Molecular tagging velocimetry

Last updated January 24, 2024

Molecular tagging velocimetry (MTV) is a specific form of flow velocimetry, a technique for determining the velocity of currents in fluids such as air and water.^[1] In its simplest form, a single "write" laser beam is shot once through the sample space. Along its path an optically induced chemical process is initiated, resulting in the creation of a new chemical species or in changing the internal energy state of an existing one, so that the molecules struck by the laser beam can be distinguished from the rest of the fluid. Such molecules are said to be "tagged".

This line of tagged molecules is now transported by the fluid flow. To obtain velocity information, images at two instances in time are obtained and analyzed (often by correlation of the image intensities) to determine the displacement. If the flow is three-dimensional or turbulent the line will not only be displaced, it will also be deformed.

Description

There are three optical ways via which these tagged molecules can be visualized: fluorescence, phosphorescence and laser-induced fluorescence (LIF). In all three cases molecules relax to a lower state and their excess energy is released as photons. In fluorescence this energy decay occurs rapidly (within $10^{-7}$ s to $10^{-9}$ s at atmospheric pressure), thus making "direct" fluorescence impractical for tagging. In phosphorescence the decay is slower, because the transition is quantum-mechanically forbidden.

In some "writing" schemes, the tagged molecule ends up in an excited state. If the molecule relaxes through phosphorescence, lasting long enough to see line displacement, this can be used to track the written line and no additional visualisation step is needed. If during tagging the molecule did not reach a phosphorescing state, or relaxed before the molecule was "read", a second step is needed. The tagged molecule is then excited using a second laser beam, employing a wavelength such that it specifically excites the tagged molecule. The molecule will fluoresce and this fluorescence is captured by means of a camera. This manner of visualisation is called laser induced fluorescence (LIF).

Optical techniques are frequently used in modern fluid velocimetry but most are opto-mechanical in nature. Opto-mechanical techniques do not rely on photonics alone for flow measurements but require macro-size seeding. The best known and often used examples are particle image velocimetry (PIV) and laser Doppler velocimetry (LDV). Within the field of all-optical techniques we can distinguish analogous techniques but using molecular tracers. In Doppler schemes, light quasi-elastically scatters off molecules and the velocity of the molecules convey a Doppler shift to the frequency of the scattered light. In molecular tagging techniques, like in PIV, velocimetry is based on visualizing the tracer displacements.

Schemes

MTV techniques have proven to allow measurements of velocities in inhospitable environments, like jet engines, flames, high-pressure vessels, where it is difficult for techniques like Pitot, hot-wire velocimetry and PIV to work. The field of MTV is fairly young; the first demonstration of implementation emerged within the 1980s and the number of schemes developed and investigated for use in air is still fairly small. These schemes differ in the molecule that is created, whether seeding the flow with foreign molecules is necessary and what wavelength of light is being used.

In gases

The most thorough fluid mechanics studies in gas have been performed using the RELIEF scheme and the APART scheme. Both techniques can be used in ambient air without the need for additional seeding. In RELIEF, excited oxygen is used as a tracer. The method takes advantage of quantum mechanical properties that prohibit relaxation of the molecule so that the excited oxygen has a relatively long lifetime.

APART is based on the "photosynthesis" of nitric oxide. Since NO is a stable molecule, patterns written with it can, in principle, be followed almost indefinitely.

Another well-developed and widely documented technique that yields extremely high accuracy is hydroxyl tagging velocimetry (HTV). It is based on photo-dissociation of water vapor followed by visualization of the resulting OH radical using LIF. HTV has been successfully demonstrated in many test conditions ranging from room air temperature flows to Mach 2 flows within a cavity.

In liquids

In liquids, three MTV approaches have been classified:^[2] MTV by direct phosphorescence (using a phosphorescent dye), absorbance (using a photochromic dye), and photoproduct fluorescence (typically using a caged dye).

MTV based on direct phosphorescence is the easiest technique to implement because a single laser is needed to produce a luminescent excited molecular state.^[3] The phosphorescence signal is generally weaker and harder to detect than fluorescence.

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. More recently, a rhodamine-based caged dye was successfully used with pulsed UV and green lasers.^[8]

Related Research Articles

Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, than the absorbed radiation. A perceptible example of fluorescence occurs when the absorbed radiation is in the ultraviolet region of the electromagnetic spectrum, while the emitted light is in the visible region; this gives the fluorescent substance a distinct color that can only be seen when the substance has been exposed to UV light. Fluorescent materials cease to glow nearly immediately when the radiation source stops, unlike phosphorescent materials, which continue to emit light for some time after.

In molecular biology and biotechnology, a fluorescent tag, also known as a fluorescent label or fluorescent probe, is a molecule that is attached chemically to aid in the detection of a biomolecule such as a protein, antibody, or amino acid. Generally, fluorescent tagging, or labeling, uses a reactive derivative of a fluorescent molecule known as a fluorophore. The fluorophore selectively binds to a specific region or functional group on the target molecule and can be attached chemically or biologically. Various labeling techniques such as enzymatic labeling, protein labeling, and genetic labeling are widely utilized. Ethidium bromide, fluorescein and green fluorescent protein are common tags. The most commonly labelled molecules are antibodies, proteins, amino acids and peptides which are then used as specific probes for detection of a particular target.

<span class="mw-page-title-main">Phosphorescence</span> Process in which energy absorbed by a substance is released relatively slowly in the form of light

Phosphorescence is a type of photoluminescence related to fluorescence. When exposed to light (radiation) of a shorter wavelength, a phosphorescent substance will glow, absorbing the light and reemitting it at a longer wavelength. Unlike fluorescence, a phosphorescent material does not immediately reemit the radiation it absorbs. Instead, a phosphorescent material absorbs some of the radiation energy and reemits it for a much longer time after the radiation source is removed.

Laser-induced fluorescence (LIF) or laser-stimulated fluorescence (LSF) is a spectroscopic method in which an atom or molecule is excited to a higher energy level by the absorption of laser light followed by spontaneous emission of light. It was first reported by Zare and coworkers in 1968.

Particle image velocimetry (PIV) is an optical method of flow visualization used in education and research. It is used to obtain instantaneous velocity measurements and related properties in fluids. The fluid is seeded with tracer particles which, for sufficiently small particles, are assumed to faithfully follow the flow dynamics. The fluid with entrained particles is illuminated so that particles are visible. The motion of the seeding particles is used to calculate speed and direction of the flow being studied.

Förster resonance energy transfer (FRET), fluorescence resonance energy transfer, resonance energy transfer (RET) or electronic energy transfer (EET) is a mechanism describing energy transfer between two light-sensitive molecules (chromophores). A donor chromophore, initially in its electronic excited state, may transfer energy to an acceptor chromophore through nonradiative dipole–dipole coupling. The efficiency of this energy transfer is inversely proportional to the sixth power of the distance between donor and acceptor, making FRET extremely sensitive to small changes in distance.

Laser Doppler velocimetry, also known as laser Doppler anemometry, is the technique of using the Doppler shift in a laser beam to measure the velocity in transparent or semi-transparent fluid flows or the linear or vibratory motion of opaque, reflecting surfaces. The measurement with laser Doppler anemometry is absolute and linear with velocity and requires no pre-calibration.

Velocimetry is the measurement of the velocity of fluids. This is a task often taken for granted, and involves far more complex processes than one might expect. It is often used to solve fluid dynamics problems, study fluid networks, in industrial and process control applications, as well as in the creation of new kinds of fluid flow sensors. Methods of velocimetry include particle image velocimetry and particle tracking velocimetry, Molecular tagging velocimetry, laser-based interferometry, ultrasonic Doppler methods, Doppler sensors, and new signal processing methodologies.

Hydroxyl tagging velocimetry (HTV) is a velocimetry method used in humid air flows. The method is often used in high-speed combusting flows because the high velocity and temperature accentuate its advantages over similar methods. HTV uses a laser to dissociate the water in the flow into H + OH. Before entering the flow optics are used to create a grid of laser beams. The water in the flow is dissociated only where beams of sufficient energy pass through the flow, thus creating a grid in the flow where the concentrations of hydroxyl (OH) are higher than in the surrounding flow. Another laser beam in the form of a sheet is also passed through the flow in the same plane as the grid. This laser beam is tuned to a wavelength that causes the hydroxyl molecules to fluoresce in the UV spectrum. The fluorescence is then captured by a charge-coupled device (CCD) camera. Using electronic timing methods the picture of the grid can be captured at nearly the same instant that the grid is created.

In chemistry, quenching refers to any process which decreases the fluorescent intensity of a given substance. A variety of processes can result in quenching, such as excited state reactions, energy transfer, complex-formation and collisions. As a consequence, quenching is often heavily dependent on pressure and temperature. Molecular oxygen, iodine ions and acrylamide are common chemical quenchers. The chloride ion is a well known quencher for quinine fluorescence. Quenching poses a problem for non-instant spectroscopic methods, such as laser-induced fluorescence.

Particle tracking velocimetry (PTV) is a velocimetry method i.e. a technique to measure velocities and trajectories of moving objects. In fluid mechanics research these objects are neutrally buoyant particles that are suspended in fluid flow. As the name suggests, individual particles are tracked, so this technique is a Lagrangian approach, in contrast to particle image velocimetry (PIV), which is an Eulerian method that measures the velocity of the fluid as it passes the observation point, that is fixed in space. There are two experimental PTV methods:

Flow visualization or flow visualisation in fluid dynamics is used to make the flow patterns visible, in order to get qualitative or quantitative information on them.

Planar Doppler Velocimetry (PDV), also referred to as Doppler Global Velocimetry (DGV), determines flow velocity across a plane by measuring the Doppler shift in frequency of light scattered by particles contained in the flow. The Doppler shift, Δf_d, is related to the fluid velocity. The relatively small frequency shift is discriminated using an atomic or molecular vapor filter. This approach is conceptually similar to what is now known as Filtered Rayleigh Scattering.

Planar laser-induced fluorescence (PLIF) is an optical diagnostic technique widely used for flow visualization and quantitative measurements. PLIF has been shown to be used for velocity, concentration, temperature and pressure measurements.

Turbulent diffusion is the transport of mass, heat, or momentum within a system due to random and chaotic time dependent motions. It occurs when turbulent fluid systems reach critical conditions in response to shear flow, which results from a combination of steep concentration gradients, density gradients, and high velocities. It occurs much more rapidly than molecular diffusion and is therefore extremely important for problems concerning mixing and transport in systems dealing with combustion, contaminants, dissolved oxygen, and solutions in industry. In these fields, turbulent diffusion acts as an excellent process for quickly reducing the concentrations of a species in a fluid or environment, in cases where this is needed for rapid mixing during processing, or rapid pollutant or contaminant reduction for safety.

Fluorescence is used in the life sciences generally as a non-destructive way of tracking or analysing biological molecules. Some proteins or small molecules in cells are naturally fluorescent, which is called intrinsic fluorescence or autofluorescence. Alternatively, specific or general proteins, nucleic acids, lipids or small molecules can be "labelled" with an extrinsic fluorophore, a fluorescent dye which can be a small molecule, protein or quantum dot. Several techniques exist to exploit additional properties of fluorophores, such as fluorescence resonance energy transfer, where the energy is passed non-radiatively to a particular neighbouring dye, allowing proximity or protein activation to be detected; another is the change in properties, such as intensity, of certain dyes depending on their environment allowing their use in structural studies.

Photoelectrochemical processes are processes in photoelectrochemistry; they usually involve transforming light into other forms of energy. These processes apply to photochemistry, optically pumped lasers, sensitized solar cells, luminescence, and photochromism.

Matched Index of Refraction is a facility located at the Idaho National Laboratory built in the 1990s. The purpose of the fluid dynamics experiments in the MIR flow system at Idaho National Laboratory (INL) is to develop benchmark databases for the assessment of Computational Fluid Dynamics (CFD) solutions of the momentum equations, scalar mixing, and turbulence models for the flow ratios between coolant channels and bypass gaps in the interstitial regions of typical prismatic standard fuel element or upper reflector block geometries of typical Very High Temperature Reactors (VHTR) in the limiting case of negligible buoyancy and constant fluid properties.

Magnetic resonance velocimetry (MRV) is an experimental method to obtain velocity fields in fluid mechanics. MRV is based on the phenomenon of nuclear magnetic resonance and adapts a medical magnetic resonance imaging system for the analysis of technical flows. The velocities are usually obtained by phase contrast magnetic resonance imaging techniques. This means velocities are calculated from phase differences in the image data that has been produced using special gradient techniques. MRV can be applied using common medical MRI scanners. The term magnetic resonance velocimetry became current due to the increasing use of MR technology for the measurement of technical flows in engineering.

Thermally activated delayed fluorescence (TADF) is a process through which a molecular species in a non-emitting excited state can incorporate surrounding thermal energy to change states and only then undergo light emission. The TADF process usually involves an excited molecular species in a triplet state, which commonly has a forbidden transition to the ground state termed phosphorescence. By absorbing nearby thermal energy the triplet state can undergo reverse intersystem crossing (RISC) converting it to a singlet state, which can then de-excite to the ground state and emit light in a process termed fluorescence. Along with fluorescent and phosphorescent compounds, TADF compounds are one of the three main light-emitting materials used in organic light-emitting diodes (OLEDs). Although most TADF molecules rely on the RISC from a triplet state to a singlet state, some of them take advantage of RISC processes between states with other spin multiplicities instead, for example from a quartet state to a doublet state.

References

↑ Koochesfahani, Manoochehr (1999). "Molecular Tagging Velocimetry (MTV) - Progress and applications". 30th Fluid Dynamics Conference. CiteSeerX 10.1.1.456.1991 . doi:10.2514/6.1999-3786.
↑ Koochesfahani, M.M.; Nocera, D.G. (2007). Tropea, Cameron; Yarin, Alexander L; Foss, John F (eds.). "Molecular tagging velocimetry". Handbook of Experimental Fluid Dynamics. doi:10.1007/978-3-540-30299-5. ISBN 978-3-540-25141-5.
↑ Gendrich, C.P.; Koochesfahani, M.M.; Nocera, D.G. (1997). "Molecular tagging velocimetry and other novel applications of a new phosphorescent supramolecule". Experiments in Fluids. 23 (5): 361–372. Bibcode:1997ExFl...23..361G. doi:10.1007/s003480050123. S2CID 121306156.
↑ Popovich, A.T.; Hummel, R.L. (1967). "A new method for non-disturbing turbulent flow measurements very close to a wall". Chemical Engineering Science. 22 (1): 21–25. doi:10.1016/0009-2509(67)80100-3.
↑ Homescu, D.; Desevaux, P. (2004). "Laser photochromic dye activation technique for the measurement of liquid free surface velocity on curved surfaces". Optics and Lasers in Engineering. 41 (6): 879–888. Bibcode:2004OptLE..41..879H. doi:10.1016/S0143-8166(03)00064-2.
↑ Rosli, N.B.; Amagai, K. (2014). "Measurement of liquid sheet using laser tagging method by photochromic dye". Experiments in Fluids. 55 (12): 1843. Bibcode:2014ExFl...55.1843R. doi: 10.1007/s00348-014-1843-0 .
↑ Lempert, W.R.; Ronney, P.; Magee, K.; Gee, K.R.; Haugland, R.P. (1995). "Flow tagging velocimetry in incompressible flow using photo-activated nonintrusive tracking of molecular motion (PHANTOMM)". Experiments in Fluids. 18 (4): 249–257. Bibcode:1995ExFl...18..249L. doi:10.1007/BF00195095. S2CID 122228370.
↑ Fort, C.; André, M.A.; Bardet, P.M. (2020). Development of long distance 2D micro-molecular tagging velocimetry (μMTV) to measure wall shear stress. AIAA Scitech 2020 Forum. Orlando, FL. doi:10.2514/6.2020-1274.