Phase-separated Liquid Mixture Research Articles

Near-critical spreading of droplets

We study the spreading of droplets in a near-critical phase-separated liquid mixture, using a combination of experiments, lubrication theory and finite-element numerical simulations. The classical Tanner’s law describing the spreading of viscous droplets is robustly verified when the critical temperature is neared. Furthermore, the microscopic cut-off length scale emerging in this law is obtained as a single free parameter for each given temperature. In total-wetting conditions, this length is interpreted as the thickness of the thin precursor film present ahead of the apparent contact line. The collapse of the different evolutions onto a single Tanner-like master curve demonstrates the universality of viscous spreading before entering in the fluctuation-dominated regime. Finally, our results reveal a counter-intuitive and sharp thinning of the precursor film when approaching the critical temperature, which is attributed to the vanishing spreading parameter at the critical point.

Enhanced Displacement of Phase Separating Liquid Mixtures in 2D Confined Spaces

Displacing liquids in a confined space is important for technological processes ranging from porous membrane separation to CO2 sequestration. The liquid to be displaced usually consists of multiple components with different solubilities in the displacing liquid. Phase separation and chemical composition gradients in the liquids can influence the displacement rate. In this work, we investigate the effects of liquid composition on the displacement process of ternary liquid mixtures in a quasi-2D microchannel where liquid–liquid phase separation occurs concurrently. We focused on model ternary mixtures containing 1-octanol (a model oil), ethanol (a good solvent), and water (a poor solvent). These mixtures are displaced with water or with an ethanol aqueous solution. As a comparison, for some experiments, water was displaced by ternary mixtures. The bright-field and fluorescence imaging measurements reveal distinct phase separation behaviors. The spatial distribution of subphases arising from phase separation and the displacement rates of the solution are impacted by the initial ternary solution composition. The boundary between the solution and displacing liquid changes from a defined interface to a diffusive interface as the initial 1-octanol composition in the solution is reduced. The displacement rate also varies non-linearly with the initial 1-octanol composition. The slowest displacement rate arises at intermediate 1-octanol concentration, where a stable three-zone configuration forms at the boundary. At very low 1-octanol concentration, the displacement rate is fast, associated with droplet formation and motion driven by the chemical concentration gradients formed during phase separation. The excess energy provided from phase separation may contribute to the enhanced displacement at intermediate to high 1-octanol concentrations but not at low 1-octanol concentration with enhancement from induced flow in confinement. The knowledge gained from this study highlights the importance of manipulating phase separation to enhance mass transport in confinement for a wide range of separation processes.

Nonlinear deformations of liquid-liquid interfaces induced by electromagnetic radiation pressure

The idea of working with a near-critical phase-separated liquid mixture whereby the surface tension becomes weak, has recently made the field of laser manipulation of liquid interfaces a much more convenient tool in practice. The deformation of interfaces may become as large as several tenths of micrometers, even with the use of conventional laser power. This circumstance necessitates the use of nonlinear geometrical theory for the description of surface deformations. The present paper works out such a theory, for the surface deformation under conditions of axial symmetry and stationarity. Good agreement is found with the experimental results of Casner and Delville [A. Casner and J. P. Delville, Phys. Rev. Lett. 87, 054503 (2001); Opt. Lett. 26, 1418 (2001); Phys. Rev. Lett. 90, 144503 (2003)], in the case of moderate power or a broad laser beam. In the case of large power and a narrow beam, corresponding to surface deformations of about 50 micrometers or higher, the theory is found to over-predict the deformation. Possible explanations of this discrepancy are discussed.

Manipulation d'interfaces liquides induite par la pression de radiation

Generally very intense fields are needed to bend a liquid interface with the optical radiation pressure. To enhance the coupling, we investigate this effect in a near-critical phase-separated liquid mixture and observe huge stationary deformations at low beam powers by tuning the softness of the meniscus with variations in temperature. This offers us the opportunity to present an universal description of both the bending and the resultant soft lensing. Non-linear behaviours are also presented and applications in optical wave-guiding are anticipated.

Adaptative lensing driven by the radiation pressure of a continuous-wave laser wave upon a near-critical liquid–liquid interface

The bending of a liquid interface by the radiation pressure of a cw laser wave is generally weak. To strongly enhance the coupling, we investigate lensing in a near-critical phase-separated liquid mixture. By continuously tuning the softness of the meniscus by varying the temperature, we observed huge stationary interface deformations at low beam power that led to an important variation of the resultant soft lensing. We also illustrate the crucial role played by temperature in this process to demonstrate its potential use in lens adaptation.

Giant deformations of a liquid-liquid interface induced by the optical radiation pressure.

Because of the small momentum of photons, very intense fields are generally required to bend a liquid interface with the optical radiation pressure. We explore this issue in a near-critical phase-separated liquid mixture to vary continuously the meniscus softness by tuning the temperature. Low power continuous laser waves become sufficient to induce huge stationary bulges. Using the beam size to build an "optical" Bond number, Bo, we investigate the crossover from low to large Bo. The whole set of data collapses onto a single master curve which illustrates the universality of the phenomenon.

Spin-wave reflection from an interface of two Fermi liquids

The boundary condition for the magnetization jump at an interface of two Fermi liquids and the spin current through the interface is rederived. With the use of the derived condition the coefficient of reflection of a spin wave from a boundary of two Fermi liquids for normal incidence is found and applied to the case of interface of phase separated liquid mixture of 3He in 4He. The effect of the phase separation boundary on the coherently precessing spin pattern is discussed.

Nucleation and early stage growth in phase-separating liquid mixtures under weak time-dependent supersaturation

In the standard description of phase separation, quenching from an initial equilibrium state to a final metastable state in the two-phase region is usually assumed to be instantaneous. Such an artificial situation is nevertheless intrinsically at variance with experiments because the quench rate is finite due to the continuous changes in thermodynamic parameters between the initial and final states. We experimentally explore this issue in near-critical micellar phases of microemulsion with induced transient grating techniques, focusing our attention on the very early stage of droplet growth, where the influence of the time dependence of supersaturation is the strongest. The experiment makes use of laser-induced concentration variations to locally quench the mixture with two intersecting pump beams, whose interference pattern optically traps the nucleated droplets on fringes. Due to the slow mass diffusion kinetics of quenches in composition, the time-resolved reflectivity of a third probe beam on the resulting droplet grating allows us to determine the mean nucleation time and the mean quench depth at the beginning of the decay of the metastable state. By varying the amplitudes of the control parameters (beam power, beam radii), we are able to characterize the dynamic properties of nucleation onset during continuous quenching. The results are interpreted in the light of very simple scaling arguments. We show in particular that for a weak linear temporal variation of the supersaturation, where and are, respectively, the measured critical radius and nucleation time.

Early Stage Kinetics of Phase-Separating Liquid Mixtures

Observation of the very early stage of phase separation is made difficult by the large number of nucleated domains and the polydispersity of the droplet distribution. We investigate this issue using laser waves to locally quench a liquid mixture with two intersecting pump beams whose interference pattern traps the nuclei on the fringes. The time-resolved reflectivity of a third probe wave on the induced droplet grating (i) confirms that nucleation is heterogeneous even in the bulk, and (ii) surprisingly shows that impurities lead to an unexpected reduction of the growth onto a single ``medium-dependent master curve'' when samples are synthesized with the same lots of constituents.

Induced transient gratings as a probe of the early-stage kinetics of phase-separating liquid mixtures

Because of the large number of nucleated domains and the polydispersity of the droplet distribution, the early-stage kinetics of a first-order phase transition remains difficult to explore. We investigate this issue by using laser-induced concentration variations to drive a transition locally in a liquid mixture with two intersecting pump beams whose interference pattern traps the nucleated droplets on the optical fringes. The time-resolved reflectivity of a third probe wave on the induced droplet grating allows us to characterize the droplet growth at its early stage, which illustrates the efficiency of transient grating methods in quantifying the kinetics of out-of-equilibrium processes.

Kinetics of Crossover in Phase-Separating Liquid Mixtures Induced by Finite-Size Effects

Classically, wetting phenomena prevent any experimental analysis of the intrinsic kinetics of phase-separating liquids in reduced geometry. We investigate this issue using a cw laser wave to quench a liquid mixture inside the beam (absence of wetting) and to trap the generated droplets (absence of droplet flow). The Gaussian wave behaves as a box with ``soft walls'' in which the diffusional growth of a single droplet can be analyzed during the crossover between the bulk behavior and its modification induced by finite-size effects. The resulting slowing down is reported and a model which describes these behaviors is presented.

Phase equilibria and solvation forces for fluids confined between parallel walls

The thermodynamics of fluids confined between two adsorbing solid substrates (walls) is revisited. Attention is focused on the phase equilibria of an open system characterized by the variables μ (chemical potential), T (temperature), and H (wall separation). Clapeyron equations for the shape of lines of coexistence are derived and used to interpret the results of earlier calculations of two first-order transitions, namely capillary condensation of an undersaturated ‘‘gas’’ to ‘‘liquid’’ and prewetting (thick–thin film transition) at finite H. At such transitions the adsorption Γ and the solvation force f jump discontinuously. Criticality of a confined fluid is associated with the divergence of the derivatives (∂Γ/∂μ)T,H and (∂2Γ/∂μ2)T,H or equivalently, with the divergence of (∂f/∂H)T,μ and (∂2f/∂H2)T,μ. The presence of the additional field variable H, and its conjugate density f, implies that the phase equilibria of a confined fluid can be much richer than those of a bulk fluid or of a single interface (H=∞). By extending the formalism to multicomponent systems Clapeyron equations are derived for the coexistence of phases in confined fluid mixtures. An equation for the shift in chemical potential (or concentration) of the phase separation curve of a binary liquid mixture resulting from confinement at constant pressure and temperature is presented. This equation, which becomes exact for large separations H, is the appropriate analog for mixtures of the Kelvin equation used to describe capillary condensation in pure fluids; it can also be regarded as a generalization to nonzero concentrations of the Ostwald–Freundlich formula for the dependence of solubility on particle size. Our analysis provides a framework for interpreting recent solvation force measurements on phase-separating liquid mixtures.