Evolution Of Foams Research Articles

Correlation between Transient Shear Experiments and Structure Evolution of Aqueous Foams

This experimental work deals with rheological properties of aqueous foams and slip phenomena. Rheological measurements are performed on a stable foam with a parallel plate rheometer. When a constant shear rate is applied to foam, two regimes can be identified in the recorded stress vs time curve: a transient regime where the structure evolves and where the recorded stress varies, followed by a steady state regime where the stress is stabilized. Measurements, modeling, and elimination of the slip velocity are performed. Experiments with grooved surfaces allow elimination of slip at the wall. From measurements at two gaps with smooth surfaces, we use two slip correction methods and check their validity by a direct comparison with actual measurements (with grooved surfaces). A foam rheological equation can be determined from the measurements. Finally, using an optical device coupled with image analysis software, foam texture is investigated on the basis of its evolution with shear rate and time. Evolution of the bubbles size and arrangement into the gap with time of shearing are shown. The transient regime is identified as a regime where the intimate structure of the foam evolves. Slip velocity is also evidenced and measured with the visualization device.

Silicon carbide–titanium carbide composite foams produced using a polymeric precursor

A polysilane solution used as a silicon carbide (SiC) precursor was mixed with different amounts of titanium carbide (TiC) powder and open cell polyurethane (PU) foams were dipped in these suspensions. The resulting pre-foams were pyrolyzed at 900°C in nitrogen and then heated further at various temperature between 1100 and 1600°C in the same atmosphere to produce SiC–TiC composite foams. The evolution of the composite foams has been studied using thermogravimetry and X-ray diffraction. The PU foam, pre-foams and the SiC–TiC composite foams were characterized by optical and scanning electron microscopy. These studies show that the pre-foams retained their shape well during pyrolysis and the composite foams produced consist of an open cell structure and hole-free solid struts. Prevention of cracking in the foams were dependent on the TiC content. The shrinkage observed during the conversion of the pre-foams to the composite ceramic foams can be controlled by varying the TiC content and the sintering temperature.

A study of aluminium foam formation—kinetics and microstructure

Aluminium foams were produced by applying the powder compact melting method, i.e. by mixing metal powders and powdered gas-releasing blowing agents and pressing them to a foamable precursor material after this. The resulting precursor was then foamed by heating it up to above its melting point inside an “expandometer”, which allowed for the volume and temperature to be measured throughout the entire process. The present studies comprise the effects of the aluminium alloy composition (AlSi7 and 6061), some of the pressing parameters of the foamable precursor material, the foaming temperature and the heating rate during foaming on the expansion behaviour of the foam. Moreover, the morphological and microstructural evolution of metal foams is investigated.

The structure of basaltic scoria and reticulite and inferences for vesiculation, foam formation, and fragmentation in lava fountains

In this investigation pyroclast structures are used to constrain degassing in basaltic lava fountains. Vesicle size, shape, number density, interconnectedness and packing character are quantified and related to (1) the kinetics of bubble nucleation and growth, (2) the structural evolution of magmatic foams and (3) the influence of vesiculation rate on magma fragmentation. Measurements made on a diverse suite of pyroclasts from Kilauea volcano indicate that basaltic foams evolve through an initially disordered, closed-celled, spherical state to a well-ordered, open-celled, polyhedral state as the vesicularity rises from ~ 75 to 98%. The structural changes occur rapidly (< 10 s) in the conduit and fountain in response to an intense vesiculation burst. Vesicle size distribution systematics indicate bubble nucleation rates (~ 2 × 10 4 events cm −3s −1) that are approximately three orders of magnitude greater than those found for effusive eruptive activity. Bubble growth rates (~ 9 × 10 −4 cm/s) exceed effusive estimates by a factor of 3. The observed “runaway” rate of bubble production indicates strong supersaturations at the onset of nucleation. We speculate that the rise speed of the magma, as it reaches the level where significant volatile exsolution begins, determines the intensity of the vesiculation burst, and hence the vigor of the eruption. Rapid expansion and acceleration of the magma under these conditions may provide the impetus for fragmentation.

Formation, evolution, and rheology of two-dimensional foams in spread monolayers at the air—water interface

We discuss the properties of two-dimensional (2-D) foams formed in the gas-liquid coexistence region of spread monolayers on the basis of observations on a surface-active fluorescent dye. Their appearance and general behavior are analogous to those of three-dimensional (3-D) foams. The line tension between the gas and liquid 2-D phase regions plays a role similar to that of the surface tension in conventional foams. It determines the thinning of the strips of liquid monolayer phase which separate the gas cells in the 2-D polygonal surface structure: a lower surface pressure in the corner regions causes the equivalent of the Plateau-border suction. The rate of strip thinning can be slowed down by several orders if a gradient in line tension opposes monolayer flow. Such a gradient presupposes the existence of the unidimensional equivalent of surface activity, for which there is so far only circumstantial evidence. The line tension also controls cell disproportionation, i.e., the growth of large cells at the expense of smaller ones, and “line activity” can also reduce the rate of this process. Finally, the rearrangement process, in which contact between four strips changes into two three-strip contacts, is based on the minimization of the total line energy. It was observed that this rearrangement actually occurs before a small central cell has disappeared to cause the four-strip contact. This phenomenon was shown to be theoretically expected.

Flammability and thermal properties of rigid polyurethane foams with additionally introduced cyclic structures

Flammability, smoke evolution, thermal, and thermomechanical properties of low-density rigid polyurethane foams obtained from different aromatic polyols were investigated. The foams were prepared according to a standard formulation ensuring the same foam phosphorus content. Cellular polyurethanes with the best fire resistance were obtained from polyols containing disubstituted naphthalene and biphenyl rings. A linear equation was proposed to describe the influence of various structural units of the polyurethane (the content of cyclic structures Cc, nitrogen content CN, and crosslinking equivalent Mc) upon its flammability, expressed in terms of its oxygen index (OI) Thermal stability of crosslinked polyurethanes was not found to influence significantly their thermomechanical properties, while crosslink density and the type and quantity of cyclic structures additionally introduced did have a pronounced effect upon these properties.