Hyunjoo Park, Ming-Tzo Wei, H. Daniel Ou-Yang
Optical trapping-based force spectroscopy was used to measure the frequency dependent dielectrophoresis (DEP) forces and DEP crossover frequencies of colloidal PMMA spheres and clusters. A single sphere or cluster, held by an optical tweezer, was positioned near the center of a pair of gold-film electrodes where ACEO flow was negligible. Use of amplitude modulation and phase-sensitive lock-in detection for accurate measurement of the DEP force yielded new insight into dielectric relaxation mechanisms near the crossover frequencies. On one hand, the size dependence of the DEP force near the crossover frequencies indicates that the dominant polarization mechanism is a volume effect. On the other, the power-law dependence of the crossover frequency on the particle radius with an exponent -2 indicates the dielectric relaxation is more likely due to ionic diffusion across the particle surface, suggesting the dominant polarization mechanism may be a surface polarization effect. Better theories are needed to explain the experiment. Nevertheless, the strong size dependence of the crossover frequencies suggests the use of DEP for size sorting of micron-sized particles.
DOI
Optical trapping-based force spectroscopy was used to measure the frequency dependent dielectrophoresis (DEP) forces and DEP crossover frequencies of colloidal PMMA spheres and clusters. A single sphere or cluster, held by an optical tweezer, was positioned near the center of a pair of gold-film electrodes where ACEO flow was negligible. Use of amplitude modulation and phase-sensitive lock-in detection for accurate measurement of the DEP force yielded new insight into dielectric relaxation mechanisms near the crossover frequencies. On one hand, the size dependence of the DEP force near the crossover frequencies indicates that the dominant polarization mechanism is a volume effect. On the other, the power-law dependence of the crossover frequency on the particle radius with an exponent -2 indicates the dielectric relaxation is more likely due to ionic diffusion across the particle surface, suggesting the dominant polarization mechanism may be a surface polarization effect. Better theories are needed to explain the experiment. Nevertheless, the strong size dependence of the crossover frequencies suggests the use of DEP for size sorting of micron-sized particles.
DOI
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