KEY CONCEPTS
• The single bubble/drop test evaluates interactions between individual bubbles and the dynamics of thin liquid films between bubbles.
• Three types of single bubble/drop test experiments have been reported in the literature. 
• Single bubble/drop tests offer the convenience of using a single setup to characterize foams and emulsions. 

Lubricants come in contact with surfaces between liquids and solids and interfaces between liquids and gases during operation. Performance is judged by how well a specific lubricant can handle specific operation conditions at these surfaces and interfaces.

One issue that inhibits the performance of lubricants is the presence of foam or entrained air. In a previous TLT article,¹ the amount of foam sustained in oil-based lubricants was determined to be dependent upon the type of base oil used. Group I base oils exhibited greater foam stability than Group II-Group V base oils. Researchers performing the study showed that the increased foam stability in Group I base oils resulted from the differential evaporation of species from Group I base oils, which usually have a broad distribution of components.

A second previous TLT article² discussed a study on how filtration affects foam stability. It is known that antifoam additives used to depress foam operate at the liquid-air interface and are susceptible to removal in filters. The researchers determined that foam stability in filtered lubricants is positively correlated to the number of filtration cycles, and inversely correlated to the filter pore size and the initial antifoam concentration in the lubricant. They also found that the time required to rupture foam bubbles is directly related to the size distribution of the antifoams used.

The common factor from both previous TLT articles is that the analytical technique used to evaluate base oils and to evaluate effects of filtration is the single bubble/drop test. STLE member Vineeth Chandran Suja, graduate student in the department of chemical engineering at Stanford University in Palo Alto, Calif., says, “The single bubble/drop test examines various properties of a single foam bubble including its stability, how the bubble interacts with asymmetric interfaces and the bubble’s rheological properties. This procedure also allows the researcher to independently control the size and velocity of the bubble. In contrast to bulk foam tests, the single bubble/drop test provides a more simplified approach to prove the physical mechanisms that dictate bubble stability.”

The classic foam tests such as ASTM D892 mimic actual applications but are inconvenient to study the mechanism that stabilizes foams. Bulk foams are quite complex to study due to many body interactions, and the simultaneous formation and collapse of bubbles. Consequently, the interactions between individual bubbles and the dynamics of thin liquid films between bubbles that govern foam stability are difficult to isolate under these conditions. Single bubble/drop tests are precisely intended to address this challenge. Suja and his colleagues have published³ a review to discuss research done with the single bubble/drop test and how this technique can be used to characterize foam and emulsions.

A schematic single bubble/drop experimental setup is shown in Figure 3a. At the start of a typical experiment, a bubble/drop is created on a capillary. Simulating the natural motion of the bubble/drop on capillary is then moved at a fixed velocity to a predetermined position at the fluid-fluid interface. Two cameras, one at the side and another at the top, record the dynamics of the bubble and the captured liquid film over the course of the experiment.


Figure 3. (a.) Schematic of an automated single bubble setup, along with snapshots showing the side and top camera views during the course of an experiment. (b.) The three types of measurements that can be made using the single bubble measurements: (i) coalescence time distributions, (ii) thickness profiles of thin films and (iii) interfacial rheology. Figure courtesy of Stanford University.

One setup—three types of measurements
Studies using single bubble/drop test rigs have been reported in the literature since the early 1960s to examine the characteristics of bubbles and drops and evaluate the factors that affect their stability. Three types of single bubble experiments (see Figure 3b) have been reported in the literature.

Suja says, “The first type evaluates the time it takes for two bubbles to coalesce, which is commonly known as coalescence time. Results from this method can be correlated to bulk foam testing used to assess foam stability. In the second type, often performed concurrently while performing coalescence time measurements, interferometry is used to measure the thickness of the thin liquid film and determine physical mechanisms stabilizing a bubble. A third type of measurement can be used to measure interfacial tension and the dilation interfacial viscoelastic properties.”

These measurement capabilities of single bubble setups have been shown to yield important insights that are valuable for the lubricant industry. For example, it is known that the evaluation of the effectiveness of an antifoam in a specific lubricant is difficult. The coalescence time curves measured from single bubble/drop test can provide unique insights into how the antifoam functions. As noted in Ref. 2, the time required for bubbles to rupture is directly related to how quickly the film thins down to the size of the antifoam.

Foam density, a measure of the amount of liquid entrained in a bubble, is a key characteristic that the single bubble/drop test is well suited to evaluate. Suja says, “This parameter is a variable important in a number of applications relevant to the food and lubricant industries. Previous research has shown that foam density is correlated to the initial thickness of the liquid film trapped by the bubble as it comes to rest at the interface, which can be easily measured from single bubble experiments.” As shown in Ref. 1, the film thickness information also can reveal mechanisms such as Marangoni flows that stabilize bubbles.

Interfacial rheological measurements conveniently obtained from single bubble setups can reveal the presence of adsorbed species and particles at the fluid-fluid interfaces, both of which can enhance foam and emulsion stability.

Emulsions consist of droplets of one phase within a continuous phase, with both phases being incompatible with each other. An example is an oil-in-water emulsion. The single bubble/drop test has been used to determine the mechanism for how surfactants stabilize emulsions. Suja says, “Equally important is the function of demulsifiers in limiting water contamination in lubricants. The single bubble/drop test has been used to better evaluate the performance of demulsifiers.”

The convenience of utilizing a single setup for a more wholistic characterization of foams and emulsions have attracted a lot of attention among lubricant manufacturers tackling issues of air entrainment and air release. Suja indicated that with further development, the single bubble/drop could be an indispensable tool for lubricant manufacturers. He says, “Current efforts to further improve the experimental setup include improving film thickness measurement tools for studying films with low-refractive index contrast and enhancing the throughput of film thickness recovery from the acquired interference patterns. Another research objective is to develop capabilities to better understand antifoam performance and drive the development of better additives for controlling foam.”

Additional information on the single bubble/drop test can be found in the review³ or by contacting Suja at vinny@stanford.edu or Dr. Gerald Fuller, professor of chemical engineering at Stanford, at ggf@stanford.edu.

REFERENCES
1. Canter, N. (2018), “Foam generation in oil-based lubricants: Evaporation,” TLT, 74 (12), pp. 12-13.
2. Canter, N. (2020), “Effect of filtration on lubricant foaming,” TLT, 76 (5), pp. 18-19.
3. Suja, V., Hakim, M., Tajuleo, J. and Fuller, G. (2020), “Single bubble and drop techniques for
characterizing foams and emulsions,” Advances in Colloid and Interface Science, 286, 102295.
Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat can be submitted to him at neilcanter@comcast.net.