Critical Mixture Research Articles

Self-similarity and coarsening rate of a convecting bicontinuous phase separating mixture: Effect of the viscosity contrast

We present a computational study of the hydrodynamic coarsening in 3D of a critical mixture using the Cahn-Hilliard/Navier-Stokes model. The topology of the resulting intricate bicontinuous microstructure is analyzed through the principal curvatures to prove self-similar morphological evolution. We find that the self similarity exists for both systems: iso-viscous and with variable viscosity. However the two system have distinct topological character. Our simulations confirm that the predicted viscous growth regime exists in both cases. Moreover the coarsening rate is inversely proportional to an \textit{effective viscosity} that is the geometrical average of the viscosities of the two phases.

Fast-flame limit for hydrogen/methane-air mixtures

Flame acceleration experiments were performed in a 10 cm inner-diameter tube filled with evenly spaced 0.43 blockage ratio orifice plates. The critical mixture composition required for flame acceleration to a fast-flame was measured for four methane/hydrogen fuel-air mixtures at initial temperatures of 298 K, 423 K, and 573 K. These conditions provide a large range in the Zeldovich number between 12 and 28, where the Zeldovich number was calculated from the laminar burning velocity obtained from 1-D flame simulations. The data collapsed very well when the expansion ratio across the flame (calculated at the critical condition) was plotted versus the Zeldovich number. This is consistent with correlation proposed by Dorofeev [7], that was based on experimental data obtained over a narrower Zeldovich number range. For pure hydrogen fuel, the critical expansion ratio was found to be between 2 and 4, and for pure methane the critical expansion ratio was as high as 8, for an initial temperature of 573 K.

Motile bacteria in a critical fluid mixture.

We studied the swimming of Escherichia coli bacteria in the vicinity of the critical point in a solution of the nonionic surfactant C_{12}E_{5} in buffer solution. In phase-contrast microscopy, each swimming cell produces a transient trail behind itself lasting several seconds. Comparing quantitative image analysis with simulations show that these trails are due to local phase reorganization triggered by differential adsorption. This contrasts with similar trails seen in bacteria swimming in liquid crystals, which are due to shear effects. We show how our trails are controlled, and use them to probe the structure and dynamics of critical fluctuations in the fluid medium.

Performance-Graded Specifications for Asphalt Emulsions Used in Chip Seal Preservation Treatments

This paper details the development of a framework for emulsion performance-grade (EPG) specifications for chip seal treatments. Chip seals are preservation surface treatments that are designed to improve the condition of the pavement surface while mitigating deterioration of the overall pavement structure. Asphalt emulsions used in chip seals often are selected based on factors that are not necessarily related to performance. Aggregate loss and bleeding have been identified as the most critical chip seal distresses that are related to binder performance. Storage stability, sprayability, and drain-out have been determined to be the most critical constructability concerns. For this study, binder and mixture test methods were identified to reflect the failure mechanisms for each critical distress type. The emulsion residue test methods that were identified to capture chip seal performance are the multiple stress creep and recovery test for bleeding and the dynamic shear rheometer frequency sweep test for low-temperature aggregate loss. The fresh emulsion test methods that were identified to capture chip seal constructability are the three-step shear test and storage stability test. The proposed EPG specifications for the fresh emulsion properties that are related to constructability were developed using statistical analysis of the binder test results. The proposed EPG specifications for the residual binder properties were developed by defining the temperature-independent relationships between the emulsion residue properties and mixture performance that correspond to each critical distress. Preliminary specification limits were then established based on the values of the binder properties that correspond to the critical mixture performance thresholds.

Microscopic Engine Powered by Critical Demixing.

We experimentally demonstrate a microscopic engine powered by the local reversible demixing of a critical mixture. We show that, when an absorbing microsphere is optically trapped by a focused laser beam in a subcritical mixture, it is set into rotation around the optical axis of the beam because of the emergence of diffusiophoretic propulsion. This behavior can be controlled by adjusting the optical power, the temperature, and the criticality of the mixture.

Tuning the motility and directionality of self-propelled colloids

Microorganisms are able to overcome the thermal randomness of their surroundings by harvesting energy to navigate in viscous fluid environments. In a similar manner, synthetic colloidal microswimmers are capable of mimicking complex biolocomotion by means of simple self-propulsion mechanisms. Although experimentally the speed of active particles can be controlled by e.g. self-generated chemical and thermal gradients, an in-situ change of swimming direction remains a challenge. In this work, we study self-propulsion of half-coated spherical colloids in critical binary mixtures and show that the coupling of local body forces, induced by laser illumination, and the wetting properties of the colloid, can be used to finely tune both the colloid’s swimming speed and its directionality. We experimentally and numerically demonstrate that the direction of motion can be reversibly switched by means of the size and shape of the droplet(s) nucleated around the colloid, depending on the particle radius and the fluid’s ambient temperature. Moreover, the aforementioned features enable the possibility to realize both negative and positive phototaxis in light intensity gradients. Our results can be extended to other types of half-coated microswimmers, provided that both of their hemispheres are selectively made active but with distinct physical properties.

Fatigue comparisons of mortars at different volume concentration of aggregate particles

Fatigue cracking is one of the primary failure mechanisms in asphalt pavements and it predominantly occurs within the mortar phase. For this reason, in recent years, a number of studies were carried out by various researchers to better understand the fatigue mechanism in such a critical mixture phase. In the present work, time sweep tests were performed in strain control mode on asphalt mortars prepared with three volume percentages of fine aggregate at different aging conditions. In particular, two different asphalt mortars were used: one containing Recycled Asphalt Pavement (RAP) materials and the other one composed of the same RAP aggregate skeleton without the aged binder. The influence of the different aging conditions, the presence of the aged binder and the addition of fine aggregate particles on the fatigue resistance of the mortars were evaluated. Moreover, a relationship between the parameters of the obtained fatigue laws and the different aging and mix design conditions was found. The proposed relationship can be easily used to predict the fatigue resistance of a mortar composed of a specific volume concentration of aggregate particles and recycled material. The potential extension of the proposed relationship to mixtures may eventually result in the implementation of a simple analysis tool for practitioner limiting the need for more sophisticated and expensive fatigue tests on asphalt mixtures.

Prediction method of both azeotropic and critical points of the binary refrigerant mixtures

The prediction of the azeotropic point and the determination of the critical point inbinary mixtures is very important in refrigerating industry. In this task, a simple method ispresented in order to predict and determine both azeotropic and critical points. Our methodemploys experimental data and a thermodynamic model. 1,1,1,2‐tetrafluoroethane (R134a) in themixture presents the possibility of inventing the azeotropic and critical properties. The mixturesstudied in this work are: Cyclopropane (RC270), Propylene (R1270), Hexafluoroethane (R116) andCarbon dioxide (R744) based on 1,1,1,2‐tetrafluoroethane (R134a). The model consists of thePeng–Robinson equation of state (EoS), the Mathias–Copeman alph a function and the Wong‐Sandler mixing rules involving the NRTL model. The results proved that there is a good agreementbetween the predicted values and the experimental data. The presented methods are able topredict the azeotropic and determine critical positions.

Phase behavior of binary and ternary mixture for the poly(TBAEMA) and TBAEMA in supercritical solvents

The cloud-point pressure of poly(t-butylaminoethyl methacrylate) [Poly(TBAEMA)] in various solvents such as supercritical carbon dioxide (CO2), dimethyl ether (DME) and t-butylaminoethyl methacrylate (TBAEMA) was measured to maximum pressure and temperature of 218.79 MPa and 452.9 K, respectively. The phase behavior for the Poly(TBAEMA)+CO2+TBAEMA mixture was investigated according to the various contribution factors, such as pressure, temperature and concentration with TBAEMA mass fraction of 9.9 wt%, 10.4 wt%, 14.9 wt%, 24.4 wt% and 35.2 wt%. The cloud point curves for the Poly(TBAEMA)+CO2+DME (15.6–78.7 wt%) systems show the variation of the (p, T) curve from upper critical solution temperature (UCST) region to lower critical solution temperature (LCST) region as DME concentration increases. The experimental data for the CO2+TBAEMA system were reported at the broad temperature range of 313.2 K to 393.2 K and the pressure range of 3.70 MPa to 20.62 MPa. The CO2+TBAEMA binary system shows the type-I phase behavior with a continuous critical mixture curve, and is correlated by Peng-Robinson equation of state with the critical properties for TBAEMA obtained by Joback and Lyderson group contribution method.

On the critical Casimir interaction between anisotropic inclusions on a membrane.

Using a lattice model and a versatile thermodynamic integration scheme, we study the critical Casimir interactions between inclusions embedded in a two-dimensional critical binary mixtures. For single-domain inclusions we demonstrate that the interactions are very long range, and their magnitudes strongly depend on the affinity of the inclusions with the species in the binary mixtures, ranging from repulsive when two inclusions have opposing affinities to attractive when they have the same affinities. When one of the inclusions has no preference for either of the species, we find negligible critical Casimir interactions. For multiple-domain inclusions, mimicking the observations that membrane proteins often have several domains with varying affinities to the surrounding lipid species, the presence of domains with opposing affinities does not cancel the interactions altogether. Instead we can observe both attractive and repulsive interactions depending on their relative orientations. With increasing number of domains per inclusion, the range and magnitude of the effective interactions decrease in a similar fashion to those of electrostatic multipoles. Finally, clusters formed by multiple-domain inclusions can result in an effective affinity patterning due to the anisotropic character of the Casimir interactions between the building blocks.

Wrinkled labyrinths in critical demixing ferrofluid.

A thin film of a critical ferrofluid mixture undergoes a sequence of transitions in a magnetic field. First the application of a field induces a critical demixing of the fluid into cylindrical droplets of the minority phase immersed in an extended majority phase. At a second critical field the cylindrical shape is destabilized and transforms into a labyrinth pattern. A third wrinkling transition occurs at even higher field if the liquid has a liquid/air interface. The wrinkling is absent if the droplet has a cover-slide on top. We explain the wrinkling by the wetting behavior of the liquid/air interface that shifts the surface region away from a critical demixing point.

Performance-Related Specifications for Asphalt Emulsions Used in Microsurfacing Treatments

This paper details the development of a framework for emulsion performance grade (EPG) specifications for microsurfacings. Microsurfacings are preservation surface treatments designed to improve the condition of the pavement surface while mitigating deterioration of the overall pavement structure. Asphalt emulsions used in microsurfacings are often selected on the basis of factors that are not necessarily related to performance. Rutting and thermal cracking have been identified as the most critical microsurfacing distresses related to binder performance. In the assessment of fresh emulsion properties, storage stability and mixability have been determined to be the most critical constructability concerns. For this study, binder and mixture test methods were identified to reflect the failure mechanisms for each critical distress type. The two emulsion residue test methods that were identified to capture microsurfacing performance were ( a) the multiple stress creep and recovery test for rutting and ( b) the dynamic shear rheometer frequency sweep test for thermal cracking. The identified critical fresh emulsion properties that related to constructability included storage stability and viscosity measured at a low shear rate. The proposed EPG specifications for fresh emulsion properties related to constructability were developed by using statistical analysis of the binder test results. The EPG specifications for residual binder were developed by defining the temperature-independent relationships between the emulsion residue properties and the mixture performance that corresponded to each critical distress. Preliminary specification limits were then established on the basis of the values of the binder properties that corresponded to the critical mixture performance thresholds.

Double-spike inversion for three-isotope systems

Double spiking is conventionally used to make accurate determinations of natural mass-dependent isotopic fractionations for elements with four or more stable isotopes. Here we document a methodology which extends the effective application of double spiking to three isotope systems. This approach requires making a mixture with isotope ratios that lie on a ‘critical curve’ where the sample – double-spike mixing line and the tangent to the instrumental mass-bias curve are coincident. Inversion of the mixing equations for such a mixture leads to a solution for the sample fractionation which is independent (to first order) of the uncertainty in the instrumental mass-bias and, hence, independent of any mass-dependent artefacts in the measurement such as those produced by residual matrix not completely removed by prior chemical purification. In practice, mixtures can be made which yield an accuracy conservatively estimated to be ∼ 0.005‰/amu. The precision of the method is explored as a function of double-spike composition for Mg, Si and K isotope systems. We show that for Mg and Si measurement precision is not compromised by the compositions of viable critical mixtures nor by uncertainty magnification during inversion of the equations. Thus, double spiking provides a valuable means to obtain robust, high precision isotopic measurements of Mg and Si. For K, however, the low abundance of 40K in the optimal critical mixture places a significant practical limitation on the application of double spiking to analyses of this element.

Equilibrium and non-equilibrium concentration fluctuations in a critical binary mixture.

When a macroscopic concentration gradient is present across a binary mixture, long-ranged non-equilibrium concentration fluctuations (NCF) appear as a consequence of the coupling between the gradient and spontaneous equilibrium velocity fluctuations. Long-ranged equilibrium concentration fluctuations (ECF) may be also observed when the mixture is close to a critical point. Here we study the interplay between NCF and critical ECF in a near-critical mixture aniline/cyclohexane in the presence of a vertical concentration gradient. To this aim, we exploit a commercial optical microscope and a simple, custom-made, temperature-controlled cell to obtain simultaneous static and dynamic scattering information on the fluctuations. We first characterise the critical ECF at fixed temperature T above the upper critical solution temperature Tc, in the wide temperature range [Formula: see text] °C. In this range, we observe the expected critical scaling behaviour for both the scattering intensity and the mass diffusion coefficient and we determine the critical exponents [Formula: see text], [Formula: see text] and [Formula: see text], which are found in agreement with the 3D Ising values. We then study the system in the two-phase region (T < T c). In particular, we characterise the interplay between ECF and NCF when the mixture, initially at a temperature Ti, is rapidly brought to a temperature T f > T i. During the transient, a vertical diffusive mass flux is present that causes the onset of NCF, whose amplitude vanishes with time, as the flux goes to zero. We also study the time dependence of the equilibrium scattering intensity I eq, of the crossover wave vector q co and of the diffusion coefficient D during diffusion and find that all these quantities exhibit an exponential relaxation enslaved to the diffusive kinetics.

Investigation of Critical Sand-Deposition Velocity in Horizontal Gas/Liquid Stratified Flow

SummaryExperimental and theoretical investigations have been conducted on gas/liquid/sand stratified flow in horizontal pipes at low sand concentrations. A 4-in. experimental facility was designed and constructed, and data were acquired using air/water/glass-bead flow. The data include measurements of critical sand-deposition velocities—namely, the transition between moving and stationary beds. The data reveal that for a constant superficial liquid velocity, the critical sand-deposition mixture and liquid velocities increase with increasing sand concentrations. Moreover, for a given sand concentration, the critical liquid velocity is nearly the same for different superficial liquid velocities. The sand-deposition correlations of Oroskar and Turian (1980) for single-phase flow and Salama (2000) for two-phase flow were modified and extended to develop a new correlation. The developed correlation enables the prediction of critical sand-deposition mixture velocity for horizontal stratified flow as a function of sand concentration, along with other parameters. Comparison between the predictions of the developed correlation and experimental data from the literature (Angelsen et al. 1989; Najmi et al. 2015) reveals a good agreement, especially for a pipe diameter of 0.1 m.

High pressure phase equilibria for the binary mixture of CO2 + 3-phenyl propionitrile and CO2 + 2-phenyl butyronitrile systems

Nitrile groups have strong polarity and show the non-ideal phase equilibria. Phase equilibria for the 3-phenyl propionitrile and 2-phenyl butyronitrile play an important role as organic solvents in several industrial processes. The solubility curves of binary mixture for 3-phenyl propionitrile and 2-phenyl butyronitrile in supercritical CO2 are investigated using a synthetic method at five temperatures of (313.2, 333.2, 353.2, 373.2 and 393.2)K and pressure up to 34.45MPa. Both CO2+3-phenyl propionitrile and CO2+2-phenyl butyronitrile systems have critical mixture curves that show maximums in pressure-temperature space between the critical temperatures of CO2 and 3-phenyl propionitrile or 2-phenyl butyronitrile. The solubility of CO2 for two systems at a constant pressure decreases with the increase of temperature. The CO2+3-phenyl propionitrile and CO2+2-phenyl butyronitrile systems display type-I phase behavior within scope of this work. The experimental results for the CO2+3-phenyl propionitrile and CO2+2-phenyl butyronitrile binary systems are correlated with Peng-Robinson equation of state using a mixing rule including two adjustable parameters (kij, ηij). The critical properties (pc, Tc and ω) and vapor pressure of 3-phenyl propionitrile and 2-phenyl butyronitrile were estimated with the Joback–Lyderson group contribution and Lee-Kesler method.

Dynamics of Self-Propelled Janus Particles in Viscoelastic Fluids.

We experimentally investigate active motion of spherical Janus colloidal particles in a viscoelastic fluid. Self-propulsion is achieved by a local concentration gradient of a critical polymer mixture which is imposed by laser illumination. Even in the regime where the fluid's viscosity is independent of the deformation rate induced by the particle, we find a remarkable increase of up to 2 orders of magnitude of the rotational diffusion with increasing particle velocity, which can be phenomenologically described by an effective rotational diffusion coefficient dependent on the Weissenberg number. We show that this effect gives rise to a highly anisotropic response of microswimmers in viscoelastic media to external forces, depending on its orientation.

Fluctuating Lipid Nanodomains Near Critical Transitions

Lipid organization in cellular membranes is fundamental to many cellular processes, and in recent years the hypothesis of lipid “rafts” has grown from one of simple macroscopic phase co-existance to a more dynamic description, encompassing modulated phases, compositional fluctuations and micro-emulsions. The static and dynamic behaviour of critical lipid mixtures have been characterised by fluorescence microscopy of GUV's, but at length scales of >1 μm due to optical resolution limits. Here we present High-Speed AFM data of the critical phase behaviour of a model cell, imaging with nanometre lateral resolution and at up to 8 frames per second. Below the critical temperature (in the macroscopic phase separation regime) the boundaries of micron-sized domains were observed fluctuating with amplitudes of 10's of nm's. This motion has been quantitatively analysed to give line tensions in the range 1 pN (similar to that observed in GUV's), reducing linearly with temperature down to a low of 0.05 pN close to the critical point. As temperature is increased whilst imaging we show the crossing of the critical temperature (TC) resulting in the breakdown of large domains into much smaller nanoscale fluctuations. The fluctuations at and above TC result in unstable 5 - 100 nm domains that have a lifetime of seconds to almost 1 minute, again dependent upon temperature. Domains persist up to 10 degrees above TC. For direct comparison with our results we perform simple Monte Carlo 2D Ising model simulations which show closely analogous behaviour for line tensions, phase structure break down and domain lifetimes. Our results give the first direct insight into membrane dynamics at the nanoscale.

Bubble-point measurement for the binary mixture of propargyl acrylate and propargyl methacrylate in supercritical carbon dioxide

Acrylate and methacrylate (acrylic acid type) are compounds with weak polarity which show a non-ideal behaviour. Phase behaviour of these systems play a significant role as organic solvents in industrial processes. High pressure phase behaviour data were reported for binary mixture of propargyl acrylate and propargyl methacrylate in supercritical carbon dioxide. The bubble-point curves for the (carbon dioxide+propargyl acrylate) and (carbon dioxide+propargyl methacrylate) mixtures were measured by static view cell apparatus at temperature range from 313.2K to 393.2K and at pressures below 19.14MPa. The (carbon dioxide+propargyl acrylate) and (carbon dioxide+propargyl methacrylate) systems exhibit type-I phase behaviour. The (carbon dioxide+(meth)acrylate) systems had continuous critical mixture curves with maximums in pressure located between the critical temperatures of carbon dioxide and propargyl acrylate or carbon dioxide and propargyl methacrylate. The solubility behaviour of propargyl (meth)acrylate in the (carbon dioxide+propargyl acrylate) and (carbon dioxide+propargyl acrylate) systems increases as the temperature increases at a fixed pressure. The experimental results for the (carbon dioxide+propargyl acrylate) and (carbon dioxide+propargyl methacrylate) systems correlate with the Peng–Robinson equation of state using a van der Waals one-fluid mixing rule. The critical properties of propargyl acrylate and propargyl methacrylate were predicted with the Joback–Lyderson group contribution and Lee–Kesler method.

Microscopic segregation of hydrophilic ions in critical binary aqueous solvents.

Solid surfaces suspended in critical aqueous binary mixtures containing hydrophilic salt have recently been found to exhibit anomalous interactions, and a possible mechanism is provided by the asymmetric solvation preferences of weakly and strongly hydrophilic cations and anions, respectively. Here we address this mechanism by studying interfacial ion distributions in a critical binary mixture of water and 2,6-dimethylpyridine containing potassium chloride at temperatures below the lower critical point, using grazing-incidence X-ray fluorescence from the liquid-vapour interface. Our data provide direct and unambiguous experimental evidence for microscopic segregation of hydrophilic ions in critical aqueous binary mixtures, thereby supporting the important role of asymmetric ion solvation in the above mentioned anomalous force. However, the experimental data are only qualitatively reproduced by state-of-the-art theoretical calculations, demonstrating the need of a microscopic theoretical model including asymmetric ion solvation.