Wet Foams Research Articles

Extended large- Q Potts model simulation of foam drainage

We study the vertical liquid profile of foam drainage using the threedimensional large- Q Potts model extended to include gravity. Forced drainage with constant-rate liquid input from the top of the foam produces a constant profile. In free drainage, without liquid input from the top, homogeneously distributed liquid drains to the bottom of the foam until capillary effects and gravity balance. For pulsed drainage, as liquid drains from the top of the foam into the dry foam, a sharp interface between the wet and dry foam develops. The fixed profile moves downwards at a constant velocity with a flat interface. The results of our simulations are suggested in both experiments and simplified meanfield analytical results.

Foam Drainage: Extended Large-Q Potts Model Simulation

ABSTRACTWe study foam drainage using the large-Q Potts model extended to include gravity on a three dimensional lattice. Without adding liquid, homogeneously distributed liquid drains to the bottom of the foam until equilibrium between capillary effects and gravity is reached, while in an ordered dry foam, if a fixed amount of liquid is added from the top, a sharp fiat interface between the wet and dry foam develops. The wetting front profile forms a downward moving pulse, with a constant velocity. The pulse decays over time while its leading edge for a brief time behaves like a solitary wave. With continuous liquid addition from the top, the pulse does not decay and we observe a soliton front moving with a constant velocity. Continuously adding liquid to an initially wet foam keeps the liquid profile constant. Our simulations agree with both experimental data and simplified mean field analytical results for ordered foams but predict an unstable interface for disordered foams.

Interaction of rarefaction waves with wet foam

The interaction of rarefaction waves of different shapes with wet water foams is studied experimentally. It is found that the observed values of the pressure are greater, while the surface velocity is lower than the corresponding values predicted by the pseudogas model. The foam breakdown starts as the pressure decreases by 0.3 atm relative to the initial pressure. During downstream propagation of the rarefaction-wave leading edge the propagation velocity decreases. Using of water-based foams as effective screens for damping blast waves in different technological processes has caused considerable interest in studying wave propagation in such systems. The pressure wave dynamics in a foam have been investigated in much detail, both experimentally and theoretically [1–3]. However, the interaction of rarefaction waves with foam has practically never been studied, although it was mentioned in [4] that the unloading phase following the compression wave phase is one of the factors defining the damaging action of blast waves. Besides blast-wave damping, rarefaction wave propagation takes place if such waves are used to breakup foam in oil-producing wells [5]. Below, the interaction of rarefaction waves of different shapes with wet water foams is studied. The vertical shock tube described in detail in [3] was used in these experiments.

Embryonic and Larval Respiration In the Arboreal foam Nests of the African Frog Chiromantis Xerampelina

In Zimbabwe, female Chiromantis xerampelina construct spherical foam nests that are suspended above temporary water. The nests average 624 ml in volume and contain 854 eggs. The 1.7 mm ova have exceptionally thin jelly capsules and are dispersed in the foamy core of the nest, which is surrounded by a layer of eggless foam. At 25 degrees C, each embryo requires 3.5 days to reach hatching at developmental stage 22, during which it consumes 30 microliters of oxygen. After hatching, each larva remains in the nest for 2 more days and consumes a further 123 microliters of oxygen. The fresh foam contains 77% air, which is sufficient to supply all of the oxygen requirements of the embryos until well after they hatch. Therefore, the size of the egg mass is not limited by oxygen availability as it is in many other anurans. Oxygen also diffuses into the nest from the atmosphere, but the rate is severely restricted by the wet foam, despite the presence of bubbles. Drying of the outer layer of foam greatly increases its oxygen conductance, but the larvae remain in the inner core of wet foam, where they compete for oxygen at the periphery. With further drying of the nest, the wet foam diminishes in volume and concentrates the larvae at a time when their oxygen demands are approaching the maximum. Oxygen pressures within the wet foam drop below 10 kPa and oxygen uptake by the larvae becomes progressively limited, possibly stimulating their emergence from the nest. The delay between hatching and escape from the nest permits the larvae to grow and mature to a stage at which all of the clutch can emerge simultaneously.

Interaction of pressure waves with a wet foam

Interaction of pressure waves with a wet foam

A relation between dynamic foam stability and surface elasticity

In our previous papers, we described a method for measuring the retention time which is a measure of the foam stability. In addition, the dependence of this retention time on surface elasticity was considered. On the basis of these results a simple mathematical formula for this dependence has now been derived and is tested by a fitting procedure. The formula is not very exact, however, but it can be used to explain the main influence of the adsorption properties on the stability of a wet foam.

Steady-state drainage of an aqueous foam.

The process of wetting of a monodisperse foam by the continuous addition of liquid from the top has been investigated for a cylindrical column. The velocity of the interface between dry and wet foam was found to vary as the square root of the rate of addition of liquid. Evidence of a structural transformation was found around gas fraction \ensuremath{\Phi}=0.87. It was interpreted as arising from the collapse of the four-sided faces of the (Kelvin) polyhedra which constitute the foam cells, and this was directly observed when large bubbles were used. There is a second transition at around \ensuremath{\Phi}=0.6, with the onset of bubble motion.

Experimental verification of a heat pump assisted continuous dryer simulation model

The experimental and predicted performance of a prototype heat pump assisted continuous dryer is reported. The dryer was shown to be capable of specific moisture extraction rates (SMERs) of between 1.5 and 2.5 kg/kWh using wetted foam rubber as the test material being dried. The results highlight the importance of maintaining conditions of high relative humidity within the air stream entering the evaporator; an increase in the relative humidity from 30 to 80% was shown to give a two‐fold increase in the SMER. An optimum evaporator bypass air ratio of between 60 and 70% was observed for this dryer. The effects on performance of deviations from this optimum condition were found to be less significant than had been indicated by earlier models. The predicted performance of the dryer using a simulation model developed previously by the authors was in good agreement with the corresponding measured values.

Wet foams: Formation, properties and mechanism of stability

Overall picture of phenomena occuring during formation and existence of the wet foams is presented. Properties and mechanism of stability are discussed on the example of the wet foams obtained from solutions of two homologous series of surface active substances; the fatty acids and n-alkanols. In general three physical processes which contribute to foam stability can be distinguished: drainage of liquid out of the foam, coalescence and/or rupture of bubbles, and disproportionation (which may be called Ostwald ripening or gas diffusion from one bubble to another). Dynamic and non-equilibrium character of the wet foams is stressed. Motion of a bubble through the solution causes disequilibration of the surface concentration alongside the bubble surface. The surface concentration on the upstream part of the bubble is much smaller than the equilibrium concentration. Thus, the bubbles arrive at the solution surface with non-equilibrium surface concentration, and these actual non-equilibrium surface coverages determine possibility of formation and properties of the foams. Solution content ϕ in the volume of wet foam is high (of an order 307.), while in top foam layer it is much smaller (ϕ≅5%) . It shows that rupture of the wet foam takes place practically only in the top layer of bubbles and durability of these top foam films determine stability and volume of the whole foam column. On the basis of measurements of liquid content ϕ and lifetimes of bubbles in the top foam layer it was estimated that thicknesses of rupture of these top films were of an order of a few micrometers. At such thicknesses the force of disjoining pressure do not attain yet any meaningful value. Influence of kinetics of adsorption, frequency of external disturbances, surface activity of the solute and lifetime of the foam films on magnitude of the surface elasticity forces induced in the systems studied is discussed. It is shown that stability of the wet foams can be explained in terms of the effective elasticity farces, i.e. the surface elasticity forces which are induced at an actual non-equilibrium surface coverage. There is agreement between the courses of the dependences of the foamability parameter (retention time, rt) and the effective elasticity forces as a function of the number n of carbon atoms in the fatty acid and n-alkanol molecule. This shows that the effective elasticity forces are decisive parameter in formation and stability of the wet foams. It also explains why the foamability of a substance with a stronger surface activity can be lower than that of a substance with a weaker surface activity. The foamability, especially under dynamic conditions, cannot simply be correlated with the surface activity.

Preparation and Properties of Low Density Replica Carbon Foams

ABSTRACTLow density foams are used for a variety of applications, including catalytic supports, battery anodes, microporous membranes, and laser fusion targets. The technique for making replica carbon foams described in this paper has been previously reported[I] and involves a process in which an inorganic substrate (sodium chloride) is infused with a carbonizable polymer. After carbonization, the substrate is removed by a leaching process and the wet foam is dried; the resultant foam is referred to as replica carbon. This paper describes improvements in the processing which result in a smaller pore size and improved foam homogeneity.The original substrate is the single most important factor affecting the resultant structure. Techniques to improve the uniformity of the substrate and the translation of substrate anomalies into the final product are described.

Relationship between foam stability and surface elasticity forces: Fatty acid solutions

The stability of wet foams of fatty acid solutions (n-pentanoic to n-decanoic acid) in 0.005 N HCl was studied. Retention time values, being a measure of the stability of foams under steady-state conditions, equilibrium surface tension - concentration isotherms σ(log c 0 ), values of the surface dilational modulus, and effective elasticity forces E eff were determined. The mechanism of foam formation is discussed and the importance of dynamic effects in foam stability is stressed. It is shown that stabilizing forces are connected with actual non-equilibrium coverages of interfaces, and changes in foamability can be correlated with values of the effective elasticity forces. Reasons why the foamability of solutions of the higher members of a homologous series of fatty acids starts to decrease even though their surface activity is successively increasing are also explained.

Foam dilatational rheology: I. Dilatational viscosity

The dilatational viscosity of a three-dimensional model, spatially periodic foam, composed of tetrakaidecahedron unit cells is determined by considering the local flow induced within planar thin liquid films during expansion or contraction of foam cells. From this fine-scale hydrodynamic motion, a global foam dilatational viscosity is calculated by equating local and global energy dissipation. The calculation is intended to illustrate the nature of dilatational viscous effects in ordered “dry foam” systems, as opposed to an earlier result (1) aimed at the “wet foam,” or lower dispersed volume fraction range. Unlike the “wet” foam calculation where primary viscous stresses are localized within Plateau border regions and derive from interfacial viscous properties, the local viscous stress for the dry foam is distributed throughout the planar thin liquid films, and the two primary contributions to the global dilatational viscosity are found to be liquid-phase viscosity and interfacial tension gradients.

On the shearing flow of foams and concentrated emulsions

Shearing flow of an idealized, two-dimensional foam with monodisperse, spatially periodic cell structure is examined. Viscous effects are modelled by the film withdrawal mechanism of Mysels, Shinoda & Frankel. The primary flow occurs where thin films with inextensible interfaces are withdrawn from or recede into quasi-static Plateau borders, film junctions that contain most of the liquid. The viscous flow induces an excess tension that varies between films and alters the foam structure. The instantaneous structure and macroscopic stress for a foam of arbitrary orientation are determined for simple shearing and planar extensional flow. As the foam flows, the Plateau borders coalesce and separate, which leads to switching of bubble neighbours. The quasi-steady asymptotic analysis of the flow is valid for small capillary numbers Ca based on the macroscopic deformation rate. This requires the foam to be wet, i.e. the liquid volume fraction must be large enough that structure varies continuously with strain. The viscous contribution to the instantaneous stress is ; by contrast, the first normal stress difference is O(1). Even though wet foams are elastic for small but finite deformations, the time-averaged shear stress does not evidence a yield stress.

Theory of mobilization pressure gradient of flowing foams in porous media: I. Incompressible foam

A theory is presented for the minimum pressure gradient (∇ p) min required to keep a foam flowing through a porous medium. The theory accounts for pore shape, foam texture, contact-angle hysteresis, and bubble separation at pore throats, but not for the effects of gas compressibility or of the Plateau borders where liquid lamellae contact the pore wall. Quantitative estimates of (∇ p) min depend on pore shape, foam texture, and surface tension. For medium-textured foams (bubbles 400 μm in diameter in bulk), reasonable parameter values give (∇ p) min = 18–32 kPa/m (0.8–1.4 psi/ft) for CO 2 foams and 125–240 kPa/m (5–11 psi/ft) for others foams. Lower gas/liquid surface tension causes the lower estimate for CO 2 foams. For most foams, foam texture must coarsen substantially as foam flows away from the well in oilfield application, or formation plugging results. Further, to flow at 20–45 kPa/m (1–2 psi/ft), foam bubbles must be at least 1 or more cm, or hundreds of pores, in length. CO 2 foams, on the other hand, can still flow if bubbles are somewhat smaller. The highest values of (∇ p) min are predicted for foams in pores with high aspect ratio (body radius/throat radius), sharp corners, and narrow throats, and at low capillary pressures (wet foams).

On Mechanism of Dynamic Stability of Wet Foams

On Mechanism of Dynamic Stability of Wet Foams

Surface elasticity and dynamic stability of wet foams

Solutions of n-butanol, n-pentanol, n-hexanol, n-heptanol, n-octanol, n-nonanol and n-decanol were studied. The frothability of the solutions was determined by a pneumatic method, surface elasticity forces were measured using the oscillating bubble technique, and the average lifetimes of gas bubbles in a top foam layer were determined by cinephotography. The frothability of solutions was described in terms of the retention time ( rt) values. The rt values depend strongly on the kind of surface-active substance and on its concentrations. The frothabilities of the various n-alkanols were compared at the same values of their reduced concentrations c 0/ a, where a is the concentration of the half surface coverage. The rt values initially increase with increasing number ( n) of carbon atoms in n-alkanol molecules, reach a maximum for C 6C 8, and then rapidly decrease for n-nonanol and n-decanol. The surface activity of the solute, its adsorption kinetics, the lifetime of the foam films, and the frequency of the external disturbances are the main factors determining the magnitude of the effective elasticity forces ( E eff). Similar to the rt values, the E eff values initially increase with increasing n, reach a maximum for C 6C 8, and then rapidly decrease for higher members of the n-alkanol homologous series. The magnitude and the position of these maxima are a function of the solute concentration, adsorption times, and frequency of external disturbances. The results obtained show that the effective elasticity forces are the major factors determining the stability of wet dynamic foams.

Development of Pectic Projections on the Surface of Wound Callus Cells of Daucus carota L.

During studies of wound healing in carrot roots, callus cells with prominent external projections were frequently observed. Protruberances of various sizes and shapes have previously been described on the outer wall surface of parenchyma cells in a number of vascular plants (Kisser, 1928; Carlquist, 1956), but were notably absent on wound callus cells of Daucus carota L. which developed on healed tissues of swollen tap roots (Sirtautaite and Sokolova, 1965) and carrot callus cells which had been grown axenically in various culture media (Fosket and Roberts, 1965; Israel and Steward, 1966; Halperin and Jensen, 1967). Carrots of the cultivars Bertina and Spalding Red Cored Improved were grown for 176 days under typical cultivation conditions in light peat soil, or in a coarse sand loam, as described previously (Davies and Lewis, 1980). Samples of 40 freshly-harvested roots of each cultivar were cut transversely into 5 cm thick discs and arranged in a single layer on nylon mesh over wet polystyrene foam in closed polythene boxes (80 x 45 x 8 cm) where the humidity, measured with lithium chloride sensors ('Hygrodynamics', V. A. Howe Ltd) was 0*95-0-98aw;. After periods of up to 10 days at 25 + 1 °C, sections of callus tissue 10-60 /?m thick were cut with a freezing microtome from the uppermost healed wound surface and examined with a Zeiss 'Universal' or 4R.A. 38' microscope with bright field, phase contrast and Nomarski interference systems. Histochemical tests with 0-5 per cent ammonium oxalate, zinc-chloro-iodine, Sudan IV, IKI-H2S04, Nile blue sulphate or osmium tetroxide were those described by Jensen (1962): hydroxylamine-ferric chloride by Reeve (1959); ruthenium red by Carlquist (1956); and methylene blue by Johansen (1940). For scanning electron microscopy (SEM) blocks (2x5 mm2) of callus tissue were carefully excised and mounted on aluminium stubs with silver 'Dag' (Acheson Colloid Company), and each stub was inserted into the surface of an aluminium block which had been precooled in liquid nitrogen. The rest of the procedure was as described by Lewis and Day (1972), and the material was examined with a 'Stereoscan Mk. II' (Cambridge Instruments Ltd) microscope. Specimens for transmission electron microscopy (TEM) were prepared by soaking the tissue either in unbuffered 2*5 per cent aqueous potassium permanaganate for 1 h, then washed in distilled water (Israel and Steward, 1966); or in 3 per cent gluteraldehyde containing 5 per cent sucrose, buffered with sodium cacodolyte at pH 7 0 (Jordan and Chapman, 1973), then in 2 per cent osmium tetroxide for 4 h. The material was then

A Noncoulombic Model for Adsorbing Colloid Flotation

Abstract A model for floc foam flotation is examined in which the driving force for bubble attachment to a floc particle derives from a nonzero contact angle of the air–water interface with the solid surface. Binding energies are calculated and found to be sufficient to overcome random thermal forces and also the viscous drag forces occurring in wet foams in stripping columns. Adsorption isotherms of floc particles at the air–water interface are calculated; a Langmuir isotherm is found to be an excellent approximation in most cases.