A device having: a laminar flow channel for liquids; two or more electrodes; a confining fluid inlet; a sample inlet; and a meter for measuring the impedance of any fluid between the electrodes. The device may have one or more specific binding sites, or it may have sheathing and unsheathing fluid transporting structures. A method of: providing the device; flowing a confining fluid and a conductive liquid that may contain cells or particles through the channel as described herein; and measuring the impedance between the electrodes.