Dynamic fluid-film interferometry as a predictor of bulk foam properties

Abstract

Understanding and enabling the control of the properties of foams is important for a variety of commercial processes and consumer products. In these systems, the role of surface active compounds has been the subject of many investigations using a wide range of techniques. The study of their influence on simplified geometries such as two bubbles in a liquid or a thin film of solution (such as in the well-known Scheludko cell), has yielded important fundamental understanding. Similarly, in this work an interferometric technique is used to study the dynamic evolution of the film formed by a single bubble being pressed against a planar air-liquid interface. Here interferometry is used to dynamically measure the total volume of liquid contained within the thin-film region between the bubble and the planar interface. Three different small-molecule, surfactant solutions were investigated and the data obtained via interferometry were compared to measurements of the density of bulk foams of the same solutions. The density measurements were collected with a simple, but novel technique using a conical-shaped bubbling apparatus. The results reveal a strong correlation between the measurements on single bubbles and complete foams. This suggests that further investigations using interferometric techniques can be instrumental to building a more detailed mechanistic understanding of how different surface-active compounds influence foam properties. The results also reveal that the commonly used assumption that surfactant-laden interfaces may be modeled as immobile, is too simplistic to accurately model interfaces with small-molecule surfactants.

Evaporation-induced foam stabilization in lubricating oils

“…2 A, i as an almost instantaneous change in foam volume. It is also worth noting that this foam collapse behavior is different from the uniform foam collapse observed in the surfactant-stabilized aqueous foams (1).…”

“…2 A, i), with the group I base oils entraining the most liquid, followed by groups II, III, and IV. In addition, the evolution of the mean film thickness was seen to be different from those expected in aqueous systems (1). Notably, the tested lubricants (particularly evident in groups I and II) were seen to display the phenomenon of spontaneous dimpling.…”

“…Foam rise measurements (reported in Fig. 2 A, i and B, i) were conducted using a bulk foam apparatus developed in-house, the details of which are reported elsewhere (1). All experiments were video-recorded at 30 frames per s. The volume of foam was measured every 10 frames, about 0.3 s after the bubbling was stopped (the time taken for the last bubbles to finish rising through bulk liquid).…”

“…Such foams are generated through the accumulation of gas bubbles originating from external gas entrainment or through the release of dissolved gases in the liquid. These liquid foams are common and desirable in many applications such as food manufacturing processes, personal and health-care products, detergency, firefighting, and flotation of minerals (1)(2)(3). In contrast, excessive foam in lubricants is undesirable and detrimental as foaming leads to excessive wear in machine parts, decreased lubrication, inadequate heat removal, lubricant oxidation, and overall energy losses (4).…”

“…Existing experimental techniques like the ASTM D892 (9), Flender foam test (10), and the foam rise test (11) are bulk foam tests and only provide information on the stability and density of the aggregate foam. However, singlefilm experiments using the well-known Scheludko cell (12) can yield mechanistic insights into thin liquid films but are known to have shortcomings, particularly relevant to probing foaming mechanics (1). These shortcomings include the inability to use full bubbles and simulate bubble coalescence at flat liquid-air interfaces.…”