Paper Number
IR2 

Session
Interfacial Rheology

Title
Instability and rupture of surfactant-laden bilayer thin films

Presentation Date and Time
October 10, 2022 (Monday) 10:10

Track / Room
Track 5 / Sheraton 2

Authors (Click on name to view author profile)

1. Yang, Shu (University of Minnesota, Twin Cities, Department of Mechanical Engineering)
2. Dutcher, Cari S. (University of Minnesota - Twin Cities, Mechanical Engineering & Chemical Engineering)
3. Kumar, Satish (University of Minnesota, Department of Chemical Engineering and Materials Science)

Author and Affiliation Lines (in printed abstract book)
Shu Yang¹, Cari S. Dutcher² and Satish Kumar^3
1Department of Mechanical Engineering, University of Minnesota, Twin Cities, Minneapolis, MN 55414; ²Mechanical Engineering & Chemical Engineering, University of Minnesota - Twin Cities, Minneapolis, MN 55431; ³Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455

Speaker / Presenter 
Yang, Shu

Keywords
theoretical methods; flow-induced instabilities; interfacial rheology; surfactants

Text of Abstract
The stability of thin liquid films containing surface-active additives is important for a range of processes, from curtain coating, to agricultural flat-fan sprays to fire suppression. In this talk, we explore a surfactant-laden bilayer thin film stability where the top layer is subject to van-der-Waals-driven breakup. The lubrication approximation is applied to derive coupled evolution equations describing the perturbations of the liquid-liquid and liquid-air interfaces and the surfactant concentrations. In the absence of surfactants, linear stability analysis shows that a higher viscosity, larger thickness, and higher interfacial tension of the top layer relative to the bottom layer each result in a more stable film. When insoluble surfactants are present at the interface of a monolayer thin film, the perturbations to the interfacial surfactant concentration and the film thickness are in phase, so that surfactant molecules tend to accumulate at film peaks. The resulting concentration gradient leads to Marangoni stresses that stabilize the film. For bilayer films with insoluble surfactants at the liquid-liquid interface, where the surfactant molecules accumulate as well as the strength of the resultant Marangoni stresses depends strongly on the viscosity ratio of the layers. For both high and low viscosity ratios, the Marangoni stress always acts to stabilize the film. In cases where the top liquid has a higher viscosity than the bottom layer, the stabilizing effect of increasing Marangoni number is more pronounced. Results are also presented for the cases where insoluble surfactants are present at the liquid-air interface and at both interfaces. The findings from these studies suggest strategies for the design of more effective fire-suppressing foams and liquids.