Optically induced variations of the electrical conductivity of a water in oil microemulsion near the percolation threshold

Water in oil microemulsions are largely used to study critical behaviour of binary mixtures since they are essentially made up of water microspheres coated by a monomolecular layer (surfactant) immersed in a continuous medium (oil). The water-AOT-decane microemulsion is particularly interesting since, for some values of temperature and droplet concentration, it exhibits the phenomena of cluster formation and percolation affecting electrical conductivity but not light scattering. In this paper we experimentally show that a nanosecond laser beam pulse travelling in such a microemulsion, near the percolation threshold, induces local variations of the de electrical conductivity. Moreover we show that, despite the short times involved, the effect cannot be explained as being produced only by a local temperature increase due to light absorption whilst neglecting other effects such as concentration variations induced by electrostriction and thermodiffusion.