Short Time Scale Phenomena Research Articles

Multi-frequency passive and active microrheology with optical tweezers

Optical tweezers have attracted significant attention for microrheological applications, due to the possibility of investigating viscoelastic properties in vivo which are strongly related to the health status and development of biological specimens. In order to use optical tweezers as a microrheological tool, an exact force calibration in the complex system under investigation is required. One of the most promising techniques for optical tweezers calibration in a viscoelastic medium is the so-called active–passive calibration, which allows determining both the trap stiffness and microrheological properties of the medium with the least a-priori knowledge in comparison to the other methods. In this manuscript, we develop an optimization of the active–passive calibration technique performed with a sample stage driving, whose implementation is more straightforward with respect to standard laser driving where two different laser beams are required. We performed microrheological measurements over a broad frequency range in a few seconds implementing an accurate multi-frequency driving of the sample stage. The optical tweezers-based microrheometer was first validated by measuring water, and then exemplarily applied to more viscous medium and subsequently to a viscoelastic solution of methylcellulose in water. The described method paves the way to microrheological precision metrology in biological samples with high temporal- and spatial-resolution allowing for investigation of even short time-scale phenomena.

Inclusion of Coulomb effects in laser-atom interactions

We investigate the role of the Coulomb interaction in strong field processes. We find that the Coulomb field of the ion makes its presence known even in highly intense laser fields, in contrast to the assumptions of the strong field approximation. The dynamics of the electron after ionization is analyzed with four models for an arbitrary laser polarization: the Hamiltonian model in the dipole approximation, the strong field approximation, the Coulomb-corrected strong field approximation and the guiding center. These models illustrate clearly the Coulomb effects, in particular Coulomb focusing and Coulomb asymmetry. We show that the Coulomb-corrected strong field approximation and the guiding center are complementary, in the sense that the Coulomb-corrected strong field approximation describes well short time scale phenomena (shorter than a laser cycle) while the guiding center is well suited for describing long time scale phenomena (longer than a laser cycle) like Coulomb-driven recollisions and Rydberg state creation.

Numerical Simulation of Laser Ablation with Short and Ultra-short Pulses for Metals and Semiconductors

Laser ablation is widely used for micro-fabrication in electronic industry and multi-physical numerical simulations allow a better understanding of such processes and can be used to predict the ablation conditions.The recent use of short and ultra-short pulses discloses a new scope of applications. In order to simulate the new conditions arising when using short as well as ultra-short pulses a complex model comprising the physics of laser assisted manufacturing was employed. The model has been extended by means of a two-temperature model that enables the coupling of electron- and lattice-temperature, which is considered essential to describe such short time-scale phenomena, since electrons and lattice of the workpiece are not in thermal equilibrium during the pulse. A comparison of simulation results with available experimental data is finally provided for the case of metals and semiconductors.

Straining and relaxation properties of wet paper during heating

The influence of increasing temperature on the strength and relaxation of wet press-dry paper was studied using a tensile tester equipped with a special heating chamber. The heating chamber made fast heating possible without detectable moisture loss. The results showed that temperature had a significant influence on the straining, relaxation and re-straining behavior of wet paper. The majority of observed changes due to increased temperature seem to originate from the softening of wet fibers. The observed short time scale phenomena in wet paper have practical significance for fiber webs dried under tension in paper machines. Straining–relaxation–de-straining cycles were used to analyze the effect of heating on the work of straining and apparent plastic and elastic work. Heating affected the amount of mechanical energy absorbed by the sample and the amount of elastic energy recoverable in a straining–relaxation–de-straining cycle. Increased temperature reduced the work of straining and both elastically and plastically absorbed energy. The hysteresis work of the examined wet papers was estimated to correspond to a 1–22 mK temperature change. This suggests that temperature changes in wet paper induced by straining play no role in practice. After mechanical conditioning, tensile stiffness in the re-straining of wet paper depended only marginally on temperature, whereas in initial straining the effect of temperature was clearly stronger. The linear thermal expansion coefficient of wet paper in the machine direction was estimated and the influence of moisture content on the linear thermal expansion coefficient of paper was found to be relatively small.

Evaluation of Ultrafast 2D NMR for Quantitative Analysis

Recent ultrafast methods make it possible to obtain two-dimensional (2D) nuclear magnetic resonance (NMR) spectra in a fraction of a second. This paper presents the first evaluation of ultrafast 2D NMR for quantitative analysis. On the basis of optimized conditions presented in recent studies, two homonuclear ultrafast techniques, J-resolved spectroscopy and TOCSY, are evaluated on model mixtures in terms of repeatability, long time stability, and linearity. The results are compared to conventional 1D (1)H NMR spectroscopy. Repeatabilities better than 1% for ultrafast J-resolved spectra and better than 7% for TOCSY spectra are obtained. The long-term stability is better than 4% for J-resolved spectroscopy and between 2% and 11% for TOCSY. Moreover, both methods are characterized by excellent linearities. This new analytical method opens important perspectives for fast, precise, and accurate quantitative analysis of complex mixtures and for the quantitative study of short time scale phenomena.

The Deep Lens Survey Transient Search. I. Short Timescale and Astrometric Variability

We report on the methodology and first results from the Deep Lens Survey transient search. We utilize image subtraction on survey data to yield all sources of optical variability down to 24th magnitude. Images are analyzed immediately after acquisition, at the telescope and in near-real time, to allow for followup in the case of time-critical events. All classes of transients are posted to the web upon detection. Our observing strategy allows sensitivity to variability over several decades in timescale. The DLS is the first survey to classify and report all types of photometric and astrometric variability detected, including solar system objects, variable stars, supernovae, and short timescale phenomena. Three unusual optical transient events were detected, flaring on thousand-second timescales. All three events were seen in the B passband, suggesting blue color indices for the phenomena. One event (OT 20020115) is determined to be from a flaring Galactic dwarf star of spectral type dM4. From the remaining two events, we find an overall rate of \eta = 1.4 events deg-2 day-1 on thousand-second timescales, with a 95% confidence limit of \eta < 4.3. One of these events (OT 20010326) originated from a compact precursor in the field of galaxy cluster Abell 1836, and its nature is uncertain. For the second (OT 20030305) we find strong evidence for an extended extragalactic host. A dearth of such events in the R passband yields an upper 95% confidence limit on short timescale astronomical variability between 19.5 < R < 23.4 of \eta_R < 5.2. We report also on our ensemble of astrometrically variable objects, as well as an example of photometric variability with an undetected precursor.

(KA-4) Optical Diagnostics in Engines

The science of optics has always played an important role in the measurement and understanding of combustion phenomena, including not only laboratory flames but also practical combustion devices such as the internal combustion engines. In particular, being the combustion in direct injection systems a two-phase, turbulent mixing-controlled process that includes short time scale phenomena such as turbulence production and dissipation, spray breakup and evaporation, and pollutants formation it is appropriate to make investigation by non intrusive diagnostic techniques for their intrinsic high temporal and spatial resolution. The paper reviews the optical techniques currently under application to investigate the in-cylinder fluid dynamics, combustion, pollutant formation processes as well as to characterize the exhaust emissions of current internal combustion engines. The review starts with the laser Doppler based techniques for measuring the fluid dynamic field both on air flow test rig and in engines operating under motored conditions as well as the droplet size and velocity of high pressure jet for direct injection engines. Then, it proceeds discussing the potential of polychromatic light scattering, extinction and absorption techniques (Fig. 1) as diagnostic tool for engine investigation alternative to laser induced incandescence (LII) and laser induced fluorescence (LIF). In the second part, polychromatic light extinction and absorption for liquid and vapor distribution measurements in an engine with large optical access is discussed. In the third part, simultaneous extinction, scattering and flame chemiluminescence measurements from ultraviolet (UV) to visible for obtaining detailed information about combustion precursors species during early soot formation and chemical properties of nanometric carbonaceous particles are introduced. In the last part, the potential of broadband (190-500 nm) extinction and scattering spectroscopy is demonstrated to detect and evaluate in real time the size and the number concentration of the exhaust soot particles.

Microstructure and other short time-scale events in millisecond pulsars

AbstractMSP have much more various short time-scale phenomena than ”classical” microstructure in NP. Some of them have a long life-time and are observable not only in single pulses.

Implications of VERITAS on AGN astrophysics

The EGRET detector and existing ground-based γ-ray telescopes have greatly impacted our understanding of blazar-like active galactic nuclei (AGN), but they have also left many questions unanswered. Among these are the nature of the particles which produce the γ-rays, and the mechanisms for accelerating the particles and producing the γ-rays. VERITAS will help answer these questions through precise measures of short time scale phenomena, accurate energy spectra spanning 50 GeV-50 TeV, and the detection of more sources. In this paper, we discuss how VERITAS will contribute to our understanding of AGN.

Using evolutionary computational techniques in environmental modelling

Evolutionary Computation (EC) is a field of computer science that borrows concepts such as natural selection and the genotype-phenotype distinction from biology in order to solve a wide range of complex problems, such as robot controller design, job-shop schedule optimization, pattern recognition, electronic circuit design and many more. In addition, EC techniques in combination with individual-based modelling can be applied in their domain of origin, biology, to investigate the emergence and evolution of natural phenomena. This paper describes the use of EC as both (a) an empirical supplement to analytical approaches to mathematically tractable biological problems, and (b) a vital tool for analyzing highly complex systems of interacting species in heterogeneous environments. Three EC applications, two tractable and one complex, are used to illustrate these points. In general, this work introduces environmental modellers to a cutting-edge computer-science technique that can be of considerable utility, especially in a modern world in which accelerated rates of large-scale environmental change heighten the need for evolutionary considerations in analyses of relatively short time-scale phenomena.

An acoustic imaging system for the concurrent measurement of suspended sediment concentration and particle velocity

This paper presents work on a system providing concurrent concentration and two-dimensional (2-D) particle velocity measurements on suspended sediments in water. The paper shows how one-dimensional methods for independent measurement of sediment concentration and particle velocity can be combined and extended to provide the concurrent 2-D information. An experimental system is described in which suspensions of 0.5-mm diameter sand are illuminated by acoustic burst signals of 1-MHz frequency from two transducers. The relevant theory for determining sediment concentration and particle velocity is presented. The system geometry provides for the illumination of a 5-cm3 volume at ranges of up to 1.1 m, although longer ranges are possible. Results are presented for concurrent concentration and particle velocity measurements for a falling curtain of sediment, and these are shown to be consistent with predicted values. The measurements have sufficient resolution, and can be made sufficiently rapidly, to investigate short time-scale phenomena in hydraulics, such as the effect of wave dynamics on sediment transport. The method presented could form the basis of an instrument giving sequenced acoustic images of the transport phenomena occurring within an observed volume.

Mechanisms Governing the Formation and Destruction of NOxand Other Nitrogenous Species in Low NOxCoal Combustion Systems1

Abstract The salient fundamental factors that influence the design of low NO x burners and of NO x abatement schemes are reviewed. They have been examined through laboratory combustor studies, which simulate some of the salient features of practical systems, while still preserving flexibility and the ability to extract mechanisms. These studies have shown that coal ignition properties and flame attachment to the burner have great influence on coal nitrogen convention to NO x in axial turbulent diffusion flames. They have also shown that distributed air addition is more effective at reducing NO x formation than simple air staging, and these results conform to field data from full scale applications Short time scale phenomena in the ignition zone involve heterogeneous and micro-scale diffusion phenomena, and the total volatile nitrogen released is only partially converted to NO x even in the presence of excess quantities of oxygen. Micro-mixing also complicates predictions of short time scale phenomena duri...

Mechanics of Faulting

Brittle tectonics may be considered on two timescales, in which earthquakes are the short timescale phenomena and faulting is the long timescale process. Faults grow and develop by the cumulative action of earthquakes, and the faults therefore contain the history of past seismicity. In this chapter we discuss the mechanics of faults, which are treated as quasi-static cracks with friction. We begin with a discussion of the elementary theory of faulting, followed by a more modern treatment of the formation and growth of faults and a description of the rocks and structures formed by faulting. Here we rely more heavily on geological observations than elsewhere. We summarize with a discussion of the strength and rheology of faults, finishing with the topic of heterogeneity and its role in faulting, which continues a subtheme to be found throughout this book. Mechanical framework Anderson's theory of faulting In his seminal paper of 1905 and in his memoir of 1951, E. M. Anderson developed the modern mechanical concepts of the origin of faults and emphasized their important role in tectonics. His key contribution was to recognize that faults result from brittle fracture and to apply the Coulomb criterion to this problem. This led him to expect that faults should sometimes form conjugate sets, with planes inclined at acute angles on either side of the maximum principal stress and which include the intermediate principal stress direction (Section 1.1.4, Equation (1.32)).

Solar Radio Bursts with Special Regard to Particle Acceleration and Mass Ejection

Radio data on solar radio bursts imply that acceleration occurs in more than the traditional two phases. Short timescale phenomena, called spike bursts, may be the most direct signatures of acceleration, but they are inadequately understood for firm conclusions to be drawn. Data on events in the solar wind provide direct evidence on particle accelerations. It is suggested that phenomena may be classified according to the type of acceleration involved, with four types being identified: bulk energization, stochastic acceleration, diffusive shock acceleration and shock drift acceleration.

Geodynamic implications of short time scale changes in the geomagnetic dynamo

The discovery of the geomagnetic jerk of 1969 has again focussed attention on short time scale phenomena in the core. The irregular fluctuations in the length of the day and the excitation of the Chandler wobble have characteristics strongly suggesting ‘impulsive’ torques on the mantle. These are believed to result from electromagnetic phenomena associated with disturbances in the core dynamo, the time scales of which depend on Alfven waves. Search for correlations between short lived magnetic phenomena arising in the core and similar short term geodynamic phenomena have yielded some interesting results. Future satellite surveys of the geomagnetic field need to be designed to distinguish such short term core phenomena from external magnetospheric ones. Future geomagnetic charts of the world require much better understanding of discontinuities in the first and second derivatives of the geomagnetic field elements.

Cross Correlation Studies of Stellar Variability using two Telescopes

The study of short time scale phenomena of astronomical interest is often handicapped by the presence of wide band noise. In particular, in the study of the optical variability of stars, such noise arises from atmospheric scintillation, extinction variations, sky radiance variations and photon statistics. For example if in order to resolve rapid stellar variability we reduce the length (T) of the time intervals over which a photometric record is integrated and sampled, we find that the fractional noise fluctuations increase, eventually obscuring any intrinsic variability.

A Flexible Fast-Photometry System

A considerable amount of observational interest in short timescale phenomena has developed in optical astronomy in recent years. This interest has stemmed primarily from search efforts directed towards optical pulsars and the study of rapid fluctuations in cataclysmic variable and flare stars. Such studies have been made feasible by the development of the necessary photon pulse-counting technology required for rapid data sampling.