Digital microfluidics system with electrodes attached to a PCB has control unit for manipulating liquid droplets by electrowetting, cartridge accommodation site for taking up a disposable cartridge having a working gap in-between two hydrophobic surfaces, and magnetic conduit/backing combination. A barrier element on an individual electrode of the PCB for narrowing the working gap. A disposable cartridge is positioned at the cartridge accommodation site, its flexible working film touching there and of the barrier element an uppermost surface. In the working gap and above a path of selected electrodes a liquid portion or liquid droplet with magnetically responsive beads moves by electrowetting on the electrode path until a magnetic field of the magnetic conduit is reached. The backing magnet is activated before and during the moving to thereby attract and remove magnetically responsive beads therefrom.