A digital microfluidics system manipulates samples in liquid droplets within disposable cartridges and has disposable cartridges each with a bottom layer, a top layer and a gap therebetween. A base unit with cartridge accommodation sites and at least one electrode array with electrodes works with a cover plate at the sites and a control unit for controlling selection of the electrodes and for providing them with voltage pulses for manipulating liquid droplets within the cartridges by electrowetting. The cover plate has an electrically conductive material that extends parallel to the array. A selection of disposable cartridges and a method for manipulating samples in liquid droplets that adhere to a hydrophobic surface can be used with the system.