Transient SANS Studies on p-Tetrafluorethylen (p-TFE)

A temperature-controlled electric field sample environment for small-angle neutron scattering experiments

A new sample environment is introduced for the study of soft matter samples in electric fields using small-angle neutron scattering instruments. The sample environment is temperature controlled and features external electrodes, allowing standard quartz cuvettes to be used and conducting samples or samples containing ions to be investigated without the risk of electrochemical reactions occurring at the electrodes. For standard 12.5 mm quartz cuvettes, the maximum applied field is 8 kV/cm, and the applied field may be static or alternating (up to 10 kHz for 8 kV/cm and up to 60 kHz for 4 kV/cm). The electric fields within the sample are calculated and simulated under a number of different conditions, and the capabilities of the setup are demonstrated using a variety of liquid crystalline samples. Measurements were performed as a function of temperature and time spent in the electric field. Finally, the advantages, drawbacks, and potential optimization of the sample environment are discussed with reference to applications in the fields of complex soft matter, biology, and electrorheology.

Electric birefringence anomaly of solutions of ionically charged anisometric particles

The term "electric birefringence anomaly" is known as the electric birefringence (EB) signal that occurs in solutions of ionically charged anisometric particles in a narrow concentration region. The signal is of opposite sign to the normal birefringence that occurs below and above this narrow concentration region. The normal electric birefringence signals in the dilute and more concentrated regions are due to the orientation of the particles in the direction of the applied electric field. The origin for the anomalous signal was not completely understood until now. The article summarises previous results in which the anomalous results had been observed but not well understood. It shows that the birefringence anomaly occurs in systems as diverse as micellar solutions, polyelectrolytes, solutions of clays, viruses and fibres. In all these systems the anomaly signals are present at the concentration when the length of the colloidal particles including the thickness of the electric double layer are about the same as the mean distance between the colloidal particles. Under these conditions the electric double layers of the particles overlap along the main axis of the particles but not in the direction across the particles. As a consequence of this situation a dipole is built up across the particles by the migration of the counter-ions of the particles in the electric field and this dipole leads to an orientation of the particles perpendicular to the electric field. The anomalous signal can usually be observed simultaneously with the normal signal. The amplitude of the anomalous signal can be larger than the amplitude of the normal signal. As a consequence the total birefringence changes its sign in the anomalous concentration region. The anomaly signal of the clays can also be explained by a fluctuating dipole around the particles, which is due to the fact that the centre of the ionic charges of the particles does not fall on the centre of the ionic charge of the counter-ions. The resulting dipole could also explain the anomaly signal.

Electric polarization of polyelectrolyte and colloid media: dielectric versus electro-optic approach

The theories of dielectric dispersion and of electric birefringence as a representative of electro-optic methods are considered and it is shown that they both depend in a similar way simply on the real part of the complex electric polarizability of the macromolecules or the particles. The latter also contains the permanent dipole moment. Experimental data on dielectric dispersion, electric birefringence and electric light scattering of strongly elongated, rod-like poly(tetrafluoroethylene) particles are compared and an attempt is made to extend the dielectric dispersion curve to lower frequencies using electric birefringence and electric light scattering data. Further, the experimental data on dielectric dispersion, electric light scattering, electro-orientation and dipolophoresis for the more complicated Escherichia coli particles are compared. Again, the possibility to extend the 10 kHz-100 MHz dielectric dispersion curve down below 1 Hz by using electric light scattering data is examined. The good matching of the dielectric dispersion and electric light scattering frequency curves found in the overlapping frequency range (10 kHz-5 MHz) essentially enhances the chance that dielectric dispersion below 1 MHz is related to alpha dispersion and not to electrode polarization. Thus it is not only possible to obtain additional information on the mechanism of polarization at lower-frequency dielectric dispersion, but also to extend our knowledge about the effective dielectric properties of biological complex fluids to frequencies essentially below 1 MHz. This could be important for the understanding of the effect of low-frequency electromagnetic fields on living matter.