I put cooking oil into a Petri dish and sprinkled on some semolina. I used some electrodes made from bent coat hanger and connected it up to 2000V. The pieces of semolina touch the electrodes and gain a charge. They are then pulled along the electric field lines. I suspect that they join in clear lines from one side to the other because each charged piece attracts the next. That probably means that one side of the piece of semolina becomes positive and the other side negative - charge separation. Notice that no pattern is shown outside the gap between the electrodes presumably because there is no possibility of a continuous chain of + and - all the way to the other plate.
