Colloids and Surfaces A: Physicochem. Eng. Aspects 262 (2005) 23–32 Surface foam film waves studied with high-speed linescan camera Stoyan I. Karakashev, Anh V. Nguyen ∗ , Emil D. Manev 1 , Chi M. Phan Discipline of Chemical Engineering, School of Engineering, The University of Newcastle, University Drive, Callaghan, NSW 2308, Australia Received 22 November 2004; received in revised form 4 April 2005; accepted 9 April 2005 Available online 31 May 2005 Abstract Dynamic foam films have been investigated using an improved experimental set-up with a CCD high-speed linescan camera in conjunction with the Scheludko micro-interferometric cell for studying the drainage and rupture of liquid foam films. The improved experimental set-up increased the sensibility of detection of the local thickness heterogeneities and domains during the film evolution. The evolution of the foam films up to the formation of black spots was recorded in the time intervals of 50ms. The wavelengths of the propagating surface waves and their frequencies were determined experimentally. The experimental results show that the current quasi-static hydrodynamic theory does not properly describe the wave dynamics with inter-domain channels. However, the thermodynamic condition for formation of black spots in the foam films was met by the experimental results. © 2005 Elsevier B.V. All rights reserved. Keywords: Foam films; Film dynamics; Marangoni effect; Film evolution 1. Introduction Foam is a dispersion of gas bubbles and a relatively small volume fraction of liquid. Surfactants are used to control the foam behaviour and stability. Typically, foam consists of liquid films residing between (polyhedral) bubbles, Plateau borders, where three films meet, and vertices, which are the junctions of four Plateau borders. The foam usually collapses due to the self-destruction of the liquid films. The film self- destruction is determined by the phenomena occurring at the film interfaces covered by the surfactant molecules. The rel- evant research related to the film transient stability and self- destruction aims to investigate: • The state of the adsorption layers [1–6]. • The rheology of the phase boundaries [7–9]. • The dynamic interaction of the two surfaces of a thinning film [10–14]. ∗ Corresponding author. Tel.: +61 2 4921 6189; fax: +61 2 4921 6920. E-mail address: Anh.Nguyen@newcastle.edu.au (A.V. Nguyen). 1 Visiting Professor from the University of Sofia, Bulgaria. All of these features are related to the dynamic behaviour of the film surfaces, caused by the propagating capillary sur- face waves. In 1908, von Smoluchowski related the liquid surface light scattering to surface corrugations. In 1913, Man- delstam [15] described the thermal fluctuation of the liquid surface as a spectrum of surface waves. Subsequently, the dynamics of the surface waves has been investigated by a number of researchers. The focus of this paper is on the relationship between the surface waves and liquid drainage. In particular, our focus is on the surface waves and the kinetics of thinning, life- time and critical thickness of the liquid films. Scheludko [12] was the first to relate the critical thickness to the sur- face waves causing the rupture of the foam films. According to Scheludko, the whole spectrum of wavelengths is limited by the film size and can be expressed by a specific for the films probability distribution of the wavelengths of the sur- face waves. There exists a most probable wave, which can cause the film rupture. However, the wavelength distribution in the Scheludko theory (as well as in the Vrij theory) is the white noise and the film is ruptured not by the most proba- ble wave but by the fastest wave. In the Scheludko theory, this wavelength is a user-defined parameter. In 1964, Vrij 0927-7757/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2005.04.012