Phase rainbow refractometry for accurate droplet variation characterization.

We developed a one-dimensional phase rainbow refractometer for the accurate trans-dimensional measurements of droplet size on the micrometer scale as well as the tiny droplet diameter variations at the nanoscale. The dependence of the phase shift of the rainbow ripple structures on the droplet variations is revealed. The phase-shifting rainbow image is recorded by a telecentric one-dimensional rainbow imaging system. Experiments on the evaporating monodispersed droplet stream show that the phase rainbow refractometer can measure the tiny droplet diameter changes down to tens of nanometers. This one-dimensional phase rainbow refractometer is capable of measuring the droplet refractive index and diameter, as well as variations.

Characterizations of transparent particle holography in near-field using Debye series.

The effects of the individual scattering process on the formations of both the particle hologram and its corresponding reconstructed three-dimensional particle image are investigated using the Debye series. A particle hologram model using the Debye series decomposes the object wave into different scattering modes and thus permits evaluating the effects of the individual scattering process [diffraction, reflection, transmission, refractions with (p-1) internal reflections] on the particle holography quantitatively. In the Gabor inline holography of a transparent droplet, the transmission light causes small discrepancies between the hologram fringes of an opaque particle (diffraction) and a transparent particle near the zero point of the Bessel-like modulation function, eventually giving rise to the glory spot in the center of the reconstructed dark particle image. For off-axis holography, this paper reveals the effects of reflection, particularly total reflection by bubbles, transmission, and refractions with (p-1) internal reflections of the scattered light on the formation and the reconstructed glory spot images of typical forward and backward off-axis holography.

One-dimensional rainbow technique using Fourier domain filtering.

Rainbow refractometry can measure the refractive index and the size of a droplet simultaneously. The refractive index measurement is extracted from the absolute rainbow scattering angle. Accordingly, the angular calibration is vital for accurate measurements. A new optical design of the one-dimensional rainbow technique is proposed by using a one-dimensional spatial filter in the Fourier domain. The relationship between the scattering angle and the CCD pixel of a recorded rainbow image can be accurately determined by a simple calibration. Moreover, only the light perpendicularly incident on the lens in the angle (φ) direction is selected, which exactly matches the classical inversion algorithm used in rainbow refractometry. Both standard and global one-dimensional rainbow techniques are implemented with the proposed optical design, and are successfully applied to measure the refractive index and the size of a line of n-heptane droplets.

Recovering the size of nanoparticles by digital in-line holography.

The development of methods to measure the size of nanoparticles is a challenging topic of research. The proposed method is based on the metrology of the stable vapor bubble created by thermal coupling between a laser pulse and the nanoparticle in a droplet. The measurement is realized by digital in-line holography. The size of the nanoparticle is deduced from numerical simulations computed with a photo-thermal finite element method.

Characterization of atomization and breakup of acoustically levitated drops with digital holography.

A digital holographic particle tracking velocimetry system is applied to quantitatively study the drop atomization induced by capillary waves, and the breakup caused by increased sound pressure levels. A wavelet-based algorithm is used for particle detection and autofocusing with a wide size range of 20 µm-2 mm. To eliminate the influence of large particles on small particles, a two-step detection method is adopted. Large drops are first characterized and simulated by a diffraction-based model. Then the contributions of the drops are subtracted from the original hologram followed by the detection of small droplets. Finally, the velocity and size distribution of the secondary droplets are obtained from the experimental holograms. The results demonstrate the validity of the digital in-line holographic technique for the atomization and breakup study of acoustically levitated drops.

Direct measurement of particle size and 3D velocity of a gas-solid pipe flow with digital holographic particle tracking velocimetry.

The 3D measurement of the particles in a gas-solid pipe flow is of great interest, but remains challenging due to curved pipe walls in various engineering applications. Because of the astigmatism induced by the pipe, concentric ellipse fringes in the hologram of spherical particles are observed in the experiments. With a theoretical analysis of the particle holography by an ABCD matrix, the in-focus particle image can be reconstructed by the modified convolution method and fractional Fourier transform. Thereafter, the particle size, 3D position, and velocity are simultaneously measured by digital holographic particle tracking velocimetry (DHPTV). The successful application of DHPTV to the particle size and 3D velocity measurement in a glass pipe's flow can facilitate its 3D diagnostics.

Controllable and enhanced photonic jet generated by fiber combined with spheroid.

Dielectric microparticles are used as simple and low-cost means to achieve strong intensity confinement below the standard diffraction limit. Here we report the demonstration of enhanced light intensity in the vicinity of optical fiber combined with dielectric spheroidal particles. Specific attention is paid to the study of the influences of the spheroid's ellipticity (ratio of horizontal length to vertical length) as well as the refractive index on the intensity enhancement and focal distance. It reveals that simply varying the ellipticity, it is possible to obtain localized photon fluxes having different characteristics. This could yield a new superenhanced intensity device with excellent optical properties and low manufacturing cost for using visible light in many areas of biology, material sciences, chemistry, medicine, and tissue engineering.

Wavelet-based depth-of-field extension, accurate autofocusing, and particle pairing for digital inline particle holography.

Depth-of-field extension and accurate 3D position location are two important issues in digital holography for particle characterization and motion tracking. We propose a method of locating the axial positions of both opaque and transparent objects in the reconstructed 3D field in the wavelet domain. The spatial-frequency property of the reconstructed image is analyzed from the viewpoint of the point spread function of the digital inline holography. The reconstructed image is decomposed into high- and low-frequency subimages. By using the variance of the image gradient in the subimages as focus metrics, the depth-of-field of the synthesis image can be extended with all the particles focalized, and the focal plane of the object can be accurately determined. The method is validated by both simulated and experimental holograms of transparent spherical water droplets and opaque nonspherical coal particles. The extended-focus image is applied to the particle pairing in a digital holographic particle tracking velocimetry to obtain the 3D vector field.

One-dimensional rainbow thermometry system by using slit apertures.

A new rainbow thermometry system by using slit apertures and a laser light sheet, called a one-dimensional rainbow thermometry (ORT) system, has been developed as an extension of global rainbow thermometry (GRT). This system is capable of one-dimensional or line measurements of the size and refractive index of droplets in the spray space, while the conventional GRT system is normally considered a typical "single-point" or "small volume" measurement method. The performance of this new system was tested and verified with both water and ethanol spray. The results show the feasibility and potential of ORT in simultaneous and one-dimensional measurement of the size and refractive index of liquid droplets, especially in the research field of spray evaporation and combustion.

Asymmetric wavelet reconstruction of particle hologram with an elliptical Gaussian beam illumination.

We propose an asymmetric wavelet method to reconstruct a particle from a hologram illuminated by an elliptical, astigmatic Gaussian beam. The particle can be reconstructed by a convolution of the asymmetric wavelet and hologram. The reconstructed images have the same size and resolution as the recorded hologram; therefore, the reconstructed 3D field is convenient for automatic particle locating and sizing. The asymmetric wavelet method is validated by both simulated holograms of spherical particles and experimental holograms of opaque, nonspherical coal particles.

Modified convolution method to reconstruct particle hologram with an elliptical Gaussian beam illumination.

Application of the modified convolution method to reconstruct digital inline holography of particle illuminated by an elliptical Gaussian beam is investigated. Based on the analysis on the formation of particle hologram using the Collins formula, the convolution method is modified to compensate the astigmatism by adding two scaling factors. Both simulated and experimental holograms of transparent droplets and opaque particles are used to test the algorithm, and the reconstructed images are compared with that using FRFT reconstruction. Results show that the modified convolution method can accurately reconstruct the particle image. This method has an advantage that the reconstructed images in different depth positions have the same size and resolution with the hologram. This work shows that digital inline holography has great potential in particle diagnostics in curvature containers.

Droplet characteristic measurement in Fourier interferometry imaging and behavior at the rainbow angle.

This paper presents the possibility of measuring the three-dimensional (3D) relative locations and diameters of a set of spherical particles and discusses the behavior of the light recorded around the rainbow angle, an essential step toward refractive index measurements. When a set of particles is illuminated by a pulsed incident wave, the particles act as spherical light wave sources. When the pulse duration is short enough to fix the particle location (typically about 10 ns), interference fringes between these different spherical waves can be recorded. The Fourier transform of the fringes divides the complex fringe systems into a series of spots, with each spot characterizing the interference between a pair of particles. The analyses of these spots (in position and shape) potentially allow the measurement of particle characteristics (3D relative position, particle diameter, and particle refractive index value).

Interferometric laser imaging for droplet sizing revisited: elaboration of transfer matrix models for the description of complete systems.

We report the development of an interferometric laser imaging for droplet sizing (ILIDS) numerical simulator. It is based on the use of generalized Huygens-Fresnel integrals associated to transfer matrices that describe the whole imaging setup. This simulator allows easy simulation of any kind of ILIDS setup. Simulations are shown to be in good agreement with experimental results. This simulator offers important perspectives in the design, realization, and calibration of ILIDS instruments, as airborne instruments, or in situ measurements in flows.

Measurement of microchannel flow with digital holographic microscopy by integrated nearest neighbor and cross-correlation particle pairing.

A micro digital in-line holographic particle tracking velocimetry (micro-DHPTV) system has been developed and applied to investigate the three-dimensional flow field in straight and Y-junction microchannels. The micro-DHPTV system comprises a cooled frame-transfer CCD camera and a double-pulsed laser. The processing algorithm introduced to evaluate the three-dimensional velocity is based on the combination of integrated cross-correlation and nearest neighbor matching algorithms, taking advantage of information from both the reconstructed particle field and the original holograms fringes patterns. Tests on simulated pairs of holograms show that the particles can be detected, located, and paired with high probability and accuracy. Results obtained in the straight and Y-junction microchannels show that the superimposed vector field is physically reasonable.

Measurements of 3D relative locations of particles by Fourier Interferometry Imaging (FII).

In a large number of physical systems formed of discrete particles, a key parameter is the relative distance between the objects, as for example in studies of spray evaporation or droplets micro-explosion. This paper is devoted to the presentation of an approach where the relative 3D location of particles in the control volume is accurately extracted from the interference patterns recorded at two different angles. No reference beam is used and only ten (2 + 8) 2D-FFT have to be computed.

Nanoshells for photothermal therapy: a Monte-Carlo based numerical study of their design tolerance.

The optimization of the coated metallic nanoparticles and nanoshells is a current challenge for biological applications, especially for cancer photothermal therapy, considering both the continuous improvement of their fabrication and the increasing requirement of efficiency. The efficiency of the coupling between illumination with such nanostructures for burning purposes depends unevenly on their geometrical parameters (radius, thickness of the shell) and material parameters (permittivities which depend on the illumination wavelength). Through a Monte-Carlo method, we propose a numerical study of such nanodevice, to evaluate tolerances (or uncertainty) on these parameters, given a threshold of efficiency, to facilitate the design of nanoparticles. The results could help to focus on the relevant parameters of the engineering process for which the absorbed energy is the most dependant. The Monte-Carlo method confirms that the best burning efficiency are obtained for hollow nanospheres and exhibit the sensitivity of the absorbed electromagnetic energy as a function of each parameter. The proposed method is general and could be applied in design and development of new embedded coated nanomaterials used in biomedicine applications.

Sizing of particles smaller than 5 microm in digital holographic microscopy.

The holographic microscopy technique is a strong contender for dynamic three-dimensional (3D) measurement of small particles (typically smaller than 5 microm) in microchannels. However, there is a big challenge to accurately measure the size of such small particles. The traditional hologram reconstruction method was numerically investigated. It is found that the error level, especially for the size measurement, is higher than expected, even in an ideal situation without consideration of noise. An alternative way based on Lorenz-Mie (LM) calculations was then presented. The intensity distribution of the fringe pattern on the particle hologram is directly used and compared with the ones calculated using an LM-based program. Various cases for particle sizes from 0.5 to 5 microm and recording distances from 5 to 500 microm are tested. The results show that the accuracy in particle sizing can be significantly improved.

Design of nanostructures for imaging and biomedical applications by plasmonic optimization.

An adapted method of optimization of coated metallic nanoparticles is introduced to perform the optimal choice of material and sizes for better scattering or absorption efficiency. This design of nanoshells, involving plasmon resonance, is achieved to maximize the efficiency factors. The presented method is turned to tune the efficiency of nanoshells for biomedical applications and an increasing of the efficiency factors by 1 or 2 orders of magnitude is predicted with realistic materials.

Sizing of irregular particles using a near backscattered laser Doppler system.

A near backscattered laser Doppler system was presented to carry out velocity and size distribution measurements for irregular particles in two-phase flows. The technique uses amplitudes of particles Doppler signals to estimate the particle size distribution in a statistical manner. Holve's numerical inversion scheme is employed to unfold the dependence of the scattered signals on both particle trajectory and orientation through the measurement volume. The performance and error level of the technique were simulated, and several parameters including the number of particle samples, the fluctuation of irregular particle response function, inversion algorithms, and types of particle size distribution were extensively investigated. The results show that the size distributions for those irregular particles even with strong fluctuations in response function can be successfully reconstructed with an acceptable error level using a Phillips-Twomey-non-negative least-squares algorithm instead of a non-negative least-squares one. The measurement system was then further experimentally verified with irregular quartz sands. Using inversion matrix obtained from the calibration experiment, the average measurement error for the mixing quartz sands with a size range of 200-560 microm are found to be about 23.3%, which shows the reliability of the technique and the potential for it to be applied to industrial measurement.

Theoretical prediction of radiation pressure force exerted on a spheroid by an arbitrarily shaped beam.

A rigorous theory is developed to predict the radiation pressure force (RPF) exerted on a spheroid by an arbitrarily oriented and located shaped beam. Analytical expressions of RPF are derived for a homogeneous spheroid, which can be prolate or oblate, transparent or absorbing. Exemplifying calculations are performed and RPF calculations for spheroids are compared to RPF calculations for spheres. The "Optical Stretcher" is also numerically simulated to study the RPF exerted on a red blood cell during its deformation.