An assay device includes: a liquid sample receiving zone; a capture zone in fluid communication with the sample receiving zone having capture elements bound thereto, the capture zone having a substrate and projections which extend substantially vertically from the substrate, the projections having a rhombus-shaped cross-section and the projections arranged on the substrate with the corners of the projections facing upstream in a direction toward the sample receiving zone. The projections have a height, cross-section and a distance between one another that defines a capillary space between the projections capable of generating capillary flow parallel to the substrate surface; and a sink in fluid communication with the capture zone having a capacity to receive liquid sample flowing from the capture zone. The sample receiving zone, the capture zone and the sink define a fluid flow path. Preferably, the rhombus-shaped cross-section is a square or a diamond.