Good Afternoon. I’m Kenta Asai from Sakai Sakai laboratory. My presentation topic is 'Foam Stability and Interfacial Rheology of Amino Acid-Based Surfactant/Gelatin Mixtures.’

Amino acid-based surfactants are widely used in industrial applications for their mildness and high biodegradability. So, their foam properties are regarded as important. Foam destabilization factors include bubble coalescence and drainage.To suppress these phenomena, appropriate viscoelasticity of the interfacial film is required. Interfacial rheology is considered a powerful tool for evaluating viscoelasticity, as it enables direct measurement of viscosity and elastic modulus. On the other hand, achievements at the air–liquid interface using bicone-type geometry have been limited So, we focused on the foam stability of amino acid-based surfactant and interfacial rheology using bicone-type geometry.

We used this amino acid-based surfactant, which contains two carboxyl groups. So, its degree of dissociation depends on the solution pH. In addition, we used acid-treated gelatin as the polymer additive. Based on our zeta potential measurements, we found that the isoelectric point of gelatin is around pH 9.5. So, at pH7, gelatin is positively charged, and surfactant is negatively charged.

First, I’ll explain the evaluation of foam stability.The photos on the left show the visual observations of surfactant-only system and gelatin-added system just after shaking and after 24 hours. we can see that gelatin enhances foam stability even at low concentrations of surfactant. The graph on the right shows the foam stability measured 24 hours After shaking. Blue line represents the foam stability of the surfactant-only system, whereas orange line represents that of the gelatin-added system. These data were plotted at different concentrations of surfactant.As shown in this graph, the gelatin-added system reached maximum foam stability at 5 mM of surfactant

Next, I’ll explain the visual observations of foam morphology using the foam analyzer.Here is a video showing the measurement in progress. In this experiment, gas was injected from bottom of the cell, and once the foam height reached 15 cm, we stopped the gas flow and observed the foam morphology.The camera was placed approximately 7 cm above the bottom of the cell to capture the lower part of the foam, where the interfacial viscoelasticity is expected to be more pronounced.

These photos show the foam morphology of the gelatin-added system. The concentration of the surfactant increases from top to bottom: 1, 5, and 50 mM. And, we observed foam morphology at 1, 10, and 60 minutes after foaming.At 1 mM and 5 mM, these foams were stable even after 60 minutes, and we observed densely packed and wet foams.On the other hand, at 50 mM, these foams were not stable, and we observed large, dry foams.

Next, I’ll explain the static surface tension of the surfactant only system and the gelatin-added system at different concentrations of surfactant. As shown in this graph, in the gelatin-added system, the surface tension decreased in two distinct steps. The first plateau region is around 5 to 12 mM. In this region, surfactant-gelatin complexes saturate the air/liquid interface. The important thing is the excellent foam stability was observed in this first plateau region.

Finally, I’ll explain the results of the interfacial rheology measurements using a bicone-type geometry. We conducted the measurement by first bringing the bicone-geometry into contact with the air-liquid interface, and then applying shear deformation. This graph shows the frequency sweep data of the gelatin-added system.The x-axis represents frequency, and the y-axis represents two-dimensional G’ which is the interfacial storage modulus. This is an indicator of the elastic (or solid-like)properties of the interface As shown in this graph, the maximum G' was observed at 5 mM surfactant.This result indicates a strong correlation with foam stability.