Oblique Excitation Research Articles

Fluorescent Imaging of Single Nanoparticles and Viruses on a Smart Phone

Optical imaging of nanoscale objects, whether it is based on scattering or fluorescence, is a challenging task due to reduced detection signal-to-noise ratio and contrast at subwavelength dimensions. Here, we report a field-portable fluorescence microscopy platform installed on a smart phone for imaging of individual nanoparticles as well as viruses using a lightweight and compact opto-mechanical attachment to the existing camera module of the cell phone. This hand-held fluorescent imaging device utilizes (i) a compact 450 nm laser diode that creates oblique excitation on the sample plane with an incidence angle of ~75°, (ii) a long-pass thin-film interference filter to reject the scattered excitation light, (iii) an external lens creating 2× optical magnification, and (iv) a translation stage for focus adjustment. We tested the imaging performance of this smart-phone-enabled microscopy platform by detecting isolated 100 nm fluorescent particles as well as individual human cytomegaloviruses that are fluorescently labeled. The size of each detected nano-object on the cell phone platform was validated using scanning electron microscopy images of the same samples. This field-portable fluorescence microscopy attachment to the cell phone, weighing only ~186 g, could be used for specific and sensitive imaging of subwavelength objects including various bacteria and viruses and, therefore, could provide a valuable platform for the practice of nanotechnology in field settings and for conducting viral load measurements and other biomedical tests even in remote and resource-limited environments.

Hydrodynamic forces induced by transient sloshing in a 3D rectangular tank due to oblique horizontal excitation

A time-independent finite difference method is developed to simulate fluid sloshing in a three-dimensional tank. The developed numerical scheme is verified by the rigorous benchmark tests. The experiment measurement of liquid sloshing in a 3D tank was also carried out in this study to further validate the accuracy of the present numerical results. Transient waves can change their types naturally in the time domain, especially for a tank excited by resonant frequencies. In this study, if the excitation frequencies are far from the fundamental natural frequency and there are four types of stable sloshing waves discovered due to the oblique horizontal excitation: “diagonal”, “single-directional”, “square-like”, and “irregular” waves. Besides, the swirling waves can only be generated for a partially-filled tank excited at near resonant frequency with oblique horizontal excitation. The evolution of forces induced by different sloshing waves acting on the tank walls is calculated and discussed in this work. In addition, the dynamics of sloshing force induced by swirling waves are explored in detail. The force of the single-directional waves acting on the tank bottom is time-invariant but the other types of sloshing waves show a beating phenomenon which is attributed to the momentum flux across the free surface and the vertical inertia of sloshing fluid. The effect of various oblique excitation directions of the tank on liquid sloshing is discussed as well. The horizontal hydrodynamic force of sloshing waves acting on the mid-section of the left wall of the tank is dominated by the added mass effect if the external excitation frequency is larger than 4 times the lowest natural frequency (ω1) of the tank with partially-filled fluid. On the other hand, the wave elevation of sloshing waves plays a key effect on the horizontal sloshing-induced force when the excitation frequency of the tank is less than 4ω1. A novel mechanism is presented to describe the phenomenon of alternate switch directions of the swirling waves. The relationship between the external force and the sloshing hydrodynamic force is the major factor to trigger the switch direction of the swirling waves. The influence of different base ratios of a rectangular tank on kinematic and dynamic responses of sloshing fluid is also explored in this work.

Measurement of refractive index of luminescent photoelastic coating

The refractive index of the coating is one of the essential parameters used for principal strain separation in luminescent photoelastic coating (LPC) method by oblique incidence technique. According to the theoretical analysis of the amplitude of the emitted light exciting the coating, which returns along the same path as the oblique incident excitation light, we propose a new online method of measuring the refractive index of the LPC based on optical Fresnel response of the coating. The refractive index of the coating containing Rhodamine B as luminescent dye, is measured under the excitation at a wavelength of 465 nm and an incident angle of 60. The experimental result proves the feasibility of the new method, and the present method is also applicable to the refractive index measurement of other luminescent coating.

Analysis on Shift of Nature Modes of Liquid Sloshing in a 3D Tank Subjected to Oblique Horizontal Ground Motions with Damping Devices

The extended study of Wu et al. (2012) of sloshing fluid in tanks with internal structures from 2D to 3D is presented in the paper. The phenomenon of liquid sloshing in a 3D tank with various damping devices is solved by the time-independent finite difference method combined with the ghost (fictitious) cell approach. Two types of damping devices, a tank bottom-mounted baffle and a vertically surface-piercing plate, are considered in the study. In this work, the experimental measurement of liquid sloshing in a 3D tank with the baffle is carried out to further validate the present simulation. The comparison of the results between the experimental measurement and the present computation shows good accuracy. The effect of the vertically tank bottom-mounted baffle or the vertically surface-piercing plate on various sloshing waves for the tank under horizontal oblique excitation is discussed and investigated. The phenomena of the shift of the nature frequency of the tank with damping devices due to various oblique horizontal excitations under different sloshing waves are presented in detail. The sloshing wave type is varied due to the influence of the baffle or the plate, and the coexistence of two types of sloshing waves is found for the tank under larger excitation frequencies.

Full-Wave Analysis of Periodic Dielectric Frequency-Selective Surfaces Under Plane Wave Excitation

A novel full-wave method for the modal characterization of dielectric electromagnetic band-gap structures under oblique plane wave excitation is presented, showing the dielectric lattice periodicity in both the longitudinal and the transverse propagation directions. In this method, an eigenvalue problem is obtained in terms of the generalized ABCD matrix of the periodic cell, whose solution provides not only the propagation constant of the fundamental Floquet mode, but also of the higher order modes of the global periodic structure. The proposed analysis technique has been successfully validated with a commercial software based on the frequency domain finite element method. The transmittance of a finite periodic material with a finite number of unit cells has also been successfully verified through comparisons with other theoretical results reported in the technical literature. Then, our new algorithm has been used to study band-gap materials made of rectangular dielectric parallel rods, investigating the variation of the dispersion diagram and the transmission and reflection transfer functions when modifying both geometrical and electrical parameters of this structure. Finally, the response differences when a periodical defect is considered in a finite periodic arrangement have also been highlighted.

Chronic Cervical Spinal Cord Injury: DTI Correlates with Clinical and Electrophysiological Measures

Diffusion tensor imaging (DTI) is rarely applied in spinal cord injury (SCI). The aim of this study was to correlate diffusion properties after SCI with electrophysiological and neurological measures. Nineteen traumatic cervical SCI subjects and 28 age-matched healthy subjects participated in this study. DTI data of the spinal cord were acquired with a Philips Achieva 3 T MR scanner using an outer volume suppressed, reduced field of view (FOV) acquisition with oblique slice excitation and a single-shot EPI readout. Neurological and electrophysiological measures, American Spinal Injury Association (ASIA) impairment scale scores, and motor (MEP) and somatosensory evoked potentials (SSEP) were assessed in SCI subjects. Fractional anisotropy (FA) values were decreased in the SCI subjects compared to the healthy subjects. In upper cervical segments, the decrease in FA was significant for the evaluation of the entire cross-sectional area of the spinal cord, and for corticospinal and sensory tracts. A decreasing trend was also found at the thoracic level for the corticospinal tracts. The decrease of DTI values correlated with the clinical completeness of SCI, and with SSEP amplitudes. The reduced DTI values seen in the SCI subjects are likely due to demyelination and axonal degeneration of spinal tracts, which are related to clinical and electrophysiological measures. A reduction in DTI values in regions remote from the injury site suggests their involvement with wallerian axonal degeneration. DTI can be used for the quantitative evaluation of the extent of spinal cord damage, and eventually to monitor the effects of future regeneration-inducing treatments.

Scattering focusing and localized surface plasmons in a single Ag nanoring

We investigate the far-field scattering property of a single Ag nanoring. Under oblique excitation, two-focus scatterings with distinct intensities were observed. We show that the two-focus scatterings result from the interference of far-field scattering light from the ring circumference, and the local field enhancement effect of surface plasmons plays the key role in the focus intensity. By finite difference time domain and numerical integer methods, we calculated far-field scattering and surface plasmons’ distributions, and the results are in good agreement with the experiment.

Luminescent photoelastic coating image analysis and strain separation on a three-dimensional grid

The luminescent photoelastic coating technique is an optical technique to measure the full-field strain on 3-D structural components. A luminescent dye within a photoelastic binder is excited with circular polarized light, and the corresponding coating emission intensity is detected via a digital camera for loaded and unloaded states of the specimen to which the coating is applied. Images are processed to find the relative change in emission with respect to camera analyzer position and subsequently analyzed to determine maximum in-plane shear strain and the principal strain directions. For 3-D structures with moderate movement or deflection in the field of view, especially when implementing an oblique excitation approach to separate the principal strains while accounting for non-strain-related polarization changes due to surface inclination, the image analysis is preferably performed on a 3-D grid. This study describes such an approach and discusses the analysis procedures to separate the principal strains and to obtain full-field strain distribution. The theoretical results are compared to experimental data from both a 2-D and a 3-D test specimen.

A note on the foreshortening effect of a flexible beam under oblique excitation

In Li et al. (Multibody Syst. Dyn. 21:249–260, 2009), we described the geometric nonlinear effects of a pivoted beam under point-surface impact. We used the floating frame of reference (FFR) formulation and the ABAQUS/Explicit model for comparative studies. The numerical solutions showed that the two approaches agree well with each other only if the FFR formulation includes proper geometric nonlinear terms. We were, however, not able to do convergence studies and critically find out the obscure stress stiffness term that reflects the negative bending stiffness of the flexible beam during oblique impact. In this paper, we rederive the foreshortening formulation with oblique excitation on a more original foundation, and extract the stress stiffness term right from the generalized active force vector. The stress stiffness effect is shown by a simple example of a cantilever beam under oblique excitation. As a partial comment to Li et al. (Multibody Syst. Dyn. 21:249–260, 2009), no extra effort is needed to account for the stress stiffness effect in the oblique impact case of the pivoted beam while using the foreshortening formulation, because it is a part of the foreshortening effect. Convergence studies are carried out to confirm the numerical solutions obtained using different approaches. As a result, the foreshortening formulation solution obtained using a sufficient number of modes can mostly agree with the benchmark in ABAQUS/Explicit, no matter if different sets of modes corresponding to different reference conditions are used.

Temperature dependence of the luminescent photoelastic coating technique

This paper presents the optical strain response results of a luminescent photoelastic coating (LPC) at elevated temperatures up to 100 °C. The LPC technique is used to acquire full-field stress and strain measurements on structural components. Differing from traditional reflective photoelastic coatings, an LPC uses a luminescent dye that partially retains the induced polarization changes in the illumination field through the coating and provides a relatively high signal response at oblique incidence angles. These characteristics enable the LPC technique to measure the full-field separated principal strains and their directions using oblique excitation. The strain-dependent response of the coating is affected by several parameters such as the luminescent polarization efficiency, optical sensitivity, coating absorptivity, and effective excitation or emission wavelength. The effect of temperature elevation on these parameters is important to characterize if the technique is extended to high-temperature or cyclic-temperature environments. Temperature sensitivity is documented and measurement uncertainty is addressed. Overall, the measured optical strain response decreased by 77 per cent between 24 °C and 92 °C with a steepest decent of –8 per cent/°C at 65 °C.

Strain Separation on a Nonplanar Object Using a Luminescent Photoelastic Coating

This paper presents the theoretical modeling and corresponding experimental results of the oblique incidence response of a luminescent photoelastic coating (LPC) applied to a cylinder under load. LPC is a measurement technique to acquire full-field maximum shear strain and its principal strain direction. The technique uses an absorption dye and a luminescent dye within a photoelastic coating, and the coating is applied on the surface of the specimen using conventional aerosol techniques. On 3D objects, the response of the emission field is dependent on the excitation orientation due to the surface inclination of the structural component and the out-of-plane strain component within the coating. Full-field strain separated results have been previously demonstrated on a 2D specimen. The extension of the strain separation technique to a 3D specimen—a cylinder in bending—is the focus of this investigation. Two different responses were obtained from normal and oblique excitation. As a result, the principal strain was separated over ±56° of circumference of the cylinder with RMS error relative to the theoretical result of 87 μɛ for maximum principal strain and 78 μɛ for minimum.

Effect of the oblique excitation and detection on the cathodoluminescence spectra

We have elucidated the effect of oblique excitation and detection on cathodoluminescence (CL) spectra. A new CL system was developed, in which the light collector (detector) is set normal to the electron beam direction (excitation). The direction of excitation and detection was varied by tilting the specimen. CL spectra from free-standing GaN platelet were recorded with various tilt angles. The band edge emission has suffered serious suppression when the detector is set at the horizontal direction. We attributed it to the reabsorption of band edge emission. However, this suppression is not serious for the shallow level and deep level emissions because these lights have not suffered reabsorption effect. The gradual decrease at higher tilt angle may be explained with the geometrical effect of reducing the excitation volume due to oblique specimen surface. These findings suggest that the geometrical factor may give serious modification on the CL spectra when the specimen surface normal is tilted more than 60° from the direction of detector.

Directional sensitivity of anterior, posterior, and horizontal canal vestibulo-ocular neurons in the cat.

Neurons subserving the vestibulo-ocular reflex transform the directionality and timing of input from semicircular canals into commands that are appropriate to rotate the eyes in a compensatory fashion. In order to assess the degree to which this transformation is evident in vestibular nucleus neurons of alert cats, we recorded the extracellular discharge properties of 138 second-order vestibular neurons in the superior and medial vestibular nucleus, including 64 neurons identified as second-order vestibulo-ocular neurons by antidromic responses to oculomotor nucleus stimulation and short-latency orthodromic responses to labyrinth stimulation (1.3 ms or less). Neuronal response gains and phases were recorded during 0.5-Hz sinusoidal oscillations about many different horizontal axes and during vertical axis rotations to define neuronal response directionality more precisely than in past studies. Neurons with spatial responses similar to anterior semicircular canal afferents were found to have more diverse maximal activation direction vectors than neurons with responses resembling those of posterior or horizontal canal afferents. The mean angle from neuron response vector to the axis of the nearest canal or canal pair was 19 degrees for anterior canal second-order neurons (n=28) and 20 degrees for anterior canal second-order vestibulo-ocular neurons (n=18), compared with 11 degrees for posterior canal second-order neurons (n=43) and 11 degrees for posterior canal second-order vestibulo-ocular neurons (n=25). Only two second-order vestibulo-ocular neurons (3%) showed a marked dependence of response phase on rotation direction, which is indicative of convergent inputs that differ in both dynamics and directionality. This suggests that spatiotemporal convergence is uncommon in the three-neuron vestibulo-ocular reflex arc of the cat. Neuron vectors included many that were closely aligned with canal axes and several that were better aligned with oblique or superior rectus extraocular muscle excitation axis vectors. Only single examples of second-order vestibulo-ocular neuron vectors were approximately aligned with the pitch and roll coordinate axes. We conclude that second-order vestibulo-ocular neurons do not exclusively represent either the semicircular canal sensory coordinate frame or the extraocular muscle excitation motor coordinate frame, and instead are mostly distributed on a continuum between the input and output coordinate frames, with anterior canal neurons having the widest distribution of directionality.

Seismic Elastic Response of Earth Dams with Canyon Interaction

This paper deals with the seismic response of earth and rockfill dams, using elastic models that consider the deformability of the surrounding medium and effects of spatial variation of the seismic excitation. A finite-element-based method has been developed in which the dam is idealized as a shear beam and the surrounding medium as a halfspace. The model can simulate canyons of arbitrary shape, oblique SH-wave seismic excitation, and inhomogeneous distribution of elastic and damping properties of the dam and the sur- rounding medium. The methodology is illustrated by simulating the upstream-downstream response of the La Villita Dam, an earth and rockfill dam located near the epicenter of the 1985 Mexico earthquake, for various conditions of the surrounding foundation material. Results show that for models that include both foundation flexibility and radiated energy, the earthquake response of the dam is, overall, only a fraction of that obtained under the assumption of a rigid canyon. Exploiting these results may have a potentially beneficial effect in the retrofit of existing dams and design of new ones.

About the mechanism of whistlers absorption in the real F-layer of the Earth's ionosphere

The results of cylindrical F-Layer model [1] are generalized for the case of a flat-layer of an inhomogeneous, magnetized plasma, which seems to be more adequate for describing the Earth's ionospheric F-Layer. Two opposite models are considered: a plasma layer with sharp boundaries and an inhomogeneous plasma layer with smooth plasma density profile. The mechanism of whistlers absorption due to oblique Langmuir waves excitation or their emission outside the F-Layer is discussed. It is shown that this mechanism may be important only in the model of plasma layer with sharp boundaries.

Transmission of reverberant sound through orthotropic, viscoelastic multilayered plates

In previous work the authors have studied acoustic transmission through orthotropic multilayered plates under oblique plane wave excitation. In this paper an extension of this work to the case of reverberant sound excitation is reported. In order to compute the mean transmission loss over the wave incidence angles an analytical method has been adopted, rather than the classical double integral calculation which is very cumbersome. The influences of layering, damping, and orthotropy are discussed. A comparison between theoretical and experimental results is presented.