Umberto Perini

Optical particle sizer based on the Chahine inversion scheme

An optical particle sizer utilizing an iterative procedure, proposed originally by Chahine for the inversion of scattering data, has been developed. The optical scheme is innovative and allows the measuring range to be selected without substitution of the Fourier transform lens. Numerical computer simulations and experimental results are presented to allow both the inversion procedure validity and the instrument performance to be evaluated.

Inversion of light scattering data from fractals by the Chahine iterative algorithm.

We used the nonlinear Chahine iterative inversion scheme to analyze size distributions of fractal objects and we tested its usefulness by computer simulations. The data to be inverted are elastic light scattering measurements at a number of angles. We chose the fractal dimension of the objects equal to 1.75 to duplicate colloid aggregates growing in the diffusion limited aggregation mode. Even in the presence of a realistic level of noise, the method offers good estimates of the average radius and of the spread of the distribution.

Low-angle elastic light scattering study of diffusion-limited aggregation

We have investigated the kinetics of the diffusion-limited aggregation of colloidal silica with low-angle elastic light scattering at 31 scattering angles covering the wave vector range 3.7 102 cm-1 <q<3.3 104 cm-1. We have studied both the asymptotic behaviour at larger q, which provides a measure of the clusters fractal dimension, as well as the roll-off region at small q, which gives information on the clusters size. Each set of data (scattered intensity vs. q vectors) has been analysed by means of a nonlinear iterative inversion scheme to provide the value of the number average gyration radius Rn which is found to grow in time with a power law tα. The exponent (average over a few runs) is α = 0.58 ± 0.02, consistent with the theoretical expectations.

LIBS Instrumental Techniques

The laser induced breakdown spectroscopy (LIBS) is a laser based technique widely used in scientific and industrial applications for the elemental analysis of materials. Because of its very attractive features like e.g. the lack of sample preparation, the ability to perform multi-element real-time analysis and the possibility of in situ analysis, this technique has become very popular during last years. As a consequence a large number of LIBS systems with different experimental configuration have so far been developed. The purpose of this chapter is to provide a description of the basic components of a LIBS system and how their technical specifications as well as their design/configuration may affect LIBS measurements.

Interferometric system for precise submicrometer particle sizing

Following a recently exploited line of thought, we present an interferometric system for particle sizing in the submicrometer region. The phase of the field that results from the interference between the incident and the scattered waves is measured through a heterodyne detection scheme in a Mach-Zehnder-type interferometer. We explored the possibility of extending previous work on this subject to the case of larger particles, i.e., particles larger than 0.5 mum. Experimental results obtained with polystyrene spheres in the range of diameters 0.16-0.71 mum are reported and compared with theoretical predictions. We show that in this range univocal detection of the particle size is not successful because the phase versus particle-size plot exhibits a maximum in correspondence to diameters close to 0.5 mum.

Heterodyne readout system for dual plate speckle photography: analysis of error sources and performance evaluation

We describe the design, construction, and testing of a dual plate specklegram heterodyne readout system. The choice of the optical layout and mode of operation is based on the results of a detailed discussion on the sources of error associated both with optical noise and misalignments. Noise analysis includes the conventional scintillation noise and a recently described additional source, the speckle grain noise. Stability and reproducibility tests have been conducted and the accuracy of displacement measurement is 10(-6) of the recorded specklegram size, approximately five times worse than the ultimate noise limited value.

Remote surface contouring using a cross-correlation speckle technique

A new high-resolution technique for remote surface contouring is proposed. Two photographic recordings of the object scene are made on separate plates. Between the two recordings, either the object or the recording camera is shifted laterally with respect to the optical axis by a known amount. The resultant relative relief displacement due to surface height distribution is accurately measured by a cross-correlation speckle technique. It is shown that a surface height resolution of the order of 50 μm is achieved at a distance of 1 m. The principle involved, experimental details, results, resolution of the method, and criteria for attaining better resolution, are discussed.

Period Doubling Bifurcation Route to Chaos

A theory recently formulated by Feigenbaum1,2 predicts that the transition to chaotic behaviour via a sequence of period doubling bifurcations has a universal character. Although at this stage the extent at which the theory is applicable is not entirely clear, it is generally believed that it should hold for a large class of nonlinear systems, provided that phase trajectories remain confined in a phase region of adequately low dimension.

Speckle-Tracking Point Diffraction Interferometer For Fluid Studies

We describe a point diffraction interferometer (PDI) for use in connection with stressed fluids. Since the wavefront distortions are conspicuous, the transmitted field breaks down into a speckle distribution. A speckle chaser has been devised so that a bright speckle is always kept on top of the PDI pinhole. Adequate power for the selfgenerated reference beam is thus available, and fringe visibility remains quite satisfactory even for large optical distortions.

Onset of Chaos in Fluid Dynamics

Forced nonlinear systems exhibit deterministic behaviour when the applied stress is small enough. Let us consider a driven anharmonic oscillation with an x3 restoring force. If the driving force is small, the motion is almost sinusoidal. The spectrum is composed of a sharp peak at the driving frequency and, eventually, smaller peaks at the higher harmonics. As the force is increased, the motion becomes more complicated and additional sharp peaks are introduced in the spectrum. One of course might wonder whether by making the driving force arbitrarily large, the spectrum will continue to be in the form of instrumentally narrow peaks. The answer is no, and this holds true for the simple case we are considering as well as for a large variety of more complex nonlinear systems. Above a given threshold the spectrum will show a continuum in between the remnants of the sharp peaks and one says that the system has reached a “chaotic” state.