A hydrodynamic focusing device comprises first and second flow channels; a wall at least partially defining an envelopment region connected in-line between the first and second flow channels which collectively define a flow direction extending therethrough; and a chimney comprising a body and a sample fluid inlet, extending from the wall and into the envelopment region. The sample fluid inlet faces at least partially perpendicular to the flow direction in the envelopment region, such that the sample fluid inlet is configured to supply a sample fluid into the envelopment region in a direction that is at least partially perpendicular to the flow direction. The body and the sample fluid inlet each have an elongate profile which has a rounded leading edge facing the first flow channel and opposing long edges connecting the leading and trailing edges and tapered towards the trailing edge.