Origin of precious opal revisited: Possible quick formation of precious opal

It has been well known that a sedimentary precious opal is composed of closely packed uni-size silica spheres with voids filled by an air or water, and that an interference among reflections from the boundaries of those spheres and filler yields play-of-colour (iridescence). So, occurrence of a play-of-colour means occurrence of natural selection in size of spheres, or suppression of further nucleation after initial outburst of spheres, during its formation process. We had been exploring the possibility if we can regard a Stöber process as an analogue of the formation process of precious opal. The key is the reason why variation in size is rarely found on both precious opal and Stöber colloid. To give a clue, we examined the internal structure of Stöber particles and how those particles were formed at very initial stage of the process. The answers for evenness in shape and size are a quick supersaturation of reactive silica species, consecutive formation of large and loose polymers by fast dehydration, and their quick aggregation as the initial burst of silica spheres in highly diffusive medium. These can be achieved in nature by quick but continuous decrease in temperature on “basic” (high pH) geothermal hot water moving upward through cracks in rocks. Sedimentary precious opal can thus be formed when such naturally occurring colloid is filtered by a permeable bed.

Wave propagation in an initially stressed rotating thermo-diffusive medium with two-temperature and micro-concentrations

Purpose The purpose of this paper is to study the reflection of plane waves in an initially stressed rotating thermoelastic diffusive medium with micro-concentrations and two-temperature. Design/methodology/approach A two-dimensional model of generalized thermoelasticity is considered. The governing equations are transformed into the non-dimensional forms using the dimensionless variables. Then, potential functions are introduced for the decoupling of the waves. Further, appropriate boundary conditions are assumed to completely solve the problem. Finally, numerical computations are performed using MATLAB. Findings The problem is solved analytically and it is found that there exist five coupled waves in addition to an independent micro-concentration wave in the considered medium. The amplitude ratios and energy ratios of these reflected waves have also been computed numerically for a specific material. Originality/value The modulus values of amplitude ratios are presented graphically to exhibit the effects of angular velocity, initial stress, two-temperature, diffusion and micro-concentration parameters. The expressions of energy ratios obtained in explicit form are also depicted graphically as functions of angle of incidence. The law of conservation of energy at the free surface during reflection phenomenon is also verified.

Improving Localization of Deep Inclusions in Time-Resolved Diffuse Optical Tomography

Time-resolved diffuse optical tomography is a technique used to recover the optical properties of an unknown diffusive medium by solving an ill-posed inverse problem. In time-domain, reconstructions based on datatypes are used for their computational efficiency. In practice, most used datatypes are temporal windows and Fourier transform. Nevertheless, neither theoretical nor numerical studies assessing different datatypes have been clearly expressed. In this paper, we propose an overview and a new process to compute efficiently a long set of temporal windows in order to perform diffuse optical tomography. We did a theoretical comparison of these large set of temporal windows. We also did simulations in a reflectance geometry with a spherical inclusion at different depths. The results are presented in terms of inclusion localization and its absorption coefficient recovery. We show that (1) the new windows computed with the developed method improve inclusion localization for inclusions at deep layers, (2) inclusion absorption quantification is improved at all depths and, (3) in some cases these windows can be equivalent to frequency based reconstruction at GHz order.

A Versatile Setup for Time-Resolved Functional Near Infrared Spectroscopy Based on Fast-Gated Single-Photon Avalanche Diode and on Four-Wave Mixing Laser

In this paper, a time-domain fast gated near-infrared spectroscopy system is presented. The system is composed of a fiber-based laser providing two pulsed sources and two fast gated detectors. The system is characterized on phantoms and was tested in vivo, showing how the gating approach can improve the contrast and contrast-to-noise-ratio for detection of absorption perturbation inside a diffusive medium, regardless of source-detector separation.