Maria Rosaria Vetrano

Global rainbow thermometry: improvements in the data inversion algorithm and validation technique in liquid-liquid suspension

Improvements on the validation of a nonintrusive laser-based measurement technique are presented. This new technique, called global rainbow thermometry (GRT), is capable of determining the temperature and the size distributions of liquid droplets dispersed in a liquid or gaseous bulk. We propose a new data inversion algorithm that takes into account the whole rainbow pattern. Experimental validation of the GRT technique is performed for a liquid-liquid suspension. We performed the validation by comparing the measurements obtained with the GRT technique for the mean droplet temperature and size with the results obtained with alternative techniques.

Generalization of the rainbow Airy theory to nonuniform spheres

Rainbow techniques permit measurement of refractive indices, and hence the temperatures of liquid droplets through determination of the absolute angular position of a rainbow interference image in space. The Airy theory, which is commonly used to explain the rainbow effect, permits the determination of a unique refractive-index value, even in the presence of nonuniformities in the droplet. An extension of this theory to spheres that exhibit internal refractive-index gradients is proposed. The case of burning droplets is considered as an example of such spheres, and the results obtained are successfully compared with those presented in the literature.

Assessment of refractive index gradients by standard rainbow thermometry

Up to now the application of rainbow thermometry has been limited to particle systems possessing a uniform refractive index. This is mostly due to the absence of an appropriate data inversion algorithm that takes into account the presence of a refractive index gradient. In this paper, for the first time to our knowledge, exploiting a generalization of the Airy theory, a data inversion algorithm for a single droplet, presenting a parabolic refractive index gradient, is proposed. This data inversion algorithm is used to compute the diameter and the refractive index at the core and at the surface of a simulated burning droplet. The results are compared to the analytical solutions showing a satisfactory agreement.

Feasibility of using glory and speckle patterns for sizing spherical and irregular particles

A backscattered laser light technique for sizing spherical and irregular particles is investigated in this paper. Two different interference patterns (glory and speckle), appearing in the backscatter region when a single droplet is illuminated with a laser light source, were recorded by a CCD camera. A theoretical model, based on a geometrical optics approximation, has been first developed to retrieve particle size from the analysis of these patterns and then applied to liquid and frozen water droplets with sizes ranging from 1 to 2 mm. The satisfactory agreement obtained between the theoretical model and the experimental data encourage further development of this technique.

Measuring contact angles of small spherical particles at planar fluid interfaces by Light Extinction

An experimental technique is introduced for measuring the contact angle of small colloidal particles at planar fluid interfaces. The presented light scattering-based method relies on performing two spectral transmittance measurements: one on a particle monolayer standing at the fluid interface and the other on a dispersion of the same particles in a homogeneous medium. The observed shift between the two transmitted spectra is explained in terms of the phase shift parameter, which is then used to determine the particle position relative to the interface and hence the contact angle. The applicability of the technique is demonstrated through simulations and experiments.

Modeling and analysis of the two-dimensional polystyrene aggregation process

Small particles tend to aggregate and create large fractal-like structures which can be analysed using microscopy techniques. In this work we present an algorithm capable of measuring the basic morphological parameters of two-dimensional polystyrene layers. Our study was divided into two separate parts. The goal of the first one was to create high quality particle monolayers. Their purpose was to allow for monitoring of the two-dimensional aggregation process by means of optical microscopy. In the next step microscopy images were analysed in more detail. The size distribution function and the total number of particles were calculated. When an aggregate was larger than a specified size its fractal dimension was approximated using the box-counting technique. After retrieving the morphological parameters fractal-like aggregate models were created using the most common tunable algorithms. Our study proved that real structures resemble to geometries generated with CC (Cluster-Cluster) aggregation techniques. Initial clusters, i.e. those generated during early stages of the aggregation process, are characterized by slightly larger fractal dimension. However, its value decreases along with the aggregation time. The next step is to improve our algorithm even further and use it in a fully automatic on-line monitoring process.