Particles having a non-uniform property, such as non-uniform charge, can be used to form complex yet controlled particle assemblies. A plurality of fluids to-be-treated are respectively caused to flow from the supply portions into the channel, and the plurality of fluids to-be-treated caused to flow in are merged and subjected to a predetermined treatment, and then the treated fluid is drawn from the collection portion to the outside. Additionally, voltage differences between pairs of actuator coil voltages and angular velocity differences between pairs of angular velocities are determined.