Relationship between dielectric properties and critical behavior of the electric birefringence in binary liquid mixtures

We present experimental results on the critical exponent ${\mathrm{\psi }}_{\mathrm{EKE}}$ describing the divergence of the Kerr constant of binary liquid mixtures near the critical consolute point. We show that the measured value of ${\mathrm{\psi }}_{\mathrm{EKE}}$ agrees with the theoretical prediction only if the measurement is performed with a mixture of two liquids presenting a small mismatch in the dielectric constant, and that the measured ${\mathrm{\psi }}_{\mathrm{EKE}}$ grows as the dielectric constant mismatch increases. Such findings are consistent with a recent model which assumes that the elongation of critical fluctations along the direction of the electric field can become so strong that fluctuations in the direction perpendicular to the electric field may cross over from Ising to mean-field behavior.