Flowing thermal lens micro-flow velocimeter

A novel micro-flow velocimeter, the flowing thermal lens micro-flow velocimeter (FTL-MFV) is developed in which a photothermally generated local refractive index change (a thermal lens) is utilized as a micro-tracer of flow. Flow velocity is measured from the time required for the thermal lens to travel between two points. Generation and detection of thermal lenses are carried out optically without contacting the flow. By choosing the wavelength of the excitation beam pulse so that it coincides with the absorption band of the solvent used, thermal lenses can be generated without adding anything to the liquid. Synchronous detection using a lock-in amplifier makes detection of thermal lens with a very small temperature rise possible. Thus, with the FTL-MFV, non-contact in situ measurement of flow can be carried out with only slight disturbance to the microfluid. In order to make the sensor small, optical fibers and SELFOC™ microlenses are used in focusing the excitation and probe beams. A dynamic range of 25–300 μL/min is realized in measurement of flow rates in a microchannel.