Molecular tagging velocimetry

[1] In its simplest form, a single "write" laser beam is shot once through the sample space.

Opto-mechanical techniques do not rely on photonics alone for flow measurements but require macro-size seeding.

In molecular tagging techniques, like in PIV, velocimetry is based on visualizing the tracer displacements.

The method takes advantage of quantum mechanical properties that prohibit relaxation of the molecule so that the excited oxygen has a relatively long lifetime.

Another well-developed and widely documented technique that yields extremely high accuracy is hydroxyl tagging velocimetry (HTV).

MTV based on direct phosphorescence is the easiest technique to implement because a single laser is needed to produce a luminescent excited molecular state.

The second technique called MTV by absorbance relies on the reversible alteration of the fluorescence properties of a photochromic dye.

The scheme showed good results in alcohol[4] and oils,[5][6] but not in water in which typical dyes are not soluble.

The third variant of MTV was first deployed in liquids in 1995[7] under the name "photoactivated nonintrusive tracking of molecular motion" (PHANTOMM).

The PHANTOMM technique initially relied on a fluorescein-based caged dye excited by a blue laser.