Coarsening and mechanics in the bubble model for wet foams

Three-state active lattice gas: A discrete Vicsek-like model with excluded volume

We study a discrete-space model of active matter with excluded volume. Particles are restricted to the sites of a triangular lattice and can assume one of three orientations. Varying the density and noise intensity, Monte Carlo simulations reveal a variety of spatial patterns. Ordered states occur in the form of condensed structures, which (away from the full occupancy limit) coexist with a low-density vapor. The condensed structures feature low particle mobility, particularly those that wrap the system via the periodic boundaries. As the noise intensity is increased, dense structures give way to a disordered phase. We characterize the parameter values associated with the condensed phases and perform a detailed study of the order-disorder transition at (i) full occupation and (ii) a density of 0.1. In the former case, the model possesses the same symmetry as the three-state Potts model and exhibits a continuous phase transition, as expected, with critical exponents consistent with those of the associated Potts model. In the low-density case, the transition is clearly discontinuous, with a strong dependence of the final state upon the initial configuration, hysteresis, and nonmonotonic dependence of the Binder cumulant upon noise intensity.

Hierarchical bubble size distributions in coarsening wet liquid foams

Coarsening of two-phase systems is crucial for the stability of dense particle packings such as alloys, foams, emulsions, or supersaturated solutions. Mean field theories predict an asymptotic scaling state with a broad particle size distribution. Aqueous foams are good model systems for investigations of coarsening-induced structures, because the continuous liquid as well as the dispersed gas phases are uniform and isotropic. We present coarsening experiments on wet foams, with liquid fractions up to their unjamming point and beyond, that are performed under microgravity to avoid gravitational drainage. As time elapses, a self-similar regime is reached where the normalized bubble size distribution is invariant. Unexpectedly, the distribution features an excess of small roaming bubbles, mobile within the network of jammed larger bubbles. These roaming bubbles are reminiscent of rattlers in granular materials (grains not subjected to contact forces). We identify a critical liquid fraction [Formula: see text], above which the bubble assembly unjams and the two bubble populations merge into a single narrow distribution of bubbly liquids. Unexpectedly, [Formula: see text] is larger than the random close packing fraction of the foam [Formula: see text]. This is because, between [Formula: see text] and [Formula: see text], the large bubbles remain connected due to a weak adhesion between bubbles. We present models that identify the physical mechanisms explaining our observations. We propose a new comprehensive view of the coarsening phenomenon in wet foams. Our results should be applicable to other phase-separating systems and they may also help to control the elaboration of solid foams with hierarchical structures.

Microfluidic Investigation of Foam Coarsening Dynamics in Porous Media at High-Pressure and High-Temperature Conditions

Coarsening or Oswald ripening, induced by interbubble gas diffusion, is considered to dominate foam structure evolution in porous media. We present the first study of trapped foam coarsening dynamics under realistic deep reservoir conditions (up to 3200 psi/22 MPa of pore pressure and 100 °C of temperature) in a high-pressure and high-temperature microfluidic system. The findings are expected to help predict foam structure evolution in applications such as enhanced oil recovery and CO₂ geological sequestration. It is shown that, in porous media, larger bubbles grow at the expense of smaller bubbles. The growth rate of the average bubble area (⟨a⟩) over time shows a long-term linear increase when ⟨a⟩ is between 1/5 and 1/2 of the average pore size. The foam coarsening kinetics are determined by the liquid film permeability, gas-liquid interfacial tension, and the molar volume of the dispersed phase. In summary, foams prepared with less water-soluble gases (e.g., N₂ and air) and lower foam quality show slower coarsening kinetics due to a lower film permeability. Foam coarsening is more sensitive to surfactant concentration (than surfactant type), as it determines the interfacial tension that controls the mass transfer driving force (capillary pressure difference). The transport properties of the dispersed phase depend strongly on its density, which increases with increasing pore pressure and decreasing temperature. At the same experimental conditions, gas CO₂ foam shows a 10-fold faster coarsening rate than N₂ foam. However, dense (i.e., liquid and supercritical) CO₂ foams show a remarkable 20-500-fold reduction in coarsening kinetics compared with gas N₂ and CO₂ foams due to the significantly reduced mass transfer driving forces. In a sense, trapped CO₂ foam can be stronger than N₂ foam at high-pressure and high-temperature conditions.

An Innovatory Surgical Technique for Submacular Hemorrhage Displacement by Means of a Bioengineering Perspective

The purpose of this case report is to present a new surgical technique for the treatment of large Subretinal Hemorrhage (SRH) secondary to Age-related Macular Degeneration (AMD). Considering the biomechanics of foam evolution theory, bubble coarsening effect, and gas-liquid biphasic absorption, an SRH due to an AMD case was treated with vitrectomy. The treatment was implemented by subretinal injection of air bubbles combined with rtPA followed by air fluid exchange. The air bubbles helped mess up and remove the blood from the macula area, and no complications occurred. Two weeks postoperatively, there was no sign of hemorrhage and the Central Macular Thickness (CMT) was sharply decreased from 443 μm to 317 μm. At the five-month follow-up, the CMT remained at 267 μm and the patient's visual acuity improved from light perception to 20/70 according to the Snellen chart. The combination of injecting multiple air bubbles and submacular rtPA, followed by air fluid exchange, was able to displace more than (90%) of the subretinal blood just two weeks postoperatively. Our technique is a promising alternative surgical approach for the displacement of SMH due to AMD, with a clear visual and anatomical benefit seen in the early follow-up period.

Critical Synergistic Concentration of Binary Surfactant Mixtures

This paper presents a simple method for determination of synergism in binary surfactant mixtures. A homologous series of cationic alkyltrimethylammonium bromides (CnTAB, with n = 8, 12, 16, 18) mixed with three non-ionic surfactants (n-octanol, methyl isobutyl carbinol, tri(propylene glycol) butyl ether) was chosen as a model system. In addition to the cationic-non-ionic system, the mixture of anionic-non-ionic surfactants (sodium dodecyl sulphate and tri(propylene glycol) butyl ether) was investigated. The foam behavior of one-component solutions and binary mixtures was characterized as a function of surfactant concentration, number of carbons (n) in alkyl chain of CnTAB as well as type of surfactant. It was shown that synergism in foamability could be produced by the ionic-non-ionic systems, and the concentration below the synergism occurs, called the critical synergistic concentration (CSC), that can be easily predicted based on the surface tension data on individual components.

Different strategies of foam stabilization in the use of foam as a fracturing fluid

An attractive alternative to mitigate the adverse effects of conventional water-based fluids on the efficiency of hydraulic fracturing is to inject foam-based fracking fluids into reservoirs. The efficiency of foaming fluids in subsurface applications largely depends on the stability and transportation of foam bubbles in harsh environments with high temperature, pressure and salinity, all of which inevitably lead to poor foam properties and thus limit fracturing efficiency. The aim of this paper is to elaborate popular strategies of foam stabilization under reservoir conditions. Specifically, this review first discusses three major mechanisms governing foam decay and summarizes recent progress in research on these phenomena. Since surfactants, polymers, nanoparticles and their composites are popular options for foam stabilization, their stabilizing effects, especially the synergies in composites, are also reviewed. In addition to reporting experimental results, the paper also reports recent advances in interfacial properties via molecular dynamical simulation, which provide new insights into gas/liquid interfacial properties under the influence of surfactants at molecular scale. The results of both experiments and simulations indicate that foam additives play an essential role in foam stability and the synergic effects of surfactants and nanoparticles exhibit more favorable performance.

Order to Disorder Transition in a Coarsening Two-Dimensional Foam

We quantify the spatiotemporal transformation of a monodisperse and well-ordered monolayer of bubbles, as they undergo Ostwald ripening, by tracking the size polydispersity of the bubbles and local ordering of the foam. After nuclei of disorder appear at random locations, the transition takes place through two successive phases: first, the disordered regions grow while the value of polydispersity increases slowly, then the polydispersity grows rapidly once the disordered zones begin to merge together. The transition is captured by a modified logistic model.