A device and method for introducing a second laminar fluid layer to, or removing a second laminar fluid layer from, a first laminar fluid layer are provided. Each laminar fluid layer can contain two or more side by side laminar streams. The device includes a main flow channel, and at least one tributary channel in fluid connection with a bridge channel which is in fluid connection with main flow channel. The device can be formed in a single piece of material, which can be optically transparent. Optionally, the channels can be formed in a first plate, the first and optionally the second surfaces of which are sealed to a second and optionally a third plate. The second and third plates can be optically transparent to allow for optical detection and analysis. A first laminar fluid layer is introduced into the main flow channel. If a second laminar fluid layer is to be added to the first laminar fluid layer, then the former is introduced into the tributary channel, from whence it flows into the bridge channel and then into the main flow channel, where it flows below the first laminar fluid layer and diffusionally mixes with it. Preferably, the width of the main flow channel is relatively small, so that particles in an added second laminar fluid layer diffusionally mix into the first laminar fluid layer rapidly. If a second laminar fluid layer is to be removed from a first laminar fluid layer, then the latter is split into two portions: one portion continues flowing down the main flow channel and one portion flows into the bridge channel from whence it flows into the tributary channel.