A parallel-plate capacitor used to determine the complex permittivity of supercooled aqueous solutions in the 1 MHz range

To evaluate temperature-dependent stretching experiments of biological cells down to sub-zero temperatures in a high-frequency electric field, knowledge of the electrical parameters of the suspension is necessary. A measuring system was developed to determine the relative permittivity epsilon(r)(f,T) and the ionic conductivity gamma(f,T) of supercooled aqueous solutions and suspensions in the frequency range 0.1-2 MHz. A parallel-plate capacitor was used in connection with an impedance analyser and a cryostat. The errors caused by the cables needed between analyser and cooled capacitor were minimized. Field homogenization within the measuring space is realized by correcting the reference potential at the guard ring. The starting capacitance at each temperature was reproduced by testing the 0 degrees C value after re-assembling the capacitor. Damaging of the capacitor by ice formation is prevented. For testing, a comparison was made with literature data and first measurements of supercooled aqueous solutions.