Short-Wave Infrared Imaging for Quantifying Liquid Concentration Fields

Core Concepts

This paper introduces a novel technique using short-wave infrared (SWIR) imaging to quantify liquid concentration fields, particularly beneficial for systems where traditional dye attenuation methods are unsuitable due to dye interference.

Abstract

• Bibliographic Information: Fortune, G.T., Etzold, M.A., Landel, J.R. & Dalziel, S.B. Dye Attenuation Without Dye: Quantifying Concentration Fields with Short-wave Infrared Imaging. Meas. Sci. Technol. (Submitted).
• Research Objective: This study aims to present a novel method for measuring liquid concentration fields without using dyes, overcoming the limitations of traditional dye attenuation techniques. The authors demonstrate the effectiveness of short-wave infrared (SWIR) imaging for this purpose.
• Methodology: The researchers developed an experimental setup using a SWIR camera, IR LEDs, and optical filters. They calibrated the system by imaging water-filled gaps of known heights to determine the effective attenuation coefficient of water. Two case studies were conducted: (1) tracking the volume of an evaporating water droplet on a glass slide and (2) monitoring the absorption and evaporation of a water droplet on an absorbent hydrogel sheet. The SWIR imaging results were validated against measurements obtained using an analytical balance.
• Key Findings: The study demonstrates that SWIR imaging can accurately quantify liquid concentrations without dyes. The technique successfully tracked the volume of evaporating water droplets on both impermeable and absorbent surfaces. The results obtained from SWIR imaging were consistent with the measurements from the analytical balance.
• Main Conclusions: SWIR imaging offers a promising alternative to traditional dye attenuation methods for quantifying liquid concentration fields. This technique is particularly valuable for systems where dyes can interfere with the fluid dynamics or chemical interactions. The authors suggest potential applications in biological, chemically-driven, and reacting flow systems.
• Significance: This research introduces a valuable tool for fluid dynamics research, enabling the study of previously inaccessible systems. The dye-free approach allows for more accurate measurements in situations where dyes could alter the system's behavior.
• Limitations and Future Research: The current setup is limited to transmission-based imaging. Future research could explore reflection-based SWIR imaging to expand the range of applicable systems. Additionally, incorporating multiple wavelengths could enable the analysis of multiphase flows and chemical reactions.

Stats

The attenuation coefficient of water (µw) for the experimental setup was determined to be 0.542.
The attenuation coefficient of water absorbed by the hydrogel (µg) was found to be 0.594.
The SWIR imaging system achieved a spatial resolution of approximately 50 µm per pixel and a temporal resolution of up to 120 fps.
The system could measure fluid heights down to 0.2 mm.

Quotes

"In this paper, we demonstrate how the concepts of dye attenuation can be extended towards tracking many normally transparent bulk liquids by using parts of the electromagnetic spectrum that do not coincide with the visible spectrum."
"In this paper, we demonstrate experimentally that by imaging in the short-wave infrared region, we can track the spatial and temporal evolution of depth averaged concentration fields of a fluid without the addition of dye and illustrate this for three test cases."

Key Insights Distilled From

Dye Attenuation Without Dye: Quantifying Concentration Fields with Short-wave Infrared Imaging

by George T. Fo... at arxiv.org 11-04-2024

https://arxiv.org/pdf/2411.00740.pdf

Deeper Inquiries

How might this SWIR imaging technique be adapted for use in microgravity environments, where fluid behavior differs significantly from that on Earth?

Adapting SWIR imaging for microgravity environments, where surface tension forces dominate over gravitational forces, presents unique challenges and opportunities:
Challenges:

Fluid Containment: In microgravity, fluids tend to form spherical blobs due to surface tension. Traditional open setups used in the paper for calibration and droplet evaporation would be unsuitable. Closed microfluidic cells with transparent SWIR windows would be necessary to contain the fluids.
Fluid Handling: Precisely manipulating and positioning fluids within the microfluidic cell in a microgravity environment requires specialized pumps, valves, and potentially, electrowetting techniques.
Calibration: The calibration procedure, relying on gravity-driven filling of gaps, needs modification. Precisely controlled volumes injected into the microfluidic cell could establish known fluid thicknesses for calibration.
Adaptations:

Microfluidic Cells: Design specialized microfluidic cells with thin, transparent SWIR windows for imaging. The cell geometry should facilitate controlled fluid manipulation and minimize optical distortions.
Integrated Fluidics: Incorporate micropumps, microvalves, and potentially electrowetting-based systems for precise fluid handling and positioning within the microfluidic cell.
In-situ Calibration: Develop in-situ calibration methods. One approach could involve injecting known volumes of the fluid into the microfluidic cell and correlating the measured absorbance with the calculated fluid thickness.
Advantages in Microgravity:

Study of Surface Tension Dominated Phenomena: SWIR imaging in microgravity offers a unique opportunity to study surface tension-driven flows, Marangoni effects, and coalescence phenomena without the influence of gravity.
3D Fluid Distribution: By rotating the microfluidic cell or using multiple viewing angles, it might be possible to reconstruct 3D fluid distributions within the cell, providing insights into complex fluid behavior in microgravity.

Could the presence of impurities or variations in the composition of the liquid affect the accuracy of the SWIR imaging technique, and if so, how can these limitations be addressed?

Yes, the presence of impurities or compositional variations in the liquid can significantly impact the accuracy of SWIR imaging by altering the fluid's absorption characteristics.
How Impurities and Compositional Variations Affect Accuracy:

Changes in Absorption Bands: Impurities can introduce new absorption bands or shift the existing ones, leading to inaccurate thickness measurements if the chosen wavelength is affected.
Scattering: Suspended particles or impurities can scatter SWIR light, reducing the transmitted intensity and leading to an overestimation of fluid thickness.
Refractive Index Mismatch: Variations in composition can alter the refractive index of the fluid, leading to refraction artifacts and distortions in the SWIR images, particularly at interfaces.
Addressing the Limitations:

Spectral Characterization: Before conducting measurements, characterize the SWIR absorption spectrum of the fluid with impurities or compositional variations. Choose a wavelength region where the absorption is minimally affected.
Filtering and Purification: If possible, filter or purify the fluid to remove impurities or standardize the composition.
Multispectral Imaging: Employ multispectral SWIR imaging to acquire data at multiple wavelengths. This allows for the identification and potential correction of spectral variations caused by impurities.
Advanced Image Processing: Develop image processing algorithms to identify and potentially correct for scattering and refraction artifacts in the SWIR images.

What are the ethical implications of using SWIR imaging in biological research, particularly when studying live organisms, and how can these concerns be mitigated?

While SWIR imaging offers a powerful tool for studying biological systems, its application to live organisms raises ethical considerations:
Potential Ethical Concerns:

Unintended Tissue Heating: Prolonged or high-intensity SWIR illumination might cause unintended tissue heating, potentially harming the organism.
Behavioral Alterations: Organisms sensitive to infrared radiation might experience behavioral changes or stress due to SWIR exposure.
Unknown Long-Term Effects: The long-term effects of SWIR exposure on certain organisms might not be fully understood, raising concerns about unforeseen consequences.
Mitigating Ethical Concerns:

Power Control and Exposure Limits: Use the lowest possible SWIR illumination intensity and duration required for imaging. Establish safe exposure limits based on the organism and tissue type being studied.
Pilot Studies: Conduct pilot studies to assess the potential impact of SWIR exposure on the organism's behavior, physiology, and long-term health.
Alternative Imaging Modalities: Explore alternative imaging modalities like visible light or ultrasound if they can provide sufficient information without potential SWIR-related risks.
Ethical Review and Transparency: Subject research proposals involving SWIR imaging of live organisms to rigorous ethical review. Transparently communicate the potential risks and benefits to all stakeholders.
By carefully considering these ethical implications and implementing appropriate mitigation strategies, researchers can harness the power of SWIR imaging in biological research while ensuring the well-being of the organisms under study.