states that the phase in which the surfactant is best soluble becomes the
continuous one. The explanation presumably is thatg-gradients will not
readily form if the surfactant is in the drops, as illustrated in Figure 11.12c.
Although ag-gradient will develop upon fast approach of two drops, it
tends to become counteracted by ongoing adsorption of surfactant from the
inside of the drop, and then depletion of surfactant does not depend on the
thickness of the film between the drops. Hence there will be no Marangoni
effect, and the newly formed drops may readily coalesce. The theory for the
approach of two droplets has been worked out in more detail than can be
given here, but one result is given in Figure 11.14. The approach velocity of
two drops is given relative to the velocity in the absence of surfactant. The
FIGURE11.14 Linear rate of approach (v) of two drops, relative to the rate in the
absence of surfactant (v 0 ) as a function of the distance between the drop surfaces (h),
if the surfactant is in the disperse phase (D) or in the continuous one (C). Calculated
for a drop of 1mm diameter. (After results by I. Ivanov, P. A. Kralchevsky. Proc. 2nd
World Congress on Emulsion, Vol. 4, 1997, pp. 145–152.)