Session: 10.2.2 - Interfacial Phenomena and Flows II

Paper Number: 158194

158194 - Film Climbing in Impulsively-Driven Capillary Flows 

Abstract: 

Thin film formation in capillaries is ubiquitous across various applications from coating to multiphase channel flows. Sudden motions of a liquid slug is capable of forming thin films and the meniscus shape, film thickness and velocity has been the focus of intense study through the classic Bretherton and Taylor-bubble systems. In the presence of surfactants, slug motion may cause Marangoni stresses which may drive backflow, yet the exact physics has been debated. In this study, we report the spontaneous upward thin film motions generated by a liquid slug driven downwards by gravity, provided that a thin film first formed during the initial descent of the liquid slug. Thus, the inner walls of the glass capillary tubes were rigorously cleaned such that they were highly wetting. The relevant physics involved the interplay of the diffusion, surfactant kinetics and advection. The diffusive and surfactant kinetics were controlled by varying the surfactant concentration. However, the surfactant adsorption and desorption coefficients presumably followed Langmuirian kinetics irrespective of the surfactant concentration. The advection was controlled by varying the meniscus release height of the slug. Thus, the dominant mechanism drove the motions observed in the thin films during the drainage process. A combination of shadowgraphy, high-speed slug tracking, and interferometric techniques were utilized to track the slug and thin film spatiotemporal dynamics. In the interferometry setup, the curvature effects of the glass capillary was removed by sandwiching the glass capillary onto a glass slide with an index-matched epoxy. The experiments also consisted of mounting the capillaries vertically and connected on one end to pneumatic controls to achieve an initially static liquid slug suspended in air. Sodium dodecyl sulfate was the surfactant utilized along with methylene blue dye for contrast. The obtained image data from the videos were processed using custom-built MATLAB codes. The results demonstrated the effectiveness in surfactant gradients formed by the drainage at sufficiently low surfactant concentrations due to the dominant advection and the failure of climbing events at intermediate surfactant concentrations (e.g., 3 mM) due to diffusion dominating the dynamics. Beyond a critical surfactant concentration (e.g., 7 mM), even the formation of a thin film was suppressed, presumably due to Marangoni effects enhancing drainage of the thin film. The results of the study are of relevance to the evacuation of a capillary, coating, and slug flows. Future work will focus on the deposition dynamics of particulates in channel fouling mechanism as well as understanding the deposition pathways of bacteria.

Presenting Author: Min Pack Baylor University

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