Complex Adsorbed Substances

Motivation

Highly dynamic flows comprising bubbles and droplets with adsorbed particles, surfactants, or ions are particularly relevant for intensified technological applications, which are characterized by high gas or organic fractions and turbulent flow conditions. The hydrodynamics of bubbles and droplets is substantially modified by interfacial shear stresses and interfacial viscosity in the presence of adsorbed material. All of these phenomena are expected to be coupled to the time-dependent distribution and the nature of the material on the bubble surface, but are not fully understood yet. More complex adsorbates such as polymers, peptides, or small hydrophobic particles interact strongly with each other. These interactions have significant impact on the interfacial properties and material transport, while the resulting consequences are largely unknown.

Goals

• Adsorption of particle-surfactant mixtures and influence on interfacial and hydrodynamic properties of bubbles and drops
• Effect of ultrafine particles
• Adsorption of substances under flow conditions
• Microscale processes of attachment/detachment
• Application of ultrasound fields in order to enhance adsorption/attachment processes at interfaces

Techniques

• Particle image/tracking velocimetry (PIV/PTV) for flow measurements
• Profile analysis tensiometry (PAT), elasticity measurements, maximum bubble pressure, Langmuir trough, Wilhelmy plate technique for interfacial tension and rheology measurements
• High-speed optical microscopy
• Specifically designed flow cells for single-bubble studies
• Combination with X-Ray techniques for opaque systems

Results

The presence of negatively charged nanoparticles affects the surface activity of anionic surfactants in an aqueous phase. This effect is mainly caused by the change in ionic strength of the system resulted from the addition of nanoparticles.

Below a certain surfactant/nanoparticle ratio of oppositely charged systems, free surfactant molecules are removed from the solution by the formation of surfactant-nanoparticle complexes. Above this ratio, free surfactant monomers and nanoparticle-surfactant complexes coexist and can co-adsorb at the interface, changing both the interfacial tension and the interfacial rheology, and thus, for example, the foamability and foam stability of the system.

For bubbles exposed to flow, a direct experimental observation of the circulating flow at the interface under asymmetric shear could be shown, which prevents the formation of the typical stagnant cap. Under these conditions, the interface remains mobile regardless of the surfactant concentration. Increasing the degree of asymmetry increases the shear forces and thus the interfacial velocity.

In the presence of nanoparticle-surfactant complexes, a contiguous network of particles forms at the interface through densification of surface structures. This interconnected nanoparticle network eventually stops the interfacial flow. The immobilization of the interface is characterized by a dimensionless number, defined as the ratio of the interfacial elasticity to bulk shear forces. This number provides an estimate of the interfacial forces required to impose interfacial immobility at a defined flow field.

During the initial stages of adsorption under an ultrasonic standing wave, when the bubble surface is almost empty, non-spherical oscillations occur, which were found to significantly expedite the adsorption process of proteins. However, during later stages of the adsorption process, despite the continued presence of several sonication phenomena such as the primary radiation force and acoustic streaming, no change in adsorption behavior of the proteins could be noted. The occurrence, duration, and intensity of the non-spherical bubble oscillations appeared to be the sole contributing factors for the change of the sorption dynamics.

Publications

Matho, C., Schwarzenberger, K., Eckert, K., Keshavarzi, B., Walther, T., Steingroewer, J., & Krujatz, F. (2019). Bio-compatible flotation of Chlorella vulgaris: Study of zeta potential and flotation efficiency. Algal Research, 44, 101705.

Eftekhari, M., Schwarzenberger, K., Karakashev, S. I., Grozev, N. A., & Eckert, K. (2024). Oppositely charged surfactants and nanoparticles at the air-water interface: Influence of surfactant to nanoparticle ratio. Journal of Colloid and Interface Science, 653, 1388-1401.

Keshmiri, A., Keshavarzi, B., Eftekhari, M., Heitkam, S., & Eckert, K. (2024). The impact of an ultrasonic standing wave on the sorption behavior of proteins: Investigation of the role of acoustically induced non-spherical bubble oscillations. Journal of Colloid and Interface Science, 660, 52-65.

Keshmiri, A., Heitkam, S., Bashkatov, A., Eftekhari, M., Eckert, K., & Keshavarzi, B. (2023). Surfactant sorption on a single air bubble in an ultrasonic standing acoustic wave field. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 665, 131210.

Eftekhari, M., Schwarzenberger, K., Heitkam, S., Javadi, A., Bashkatov, A., Ata, S., & Eckert, K. (2021). Interfacial behavior of particle-laden bubbles under asymmetric shear flow. Langmuir, 37(45), 13244-13254.

Eftekhari, M., Schwarzenberger, K., Heitkam, S., & Eckert, K. (2021). Interfacial flow of a surfactant-laden interface under asymmetric shear flow. Journal of Colloid and Interface Science, 599, 837-848.

Eftekhari, M., Schwarzenberger, K., Javadi, A., & Eckert, K. (2020). The influence of negatively charged silica nanoparticles on the surface properties of anionic surfactants: electrostatic repulsion or the effect of ionic strength?. Physical Chemistry Chemical Physics, 22(4), 2238-2248.

Matho, C., Schwarzenberger, K., Eckert, K., Keshavarzi, B., Walther, T., Steingroewer, J., & Krujatz, F. (2019). Bio-compatible flotation of Chlorella vulgaris: Study of zeta potential and flotation efficiency. Algal Research, 44, 101705.