An apparatus comprising a substrate having a surface configured to accommodate a fluid thereover. A plurality of fluid-support-structures are on the surface. Each of the fluid-support-structures has at least one dimension of less than one millimeter. A well in the substrate has an opening on the surface. A medium is locatable between the plurality of fluid-support-structures and in the well. The medium located between the fluid-support-structures is in communication with the medium in the well.