Interfacial Flow

Motivation

The mobility of a bubble or droplet interface controls momentum transfer, mass transfer, collision efficiency, coalescence, and particle attachment. In clean systems, the interface can remain mobile. In the presence of surfactants, frothers, collectors, proteins, or particles, adsorbed material can create surface-tension gradients and Marangoni stresses that immobilize parts of the interface.

Many classical descriptions are based on axisymmetric bubble rise in a quiescent liquid, where surfactants accumulate at the rear of the bubble and form a stagnant cap. IIn practice, however, real flotation cells are dominated by asymmetric shear, vortices, wall effects, neighboring bubbles, and bubble–particle interactions. Under these conditions, interfacial flow can differ strongly from the classical stagnant-cap picture.
 

Goals

• Directly visualize and quantify flow on surfactant-laden and particle-laden bubble interfaces.
• Determine how asymmetric shear changes surfactant distribution and interfacial mobility.
• Study how adsorbed substances modify the hydrodynamic boundary condition at bubbles and droplets.
• Identify conditions under which particle networks immobilize the interface.

Techniques

• Capillary-held bubbles and pendant droplets exposed to defined asymmetric shear flow.
• Microscopic particle image velocimetry and particle tracking velocimetry.
• Profile analysis tensiometry for dynamic surface tension and surface rheology under flow.
• Comparison of bulk flow, near-interface flow, and true interfacial motion.

Results

We showed that asymmetric shear can fundamentally change the interfacial dynamics of surfactant-laden bubbles. Instead of forming a classical stagnant cap, the rotational component of the surrounding flow induces a circulating motion at the bubble surface. This circulation redistributes adsorbed surfactants and keeps the interface mobile, even at high surfactant concentrations.

This result is important because it shows that interface immobilization cannot be inferred from surfactant concentration alone. The surrounding flow geometry and degree of asymmetry must also be considered. Increasing the asymmetry of the flow increases the shear acting on the interface and therefore increases the interfacial velocity.

When nanoparticles or nanoparticle–surfactant complexes are present, the behavior changes again. Under compression, adsorbed particles can form a contiguous interfacial network. Once this network becomes sufficiently dense and mechanically connected, it can suppress interfacial flow and immobilize the bubble or droplet surface. The transition to immobility can be described using a dimensionless ratio between interfacial elasticity and bulk shear forces.

These findings are relevant for flotation because interfacial mobility affects film drainage, bubble–particle collision and attachment, coalescence, and gas–liquid mass transfer. They also provide a basis for more realistic modeling of surfactant- and particle-laden bubbles under industrial flow conditions.

Publications

• Eftekhari, M., Schwarzenberger, K., Heitkam, S., & Eckert, K. Interfacial flow of a surfactant-laden interface under asymmetric shear flow. Journal of Colloid and Interface Science, 2021.
• Eftekhari, M., Schwarzenberger, K., Heitkam, S., Javadi, A., Bashkatov, A., Ata, S., & Eckert, K. Interfacial behavior of particle-laden bubbles under asymmetric shear flow. Langmuir, 2021.