Speckle correlometry as a method for evaluating the dynamics of the liquids foam

Abstract

Background and Objectives: The two-phase gas-liquid foams have been an active object of research over the past few decades. Usually, during the coarsening of the foam such physical processes as a foam syneresis (the liquid drainage along the Plateau channel and the bubble walls under the gravity), an Oswald ripening of the gas bubbles, and their coalescence are investigated. Another process that accompanies the aging of gas-liquid foams is the evaporation of the liquid component of the foam that is insufficiently described in the literature. Each of these processes is characterized by its own kinetics. The major factors that determine the dynamic and kinematic characteristics of the foams are the volume fraction of the liquid in the foam, the rheological properties of foam films, the average thickness of films between the gas bubbles, and the dispersion of the system. The modern methods for the diagnostic of the structural rearrangements and the foam aging do not allow studying the evolution of the three-dimensional foams in the real time. In this work, a comparative analysis of the behavior of the time correlations of the intensity fluctuations of the scattered by the liquid foam laser radiation on long time scales is carried out for the case of the system with mass transfer of the liquid component due to its partial evaporation and the isolated system. Such studies play an important role in the development of the coherent-optical methods for the morphofunctional diagnostic of the micro- and nanostructured multiphase systems in the real time. Materials and Methods: The analysis of the evolution of the isolated and “open” liquid foams during their aging is carried out by use of the speckle correlometry method. Two series of the experiments were performed with an isolated system and an “open” system at a temperature of 24°C. Results: The correlation time of the intensity fluctuations of the radiation scattered by the medium was calculated from the normalized correlation function, based on the criterion of its decay by a factor of e. The dependences of the correlation time of the intensity fluctuations on the aging time for isolated and “open” systems are obtained. A phenomenological model to describe the increase in the correlation time of intensity fluctuations is proposed. Within the framework of the model, the experimental data agree with the model data for an isolated foam and correlates with a power law with an exponent equal to 1.5. Conclusion: A speckle-correlation analysis as method for the analysis of the local instabilities caused by the structural rearrangements in the foams under the coarsening was considered. The phenomenological model, that establishes the relationship between the correlation time of the intensity fluctuations of the laser radiation scattered by the foam and the aging time of the foam, is proposed. The obtained results may be useful for the further development of laser methods for the diagnostic of nonstationary multiphase systems with a complex structure and dynamics. 

Keywords

• speckle correlometry
• gas-liquid foam
• evaporation
• diffusion