An apparatus for controlling the motion of a particle and a method for using the same are disclosed. The apparatus includes a channel containing liquid between first and second electrodes. The apparatus also includes an array of variable impedance elements, each variable impedance element connecting the first electrode to a corresponding location in the channel by a path having an average impedance that is continuously variable between first and second impedances when averaged over an update time interval. A controller sets the average impedance of each of the variable impedance elements such that a particle in the channel moves in a predetermined direction when voltage is applied between the first and second electrodes. At least one of the variable impedance elements has an average impedance that is intermediate between the first and second impedances.