Electrowetting with alternate voltage (AC) creates azimuthal flow vortices inside sessile droplets. These flow vortices can be controlled by introducing pinning sites at the contact line. When the frequency of the applied AC voltage is gradually ramped from a few hundreds of hertz to a few tens of kilohertz the azimuthal flow vortices contract and move towards the contact line near the pinning site. Dispersed particles in the liquid are collected in the center of these vortices leading to an increase in the local particle concentration by up to more than one order of magnitude. We provide a qualitative explanation for symmetry of the flow patterns within the drops and discuss possible scenarios explaining the particle collection and preconcentration.

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