Refractive Index Matching Research Articles

Nonadditive drag of tandem rods drafting in granular sediments.

We examine the drag experienced by a pair of vertical rods moving in tandem through a granular bed immersed in a fluid as a function of their separation distance and speed. As in Newtonian fluids, the net drag experienced by the rods initially increases with distance from the value for a single rod before plateauing to twice the value. However, the drag acting on the two rods is remarkably different, with the leading rod experiencing roughly similar drag compared to a solitary rod, while the following rod experiences far less drag. The anomalous relationship of drag and the distance between the leading and following body is observed in both dry granular beds and while immersed in viscous Newtonian fluids across the quasistatic and the rate-dependent regimes. Through refractive index matching, we visualize the sediment flow past the two rods and show that a stagnant region develops in their reference frame between the rods for small separations. Thus, the following rod is increasingly shielded from the granular flow with decreasing separation distance, leading to a lower net drag. Care should be exercised in applying resistive force theory to multicomponent objects moving in granular sediments based on our result that drag is not additive at short separation distances.

Volumetric Bioprinting of Organoids and Optically Tuned Hydrogels to Build Liver-Like Metabolic Biofactories.

Organ- and tissue-level biological functions are intimately linked to microscale cell-cell interactions and to the overarching tissue architecture. Together, biofabrication and organoid technologies offer the unique potential to engineer multi-scale living constructs, with cellular microenvironments formed by stem cell self-assembled structures embedded in customizable bioprinted geometries. This study introduces the volumetric bioprinting of complex organoid-laden constructs, which capture key functions of the human liver. Volumetric bioprinting via optical tomography shapes organoid-laden gelatin hydrogels into complex centimeter-scale 3D structures in under 20 s. Optically tuned bioresins enable refractive index matching of specific intracellular structures, countering the disruptive impact of cell-mediated light scattering on printing resolution. This layerless, nozzle-free technique poses no harmful mechanical stresses on organoids, resulting in superior viability and morphology preservation post-printing. Bioprinted organoids undergo hepatocytic differentiation showing albumin synthesis, liver-specific enzyme activity, and remarkably acquired native-like polarization. Organoids embedded within low stiffness gelatins (<2kPa) are bioprinted into mathematically defined lattices with varying degrees of pore network tortuosity, and cultured under perfusion. These structures act as metabolic biofactories in which liver-specific ammonia detoxification can be enhanced by the architectural profile of the constructs. This technology opens up new possibilities for regenerative medicine and personalized drug testing.

Visualization by optical fluorescence of two-phase flow in a three-dimensional porous medium

Slow flow of a single fluid through a porous medium is well understood on a macroscopic level through Darcy’s law, a linear relation between flow rate and a combination of pressure differences, viscosity, and gravitational forces. Two-phase flow is complicated by the interface separating the fluids, but understanding of two-dimensional, two-phase flow has been obtained from experiments using transparent cells. In most three-dimensional media, however, visual observation is difficult. Here, we present preliminary results of experiments on a model medium consisting of randomly packed glass spheres, in which one fluorescent liquid invades another. By refractive index matching and scanning with a sheet-shaped laser beam, we obtain slices of the flow patterns, which we combine into three-dimensional pictures. We observe a compact region of invading fluid, surrounded by finger-like protrusions. The compact region becomes more dominant with increasing invader flow rate. The patterns are theoretically analyzed in terms of the interplay between gravitational, viscous, and capillary forces.

Time-resolved 3D visualization of liquid jet breakup and impingement behavior in a shallow liquid pool

A new three-dimensional laser-induced fluorescent (3D-LIF) technology to obtain the hydrodynamic behavior of liquid jets in a shallow pool were developed. In this technology, firstly, a refractive index matching was applied to acquire a clear cross-sectional image. Secondly, a series of cross-sectional images was obtained by using a high-speed galvanometer scanner. Finally, to evaluate the unsteady 3D interface shape of liquid jet, a method was developed to reconstruct 3D shapes from the series of cross-sectional images obtained using the 3D-LIF method. The spatial and temporal resolutions of measurement were 0.21 × 0.21 × 1.0 mm/pixel and 2 ms, respectively. The shape of a 3D liquid jet in a liquid pool and its impingement, spreading and atomization behavior were reconstructed using the proposed method, successfully. The behaviors of atomized particles detached from the jet were obtained by applying data processing techniques. Diameters distribution and position of atomized droplets after detachment were estimated from the results.

Effect of split injection on fuel adhesion characteristics under non-evaporation and evaporation conditions

Spray/wall interaction cannot be avoided in direct-injection spark-ignition (DISI) engines. This leads to the formation of fuel adhesions on the piston head and cylinder wall. The fuel adhesion has a negative impact on combustion and emissions. In the present study, the aim is to investigate the effect of split injection on fuel adhesion characteristics under non-evaporation and evaporation conditions. The fuel adhesion of single, double and triple injections was measured using the refractive index matching (RIM) method. Results show that under the non-evaporation condition, split injection decreases the adhered mass ratio from 15.6% to 12.2% at 30 ms after start of injection (ASOI). Two possible reasons can be concluded as “splashing” and “suction” effects. However, under the evaporation condition, split injection increases the adhered mass ratio slightly. The “heat transfer” has the advantage over “splashing” and “suction”. The main reason is that the fuel adhesion of split injection has short time for heat-transfer with the hot wall and high-temperature ambient gas, leading to more adhesion left on the wall. Furthermore, split injection promotes the uniformity of the fuel adhesion on the wall under non-evaporation and evaporation conditions. Finally, these results could provide data for the development of simulation models.

Ultra-thick inverted green organic light-emitting diodes for high power efficiency over 300 lm/W

We utilize an inverted green OLED architecture with a thick hole transfer layer (HTL) composed of multi-period organic heterojunctions as well as an emitter with horizontal orientation of dipole moment to suppress SP mode loss. The waveguide and substrate mode are extracted by combining the matched high refractive index hemispherical lens on high refractive index glass substrate, Maximum forward-viewing power efficiency (PE) and external quantum efficiency (EQE) of 305.5 lm/W and 71.0% are achieved in the device with 210 nm-thick-HTL, The optical simulation show that our results are very close to the out-coupling theoretical limit. Our work provides a promising strategy to construct OLED with ultra-high PE for achieving low-power environment-friendly display and lighting applications.

Studying dispersed phase holdup in a pilot plant agitated liquid–liquid mixer by developing online expanded laser beam based technique

Expanded Laser Transmission Technique (E-LTT), where a laser beam is used in conjunction with a beam expander large enough to cover the average drop size up to a few millimeters in the path of the laser in a liquid–liquid dispersion mixture, has been applied for online continuous measurement and investigation in a non-invasive manner of the drop phase volume fraction in an agitated flow mixer of a pilot plant scale. The limitations of beam scattering by the drops through the dispersion path were overcome by having liquids of matching refractive indices enabled by the temperature control system. This study reports for the first time the continuous measurements of the line averaged dispersed phase holdup for a pilot plant scale liquid–liquid mixer equipped with a commercial design mixer, where the measurements have no limitations to the geometrical aspects. Experimental results from a cubic mixing tank with a dispersion depth of 30cm were discussed. Online measurements were carried out in the presence of a revolving impeller and transmission of the expanded laser beam. The net volume of the mixer was 20.42liter, and the dispersed phase holdup that was successfully measured ranged between 0.15 and 0.75. In this work, arrangement of the laser setup was made to scan and measure continuously the line average dispersed phase holdup along the height of the pilot plant scale of the flow agitated liquid–liquid mixer. The E-LTT measurements were validated by comparing their results with those obtained from the mixer by shut-down procedure.

Influence of the difference in refractive index on the interface of an object and the surroundings in near-infrared fluorescence tomography.

The refraction of fluorescence from the inside of a sample at the surface results in fluctuations in fluorescence computed tomography (CT). We evaluated the influence of the difference in refractive index (RI) between the sample body and the surroundings on fluorescence CT results. The brightest fluorescent point is away from the correct point on the tomograms owing to the refraction. The speculated position is determined as the exact point if the RI ratio ranges between 0.97 and 1.03 by immersing the body in an RI matching liquid. The results can help in experimental settings of fluorescence CT for acquiring three-dimensional positional information.

Refractive-index matching enhanced polarization sensitive optical coherence tomography quantification in human brain tissue.

The importance of polarization-sensitive optical coherence tomography (PS-OCT) has been increasingly recognized in human brain imaging. Despite the recent progress of PS-OCT in revealing white matter architecture and orientation, quantification of fine-scale fiber tracts in the human brain cortex has been a challenging problem, due to a low birefringence in the gray matter. In this study, we investigated the effect of refractive index matching by 2,2'-thiodiethanol (TDE) immersion on the improvement of PS-OCT measurements in ex vivo human brain tissue. We show that we can obtain fiber orientation maps of U-fibers that underlie sulci, as well as cortical fibers in the gray matter, including radial fibers in gyri and distinct layers of fibers exhibiting laminar organization. Further analysis shows that index matching reduces the noise in axis orientation measurements by 56% and 39%, in white and gray matter, respectively. Index matching also enables precise measurements of apparent birefringence, which was underestimated in the white matter by 82% but overestimated in the gray matter by 16% prior to TDE immersion. Mathematical simulations show that the improvements are primarily attributed to the reduction in the tissue scattering coefficient, leading to an enhanced signal-to-noise ratio in deeper tissue regions, which could not be achieved by conventional noise reduction methods.

Examining the Transmission of Visible Light through Electrospun Nanofibrous PCL Scaffolds for Corneal Tissue Engineering.

The transparency of nanofibrous scaffolds is of highest interest for potential applications like corneal wound dressings in corneal tissue engineering. In this study, we provide a detailed analysis of light transmission through electrospun polycaprolactone (PCL) scaffolds. PCL scaffolds were produced via electrospinning, with fiber diameters in the range from (35 ± 13) nm to (167 ± 35) nm. Light transmission measurements were conducted using UV–vis spectroscopy in the range of visible light and analyzed with respect to the influence of scaffold thickness, fiber diameter, and surrounding medium. Contour plots were compiled for a straightforward access to light transmission values for arbitrary scaffold thicknesses. Depending on the fiber diameter, transmission values between 15% and 75% were observed for scaffold thicknesses of 10 µm. With a decreasing fiber diameter, light transmission could be improved, as well as with matching refractive indices of fiber material and medium. For corneal tissue engineering, scaffolds should be designed as thin as possible and fabricated from polymers with a matching refractive index to that of the human cornea. Concerning fiber diameter, smaller fiber diameters should be favored for maximizing graft transparency. Finally, a novel, semi-empirical formulation of light transmission through nanofibrous scaffolds is presented.

Transparent thermoplastic composite from a refractive index-adjustable polymer blend

Transparent glass fiber composites were prepared with a refractive-index-adjustable thermoplastic blend. The opacity of a polymer blend is affected by the phase separation and crystallization properties. Polyamide 6 (PA6)/poly ( m -xylene adipamide) (MXD6) blends are intrinsically miscible and transparent in the molten state; further, crystallization is suppressed by polymer chain disordering induced by in-situ transamidation. Thus, the transparency of the molten state was maintained after a sufficient period of melt-mixing. The refractive index was adjusted to that of the glass fiber reinforcement by altering the PA6/MXD6 blend ratio. Injection-molded glass fiber composites of PA6/MXD6 had good transparency and mechanical properties. • Injection moldable transparent thermoplastic composites are developed. • Miscible polyamide blends are utilized as refractive index adjustable matrix. • Crystallization of polyamide blends is eliminated by in situ transamination. • Refractive index matching is achieved by adjusting weight fraction of polyamides. • The transparency and mechanical properties are better than those of PC composites.

A conveyor belt experimental setup to study the internal dynamics of granular avalanches

This paper shows how a conveyor belt setup can be used to study the dynamics of stationary granular flows. To visualise the flow within the granular bulk and, in particular, determine its composition and the velocity field, we used the refractive index matching (RIM) technique combined with particle tracking velocimetry and coarse-graining algorithms. Implementing RIM posed varied technical, design and construction difficulties. To test the experimental setup and go beyond a mere proof of concept, we carried out granular flow experiments involving monodisperse and bidisperse borosilicate glass beads. These flows resulted in stationary avalanches with distinct regions whose structures were classified as: (i) a convective-bulged front, (ii) a compact-layered tail and, between them, (iii) a breaking size-segregation wave structure. We found that the bulk strain rate, represented by its tensor invariants, varied significantly between the identified flow structures, and their values supported the observed avalanche characteristics. The flow velocity fields’ interpolated profiles adjusted well to a Bagnold-like profile, although a considerable basal velocity slip was measured. We calculated a segregation flux using recent developments in particle-size segregation theory. Along with vertical velocity changes and high expansion rates, segregation fluxes were markedly higher at the avalanche’s leading edge, suggesting a connection between flow rheology and grain segregation. The experimental conveyor belt’s results showed the potential for further theoretical developments in rheology and segregation-coupled models.Graphic

Two-photon excitation fluorescent spectral and decay properties of retrograde neuronal tracer Fluoro-Gold

Fluoro-Gold is a fluorescent neuronal tracer suitable for targeted deep imaging of the nervous system. Widefield fluorescence microscopy enables visualization of Fluoro-Gold, but lacks depth discrimination. Though scanning laser confocal microscopy yields volumetric data, imaging depth is limited, and optimal single-photon excitation of Fluoro-Gold requires an unconventional ultraviolet excitation line. Two-photon excitation microscopy employs ultrafast pulsed infrared lasers to image fluorophores at high-resolution at unparalleled depths in opaque tissue. Deep imaging of Fluoro-Gold-labeled neurons carries potential to advance understanding of the central and peripheral nervous systems, yet its two-photon spectral and temporal properties remain uncharacterized. Herein, we report the two-photon excitation spectrum of Fluoro-Gold between 720 and 990 nm, and its fluorescence decay rate in aqueous solution and murine brainstem tissue. We demonstrate unprecedented imaging depth of whole-mounted murine brainstem via two-photon excitation microscopy of Fluoro-Gold labeled facial motor nuclei. Optimal two-photon excitation of Fluoro-Gold within microscope tuning range occurred at 720 nm, while maximum lifetime contrast was observed at 760 nm with mean fluorescence lifetime of 1.4 ns. Whole-mount brainstem explants were readily imaged to depths in excess of 450 µm via immersion in refractive-index matching solution.

On the micro-PIV accuracy and reliability utilizing non-Gaussian particle images

Optical investigations of the dynamics of concentrated suspensions, such as in blood flows (Fitzgibbon et al. in Biophys J 108(10):2601–2608, 2015. http://doi/org/10.1016/j.bpj.2015.04.013) or slurry flows (Li et al. in Ocean Eng 163(October 2017):691–705, 2018. http://doi/org/10.1016/j.oceaneng.2018.06.046), are challenging due to reduced optical accessibility. Furthermore, the suspension particle image size can strongly deviate from the optimal particle image size for PIV measurements. Optical accessibility can be achieved by refractive index matching of surface labelled suspension particles. This results in particle images that are transparent in the particle image centre, but fluoresce at the particle image rim, resulting in ring-shaped particle images. In the present study, the influence of the particle image size on the cross-correlation result of such ring-shaped particle images is compared with Gaussian and plateau-shaped particle images. Particles of Gaussian image shape result from fully labelled particles with small image diameters and are commonly used in PIV measurements. Such particles are also utilized for the determination of the continuous phase velocities in the experimental part of the present study. With increasing image diameter, fully labelled particles are observed to assume plateau-shaped particle images. Monte Carlo simulations of synthetically generated images show that ring-shaped particle images have a superior behaviour, i.e. they assume a reduced displacement estimation error for noisy as well as for noise-free image data, compared to Gaussian and plateau-shaped particle images. This is also true for large particle image diameters when particle images are intersected at interrogation window borders or when different values of nonzero particle image displacements are considered. The detectability is similar for all three particle image shapes as long as particles do not intersect with the interrogation window border. Interestingly, for intersected particles of large image diameter, ring-shaped particle images show a slightly improved detectability compared to particle images of Gaussian and plateau shape. Furthermore, the detectability is insensitive against a nonzero particle image displacement. The usage of refractive index matched, ring-shaped particle images results in a good optical accessibility of the suspension. This allows to perform simultaneous cross-correlation evaluations on large ring-shaped particle images and fluid tracers with Gaussian particle images that are two orders of magnitude smaller compared to suspension particle images. Velocity measurements are taken on a suspension containing 5 vol% surface labelled, refractive index matched 60 upmu hbox {m} polymethylmethacrylate (PMMA) particles. Simultaneously, upmuPIV measurements of the carrier liquid flow are performed utilizing 1.19 upmu text {m} fluorescent polystyrene (PS) particles. Measurement results reveal a parabolic shape of the velocity profiles of both phases with a mean slip velocity of 7.4% at the position of maximum streamwise velocity in a 580 upmu text {m} high trapezoidal channel. An error analysis confirms the presence of these slip velocities within a 68.5% confidence interval. A measurement uncertainty in the order of magnitude of {mathcal {O}}(10^{-1} mathrm{px}) is reached for both fluid tracers and suspension particles. Overall, the present study demonstrates theoretically and experimentally that the usage of suspension particles with ring-shaped images is superior compared to Gaussian and plateau-shaped particle images of the same size. Additionally, the present study demonstrates that the usage of ring-shaped particle images allows to investigate suspension bulk dynamics by measuring velocity fields of both the suspended and the continuous phase simultaneously and with an overall uncertainty that is in the same order of magnitude as for standard upmuPIV measurements.Graphic abstract

Dual photonic bandgap hollow sphere colloidal photonic crystals for real-time fluorescence enhancement in living cells

To overcome the problems of refractive index matching and increased disorder when working with traditional heterostructure colloidal photonic crystals (CPCs) with dual or multiple photonic bandgaps (PBGs) for fluorescence enhancement in water, we propose the use of a chemical heterostructure in hollow sphere CPCs (HSCPCs). A partial chemical modification of the HSCPC creates a large contrast in wettability to induce the heterostructure, while the hollow spheres increase the refractive index difference when used in aqueous environment. With the platform, fluorescence enhancement reaches around 160 times in solution, and 72 times (signal-to-background ratio ~7 times) in cells during proof-of-concept live cardiomyocyte contractility experiments. Such photonic platform can be further exploited for chemical sensing, bioassays, and environmental monitoring. Moreover, the introduction of chemical heterostructures provides new design principles for functionalized photonic devices.

Velocity measurements of dilute suspensions over and through various porous media models

This study is focused on the motion of a dilute suspension containing rigid, spherical, non-Brownian, noncolloidal particles flowing over and through porous media models. The flow is confined to very low Reynolds numbers. To examine the velocity distribution particle image velocimetry (PIV) was applied in conjunction with refractive index matching (RIM) techniques. This study is the first of its kind analyzing the interaction between two common engineering systems: suspension fluid and porous media.

Combining high-speed planar PIV and motion tracking of a flexible cylinder in cross-flow

Most modeling studies investigating the flow dynamics in vegetation canopies are limited to rigid models as proxies for vegetation elements. However, most canopies embody some degree of structural flexibility, resulting in aeroelastic mechanisms coupling the motion of the vegetation with the surrounding flow. Studies addressing flexible canopies typically quantify either the flow or the plant motion independently, thus missing the instantaneous coupling between turbulent stresses and structural deformations. Few experiments have been devoted to measuring both quantities simultaneously. Okamoto and Nezu (2009) utilized a combined PIV-PTV technique to capture both flow and canopy motion. However, only the motion of the stem tips was captured, as opposed to the deformation of the entire stem. Py et al. (2006) employed digital image correlation (DIC) to quantify the motion of crop canopies using in-field images. However, the wind itself was not measured across the domain.&#x0D; The present work presents an experimental technique that can be utilized to study the flow–structure interaction in flexible canopies, and that could be extended to other flexible and/or moving objects. High-speed PIV data of the flow surrounding an idealized canopy element, consisting of a flexible cylinder, together with the corresponding displacement field throughout the cylinder were simultaneously obtained combining fluorescent imaging and refractive index matching (RIM).

Research on Design and Mechanism of Sound Insulating Structure Based on Acoustic Metamaterials

In view of the difficult problems of low-frequency noise reduction, based on the clamping boundary-type acoustic metamaterial theory with negative effective mass density, a structure combining membrane-type and clamping boundary-type is designed, which can realize good sound insulation performance in the low-frequency region, and the theoretical calculation and numerical simulation are in good agreement. At the same time, the transmission property of the structure at zero refractive index and impedance matching is studied. The research suggests that the structure has good low-frequency broadband sound insulation performance, and can achieve high transmission performance of sound waves through structural regulation.

Preliminary analysis of the flow field in a transparent model of a SRF cavity by PIV

Niobium superconductive radio frequency cavities (SRF cavities) are commonly employed in linear accelerator; very pure, clean, and smooth internal walls are required to make the cavity working properly and with a high Q factor. Typically, to achieve a suitable surface quality the SRF cavities are subjected to a surface treatment called Buffered Chemical Polish (BCP), the latter uses a strong concentrated acids mixture flowing through the cavity to chemically etch a thin niobium layer (∼250 μm). A viable way to understand and optimize the BCP process is by using computational fluid dynamic (CFD); however, the experimental validation of the numerical and physical models is required to confidently use CFD results. The aim of our work is to investigate the fluid dynamics of the BCP process by using the Particle Image Velocimetry (PIV) and the refractive index matching (RIM) techniques, and to provide data to validate CFD simulations. The paper describes and briefly analyse the experimental setup and the preliminary results achieved so far.

Direct Stress Birefringence Distribution Measurement in Lens Using Refractive Index Matching Liquid

Stress birefringence (SBR) of optical components is significant in polarization-dependent and fine optical applications, such as optical lithography. However, the methods generally used for high-resolution birefringence distribution detection are unsuitable for measuring samples with nonplanar surfaces, such as lenses, owing to refraction and aberration. In this work, we propose and verify a method for directly measuring SBR distributions in a lens. An aqueous glycerol solution is used as the refractive index matching liquid to rectify the measuring light path. The retardation level of the matching liquid is confirmed to be considerably smaller than that of the window, thus simplifying the measurement error. The retardation levels of the optical plate and wave plate obtained using this method are consistent with that measured in air (Δ <; 0.025 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> and 0.212°, respectively), as well as with the fast axis angle of the wave plate (Δ <; 0.15 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> ). Additionally, the SBR distribution of the lens sample coincides with its physical state and is repeatable with a coefficient of variation of less than 2.585%. This method provides a convenient tool for SBR distribution measurements in a lens, which could be extended to the measurement of optical components exhibiting any surface shape.