Abstract: A method comprises providing a microfluidic device including a reservoir; an injection nozzle at the bottom of the reservoir, the injection nozzle being wider at its top than at its bottom; an injection channel below the injection nozzle; and a microfluidic channel below the injection channel. The method further comprises placing fluid in the reservoir, and allowing the fluid to passively flow through the injection nozzle and the injection channel.

Abstract: In one implementation, a microfluidic probe has a non-planar processing surface and an inlet aperture. The shape of the surface may be selected to produce a specific velocity gradient profile across a surface onto which fluid is deposited using the microfluidic probe, for example a constant velocity gradient or a velocity gradient that decreases linearly with distance from the inlet aperture. The microfluidic probe may define and overflow notch in a perimeter edge of the processing surface.