of protein amino acid side chains using NMR relaxation in solution.

The physical basis of protein structure, dynamics and function has been intensely studied for several decades. Indeed since the new millennium there has been a tremendous expansion in the number of unique topological folds that have been characterized at high resolution by crystallographic and nuclear magnetic resonance (NMR) based methods. In the midst of this rush towards a grand scale structural genomics effort, a quieter effort dedicated to the experimental characterization of protein conformational heterogeneity has also emerged. The influence of atomic scale structure on molecular recognition and catalysis by proteins is often the focus of attention while the role of dynamics is largely unknown and frequently ignored. Nevertheless, it has long been recognized that proteins are indeed dynamic systems. Early insights into the time scale and character of protein internal motion largely employed local optical probes, unresolved hydrogen exchange, and one dimensional NMR techniques that, though limited, revealed a startling complexity and richness in the internal motion of proteins.1–5 These initial views contributed significantly to the development of current treatments of protein dynamics and thermodynamics.6 The connection to biological function rather than just biological form is more recent. Internal protein dynamics can potentially affect protein function through a variety of mechanisms, some of which are tautological or obvious in nature while others are subtle and remain to be fully explored and appreciated. There are now several examples of protein-protein and protein-ligand interactions that illustrate that dynamics may be fundamentally linked to function in several ways. Nuclear magnetic resonance (NMR) spectroscopy is very much at the center of current efforts to illuminate the nature of protein dynamics and their role in biological function. Here we will focus on the use of solution NMR methods to provide fast subnanosecond dynamics of protein side chains. The interested reader is referred to separate reviews in this issue by Goehlert and Stone,7 Tolman8 and Palmer and Massi9 discussing dynamics of the polypeptide backbone. The emerging success of NMR spectroscopy in the arena of protein dynamics rests on four general areas of development over the past two decades. First and perhaps foremost, triple resonance NMR spectroscopy now provides an efficient and robust set of tools for the comprehensive resonance assignment of proteins of significant size.10,11 These methods, in turn, derive much of their power from companion isotopic enrichment strategies,12,13 which have been subsequently refined to allow for isotopic labeling patterns that are optimized for NMR relaxation studies (vide infra). Two-dimensional sampling of relaxation has allowed for comprehensive studies to be efficiently undertaken, albeit with great instrumental cost.14–16 A variety of technical issues such as the effects of macromolecular tumbling and the influence of competing relaxation mechanisms have also been largely resolved (vide infra). These advances have positioned solution NMR spectroscopy to efficiently and comprehensively characterize the fast internal dynamics of proteins of significant size. This review seeks to provide a compact but reasonably complete description of the theoretical and technical foundation for solution NMR relaxation methods that are currently being brought to bear on fast sub-nanosecond protein side chain dynamics and to present a summary of current findings and their possible significance. A survey of basic observations about side chain dynamics derived from NMR-relaxation studies is presented along with several analyses meant to dispel commonly held but apparently inaccurate correlations between dynamics, structure and function. How dynamics can enter into fundamental thermodynamic and kinetic aspects of protein function is also reviewed and illustrated with intriguing results from several systems that point to a promising future for this area of inquiry.

Towards Rapid DNA Sequencing: Detecting Single‐Stranded DNA with a Solid‐State Nanopore

The rapid developments in nanotechnology have led to exciting progress in the investigation of biological events at the single-molecule level. When studying systems in which molecular individuality matters, single-molecule experiments offer several important advantages over ensemble measurements. First, single-molecule measurements provide important information that is “averaged out” in ensemble results. Second, by conducting many sequential measurements, they allow one to determine the distribution of molecular properties and investigate the inhomogeneous systems. Finally, they permit observation of rarely populated transients that are difficult or impossible to capture using bulk measurements. Traditional single-molecule detection techniques can be roughly grouped into two categories: applied optical spectroscopies, which provide optical and spectroscopic information, and scanning probe microscopies, which yield force and electrical current data. High-speed and reliable genome sequencing is one of the grand challenges in the 21st century. Recently, a number of groups have started to use nanopores for rapid detection of single DNA molecules and their sequences. Two types of nanopores have been used for this purpose: nanopores embedded in an insulating membrane, for example, biological nanopores such as a-hemolysin protein nanopores in lipid membranes; or solid-state nanopores in Si3N4 [9,10] and SiO2. [11,12] While most of the research has focused on double-stranded DNA (dsDNA) translocations, rapid DNA sequencing with nanopores requires single-stranded DNA (ssDNA) molecules to pass through the nanopores and be detected with single-base resolution. Solid-state nanopores possess advantageous features for robust DNA detection and sequencing. They are chemically, thermally, and mechanically stable, which makes it possible to denature DNA under conditions of high pH and/or increased temperature. It is also possible to incorporate solid-state nanopores with local electrical and optical probes to form an integrated circuit. In a recent paper, Li and co-workers demonstrated an exciting experimental investigation for detecting ssDNA using voltage-biased solid-state Si3N4 nanopores. A single nanopore was fabricated using focused ion-beam (FIB) milling followed by feedback-controlled ion-beam sculpting in an insulating Si3N4 membrane. [9] The membrane was then immersed in an ionic solution, which was then divided into two isolated reservoirs with a Ag/AgCl electrode inserted in each of the two reservoirs. By applying a bias voltage over the two electrodes, an electric field was established in the area between the electrodes, ideally, in the nanopore and near it. When DNA molecules were introduced to the reservoir with the negative electrode they diffused toward the nanopore and were captured by the local electric field near it. The electric field within the nanopore then forced the DNA molecules to pass through the pore and translocate into the positive electrode reservoir. The ionic current through the nanopore during the translocation event was monitored to study the history of the molecule=s interaction with the nanopore (Figure 1A). In the design by Li and co-workers, they studied both single dsDNA and ssDNA molecule translocations by performing the experiments under different pH values at room temperature. Figure 1B and C are two-dimensional (translocation time and mean ionic current blockage) histograms of the translocation event density at pH 7 (dsDNA, Figure 1B) and pH 13 (ssDNA, Figure 1C) environments. The DNA molecules used in the experiments are the same length (3kilobase long) and the bias voltage applied was fixed at 120 mV. At pH 7, as Figure 1B shows, a well-defined peak reveals a translocation time of about 170 ms and a current [*] Prof. H. Yan Department of Chemistry and Biochemistry and The Biodesign Institute Arizona State University, Tempe, AZ 85287 (USA) Fax: (+1)480-965-2747 E-mail: hao.yan@asu.edu

Ion sensitive probe measurement in the linear plasma device PSI-2

The suitability of an ion sensitive probe (ISP) with respect to ion temperature (Ti) measurements has been tested in the linear plasma generator in PSI-2. Current–voltage (I–V) characteristics were recorded as a function of two key parameters: shielding height (h) and potential differences between inner and outer electrodes (VB). It could be experimentally confirmed that the current within the electron saturation region is reduced with increasing h. Furthermore, the I–V characteristics change drastically when VB is varied. Radial profiles of the plasma parameters including plasma and floating potentials could be taken. Whereas the Ti values determined by ISP in the plasma core region agree fairly with those obtained by using a local optical probe, severe interpretation difficulties occurred for the outer regions.

Near-field microscopy: throwing light on the nanoworld.

Optical microscopy with nanoscale resolution, beyond that which is possible with conventional diffraction-limited microscopy, may be achieved by scanning a nanoantenna in close proximity to a sample surface. This review will first aim to provide an overview of the basic principles of this technique of scanning near-field optical microscopy (SNOM), before moving on to consider the most widely implemented form of this microscopy, in which the sample is illuminated through a small aperture held less than 10 nm from the sample surface for optical imaging with a resolution of ca. 50 nm. As an example of the application of this microscopy, the results of SNOM measurements of light-emitting polymer nanostructures are presented. In particular, SNOM enables the unambiguous identification of the different phases present in the nanostructures, through the local analysis of the fluorescence from the polymers. The exciting new possibilities for high-resolution optical microscopy and spectroscopy promised by apertureless SNOM techniques are also considered. Apertureless SNOM may involve local scattering of light from a sample surface by a tip, local enhancement of an optical signal by a metal tip, or the use of a fluorescent molecule or nanoparticle attached to a tip as a local optical probe of a surface. These new optical nanoprobes offer the promise of optical microscopy with true nanometre spatial resolution.

Flow phenomena in the exit zone of a circulating fluidized bed

An experimental investigation of the gas and solids flow in a pilot-scale circulating fluidized bed (CFB) cold model with two different abrupt exit configurations (L-shape and extended top) has been carried out. Measurements of axial pressure profiles, high-speed video images of the flow phenomena at the wall as well as local optical probe measurements inside the exit zone are presented. Contrary to published results obtained in bench-scale CFB risers the axial profiles of the apparent solids volume concentration obtained by pressure measurements showed no indication of an increased solids hold-up in the vicinity of the exit, which confirms the conclusion by Pugsley et al. (Can. J. Chem. Eng. 75 (1997) 1001) that this is a scale effect. The local probe measurements showed the well-known core–annulus flow structure prevailing until the riser top. In the vicinity of the exit this flow structure is superimposed by a strong horizontal velocity component directed to the exit duct. In comparison to the conventional L-shaped abrupt exit the extended top does not increase the solids inventory in the riser.

Nanopolar reorientation in ferroelectric thin films

The influence of varying oxygen pressure P(O2) during the growth of Ba0.4Sr0.6TiO3 thin films is investigated using dielectric and local optical probes. A transition from in-plane to out-of-plane ferroelectricity is observed with increasing P(O2). Signatures of in-plane and out-of-plane ferroelectricity are identified using dielectric response and time-resolved confocal scanning optical microscopy (TRCSOM). At the crossover pressure between in-plane and out-of-plane polarization (Pc=85 mTorr), TRCSOM measurements reveal a soft, highly dispersive out-of-plane polarization that reorients in plane under modest applied electric fields. At higher deposition pressures, the out-of-plane polarization is hardened and is less dispersive at microwave frequencies, and the dielectric tuning is suppressed. Nanopolar reorientation is believed to be responsible for the marked increase in dielectric tuning at P(O2)=Pc.

A local optical probe using fluorescence and reflectance formeasurement of volume fractions in multi-phase flows

Three-phase volume-fraction measurement can be obtained by acombination of reflectance and fluorescence methods. Fluorescence from oil canbe detected by an optical fibre probe placed in the flow, while reflectance atthe end of the same fibre probe is also detected. The method is demonstratedby preliminary experiments.

An Experimental Study of the Two-Phase Flow Adjacent to a Wire in Nucleate Boiling Under an Electric Field

An experimental study of the two-phase flow near a heated wire in nucleate pool boiling is presented. A nonuniform electric field of cylindrical geometry was imposed to the heater. The two-phase flow parameters were measured using specifically developed local phase-detection optical probes. The study includes experiments varying the vertical distance from the heater (z) and measurements of the void fraction and the impact rate in a plane perpendicular to the heater (y-z). The results show that the void fraction decreases with applied voltage and with z. The dependence of the void fraction and the impact rate with heat flux is complex, with a strong decrement in the impact rate and void fraction near the critical heat flux when voltage is applied. The maximum void fraction and impact rate move from the vertical centerline (z) to a position 3 heater radii away.

Corrections of gradiomanometer data for volume fractions in two-phase flows

For typical applications of the gradiomanometer in laboratory two-phase flows, we analyse major contributions to the measured differential pressure. Application for estimation of the fluid-phase volume fraction is valid only when terms representing friction and differential momentum flux (acceleration) are negligible or can be corrected for. The importance of friction is commonly recognized, but that of acceleration in developing flows is less so. Order-of-magnitude estimates of friction and acceleration are given for typical cases of (i) air/water and (ii) kerosene/water two-phase flows. We point out that the acceleration term could be significant in developing liquid/liquid flows of weak density contrast. For the liquid/liquid case, it is therefore important that measurements be made in a fully developed region of the flow. Results for several multi-phase flows are compared with data from local optical fibre probes in a series of companion articles.