Effective manipulation and understanding of the dynamic responses of complex micelle nanocolloids under ac-electric fields is of great scientific and technological importance due to the emerging interest in using rich electrokinetic phenomena for medical diagnostics and drug delivery. In this work, we employ fluorescence correlation spectroscopy (FCS) to study the ac-electrokinetic behaviors of surfactant-based micelles encapsulated with varied fluorescence and model drug probes at a single micelle level. Under applied non-uniform ac-fields of constant voltage across two adjacent microelectrodes in a quadrupole configuration, probe-encapsulated micelles exhibit positive dielectrophoresis (DEP) characteristics at the ac-frequency range of 10–500 kHz, in contrast to negative or zero DEP at higher frequency of approximately greater than 500 kHz. The transition from positive to negative or zero DEP is indicated by the field-induced decrease of micelle concentration in the FCS focal volume located in the high field regions between two adjacent microelectrodes. The estimated DEP crossover frequency from ω-dependent micelle concentration profile agrees with advanced DEP theory, which predicts the scaling of crossover frequency with Stern layer conductance. At a lower frequency range of ω = 5–10 kHz, unique and surprising response in the measured fluorescence autocorrelation functions of charged probe-encapsulated micelles is observed, but not so with neutral encapsulated probes, suggesting ac-field induced micelle instability with the resulting release of interior charged probe encapsulates. Intriguingly, the release of encapsulates from micelles by non-uniform ac-electric fields shows strong dependence on ac-frequency and encapsulate chemistry.
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