(601j) Evaporation Driven Oscillatory Marangoni Flow in Thin Films of a Binary Mixture

Marangoni flows arise due to surface tension gradients in liquid films with temperature or compositional variations. These flows are prevalent in our daily lives, from tears of wine to paint wrinkles, and, moreover, understanding how these flows impact engineering fluid applications is of great practical interest. For instance, lubricants that operate in the presence of air are prone to foaming which can cause poor, inhomogeneous heat transfer and lead to lubricant degradation. Thus, understanding and controlling these thermal-solutal-Marangoni flows, has direct application to controlling lubricant foaming.

One of the simplest systems where Marangoni flows may occur is a binary liquid film over an air bubble. Recent experiments in a dynamic fluid-film interferometer [1] show that such a system can demonstrate long lived, time dependent states that apparently stabilize the thin film. We develop and fully solve a lubrication model that describes these states. Within this model, we include the fact that the two miscible species have different temperature-dependent surface tensions and different relative evaporation rates. When exposed to air, the evaporative species leaves the curve thin film, creating temperature and concentration gradients, both in the bulk and at the interface. These gradients then draw liquid toward the high surface tension region, and the increase in local film thickness leads to an increase in film curvature. Once the film curvature grows large enough such that the capillary pressure can overwhelm the Marangoni effects, then the liquid film will discharge, thereby homogenizing the composition and temperature across the film through convective flow. Evaporation over the bubble surface then recreates the thermal-compositional gradient and the imbalance between Marangoni and capillary stresses repeats, leading to an oscillatory flow. We describe, over the parameter regime of the original experiments, how these oscillations are influenced by thermal-solutal-Marangoni stresses, capillarity, gravity, and evaporation.

We then return to the laboratory and examine these flows experimentally, however now guided by the insight of our model. Oscillatory flows of binary silicone oil mixtures over an air bubble are studied via the dynamic fluid-film interferometer. We focus on determining which characteristics of the experiments are captured by the model, and which are not.

[1] Shi, X., Fuller, G., & Shaqfeh, E. S. (2018). Evaporation Driven Oscillatory Marangoni-Capillary Flow in Thin Films of a Binary Mixture. Bulletin of the American Physical Society.