Two-mode dynamics in dispersed systems: the case of particle-stabilized foams studied by diffusing wave spectroscopy

The stabilization of aqueous foams solely by solid particles is an active field of research. Thanks to controlled particle chemistry and production devices, we are able to generate large volumes of such foams. We previously investigated some of their unique properties, especially the strongly reduced coarsening. Here we report another type of study on these foams: performing diffusing wave spectroscopy (DWS), we investigate for the first time the internal dynamics on the scales of both the particles and the bubbles. When compared to surfactant foams, unusual features are observed; in particular, two well-separated modes are found in the dynamics, both evolving with foam aging. We propose an interpretation of these specificities, taking into account both the scattering by free particles in the foam fluid (fast mode), and by the foam structure (slow mode). To validate our interpretation, we show that independent measurements of the interstitial fluid scattering length, obtained indirectly on the foam and directly on the drained liquid, are in good agreement. We have also identified the experimental conditions required to observe such two-process dynamics. Counter-intuitively, the fraction of free particles within the foam interstitial fluid has to be very low to get an optimal signature of these particles on the DWS correlation curves. This study also sheds light on the partitioning of the particles inside the foams and at the interfaces, as the foam ages. Lastly, the results shown here (obtained by analyzing the fluctuations of the transmitted light) implement the previous ones (obtained by analyzing the mean transmitted intensity), and prove that the foam structure is actually not fully frozen.     

Dynamic masking of PIV images using the Radon transform in free surface flows

Particle image velocimetry (PIV) processing of free surface flow images often requires the use of digital masks to overcome the problems caused by the interface. In cases where a large number of particle images are collected it is essential that the time-varying boundary between the two phases can be tracked automatically to produce the binary masks. The Radon transform-based technique presented in this paper allows the automatic detection of the air–water interface in a stream of particle images acquired from a single camera. It is applied to time-resolved PIV measurements in the liquid phase of a stratified multiphase flow in a circular pipe. Accuracy estimations are provided using synthetic and real wave profiles. An extension to the more complex case of an overturning wave is also discussed.     

Fundamental groupoids for simplicial objects in Mal'tsev categories

We show that the category of internal groupoids in an exact Mal'tsev category is reflective, and, moreover, a Birkhoff subcategory of the category of simplicial objects. We then characterize the central extensions of the corresponding Galois structure, and show that regular epimorphisms admit a relative monotone-light factorization system in the sense of Chikhladze. We also draw some comparison with Kan complexes. By comparing the reflections of simplicial objects and reflexive graphs into groupoids, we exhibit a connection with weighted commutators (as defined by Gran, Janelidze and Ursini).     

Classical theory of laser noise

The classical theory of laser noise treats light in a classical manner, yet agrees with quantum theory for large particle numbers. The basic concept is that laser noise is caused by atomic jumps between lower and upper levels, and that atoms subjected to classically-prescribed optical fields are independent. The treatment of amplitude noise of single-mode cavities containing resonant three-level atoms is applicable to semiconductor lasers at moderate power. At high power one must account for the dependence of the gain on optical power and for state-occupancy fluctuations. The phasor theory that attributes noise to the beat between the oscillating field and the field spontaneously emitted in the mode by excited-state atoms cannot be understood consistently in semiclassical terms.     

Sur les singularités des séries de fractions rationnelles

Rendiconti del Circolo Matematico di Palermo Series 1 -     

<Emphasis Type="Italic">m</Emphasis>-Dissipativity of Kolmogorov Operators Corresponding to Burgers Equations with Space-time White Noise

In this article, we prove new a priori estimates on the solutions of the Burgers equation driven by a space-time white noise and on the associated invariant measure. We also prove smoothing properties for the transition semi-group. This is obtained thanks to the introduction of a modified Kolmogorov operator. These results are then used to prove that the Kolmogorov operator associated to the Burgers equation is m-dissipative. This implies several properties on the Kolmogorov equation.      

Dairy aroma compounds recovery by pervaporation

Original pervaporation experiments with two dairy aroma compounds diluted in model aqueous solutions through GFT silicalite-filled silicone composite membrane and GKSS PEBA homogeneous membrane (0.1 m² effective area) were carried out on a pilot-plant. A systematic approach was done, studying the influence of various operating parameters (feed temperature and permeate pressure). The permeabilities of the membranes were calculated for each permeant, based on the estimation of the driving force as a fugacities difference. The pervaporation membranes tested showed a good selectivity for the extraction of methylthiobutanoate (hydrophobic molecule with cheese fragrance) at high dilution rate. However, these membranes proved to be less selective for the recovery of diacetyl (hydrophilic butter aroma). For this component, the coupling of pervaporation and two-stage condensation improved significantly the selectivity of the whole process. The thermodynamic properties in the liquid feed (real dairy media) as well as in the permeate (vapour-liquid equilibria at low pressure) have to be well known in order to optimize the recovery of the aroma compounds