Oscillating phase separation in microemulsions .1. Experimental observation

We examine the phase separation of a single phase of water-in-oil microemulsion droplets towards a phase of smaller water droplets coexisting with a water-rich excess phase. This transition is found to oscillate when induced by a continuous temperature increase. A periodic clouding and clearing is observed under the microscope and in the microcalorimeter, allowing to determine the oscillation period from the extinction of transmitted light and from the specific heat. The period depends on the surfactant concentration and increases as a square root of the scan speed. To the authors' knowledge this is the first time that oscillations in a temperature induced phase separation of a mixture have been observed. (C) 1997 American Institute of Physics.