Tiny Flow Sensors
Tiny flow sensors exist in chip-size packages. These are in a group of products called MEMS (micro-electro-mechanical system) devices.
Sensirion makes small 10x10mm liquid flow sensors (the LPP10 and LPG10). They are based on thermal MEMs elements. They can measure between 0 and 1000uL/min (1mL/min), with a resolution that can be as low as 0.5nL/min. It has a reponse time 40ms/sample. It is compatible with H20, methanol, ethanol, and blood.