Caged-fluorescent-dye-based studies of turbulent scalar mixing

The initialization of a flow field with distinct and spatially segregated scalar components represents a significant experimental difficulty. Many theoretical modeling efforts in turbulent mixing, however, seek to describe the temporal evolution of a scalar concentration field that begins with this type of idealized initial conditions experimentally. This technique uses photoactivatable (caged) fluorescence dyes dissolved in the flow medium. Caged fluorescent dyes differ from tradition dyes in that excitation and subsequent emission will not occur until a bond within the caged dye molecule is broken with an ultraviolet photon. The flow field is then tagged by activating or 'uncaging' the appropriate regions with an excimer laser. Mixing between the tagged and untagged regions is quantified by illuminating the points to be studied with an argon ion laser, and measuring the subsequent emission intensity using standard laser induced fluorescence techniques. The intensity of the emission is proportional to the concentration of the uncaged dye. High sensitivity photodetectors allow very low intensity fluctuation measurements to be made. This method is currently being used to study mixing in a turbulent pipe flow, and shows potential to be used in a large number of other flow situations.