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Spreading of a surfactant monolayer on a thin liquid film: Onset and evolution of digitated structures

Matar, Omar K. and Troian, Sandra M. (1999) Spreading of a surfactant monolayer on a thin liquid film: Onset and evolution of digitated structures. Chaos, 9 (1). pp. 141-153. ISSN 1054-1500. http://resolver.caltech.edu/CaltechAUTHORS:MATchaos99

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Abstract

We describe the response of an insoluble surfactant monolayer spreading on the surface of a thin liquid film to small disturbances in the film thickness and surfactant concentration. The surface shear stress, which derives from variations in surfactant concentration at the air–liquid interface, rapidly drives liquid and surfactant from the source toward the distal region of higher surface tension. A previous linear stability analysis of a quasi-steady state solution describing the spreading of a finite strip of surfactant on a thin Newtonian film has predicted only stable modes. [Dynamics in Small Confining Systems III, Materials Research Society Symposium Proceedings, edited by J. M. Drake, J. Klafter, and E. R. Kopelman (Materials Research Society, Boston, 1996), Vol. 464, p. 237; Phys. Fluids A 9, 3645 (1997); O. K. Matar Ph.D. thesis, Princeton University, Princeton, NJ, 1998]. A perturbation analysis of the transient behavior, however, has revealed the possibility of significant amplification of disturbances in the film thickness within an order one shear time after the onset of flow [Phys. Fluids A 10, 1234 (1998); "Transient response of a surfactant monolayer spreading on a thin liquid film: Mechanism for amplification of disturbances," submitted to Phys. Fluids]. In this paper we describe the linearized transient behavior and interpret which physical parameters most strongly affect the disturbance amplification ratio. We show how the disturbances localize behind the moving front and how the inclusion of van der Waals forces further enhances their growth and lifetime. We also present numerical solutions to the fully nonlinear 2D governing equations. As time evolves, the nonlinear system sustains disturbances of longer and longer wavelength, consistent with the quasi-steady state and transient linearized descriptions. In addition, for the parameter set investigated, disturbances consisting of several harmonics of a fundamental wavenumber do not couple significantly. The system eventually singles out the smallest wavenumber disturbance in the chosen set. The summary of results to date seems to suggest that the fingering process may be a transient response which nonetheless has a dramatic influence on the spreading process since the digitated structures redirect the flux of liquid and surfactant to produce nonuniform surface coverage.


Item Type:Article
Additional Information:©1999 American Institute of Physics. (Received 16 July 1998; accepted 30 December 1998) This work was supported by a National Science Foundation CAREER award through Grant No. CTS-9624776.
Subject Keywords:wetting; monolayers; liquid films; chaos; perturbation theory
Record Number:CaltechAUTHORS:MATchaos99
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:MATchaos99
Alternative URL:http://dx.doi.org/10.1063/1.166385
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:4950
Collection:CaltechAUTHORS
Deposited By: Archive Administrator
Deposited On:15 Sep 2006
Last Modified:26 Dec 2012 09:02

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