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)     

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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Nonintrusive measurements of mixture concentration fields by analyzing diffraction image patterns (point spread function) of nanoparticles

The diffraction image patterns of small particles are referred to as their point spread function (PSF); these patterns vary distinctively with the refractive index (RI) of a transparent test medium when the particles are imaged through the medium. The RI correlates directly with the mixture concentration, so proper inversion of measured PSFs can provide full-field information on the mixture concentration field. In this study, fluorescent nanoparticles of 500 nm diameter are fixed on a glass surface by means of evaporative self-assembly, and the time-varying test mixture is placed in front of the glass surface. The time-varying and full-field PSF distributions are imaged and digitally analyzed to determine the local RI values as well as the local mixture concentrations. Both immiscible water/oil mixture and miscible water/glycerol mixture are imaged. The present method shows an RI measurement to have an uncertainty of ±5 × 10⁻³ RIU and the mixture concentration measurements to have uncertainty of approximately 4%.     

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.     

Tomographic-image reconstruction using a hybrid genetic algorithm

An improved tomographic-image reconstruction method is proposed that uses a hybrid genetic algorithm (GA) that hybridizes a conventional GA and a concurrent simplex method. For the purposes of discussion, an axisymmetric phantom density field is used with an interferometric optical projection. Tomographic-image reconstruction using the hybrid GA not only improves the convergence over the pure GA but also significantly reduces the computation time.     

Direct decay of the giant dipole resonance in58,60Ni and64Zn from (e, é c) experiments

(e,e'c) coincidence experiments at a momentum transfer of q=0.27 fm^?1 to study the decay of the giant dipole resonance in^58,60Ni and⁶⁴Zn show deviations from photonuclear results and reveal strong contributions of direct decay to proton hole states.     

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.