Dielectrophoresis of Submicrometer Latex Spheres. 1. Experimental Results

Abstract

A nonuniform electric field exerts a force on a polarizable particle through the Coulomb interaction with the electric dipole induced in the particle, resulting in a motion termed dielectrophoresis. The magnitude of the force depends on the dielectric properties of both the particle and the medium it is suspended in. As a result, measurement of the dielectrophoretic force provides information about the internal and surface dielectric properties of the particle. This paper presents the first detailed measurements of the dielectrophoretic response of submicrometer particles as a function of electrolyte composition and conductivity, applied field frequency, and particle size. Comparisons are made between the experimental results and the classical theory of the dielectrophoretic force derived from Maxwell-Wagner interfacial polarization. For particles of 557 nm diameter, good agreement is obtained between the experimental results and theory of interfacial polarization taking into account the effects of surface conductance. However, the results for smaller sizes of particle (93, 216, and 282 nm diameter) demonstrate that the theory does not adequately explain the dielectric or dielectrophoretic behavior of colloidal particles. The existence of a second low-frequency dispersion is also apparent in the data, attributable to the polarization of the double layer. The data were compared with a theoretical plot generated by modeling the dispersion in terms of a single Debye relaxation.

Dielectrophoretic Behavior of Latex Nanospheres: Low-Frequency Dispersion

“…As can be seen, the model accurately predicts the behaviour both below and above the decade transition in crossover frequency at 40-50mS/m. The model also accurately predicts the behaviour of latex beads of various sizes and functionalities published in the literature (12,16) and virus particles in various conditions (18). It is interesting to note that the dispersion indicates that the total capacitance of the double layer is dominated by the capacitance of the Stern layer.…”

“…Unlike the diffuse layer, the Stern layer is of fixed size and charge, these being dictated by the surface charge density of the particle. Hence, the frequency of the dielectric dispersion would be expected to be stable over a range of medium conductivities; this is observed in the experimental data published on the subject (5,6,12,(16)(17)(18). Furthermore, the dispersion frequency has been observed to vary linearly with predicted surface conductance Ks i for latex beads with functionalised surfaces (12) and herpes simplex viruses in a variety of biophysical states (18).…”

“…Particles are observed to undergo positive dielectrophoresis at lower frequencies, with observed crossovers in the range 100kHz-1MHz. The crossover frequency is typically constant for particles of one type, but has been observed to scale with particle radius for particles with similar surface chemistry (12), and with surface conductance for particles with similar size (16).…”

“…Previous attempts to model the full dielectrophoretic response of latex beads (e.g. 16) explored the possibility that a Debye-type relaxation may be responsible for the anomalous dielectrophoretic effect in high-conductivity media, but considered the dispersion process as being related solely to ionic diffusion through the double layer. An accurate fit to all published data is a dispersion having relaxation time related to the effective resistorcapacitor (i.e.…”

The Influence of Stern Layer Conductance on the Dielectrophoretic Behavior of Latex Nanospheres

“…Analysis of the crossover frequency as a function of medium conductivity can be used to characterize the polarizability and as a result the dielectric properties of a particle. This method of dielectrophoretic analysis has been demonstrated on sub-micrometer particles such as latex beads (5,6) and viruses (7)(8)(9).…”