Introducing phase analysis light scattering for dielectric characterization: measurement of traveling-wave pumping.

Phase analysis light scattering (PALS) was applied to characterize a high-frequency traveling-wave (TW) micropump. Field strength and frequency characteristics were measured for aqueous solutions up to 40 MHz and conductivities of 16 mS/m. The TW field was generated by an ultramicroelectrode array of intercastellated electrodes, which were driven by square-topped signals. Pumping exhibited one major relaxation peak, which strongly increased for conductivities above 4 mS/m. The conductivity dependence of the peak frequency showed an unexpected nonlinear behavior. Around 20 MHz an additional peak caused by electronic resonance was found. Additional coils or capacitors shifted the resonance peak and allowed us to determine the electronic properties of the array. Analysis of distortions in the pump spectra caused by the harmonic content of the driving signals showed that the pump direction is determined by the traveling direction of the field. For measurement of AC-field-induced particle translations, the advantage of PALS over the commonly used microscopic analysis is that it offers an objective method for statistically significant, computerized registration of extremely slow motions. Thus, for dielectric characterization, low field strengths can be used, which is advantageous not only for analyzing liquid pumping, but also for measuring particle translations induced by dielectrophoresis or TW dielectrophoresis.

Measurement of inherent particle properties by dynamic light scattering: introducing electrorotational light scattering.

Common dynamic light scattering (DLS) methods determine the size and zeta-potential of particles by analyzing the motion resulting from thermal noise or electrophoretic force. Dielectric particle spectroscopy by common microscopic electrorotation (ER) measures the frequency dependence of field-induced rotation of single particles to analyze their inherent dielectric structure. We propose a new technique, electrorotational light scattering (ERLS). It measures ER in a particle ensemble by a homodyne DLS setup. ER-induced particle rotation is extracted from the initial decorrelation of the intensity autocorrelation function (ACF) by a simple optical particle model. Human red blood cells were used as test particles, and changes of the characteristic frequency of membrane dispersion induced by the ionophore nystatin were monitored by ERLS. For untreated control cells, a rotation frequency of 2 s-1 was induced at the membrane peak frequency of 150 kHz and a field strength of 12 kV/m. This rotation led to a decorrelation of the ACF about 10 times steeper than that of the field free control. For deduction of ERLS frequency spectra, different criteria are discussed. Particle shape and additional field-induced motions like dielectrophoresis and particle-particle attraction do not significantly influence the criteria. For nystatin-treated cells, recalculation of dielectric cell properties revealed an ionophore-induced decrease in the internal conductivity. Although the absolute rotation speed and the rotation sense are not yet directly accessible, ERLS eliminates the tedious microscopic measurements. It offers computerized, statistically significant measurements of dielectric particle properties that are especially suitable for nonbiological applications, e.g., the study of colloidal particles.