Provided herein are cartridges, devices, and methods for carrying out multistep assays on a microfluidic scale. A cartridge includes a block frame comprising a well, wherein the well comprises an outlet, at least one inlet, and a bottom surface; a plurality of containers, wherein each container is connected to the well via a microchannel leading to the at least one inlet; and an openable cover, which cover when closed is configured to enclose an assay surface and form a gap between (i) the assay surface and the bottom surface of the well or (ii) the assay surface and the cover. The gap can be formed by a spacer that extends from the bottom surface of the well or from the outer edge of the cover. Methods include flowing liquid into the gap and/or through an opening adjacent to the assay surface.