Date of Award


Degree Type


Degree Name

Doctor of Philosophy (PhD)


Chemical Engineering


Professor D.R. Woods


This thesis is a study of the effect of surfactant on the film thinning behaviour of small, single droplets coalescing at a planar interface in oil/water systems. There are two major parts in this thesis. The first is the mathematical modelling of film thinning, and the second is the experimental measurement of the variation of film thickness during thinning.

The mathematical modelling consists of:

(i) the development of a film thinning model in which the interface mobility is calculated and not assumed arbitrarily,

(ii) the numberical solution of this film thinning model based on a polynomial representation of the pressure in the film,

(iii) the extension of the parallel disc model to describe the thinning at the barrier ring for uneven thinning, by allowing the interfaces to have varying degrees of partial mobility, and

(iv) the algebraic analysis of a postulate for the conditions for even and uneven thinning, based on the assumption that the pressure in the film is a parabolic function of the radial distance. To verify these models, their predictions are compared with experimental observations.

In the experimental part, the effect of additions of palmitic acid and the effect of drop size and bulk interface age on the film thinning behaviour of toluene drops have been investigated using the white light interference technique. A novel drop forming technique has been developed, by which one drop forming nozzle can be used to form drops of different sizes without piercing the bulk interface; this technique eliminates the possibility of surfactant transfer between the bulk and drop interfaces.

New observations of film thinning behaviour are reported. These are the uneven thinning pattern that occurs when both the drop and bulk interfaces have the same surface concentration, and new thinning patterns which occur when drops are released to the bulk interface in rapid succession.

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