  A microemulsion of decane droplets stabilized by a non-ionic surfactant film
is progressively charged by substitution of a non-ionic surfactant molecule by
a cationic surfactant. We check that the microemulsion droplets remain
identical within the explored range of volume fraction (0.02 to 0.18) and of
the number of charge per droplets (0 to 40) . We probe the dynamics of these
microemulsions by dynamic light scattering. Despite the similar structure of
the uncharged and charged microemulsions the dynamics are very different . In
the neutral microemulsion the fluctuations of polarization relax, as is well
known, via the collective diffusion of the droplets. In the charged
microemulsions, two modes of relaxation are observed. The fast one is ascribed
classically to the collective diffusion of the charged droplets coupled to the
diffusion of the counterions. The slow one has, to our knowledge, not been
observed previously neither in similar microemulsions nor in charged spherical
colloids. We show that the slow mode is also diffusive and suggest that its
possible origine is the relaxation of local charge fluctuations via local
exchange of droplets bearing different number of charges . The diffusion
coefficient associated with this mode is then the self diffusion coefficient of
the droplets.
