  The coalescence of liquid drops induces a higher level of complexity compared
to the classical studies about the aggregation of solid spheres. Yet, it is
commonly believed that most findings on solid dispersions are directly
applicable to liquid mixtures. Here, the state of the art in the evaluation of
the flocculation rate of these two systems is reviewed. Special emphasis is
made on the differences between suspensions and emulsions. In the case of
suspensions, the stability ratio is commonly evaluated from the initial slope
of the absorbance as a function of time under diffusive and reactive
conditions. Puertas and de las Nieves (1997) developed a theoretical approach
that allows the determination of the flocculation rate from the variation of
the turbidity of a sample as a function of time. Here, suitable modifications
of the experimental procedure and the referred theoretical approach are
implemented in order to calculate the values of the stability ratio and the
flocculation rate corresponding to a dodecane-in-water nanoemulsion stabilized
with sodium dodecyl sulfate. Four analytical expressions of the turbidity are
tested, basically differing in the optical cross section of the aggregates
formed. The first two models consider the processes of: a) aggregation (as
described by Smoluchowski) and b) the instantaneous coalescence upon
flocculation. The other two models account for the simultaneous occurrence of
flocculation and coalescence. The latter reproduce the temporal variation of
the turbidity in all cases studied (380 \leq [NaCl] \leq 600 mM), providing a
method of appraisal of the flocculation rate in nanoemulsions.
