Frequency domain electric birefringence study of water-in-oil microemulsion droplets

Frequency domain electric birefringence (FEB) spectroscopy has been applied to water-in-oil (W/O) microemulsion droplets in order to investigate the deformation dynamics of a single droplet by applying sinusoidal electric field. The dc component of FEB spectra shows the stationary deformation of the droplet, so that we can obtain the information on the polarization dynamics of the droplet. On the other hand, the 2ω component reflects the oscillating deformation of the droplet (oscillation frequency 2ω), which gives us the flexibility of the amphiphilic monolayer surrounding the water core. From the relaxation frequency observed in the 2ω component, we calculated the bending modulus κ of the amphiphilic layer to be 0.22 kBT, which was of the same order as the bending modulus measured by the spin echo and time-domain electric birefringence (TEB) method. Further, the temperature dependence of FEB spectra indicated that the surfactant layer softened near the phase separation point.