The objective of this work was to develop a high-speed, three-dimensional (3D) confocal imaging system to study coupled fluidic and heat transport processes for high-performance thermal management applications. However, to successfully implement these approaches, fundamental understanding of interfacial dynamics and transport processes was necessary.

The high-speed Confocal Imaging System for the investigation of fundamental thermal, fluidic, and interfacial phenomena at the micro scale.
The integrated imaging system, comprised of a swept-field confocal microscope and high-sensitivity camera, as well as high-speed and high-resolution cameras, achieves diffraction-limited 3D imaging of highly transient flows. The laser confocal scanner with selectable pinholes enables fast scanning, and when interfaced with a high-speed camera, achieves capture rates up to 1000 Hz. The combination of these features offers drastic improvements to the existing imaging systems, which are limited in spatial and/or temporal resolution.

The quantitative high-speed, diffraction- limited confocal imaging system is aimed to achieve new insights into complex liquid-heat-structure interactions and interfacial phenomena. A key component to achieving highly resolved transient flows and interfacial dynamics is the requested laser confocal scanner with selectable pinholes combined with either one of the existing cameras or the high-sensitivity camera. The integration of these components enables imaging of ultra-fast 2D flow phenomena at rates up to 1000 Hz, or 3D flow phenomena down to the diffraction limit (~200 nm) at rates of approximately 100 Hz.

Important functionality of the system was demonstrated to study interfacial phenomena on nanostructured surfaces. Specifically, the confocal microscope system was used to experimentally capture the 3D shape of the meniscus in pillar arrays. Such quantitative understanding is essential to accurately obtaining the capillary pressure and liquid propagation rates. Second, the shape of the liquid front during liquid propagation in pillar arrays was obtained, which determines the local pressure gradient. This understanding helped explain the role of microscopic dynamics in macroscopic propagation rates. Finally, the confocal microscope facilitated understanding of how microstructures can be used to pin the liquid. This is a first step towards developing methods to quantifying thermal interfacial resistances. These measurements offer new insights towards understanding fluid-thermal-structure interactions to aid in the development of high-heat-flux thermal management solutions.

The confocal imaging system is comprised of a Visitech-Infinity3 multi-beam laser confocal scanner with selectable pinholes, two solid-state lasers, a Hamamatsu EMCCD high-sensitivity camera, and a Nikon upright microscope. Together, they provide 2D and 3D capture rates of 1000 Hz and 100 Hz, respectively, down to sub-micron spatial resolutions. The modular system can also be easily integrated with two cameras previously purchased. The confocal system’s modular design allows for easy integration with a variety of cameras and lasers to achieve the ideal optimizations of combined spatial and temporal resolution and sensitivity. The confocal system operates in both fluorescence and reflective modes, and has the potential for unprecedented flow imaging capabilities through a variety of integrated components. Recent efforts have focused on using the confocal microscope system with a 405-nm and 532-nm solid-state laser to investigate interfacial phenomena and quantify meniscus shapes to predict liquid propagation behavior on engineered surfaces. Prediction and optimization of liquid propagation rates in micropillar arrays are important for various thermal management applications.

The confocal microscope system was also used to quantify the meniscus of the liquid front during propagation in micropillar arrays. Such information is important to understand how microscopic dynamics affects macroscopic propagation rates, where the shape of the liquid front determines the local pressure gradient.

This work was done by Evelyn N. Wang of the Massachusetts Institute of Technology for the Office of Naval Research. ONR-0028

This Brief includes a Technical Support Package (TSP).
Confocal Imaging System for Ultra-Fast, Three-Dimensional Transport Studies in Thermal Management Applications

(reference ONR-0028) is currently available for download from the TSP library.

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