Refractive Index Matching Research Articles

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.

Spontaneous Formation of Stable Vesicles and Vesicle Gels in Polar Organic Solvents.

The self-assembly of lipids into nanoscale vesicles (liposomes) is routinely accomplished in water. However, reports of similar vesicles in polar organic solvents like glycerol, formamide, and ethylene glycol (EG) are scarce. Here, we demonstrate the formation of nanoscale vesicles in glycerol, formamide, and EG using the common phospholipid lecithin (derived from soy). The samples we study are simple binary mixtures of lecithin and the solvent, with no additional cosurfactants or salt. Lecithin dissolves readily in the solvents and spontaneously gives rise to viscous fluids at low lipid concentrations (∼2-4%), with structures ∼200 nm detected by dynamic light scattering. At higher concentrations (>10%), lecithin forms clear gels that are strongly birefringent at rest. Dynamic rheology confirms the elastic response of gels, with their elastic modulus being ∼20 Pa at ∼10% lipid. Images from cryo-scanning electron microscopy (cryo-SEM) indicate that concentrated samples are "vesicle gels," where multilamellar vesicles (MLVs, also called "onions"), with diameters between 50 and 600 nm, are close-packed across the sample volume. This structure can explain both the elastic rheology as well as the static birefringence of the samples. The discovery of vesicles and vesicle gels in polar solvents widens the scope of systems that can be created by self-assembly. Interestingly, it is much easier to form vesicles in polar solvents than in water, and the former are stable indefinitely, whereas the latter tend to aggregate or coalesce over time. The stability is attributed to refractive index-matching between lipid bilayers and the solvents, i.e., these vesicles are relatively "invisible" and thus experience only weak attractions. The ability to use lipids (which are "green" or eco-friendly molecules derived from renewable natural sources) to thicken and form gels in polar solvents could also prove useful in a variety of areas, including cosmetics, pharmaceuticals, and lubricants.

Combining novel polyether-based ionomers and polyethylene glycol as effective toughening agents for polylactide

Eco-friendly polylactide (PLA)-based materials with excellent toughness and transparency have wide application prospects in high value-added areas. However, toughening brittle PLA without losing its attractive transparency remains a great challenge for PLA-based blends because of the indispensable phase-separated structure. Herein, we synthesized a series of imidazolium-functionalized polyether-based ionomers by facile quaternization reaction from renewable epichlorohydrin elastomer and various comprehensive imidazoles. Combining these ionomers and polyethylene glycol (PEG) as toughening agents for PLA, we successfully achieved supertough, highly transparent and sustainable blends owing to a refractive index (RI) matching mechanism. The influence of the chemical structure of the ionomers, PEG molecular weight (Mw), and blending composition on the phase structure, mechanical performance, and transparency of the blends was systematically investigated. All the ternary blends exhibit remarkably high transparency (≥80%) and strain-to-failure (≥230%) owing to the similar RIs of each component and good compatibilization. Notably, the PLA/ECO-PF6/PEG20000 blend with 20 wt % PEG exhibits the highest strain at break (~400%) and impact strength of 82.2 kJ/m2. Pervasive matrix shear yielding induced by the internal and interfacial cavitation of the ionomer effectively dissipates the break energy because of the improved interfacial adhesion and plasticization of PEG20000. Combining RI matching and noncovalent bond interactions with a facile multicomponent compounding strategy provides us with an extremely cost-effective way to develop high-performance PLA-based materials for high value-added applications.

Flow Past Mound-Bearing Impact Craters: An Experimental Study

An experimental investigation into the flow produced by mound-bearing impact craters is reported herein. Both an idealized crater and a scaled model of a real martian crater are examined. Measurements were performed using high-resolution planar particle image velocimetry (PIV) in a refractive-index matching (RIM) flow environment. Rendering the crater models optically invisible with this RIM approach provided unimpeded access to the flow around and within each crater model. Results showed that the mean flow within the idealized crater exhibits more structural complexity compared to its moundless counterpart. Second-order statistics highlighted regions of minimal and elevated turbulent stresses, the latter of which revealed a complex interaction between shear layers that are present at the upstream and downstream parts of the rim and the central mound. Periodic vortex shedding of quasi-spanwise vortices from the upstream rim was revealed by POD-filtered instantaneous flow fields. Vertical flapping of this shear layer resulted in vortices occasionally impinging on the inner wall of the downstream rim. Further, conditional averaging analysis suggested periodic lateral oscillations of wall-normal vortices within the crater rim region reminiscent of those observed for flow inside spherical dimples. These results have implications for intra- to extra-crater mass and momentum exchange, and for sediment transport processes. Lastly, experiments with the Gale Crater model showed both similarities with and differences from the primary flow features found for the idealized model.

Advances in application of tissue clearing technique in hard tissues

Tissue clearing is a novel technique developed within recent years, which could make the tissue optical transparency using physical or chemical methods by refractive index matching. Combined with fluorescence imaging and three-dimensional reconstruction technology, it could achieve three-dimensional observing and analyze the tissue structure at the cellular resolution. The tissue clearing technique is mainly applied to soft tissues, as less to hard tissues. In recent years, many researchers have modified tissue clearing methods and made them suitable for hard tissues, such as bone and teeth. The present paper reviews the recent application of tissue clearing techniques in hard tissues.

Preparation of thin films of molecule-based magnets for optical measurements

Prussian blue analogues provide a useful model system for studying how the magnetic properties of molecular-based systems can be both perturbed and probed using light. However, the study of thin films of these materials using transmission-based optical techniques is limited due to their polycrystalline (or sometimes amorphous) morphology and inconsistent homogeneity, which act to scatter light. To address this shortcoming, thin films of Cr-Cr and Fe-Cr Prussian blue analogues have been encapsulated by suitable transparent adhesives to mitigate the effects of scatter, which would be of particular benefit in experiments that rely on the polarization of transmitted light being maintained. It is observed that transmittance is appreciably improved across the visible spectrum following encapsulation, reaching a tenfold increase at certain wavelengths. This is due to a combination of the reduction of air/film interfaces and refractive index-matching between adhesive and film.

Highly transparent antioxidant high internal phase emulsion gels stabilized solely by C-phycocyanin: Facilitated formation through subunit dissociation and refractive index matching

In the work, we report a kind of novel highly transparent oil-in-water (O/W) HIPE gels stabilized solely by c -phycocyanin (CPC; an important biliprotein pigment), with a great potential to act as outstanding containers for heat or UV labile lipophilic bioactives. The CPC was solubilized in 6 M urea solution containing 40 wt% glycerol, in order to achieve the improved emulsification performance of the protein, as well as the transparency of the resultant HIPE gels through the refractive index (RI) matching between the aqueous and dispersed oil (dodacane) phases. Using 6 M urea containing 40 wt% glycerol as the aqueous phase, the transparent HIPE gels could be formed at the CPC concentration ( c ) as low as 0.1 wt%, and oil fractions in the range 0.6–0.86. In the protein-poor regime, e.g., c < 0.5 wt%, increasing the c led to a progressive decrease in emulsion size, as well as strengthening of gel network, while in the protein-rich regime ( c = 0.5–1.0 wt%), the emulsion droplet and gel stiffness were independent of the c . The as-fabricated transparent HIPEs (gels) exhibited an outstanding oxidative protection to encapsulated β -carotene against degradation upon extensive heating or UV irradiation. • A kind of novel transparent antioxidant HIPE gels stabilized by c -phycocyanin was reported. • The transparency of HIPE gels was achieved by a RI matching technique with urea and glycerol. • The emulsification performance of c -phycocyanin was remarkably improved by the presence of 6 M urea. • The chemical stability and antioxidant capacities of c-phycocyanin were unaffected by the usage of urea and glycerol. • The as-fabricated transparent HIPE gels exhibit a great potential in cosmetic and other formulations.

Lotus Seedpod Inspiration: Particle-Nested Double-Inverse Opal Films with Fast and Reversible Structural Color Switching for Information Security.

The integration of novel structures into colloidal crystals provides the possibility of constructing stimuli-responsive photonic materials. However, in most opal and inverse opal structures, replacing the interior air with an infiltrated liquid will cause partial refractive index matching, resulting in the reduction or even disappearance of the photonic band gap. Herein, inspired by the lotus seedpod, an innovative particle-nested double-inverse opal film with fast and reversible structural color switching (≈1 s) is first fabricated by introducing polystyrene (PS) spheres into an inverted opal backbone. Importantly, refractive index matching can be effectively avoided due to the existence of internal PS spheres, and optical switching from diffusive to photonic behavior is achieved by a liquid with low surface tension for the response. Furthermore, a reversible ethanol stimuli-response bilayer double-inverse opal film with multistate switching for information encryption is proposed by combining optical scattering and diffraction. The scattered light from the top layer caused by the randomly distributed and weakly scattering PS spheres within the pores makes the pattern at the bottom invisible. Simultaneously, the display and discoloration of the pattern can be realized instantaneously by ethanol response. Thus, this new preparation strategy exhibits great potential in the security fields.

Novel silicone laminating materials for improved index matching in automotive display applications

AbstractTo improve display performance, refractive index matching between a laminating material and the substrate was considered a crucial interfacial issue in optical stack design. To control the refractive index of the silicone optical laminating material, phenyl groups were introduced into the polymer component. and novel vinyl‐ and phenyl‐terminated MTQ (MviTphQ or MTPQ) resins were prepared by hydrolysis and condensation‐polymerization of a mixture of ethyl polysilicate, phenyltriethoxysilane, and divinyltetramethyl disiloxane to enhance the performance of optical laminating materials.These synthesized resins were utilized to formulate novel high‐refractive‐index silicone optical laminate compositions. The influence of refractive index on light transmittance was measured and compared with theoretical values. The influence of s‐MTPQ resin content on humidity resistance and transparency retention during aging were evaluated as crucial characteristics of an optical laminating material for automotive display applications.

A microfluidic column of water index–matched packed microspheres for label-free observation of water pollutants

A microfluidic, label-free optical sensor for water pollutants, which is based on a packed micro-column of microspheres with refractive index similar to that of water, is presented. The perfluoropolyether microspheres are synthetized by membrane emulsification followed by UV irradiation. The microfluidic channel hosting the packed column is transparent when filled with pure water as a consequence of refractive index matching, whereas it scatters light in presence of compounds with lipophilic moieties that spontaneously adsorb on the fluorinated microspheres. The device is characterized by investigating the response to cationic and anionic surfactants. Both the signal growth rate and the recovery rate measured during washing with water depend on the type and concentration of the compounds. The cationic surfactants tested display a larger signal increase, linearly scaling with concentration. A limit of detection of 1 μM is obtained in the current configuration. The water index-matched microspheres enable to access an additional analytical parameter, that is the propagation velocity of the scattering signal along the column. This parameter is also found to scale linearly with concentration, hence providing a complementary analytical tool sensitive to the adhesion kinetics.

On the spreading of high-pressure spray-generated liquid wall films

The spreading of a n-decane liquid film induced by the impingement of a spray generated by a high-pressure injector is studied experimentally. The experimental setup uses the Refractive Index Matching (RIM) method to observe the development and propagation of the wall film. The influence of injection time, injection pressure, temperature, and injector to wall distance are considered. The film evolution on the wall presents two clear distinct phases. During the injection phase the film area is growing linearly with respect to time. Once the injection is stopped, the film relaxes to its maximal expansion. Each phase is accurately described using a simple modeling based on mass and momentum balance in the liquid film.

Immunostaining of Whole-Mount Retinas with the CLARITY Tissue Clearing Method.

The tissue hydrogel delipidation method (CLARITY), originally developed by the Deisseroth laboratory, has been modified and widely used for immunostaining and imaging of thick brain samples. However, this advanced technology has not yet been used for whole-mount retinas. Although the retina is partially transparent, its thickness of approximately 200 µm (in mice) still limits the penetration of antibodies into the deep tissue as well as reducing light penetration for high-resolution imaging. Here, we adapted the CLARITY method for whole-mount mouse retinas by polymerizing them with an acrylamide monomer to form a nanoporous hydrogel and then clearing them in sodium dodecyl sulfate to minimize protein loss and avoid tissue damage. CLARITY-processed retinas were immunostained with antibodies for retinal neurons, glial cells, and synaptic proteins, mounted in a refractive index matching solution, and imaged. Our data demonstrate that CLARITYcan improve the quality of standard immunohistochemical staining and imaging for retinal neurons and glial cells in whole-mount preparation. For instance, 3D resolution of fine axon-like and dendritic structures of dopaminergic amacrine cells were much improved by CLARITY. Compared to non-processed whole-mount retinas, CLARITY can reveal immunostaining for synaptic proteins such as postsynaptic density protein 95. Our results show that CLARITY renders the retina more optically transparent after the removal of lipids and preserves fine structures of retinal neurons and their proteins, which can be routinely used for obtaining high-resolution imaging of retinal neurons and their subcellular structures in whole-mount preparation.

Transparent wood developed by introducing epoxy vitrimers into a delignified wood template

Transparent wood (TW) has shown great potential in various functional applications, including light, thermal, electromagnetic, and energy management fields. Despite significant progress in functional TW, programmable shape-memory TW (PSMTW) has not yet been reported. Epoxy vitrimers with exchangeable dynamic covalent crosslinking networks exhibit excellent stimuli-responsive, shape-memory, reprocessing, and self-healing properties, which are suitable for fabricating smart TW. In this study, a TW with programmable shape-memory ability was developed by introducing transparent, refractive index-matching, and intelligent epoxy-based vitrimers into delignified wood. The transmittance and haze of the resultant TW with a thickness of 2 mm were approximately 60% and 95%, respectively. Vitrimers with dynamic covalent crosslinking networks perfectly combine the glass transition temperature (Tg)-inducted phase change and topology freezing transition temperature (Tv)-inducted rearrangement under thermal stimulus, which endow TW with excellent programmable shape-memory properties, such as shape-recovery, shape-programming, shape-erasing, and re-programming. The programmable shape-memory properties of TW have not been investigated thus far, and development of PSMTW would be a multi-functionalized and efficient application of TW-based materials.

Methods for Laser-Induced Fluorescence Imaging of Solute Plumes at the Darcy Scale in Quasi-Two-Dimensional, Refractive Index-Matched Porous Media

A spectrum of environmental, industrial, and biomedical processes depends on solute plume spreading in laminar flows within porous media. The literature includes a number of approaches to model solute movement in this context, but fewer techniques are reported to measure it experimentally. To address this gap, this study describes novel experimental methods that permit measurement of solute plume spreading at the Darcy scale using laser-induced fluorescence in a 50 cm × 50 cm × 4 cm quasi-two-dimensional apparatus filled with refractive index-matched porous media. Fluorescence is provided by Rhodamine dye; refractive index matching is provided by borosilicate glass in glycerin. The depth-averaged porosity distribution of the porous media is determined using a novel optical method, whose results are used to investigate solute transport in two spatiotemporally varying flow fields, (1) a push–pull pumping sequence and (2) a sequence to induce stretching and folding of the solute plume. Quantitative analysis of replicate experiments reveals an extraordinary degree of reproducibility based on spatial moments, reactor ratio, and correlation coefficient.

Exploration of frequencies peaks observed on local wall pressure measurements by time-resolved velocity field measurements in complex flows

In pressurized water reactors (PWR), fluid–structure interaction provokes vibrations of the fuel rods leading to grid-to-rod fretting. To explore the origins of the local excitations of the rods by the flow, analytical experiments are performed within a 5 × 5 rod bundle maintained by spacer grids. New velocity (laser Doppler velocimetry) and pressure fluctuations measurements, taken for various Reynolds numbers, show the presence of frequency peaks in the spectra over a broad range of Reynolds numbers, i.e. 13,000 to 108,000. The Strouhal numbers obtained vary weakly with the Reynolds number and are in the range of 0.21–0.26. The phenomena being more pronounced downstream the dimples, this manuscript explores the flow at the exit of spacer grids downstream the dimples. To see through aligned rods, refractive index matching is implemented on parts of selected rods. The dilated scale of the experiments permits one to perform particle image velocimetry (PIV; grid of 51 × 89 points with a correlation window size of 0.032 Dh) to identify eddies smaller than the rods in the wake of the dimples. PIV measurements reveal the presence of multi-scale eddies with a periodic street of main eddies. The size of these eddies and the periodic length scale of the street are found to be in agreement with the integral length scale and the length scale built on the frequency peaks of pressure fluctuations. To the authors’ knowledge, this is the first experimental evidence of the existence and the role of coherent eddies streets downstream from dimples of spacer grids with geometries relevant to PWR fuel assemblies.

A Biologically Friendly, Low‐Cost, and Scalable Method to Map Permeable Media Architecture and Interstitial Flow

AbstractPorous media are ubiquitous, a key component of the water cycle and locus of many biogeochemical transformations. Mapping media architecture and interstitial flows have been challenging because of the inherent difficulty of seeing through solids. Previous works used particle image velocimetry (PIV) coupled with refractive index‐matching (RIM) to quantify interstitial flows, but they were limited to specialized and often toxic fluids that precluded investigating biological processes. To address this limitation, we present a low‐cost and scalable method based on RIM coupled PIV (RIM‐PIV) and planar laser induced fluorescence (RIM‐PLIF) to simultaneously map both media architecture and interstitial velocities. Our method uses irregularly shaped grains made of a fluorocarbon plastic with refractive index of 1.36 and specific gravity of 1.93. This allows using a water–glycerin solution for the RIM fluid. By using RIM‐PIV, we mapped media structure with 2% accuracy, which improved to 0.2% with RIM‐PLIF because of improved image contrast.

Investigation of ZnSe stability and dissolution behavior in As-S-Se chalcogenide glasses

Optical composite fibers based on active transition metal (TM)-doped semiconductor crystals are being investigated for use in mid-infrared (mid-IR) fiber laser systems. This study evaluates a candidate glass matrix system assessing its key physical properties and suitability for optical fiber drawing and examines the stability of TM-doped ZnSe crystals during processing. Results indicate that despite excellent refractive index matching between crystal and glass matrix and good fiber draw attributes, the stability of the crystalline dopant within the glass melt is severely impacted by melt conditions. The time and temperature dependence of this stability is mapped by X-ray diffraction (XRD) and second-harmonic generation (SHG) microscopy, illustrating how these tools can be used to track the phase stability and robustness throughout the melting process. Results show that in a range of sulfo-selenide based matrices, dissolution and re-precipitation of Zn-containing crystalline phases is a dominant mechanism.

Mapping the neuronal cytoskeleton using expansion microscopy.

Expansion microscopy (ExM) is a recently introduced technique that enables high-resolution imaging with conventional microscopes by using physical expansion of samples. While this technique does not require a complicated microscope setup (like in STED or STORM microscopy), sample preparation and handling require additional attention. Here we describe a workflow for imaging of the neuronal microtubule network with minimal artifacts and sample perturbations. We demonstrate that the use of custom-printed mounting chambers simplifies sample handling and facilitates stable imaging of the sample. In addition, refractive index matching between the sample and the objective greatly improves signal retention deeper in thick samples. To accurately determine the precise expansion factor and determine sample distortion, we describe how samples can be compared using STED and ExM. Together, these procedures enabled us to better resolve different microtubule subsets in neuronal soma and dendrites.

An advanced optical clearing protocol allows label-free detection of tissue necrosis via multiphoton microscopy in injured whole muscle.

Rationale: Structural remodeling or damage as a result of disease or injury is often not evenly distributed throughout a tissue but strongly depends on localization and extent of damaging stimuli. Skeletal muscle as a mechanically active organ can express signs of local or even systemic myopathic damage, necrosis, or repair. Conventionally, muscle biopsies (patients) or whole muscles (animal models) are mechanically sliced and stained to assess structural alterations histologically. Three-dimensional tissue information can be obtained by applying deep imaging modalities, e.g. multiphoton or light-sheet microscopy. Chemical clearing approaches reduce scattering, e.g. through matching refractive tissue indices, to overcome optical penetration depth limits in thick tissues.Methods: Here, we optimized a range of different clearing protocols. We find aqueous solution-based protocols employing (20-80%) 2,2'-thiodiethanol (TDE) to be advantageous over organic solvents (dibenzyl ether, cinnamate) regarding the preservation of muscle morphology, ease-of-use, hazard level, and costs.Results: Applying TDE clearing to a mouse model of local cardiotoxin (CTX)-induced muscle necrosis, a complete loss of myosin-II signals was observed in necrotic areas with little change in fibrous collagen or autofluorescence (AF) signals. The 3D aspect of myofiber integrity could be assessed, and muscle necrosis in whole muscle was quantified locally via the ratios of detected AF, forward- and backward-scattered Second Harmonic Generation (fSHG, bSHG) signals.Conclusion: TDE optical clearing is a versatile tool to study muscle architecture in conjunction with label-free multiphoton imaging in 3D in injury/myopathy models and might also be useful in studying larger biofabricated constructs in regenerative medicine.

Tissue Optical Clearing in the Ultraviolet for Clinical Use in Dentistry to Optimize the Treatment of Chronic Recurrent Aphthous Stomatitis

The immersion optical clearing (OC) treatment with a highly concentrated glycerol solution has induced three new tissue windows in the UV spectral range of gingival tissues – from 200 to 250 nm, from 250 to 300 nm and from 300 to 400 nm. By combining the immersion OC technique in human tissues with UV-spectroscopy, it was possible to verify and study the major OC mechanisms – tissue dehydration and refractive index matching, and that the OC efficiency is higher in the deep-UV than in the visible-NIR range. Since all biological tissues present high scattering in the UV range, the presented technology, which basically reduces the strong light scattering in the UV range, has a broad application area in medicine. The effectiveness of the developed technology combining UV phototherapy and OC in application to treatment of aphthous recurrent stomatitis in children was demonstrated.