The system of primary interest in this study was water-in-diluted Athabasca bitumen films. Formation of very stable water-in-oil emulsions is a major challenge for recovery of bitumen from oil sand deposits in Alberta-Canada. Emulsion studies 14-17 and works on single emulsion films 18,19 have shown that emulsion and film stability depends on type of solvent and solvent to bitumen (S/B) ratio. Furthermore, it was found that film behavior changes dramatically at so called critical (S/B) ratio 20, which coincides with formation of asphaltenes precipitates in solutions. In our previous work  we studied the stability of water-in-diluted films as a function of bitumen content at different aromaticity of the solvent. Aromaticity of the solvent was modeled by mixing heptane and toluene at different ratios. In that previous work, we characterized film stability using film lifetimes and rate of thinning. Here we report results for the same system, Athabasca bitumen diluted with toluene and heptane and their mixtures, but film stability is studied by means of electric polarization of single emulsion film.
Critical voltages of film rupture were measured for different solvent mixtures and S/B ratios. For sake of relevance to the industrial applications, electric field was applied on draining films. The effect of film thickness (time of application of ramp potential) on critical voltage is reported. Experimental results are further discussed in relation to mechanism of film rupture and implications to the electro-induced coalescence.
Theoretical background: Effect of DC Polarization on film drainage and stability
Maxwell stress. Let us consider a dielectric layer with thickness h placed in aqueous electrolyte solution considered as ideal conductor. In electric filed, the force exerted on the dielectric layer is formulated in terms of Korteweng-Helmhotz  body force density and corresponding Maxwell stress tensor . For incompressible and electrically linear liquid :
Where E is electric field, ρf is volume density of free electric charge, ε is permittivity. In Expression of the Maxwell stress tensor is Kronecker delta function.
The component in direction i of the total force that acts on the body can be calculated as a volume integral of or as a surface integral of Maxwell stress tensor over a surface enclosing that S :
We assume that ρf=0 and the electrolyte solution is considered ideal conductor. The solution of the problem shows  that equal, and oppositely directed forces, act at each interface (defined as the plane where discontinuation of permittivity occur). The forces are directed toward the media with lower permittivity. The result is an extra pressure in the film due the electric field PE. and an In terms of additional pressure
where εf is dielectric constant of film liquid, εo= 8.854 × 10-12 F/m is permittivity of free space. It is assumed also that the...