An Adsorption-Desorption-Controlled Surfactant on a Deforming Droplet

Area of Science:

• Fluid Dynamics
• Surface Science
• Computational Modeling

Background:

• Surfactants significantly alter interfacial properties and fluid behavior.
• Understanding surfactant effects on deforming drops is crucial for various industrial processes.
• Previous studies often focused on insoluble or soluble surfactant limits, with less attention to monolayer-forming surfactants.

Purpose of the Study:

• To numerically investigate the effects of sorption-controlled, monolayer-forming surfactants on drop deformation in extensional flow.
• To analyze how varying mass transfer rates and surfactant concentrations impact drop hydrodynamics.
• To quantify the drop's contribution to the stress tensor under different surfactant conditions.

Main Methods:

• Numerical simulations of drop deformation in extensional flow.
• Analysis of scaling arguments for different drop sizes (1 cm and 1 µm).
• Parametric study varying surfactant concentration (Gamma) and mass transfer rates, relative to the monolayer limit (Gamma_infinity).

Main Results:

• In the insoluble limit, surfactants increase deformation and lower the breakup capillary number (Ca), forming stagnant caps.
• High surfactant concentrations (approaching Gamma_infinity) reduce deformation and create fully stagnated surfaces due to Marangoni stresses.
• Finite mass transfer rates diminish deformation and eliminate stagnant caps, leading to non-monotonic changes in deformation with increasing mass transfer.

Conclusions:

• Surfactant behavior, particularly monolayer formation and mass transfer, critically dictates drop deformation and hydrodynamics.
• The interplay between surfactant concentration, mass transfer, and interfacial stresses (Marangoni effect) governs the observed fluid behavior.
• Drop deformation and its contribution to the stress tensor are strongly dependent on these surfactant-related parameters.