Marina Carpineti

Near-field intensity correlations of scattered light

We show that the two-point correlation function in the near field of scattered light is simply related to the scattered intensity distribution. We present a new, to our knowledge, optical scheme to measure the correlation function in the near field, and we describe a processing technique that permits the subtraction of stray light on a statistical basis. We present experimental data for solutions of latex spheres, and we show that this novel technique is a powerful alternative to static light scattering.

Light scattering studies of aggregation phenomena

We present recent results in the field of colloidal aggregation. We first present static light scattering data of aggregation in intermediate regimes between RLCA and DLCA. In particular, we show that working with dense solutions, new and unexpected features are observed. In fact, we measure intensity distributions strongly peaked at q≠0, which scale according to the same scaling law typical of spinodal decomposition. Furthermore, we show that by only varying the initial conditions, the appearance of the peak can be completely inhibited, and that this result provides new information about the intermediate regimes. Finally, we present depolarized dynamic light scattering data from aggregates grown in the RLCA regime. We show that using optically anisotropic spheres it is possible to determine in a unique experiment all the features characterizing an aggregation process. Moreover, we present evidence that the asymptotic form expected for the cluster mass distribution is attained in the very early phases of the aggregation process.

Metastability and supersaturation limit for lysozyme crystallization

We investigate the ionic strength and temperature dependence of metastability with respect to crystallization in lysozyme solution. The supersaturation limit c* shows a simple scaling relation with salt concentration and, together with the equilibrium solubility csat, approximately factorizes into two terms, respectively accounting for electrostatic and temperature effects. The supersaturation ratio S = c*/csat monotonically increases with the addition of salt. While the dependence of solution properties on the ionic strength is consistent with a recent model by Sear and Warren, temperature effects highlight the importance of hydrophobic contributions.

The NEUF-DIX space project - Non-EquilibriUm Fluctuations during DIffusion in compleX liquids

Abstract.Diffusion and thermal diffusion processes in a liquid mixture are accompanied by long-range non-equilibrium fluctuations, whose amplitude is orders of magnitude larger than that of equilibrium fluctuations. The mean-square amplitude of the non-equilibrium fluctuations presents a scale-free power law behavior q-4 as a function of the wave vector q, but the divergence of the amplitude of the fluctuations at small wave vectors is prevented by the presence of gravity. In microgravity conditions the non-equilibrium fluctuations are fully developed and span all the available length scales up to the macroscopic size of the systems in the direction parallel to the applied gradient. Available theoretical models are based on linearized hydrodynamics and provide an adequate description of the statics and dynamics of the fluctuations in the presence of small temperature/concentration gradients and under stationary or quasi-stationary conditions. We describe a project aimed at the investigation of Non-EquilibriUm Fluctuations during DIffusion in compleX liquids (NEUF-DIX). The focus of the project is on the investigation in micro-gravity conditions of the non-equilibrium fluctuations in complex liquids, trying to tackle several challenging problems that emerged during the latest years, such as the theoretical predictions of Casimir-like forces induced by non-equilibrium fluctuations; the understanding of the non-equilibrium fluctuations in multi-component mixtures including a polymer, both in relation to the transport coefficients and to their behavior close to a glass transition; the understanding of the non-equilibrium fluctuations in concentrated colloidal suspensions, a problem closely related with the detection of Casimir forces; and the investigation of the development of fluctuations during transient diffusion. We envision to parallel these experiments with state-of-the-art multi-scale simulations.Graphical abstract

Small angle light scattering studies concerning aggregation processes

Substantial work, both experimental and theoretical, has been performed on aggregation processes in dense solutions. Aggregation driven by depletion interactions has recently been studied in a variety of systems. The phase diagrams that have been obtained are extremely complex, and they include aggregation and gelation phenomena.

Microporous membrane filters: a static light scattering study

We studied some of the more employed microporous membrane filters via small-angle static light scattering under quasi-index matching conditions. We report data from both acetate of cellulose (AC) and mixed-esters of cellulose (MEC) membranes of various pore sizes ps. On short length scales, the membranes have fractal morphology. While AC filters may be described as surface fractals, MEC membranes turn out to be mass fractals. Furthermore, the scattered intensity distributions exhibit a peak at a finite wave vector qm, thus indicating the presence of a certain degree of spatial order over a characteristic length Λ = 2πqm. Λ is found to increase with ps, as expected, although it is always larger than ps and is not trivially related to it.

Static light scattering characterisation of microporous membrane filters

Abstract Some of the more commonly employed microporous membrane filters have been studied in quasi index matching solvents by means of small-angle static light scattering. We report data obtained from membranes made of esters of cellulose with rated pore size ps ranging from 0.1 to 8 μm. The high q behaviour of the scattered intensity follows a power law I(q) α q−α where α is either 1.9 or 3.8 depending on the membrane chemical composition. Such values reveal that on a short length scale the filters have a fractal nature. Furthermore, the scattered intensity distribution exhibits a peak at a finite wave vector qm, like in spinodal decomposition processes (which indicates mass anticorrelation effects), a behaviour similar to that observed in Vycor porous glasses via neutron and X-ray small angle scattering. The peak position moves towards smaller q values when increasing ps although the characteristic length Λ = 2π q m is always larger than the pore size and it is not trivially related to it. In particular, for membranes made of mixed-esters of cellulose, Λ is found to be roughly proportional to ps for small pore sizes, while for ps > 1.2 μm it increases only slightly with the pore size and eventually tends to saturate. In conclusion, these results indicates that small-angle static light scattering promises to be a very useful technique for characterising microporous membrane filter morphology, and also for quality control.

Colloidal aggregation in the intermediate regimes

We have studied salt induced aggregation of polystyrene latex in an intermediate region between DLCA and RLCA. The average cluster mass is found to grow according to a power law ( ?α tz) with an exponent z varying with the salt concentration C. Furthermore we find that at high C (DLCA) the fractal dimension d f is stable in time and equal to 1.65. On the contrary, for lower concentrations d f surprisingly decreases during each run, starting from values typical of RLCA. An interpretation of this trend is presented.