A. Vailati | Università degli Studi di Milano - State University of Milan (Italy) (original) (raw)

Papers by A. Vailati

Physical Review Letters, 2013

Convective motions in a fluid layer are affected by its orientation with respect to the gravitati... more Convective motions in a fluid layer are affected by its orientation with respect to the gravitational field. We investigate the long-term stability of a thermally-stressed layer of a binary liquid mixture and show that pattern formation is strongly affected by marginal inclinations as small as a few milliradians. At small Rayleigh numbers the mass transfer is dominated by the induced large scale shear flow, while at larger Rayleigh numbers it is dominated by solutal convection. At the transition, the balance between the solutal and shear flows gives rise to drifting columnar flows moving in opposite directions along parallel lanes in a Super-Highway configuration. Our findings suggest a potential relevance for the experimental modeling of thermohaline convection in the oceans. PACS numbers: 47.20.Bp, 92.10.af Pattern formation in non-equilibrium systems arises from symmetry breaking of an isotropic initial state [1]. Whenever two symmetry-breaking mechanisms coexist their competition gives rise to a rich phase diagram. A typical example is represented by an inclined layer of liquid under the action of a temperature difference. Tilting the layer can determine a large scale shear flow (LSF). For fairly large inclinations theoretical [2-4] and experimental studies [5-9] reveal rich spatio-temporal dynamics in the phase diagram, characterized by the presence of Busse oscillations, subharmonic oscillations as well as longitudinal and cross rolls. The transitions between different regimes occur at angles of the order of tens of degrees, and pattern formation does not appear to be influenced by small inclination angles of the order of a few degrees or smaller. By adding a second component to the mixture, the compositional stratification also contributes to the convective behavior. A remarkable example is represented by thermohaline circulation in oceans, where both the local salinity and temperature of water contribute to the convective motions, the thickness of the layer of water being modulated by the seafloor . The thermal stress applied to a liquid mixture via a temperature gradient can be quantified by the dimensionless Rayleigh number Ra = g∆ρ T h 3 /(ηD T ) [12], where g is the gravity acceleration, ∆ρ T the density difference generated by thermal dilation of the liquid, h the liquid layer thickness, η its shear viscosity and D T the thermal diffusivity. The presence of the thermal stress determines a separating flux of the two components and, in turn, a solutal density difference ∆ρ c throughout the sample, whose value and orientation is quantified by the Soret coefficient S T . In the case of a mixture with positive S T , as the one used in our study, both ∆ρ T and ∆ρ c contribute to destabilize a sample heated from below.

Current Opinion in Colloid & Interface Science, 1997

Substantial work, both experimental and theoretical, has been performed on aggregation processes ... more 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.

Applied Optics, 2001

We show that the two-point correlation function in the near field of scattered light is simply re... more 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.

Physica A: Statistical Mechanics and its Applications, 1995

We present recent results in the field of colloidal aggregation. We first present static light sc... more 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

Physical Review Letters, 2000

We show that the two point intensity correlation in the near field of the scattered light is dire... more We show that the two point intensity correlation in the near field of the scattered light is directly related to the two point density correlation g͑r͒. Preliminary measurements on two sets of calibrated random pinholes of 140 and 300 mm diameters, and on aqueous solutions of latex spheres of 5, 10, and 40 mm are reported. A discussion on the desirability of the technique as a simple and powerful alternative to low angle scattering will be presented.

Electrophoresis, 1994

Potential gravity-induced deformations of polyacrylamide matrices during gelling were investigate... more Potential gravity-induced deformations of polyacrylamide matrices during gelling were investigated in two different initiator systems based on (i) photopolymerization with 100 microM methylene blue, 1 mM sodium toluene sulfinate (reducer) and 50 microM diphenyliodonium chloride (oxidizer) (photopolymerization) and (ii) chemical polymerization, utilizing the standard persulfate N,N,N',N'-tetramethylethylenediamine. In both systems, it is seen that convective flows are imprinted in the final gel structure above a critical gelling layer thickness, set at ca. 3 mm. In both systems, progressive increments of the solution density, from normodense (density = 1.0) up to isodense with the growing polymer chains (density = 1.3) do not inhibit the appearance of strong convective flows. However, gel inhomogeneities are completely abolished even in 10 mm gelling layers if polymerization is performed in presence of density gradients, notably of sucrose, from 0 to 20%, 0 to 40% and 0 to 60%. Even the shallower gradient (0-20% sucrose) is able to completely abolish convective flows in persulfate-driven polymerization. It is hypothesized that such disturbances are not created by sedimentation of the growing polymer chains in the gravitational field, but are produced by the reaction exothermality, which produces strong buoyancy-driven flows. It is additionally demonstrated that persulfate polymerization is sensitive to oxygen absorbed from the top liquid layers, which should be carefully protected by an overlay of organic solvent. Methylene blue-induced polymerization appears to offer a series of unique advantages over chemical initiation with persulfate.

Physical Review E, Jan 1, 2000

We have performed small-angle light-scattering measurements of the static structure factor of a c... more We have performed small-angle light-scattering measurements of the static structure factor of a critical binary mixture undergoing diffusive partial remixing. An uncommon scattering geometry integrates the structure factor over the sample thickness, allowing different regions of the concentration profile to be probed simultaneously. Our experiment shows the existence of interface capillary waves throughout the macroscopic evolution to an equilibrium interface, and allows to derive the time evolution of surface tension. Interfacial properties are shown to attain their equilibrium values quickly compared to the system's macroscopic equilibration time.

A wavelet transform spectrum analyzer operating in real time within the frequency range 3 × 10 −5... more A wavelet transform spectrum analyzer operating in real time within the frequency range 3 × 10 −5 − 1.3 × 10 5 Hz has been implemented on a low-cost Digital Signal Processing board operating at 150MHz. The wavelet decomposition of the signal allows to efficiently process non-stationary signals dominated by large amplitude events fairly well localized in time, thus providing the natural tool to analyze processes characterized by 1/f α power spectrum. The parallel architecture of the DSP allows the real-time processing of the wavelet transform of the signal sampled at 0.3MHz. The bandwidth is about 220dB, almost ten decades. The power spectrum of the scattered intensity is processed in real time from the mean square value of the wavelet coefficients within each frequency band. The performances of the spectrum analyzer have been investigated by performing Dynamic Light Scattering experiments on colloidal suspensions and by comparing the measured spectra with the correlation functions data obtained with a traditional multi tau correlator. In order to asses the potentialities of the spectrum analyzer in the investigation of processes involving a wide range of timescales, we have performed measurements on a model system where fluctuations in the scattered intensities are generated by the number fluctuations in a dilute colloidal suspension illuminated by a wide beam. This system is characterized by a power-law spectrum with exponent −3/2 in the scattered intensity fluctuations. The spectrum analyzer allows to recover the power spectrum with a dynamic range spanning about 8 decades. The advantages of wavelet analysis versus correlation analysis in the investigation of processes characterized by a wide distribution of time scales and non-stationary processes are briefly discussed.

Recent experimental and theoretical works have shown that giant fluctuations are present during d... more Recent experimental and theoretical works have shown that giant fluctuations are present during diffusion in liquid systems. We use linearized fluctuating hydrodynamics to calculate the net mass transfer due to these non equilibrium fluctuations. Surprisingly the mass flow turns out to coincide with the usual Fick's one. The renormalization of the hydrodynamic equations allows us to quantify the gravitational modifications of the diffusion coefficient induced by the gravitational stabilization of long wavelength fluctuations.

Annals of The New York Academy of Sciences, 2006

Diffusion is commonly believed to be a homogeneous process at the mesoscopic scale, being driven ... more Diffusion is commonly believed to be a homogeneous process at the mesoscopic scale, being driven only by the random walk of fluid molecules. On the contrary, very large amplitude, long wavelength fluctuations always accompany diffusive processes. 1-4 In the presence of gravity, fluctuations in a fluid containing a stabilizing gradient are affected by two different processes: diffusion, which relaxes them, and the buoyancy force, which quenches them. These phenomena affect both the overall amplitude of fluctuations and their time dependence. For the case of free diffusion, the time-correlation function of the concentration fluctuations is predicted to exhibit an exponential decay with correlation time depending on the wave vector q. For large wave vector fluctuations, diffusion dominates, and the correlation time is predicted to be 1/(Dq 2 ). For small wave vector fluctuations, gravitational forces have time to play a significant role, and the correlation time is predicted to be proportional to q 2 . The effects of gravity and diffusion are comparable for a critical wave vector q c determined by fluid properties and gravity. We have utilized a quantitative dynamic shadowgraph technique to obtain the temporal correlation function of a mixture of LUDOX TMA and water undergoing free diffusion. This technique allows one to simultaneously measure correlation functions achieving good statistics for a number of different wave vectors in a single measurement. Wave vectors as small as 70 cm −1 have been investigated, which is very difficult to achieve with ordinary dynamic light-scattering techniques. We present results on the transition from the diffusive decay of fluctuations to the regime in which gravity is dominant.

Progress in Colloid & Polymer Science, 1997

We investigate both steady-state and time-dependent nonequilibrium fluctuations by means of very ... more We investigate both steady-state and time-dependent nonequilibrium fluctuations by means of very low-angle static light scattering. The system is a thin layer of a binary liquid mixture close to a critical consolution point and placed in a temperature gradient. Due to this choice, the Soret driven nonequilibrium fluctuations are large and their time evolution is slow enough to be followed at ease. We discuss the fast q divergence at steady state and its low q frustration induced by the presence of gravity. We also report measurements taken during the build-up of the concentration gradient. The data indicate that no additional contributions are present during the time-dependent part of the process beyond those calculated in the quasi-steady-state approximation.

Applied Physics Letters, 2002

We describe an optical technique based on the statistical analysis of the random intensity distri... more We describe an optical technique based on the statistical analysis of the random intensity distribution due to the interference of the near-field scattered light with the strong transmitted beam. It is shown that, from the study of the two-dimensional power spectrum of the intensity, one derives the scattered intensity as a function of the scattering wave vector. Near-field conditions are specified and discussed. The substantial advantages over traditional scattering technique are pointed out, and is indicated that the technique could be of interest for wave lengths other than visible light.

We report the results of an experimental study of the static and dynamic properties of long wavel... more We report the results of an experimental study of the static and dynamic properties of long wavelength concentration fluctuations in a mixture of glycerol and water undergoing free diffusion. The shadowgraph method was used to measure both the mean-squared amplitude and the temporal correlation function of the fluctuations for wave vectors so small as to be inaccessible to dynamic light scattering. For a fluid with a stabilizing vertical concentration gradient, the fluctuations are predicted to have a decay rate that increases with decreasing wave vector q, for wave vectors below a cutoff wave vector q C , determined by gravity and the fluid properties. This behavior is caused by buoyant forces acting on the fluctuations. We find that for wave vectors above ϳq C , the decay rate does vary in the normal diffusive manner as Dq 2 , where D is the mass diffusion coefficient. Furthermore, for q Շ q C we find that longer wavelength fluctuations decay more rapidly than do shorter wavelength fluctuations, i.e., the behavior is nondiffusive, as predicted.