Coalescence and Phase Separation During Spinodal Decomposition of Solvent Mixtures Far From the Critical Point

Cuny City College, New York NY

Investigators

Abstract

ABSTRACT CTS-9978781 R. Mauri CUNY City College In the previous stages of this work we have studied the phase transition occurring when a mixture of partially miscible liquids is deeply quenched across its miscibility curve [2][4]. This study was motivated by a previous NSF-funded investigation, where solvents [1][3]. One of the main results of this work was the discovery that, following phase transition, coalescence and phase segregation are very rapid, even when emulsifiers and surface-active compounds are dissolved within the mixture. Like most transport processes, phase separation is driven by two basic demising process is retarded by either using high-viscosity systems, such as polymer melts and alloys, or by quenching the liquid mixtures only by few millikelvins below their miscibility curve [6]-[9]. In these cases, right after the temperature of the system has crossed that of the miscibility curve, the mixture phase separates by forming well-defined patches, whose typical size grows wit time as t1/3 [6]-[10]. On the other hand, low-viscosity, deeply quenched liquid mixtures separates by convection, due to the influence of an attractive force, which tends to minimize the interfacial area. In our recent studies on spinodal decomposition, we showed that convection leads to the information of rapidly coalescing drops, whose typical size grows linearly in time. In addition, our results have shown that surface tension-driven attractive forces are far larger that repulsive electrostatic forces, thus explaining why the phase separation of low-viscosity liquid mixtures occurs rapidly, irrespectively of the presence of emulsifiers. While our investigation has produced novel and unexpected results, much work is required to achieve our ultimate goal and model the flow of liquid mixtures during phase transition in the presence of surface-active compounds. The following proposal is intended to address some of these problems. In particular, we want to focus on the following four areas of research. 1. The role of emulsion-forming and coalescence-hindering compounds and impurities during phase segregation and spinodal decomposition of partially miscible liquid mixtures. As we saw, one of the main advantages of using partially miscible solvents in extraction processes is that phase segregation is unaffected by the presence of emulsifiers. In this research we intend to better understand this phenomenon, stressing the limitations under which is ceases to occur. 2. The role of viscosity during phase separation. Since we know that phase separation in low-viscosity fluid mixtures (with Peclet numbers larger than 10,000) is very different than in high-viscosity's (with Peclet numbers smaller than 10), we propose to study the dynamics of macroscopically quiescent phase separating liquid mixtures with intermediate viscosity, i.e. with Peclet numbers ranging from 10 to 10,000. 3. The role of shear on spinodal decomposition. This investigation is motivated by the fact that in practical extraction processes a rapid cooling requires mixing, which in turn may cause emulsification. This process can be modeled in a Couette apparatus, where the fluid mixture undergoes constant shear. 4. Extension of our theoretical model to simulate the effects studied in the previous 3 points.