Schematic diagram of (a) magnetic field device structure and (b) magnetic field distribution. 1: iron plates. 2: permanent magnet array. 3: magnetic yoke. 4: flow channel. X: width of magnetic field. B: magnetic flux density. The magnetic field can be divided into two equal areas for placing the flow channel respectively. The magnetic flux density and magnetic field gradient in the channel are opposite when the channel is placed respectively in areas I and II.