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Phase Separation of Initially Non-Homogeneous Liquid Mixtures
No full textIn previous works, we showed that the spinodal decomposition of low-viscosity liquid mixtures is driven by the convection induced by chemical potential gradients. In this article, we study the impact of this driving force on the phase separation of low-viscosity liquid mixtures in which a strong initial concentration gradient is present within a few-millimeter-thick layer. This region was created by keeping an initially demixed mixture at a higher-than-critical temperature for 0.5 h and allowing it to mix by diffusion. After a deep and rapid quench within the spinodal range, the mixtures at first remained macroscopically unchanged; then micron-sized drops
appeared; and finally, after few seconds, a sharp interface suddenly formed, with droplet sizes never exceeding 10 micron. As it took hours for diffusion and only seconds for phase separation, this result shows that the latter process cannot be driven by diffusion. This conclusion was reinforced by adding glass particles to the mixture and observing the velocity pattern, which had speeds exceeding 0.5 mm/s, thus demonstrating that the process is driven by convection. The impact of gravity was ruled out, as the morphology of a density-segregated mixture as it phase separates appears to be the same as that of an isodensity mixture. These effects can be explained considering that the large initial concentration gradient induces a body force that drives small drops toward one or the other of the homogeneous phases, where they are rapidly reabsorbed, thus explaining why larger drops were not observed during the separation process. In addition, the measured bulk velocities correlate with the magnitude of the chemical potential gradients, in agreement with the theoretical model
Liquid-Liquid Extraction Using the Composition Induced Phase Separation Process
No full textThis paper describes a new separation process of liquid-liquid extraction. It consists of first mixing the system to be extracted with a primary solvent, which is soluble with the native solvent, and subsequently adding a modifier, which is insoluble with either the native or the primary solvent. This process is similar to the phase transition process which was described in a previous paper, where the liquid mixture, together with the solute to be separated, is first heated above its critical temperature, where it forms a uniform solution, and then cooled to the region below the miscibility curve, where it separates. Both processes have the advantage that the resulting separation of the solvents is very rapid, even in the presence of emulsion-forming impurities. In addition, the extraction efficiency of the new process may be 10 times higher than that of the traditional liquid-liquid extraction. The new process is thought of having significant advantages in the extraction of products from fermentation broths, plants and other natural sources
Spinodal Decomposition in Binary Mixtures
No full textWe study the early stage of the phase separation of a binary mixture far from its critical point of demixing. Whenever the mixture of two mutually repulsive species is quenched to a temperature below its critical point of miscibility, the effect of the enthalpic repulsive force prevails upon the entropic tendency to mix, so that the system eventually separates into two coexisting phases. We have developed a highly non linear model, in close analogy with the linear theory of Cahn and Hilliard, where a generalized free energy is defined in terms of two parameters, psi and a, the first describing the equilibrium composition of the two phases and the second denoting a characteristic length scale that is inversely proportional to the equilibrium surface tension. The linear stability analysis predicts that any perturbation of the initial mixture composition with wave number k smaller than sqrt(2 psi / a) will grow exponentially in time, with a maximum growth corresponding to k_max = sqrt (psi/a). A numerical solution of the equation shows that nonlinear effects saturate the exponential growth, and that the concentration distribution tends to a steady state, periodic profile with wavelength lambda = 2 pi a / sqrt(psi) corresponding to the fastest growing mode of the linear regime. The main result of our theoretical model is that this steady state does not depend on the form of thye initial perturbation to the homogeneous composition profile
Large Scale, Unidirectional Convection during Phase Separation of a Density Matched Liquid Mixture
No full textComplete phase segregation may occur on a 10 centimeter scale even in the absence of buoyancy, due to unidirectional, large scale rapid bulk flows driven by chemical potential gradients. Using a hexadecane-acetone nearly density-matched liquid mixture in a 20 cm-long condenser tube with a 1 cm diameter, we observed the rapid axial migration of the acetone-rich drops towards the warmer regions of the condenser. Conversely, the hexadecane-rich drops moved in the opposite direction, therefore ruling out thermocapillary effects as a possible explanation of the phenomenon. These flows lead to a complete phase segregation within 10 seconds, with the formation of a single interface perpendicular to the axial direction. Changing the temperature gradient along the tube from 0.25 C/cm to 1 C/cm no change was detected, with typical drop speeds up to 6 cm/s, irrespectively of the distance of the drop from the wall, showing that the phenomenon is not due to a flow instability
Phase Separation of Liquid Mixtures in the Presence of Surfactants
No full textThe presence of surface-active compounds does not influence the settling time of a partially miscible liquid mixture as it phase separates. On the other hand, when the same mixture is agitated isothermally while in its two-phase state, its settling time greatly increases if surfactants are added. This phenomenon is monitored microscopically through a series of direct visualizations and is explained theoretically by applying the model H, showing that phase separation is governed by the convective motion due to capillary forces. These forces induce a net attraction between droplets which greatly dominates the repulsive forces due to the presence of surfactants
Convection-Driven Phase Segregation of Deeply Quenched Liquid Mixtures
No full textObserving the phase separation of deeply quenched, low viscosity liquid mixtures we inferred that the process is driven by the convection due to capillary forces, and not by molecular diffusion neither by gravity, heat or surface effects. After quenching a partially miscible, initially homogeneous, off-critical liquid mixture to a temperature T deeply below its critical point of miscibility, we observed the formation of rapidly coalescing droplets of the minority phase, whose size grows linearly with time. Following the motion of isolated 10 micron droplets, we saw that they move in random directions at speeds exceeding 100 micron/s, showing that during most of the process the system is far from local equilibrium. Eventually, when their size reaches the capillary length, the nucleating drops start sedimenting as gravity becomes the dominant force. This behavior was observed for both density-segregated and density-matched systems, irrespectively whether they were kept in horizontal or vertical cells. The experiments were repeated using both untreated (i.e. hydrophilic) and modified (i.e. hydrophobic) cell walls, with identical results and, in addition, no bulk motion was observed when the mixture was replaced with water, showing that the observed convection is not induced by gravity, neither by surface or temperature effects. Using a simple dimensional analysis of the governing equations based on the diffuse interface model, we showed that convection is induced by the coalescence among drops which, in turn, is the result of a non-equilibrium capillary force that indeed dominates both diffusion and gravity forces
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