(128c) Surfactant-Retarded And Surfactant-Enhanced Marangoni-Bénard Convection | AIChE

(128c) Surfactant-Retarded And Surfactant-Enhanced Marangoni-Bénard Convection

Authors 

Hanumanthu, R. - Presenter, Johns Hopkins University
Stebe, K. J. - Presenter, University of Pennsylvania


The effects of an insoluble surfactant on Marangoni-Bénard convection are studied numerically in a two-dimensional simulation of a single unit cell to understand two regimes of surfactant behavior. First, under the usual circumstance when surfactants resist thermocapillary flow, we describe conditions in which the interface can be partially stagnated with persistent flow at steady state. Second, we explore the less usual circumstance in which surfactants enhance the coupling between surface tension and temperature. A temperature fluctuation creates thermal Marangoni stresses which can drive Marangoni-Bénard. In the absence of surfactants, the numerical simulation recovers critical conditions for instability identified by Scriven & Sternling in terms of the Marangoni number Ma for films with finite surface tension. Marangoni-Bénard flow sweeps surfactants to the cool regions at the edges of a unit cell. Surface tension typically reduces with surfactant concentration. Under these circumstances, surfactants create stresses that oppose the driving thermocapillary stress. For dilute amounts of surfactant at elevated surface Peclet number, partially stagnated interfaces can be created. All of the surfactant is swept to the edges of the unit cell, dividing the interface into a surfactant-covered region with zero velocity, and a surfactant-free region of width λfree. In the surfactant-free region, flow can decay, or persist. Flow persists in the surfactant free region only if λfree corresponds to a linearly unstable wavelength at the Ma of interest according to the marginal stability criterion. That is, the steady response of the system in the stagnant cap regime can be related to the linear marginal stability results, provided the wavelength is replaced by λfree. Prior work on surfactants in Marangoni-Bénard flow has focused on how surfactants resist thermocapillary flows. Truskett (née Nguyen) and Stebe have recently shown that insoluble surfactants in coexisting liquid expanded and liquid condensed surface states can promote thermocapillary flows. For such surfactants, linearly unstable conditions are readily established. The ensuing dynamics are described. Simulations predict that Marangoni-Bénard flow will rapidly occur and self-quench, returning the system to its initial state.