Label-free visualization of microfluidic mixture concentration fields using a surface plasmon resonance (spr) reflectance imaging

A label-free visualization technique based on surface plasmon resonance (SPR) illumination sensing is applied for the nonintrusive and real-time mapping of microscale mixture concentration fields. The key idea is that the SPR reflectance sensitively varies with the refractive index of the near-wall region of the test mixture fluid contacting the metal (Au) layer, of tuned 47.5 nm thickness. The Fresnel equation, based on Kretschmann’s theory, correlates the SPR reflectance with the refractive index, or dielectric constant, of the test medium, and then, the measured refractive index correlates with the mixture concentration. An example application is presented for the case of ethanol penetrating into water contained in a micro-channel with a rectangular cross-section of 91 μm wide and 50 μm high. The measurement sensitivity, uncertainties and detection limitations of the implemented SPR imaging sensor are carefully examined for its potential as a nonintrusive means of microscale concentration field mapping.

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Measurements of the minimum elevation of nano-particles by 3D nanoscale tracking using ratiometric evanescent wave imaging

Effect of saline concentration on the minimum elevation of nanoparticles has been examined under the electric double layer interactions with the substrate glass surface. The use of ratiometric total internal reflection fluorescence microscopy (R-TIRFM) allows three-dimensional tracking of nanoparticles in the near-wall region within less than 1 μm from the surface. The measurements of minimum elevation were made for polystyrene fluorescent nanospheres of 100, 250, and 500 nm in radii (SG = 1.05) for the salinity ranging from 0.1 to 10 mM. Special care was taken to insure cleaned surface conditions by elaborate sonication and rinsing of the glass substrate. The laser illumination intensity and duration also had to be carefully examined to minimize photobleaching of the fluorescence emission from particles. It is reported that the minimum elevation decreases with increasing saline concentration and with increasing particle sizes, for the first time experimentally and quantitatively to the authors’ knowledge.

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Study of laminar thermal boundary layers occurring around the leading edge of a vertical isothermal wall using a specklegram technique

The influence of a 45 degree leading edge on the development of a thermal boundary layer on a vertical isothermal wall has been investigated by measuring detailed temperature profiles with a He- Ne laser specklegram technique. A digital image processor was employed to improve the accuracy in evaluating the space of fringes, which were constructed from the specklegram. A matrix of five different temperatures and four different vertical locations was considered in the investigation. The leading edge effect was evident in the vicinity of the edge (x = 7.8 mm and 14.2 mm) showing a strong nonsimilarity, and the thermal boundary layers occurred around the leading edge. Classical theory did not properly describe the actual situation near the leading edge, where the theory assumed an abrupt starting of the boundary layer. In an attempt to improve the agreement in temperature profiles, the Grashof number was modified by extending x to an equivalent plate length x ₀. In the region farther from the leading edge (x = 50.0 mm), where similarity in temperature profiles was well established, agreement with theory was dramatically improved with substitution of x ₀ forx. A linear decrease in the extension ratio x ₀/x with Gr _x ^1/4 i.e., x ₀/x= -0.4 · Gr _x ^1/4 + 12.63 where the Grashof number was based on x, was observed for Grashof numbers up to 8.0 × 10⁵. For Gr _x 8.0 × 10⁵, the leading edge effect became negligibly small and the ratio x ₀/x approached unity.List of symbols a distance between the test section and the second parabolic mirror (mm) - b distance between the second mirror and the ground glass (mm) - c distance between the ground glass and the focal plane of a camera used for specklegram recording (mm) - d distance between the specklegram negative and the detector plane of a video camera used for the fringe spacing evaluation (mm) - g acceleration of gravity (9.8 m/s²) - Gr _x Grashof number based on - Gr _x0 Grashof number based on x ₀ - H plate height (254 mm) - K Gladstone-Dale constant for air (0.2257 × 10^T-3 m³/kg) - L plate width measured along the optical axis (254 mm) - m magnification of the second (parabolic) mirror (b/a) - m magnification of the conventional camera lens - n index of refraction of air - s fringe spacing measured at the image plane of the video camera (mm) - T temperature (K) - x coordinate parallel to the vertical plate (mm) - x ₀ equivalent plate-length used to improve the comparison of data with theory (mm) - y coordinate perpendicular to the vertical plate (mm) - z coordinate parallel to the optical axis (mm) - refraction angle of a light ray after passing the test section - coefficient of thermal volumetric expansion (K^-1) - incident angle of the refracted ray toward the ground glass - fringe spacing at the focal plane of the camera or the thermal boundary layer thickness (mm) - dimensionless temperature function - dimensionless similarity variable - air density (kg/m³) - wavelength of He = Ne laser (632.8 nm) - speckle translocation on the specklegram - kinematic viscosity of air (m²/s)     

Unveiling hidden complex cavities formed during nanocrystalline self-assembly

The existence of hidden complex cavities formed inside a self-assembled nanocrystalline structure is discovered in real-time by using surface plasmon resonance near-field refractive index fingerprinting. Furthermore, computer analysis of the naturally occurring R-G-B interference fringes allowed us to reconstruct the 3D cavity formation and crystallization processes quantitatively. For the case of an aqueous droplet containing 10% by volume of 47 nm Al2O3 nanoparticles, the submicrometer-scale inner cavity peak grows up to 0.5% of the entire crystallized crust height of over 150 microm. The formation of the complex inner structure was found to be attributable to multiple cavity inceptions and their competing growth during the aquatic evaporation. This outcome provides a better understanding and feasible control of the formation of nanocrystalline inner structures.     

Suggestive correctional methods for PIV image biasing caused by a rotating mirror system

A rotating mirror is widely used to generate the velocity shift that can resolve the directional ambiguities of PIV (particle image velocimetry) measurements. The rotating mirror system inevitably creates the normal displacement of the resulting PIV images and causes systematic image errors. Two corrective methods are proposed to eliminate or reduce the image biasing in PIV system. The use of two linearly traversing mirrors, instead of a single rotating mirror, shows that image biasing can be eliminated and the velocity shift well generated. As a second option, two co-rotating mirrors, instead of one, can reduce the image biasing with a maximum velocity shift available. Detailed imaging kinematics of the two suggestive methods are presented to lead to designing of practical devices that improve the PIV capabilities by reducing the systematic image errors. The authors like to acknowledge the partial financial support provided from the Turbo and Power Machinery Research Center (TPMRC) of Seoul National University, Seoul, Korea.     

The determinants of equity transmission between the new and used car markets: a hedonic analysis

Drawing on a data set containing 371082 observations on new and used cars from 2008, this study employs a hedonic model to estimate the determinants of prices in the primary and secondary car markets in Germany. We are specifically interested in identifying those vehicle attributes that are responsible for retaining the car’s value in the used car market. Beyond parameterizing the influence of technical features and brand name on the retail price, our model simultaneously generates a corresponding set of parameter estimates for the used car price, thereby allowing us to formally compare their magnitudes across the two markets. This comparison reveals that fuel consumption, in particular, is an important determinant of the price, one whose impact is higher in magnitude in the used car market than in the new car market. Large heterogeneity in how cars hold their investment value is also seen to depend on body type and brand/model name.     

Surface plasmon resonance (SPR) reflectance imaging: Far-field recognition of near-field phenomena

The surface plasmon resonance (SPR) reflectance imaging technique provides a label-free visualization tool to characterize the near-field fluidic transport properties within 100 nm from the solid surface. The key idea is that the SPR reflectance intensity varies with the near-field refractive index (RI) of the test fluid, which in turn depends on the micro- and nano-fluidic scalar properties such as concentrations, temperatures, and phase changes, occurring in the near-field. As essential knowledge to understand and implement the SPR reflectance imaging technique, this paper presents discussions on the basics of surface plasmon polaritons (SPPs), surface plasmon resonance (SPR), setup of the SPR reflectance imaging system, and the SPR reflectance imaging resolution. The second part of the paper elaborates the applications of the SPR imaging sensor technique in characterizing the near-field fingerprints of nanofluidic evaporative self-assembly.