Speaker
Description
To understand heat transfers in complex system, three-dimensional temperature measurements is a powerful source of information. Various techniques exist, optical, electromagnetic, or scanning thermal microscopy, generally falling into two categories: two-dimensional measurements together with inverse methods, or direct 3D measurements requiring fewer assumptions and potentially handling more complex geometries [1].
Semi-transparent media are ubiquitous in many practical applications used in our everyday technologies. One can cite the semiconductor materials such as silicon, or microfluidic systems used in biology, chemistry and energy, where temperature critically impacts device performance and reveals indications about the ongoing physics or chemical reactions. For instance, in polymerase chain reaction (PCR) DNA amplification, which only works under precise temperature control, the need for accurate 3D temperature mapping is evident. Achieving this in microfluidics remains challenging and needs to solve both geometrical (a few millimetres-thick) and optical (multiple light reflections and absorptions) constrains to develop a new 3D temperature measurement method [2].
This study fills this gap and proposes a system that enables 3D measurement of temperature elevation fields in a semi-transparent system: a microfluidic chip (PDMS, water, silicon). To do this, a new laminographic method based on transmission photothermal heterodyne imaging (TPHI) is used in the mid-IR range. It is based on the principle that the reflectance and absorbance of incident light shed in a medium vary with its internal temperature. The increase in signal-to-noise ratio achieved by using a heterodyne method overcomes the problem of material (PDMS, water…) refractive index sensitivity to the temperature [3]. The transmitted signal is then measured for different angular positions of the sample using a laminography device in order to reconstruct the 3D field at microscale (below 20 µm3/voxel), and using a SIRT reconstruction algorithm [4]. Compressive sensing to reduce the acquisition time during the 3D laminography is also used to speed up the measurements to less than 1 h.
In this communication, we will show the imaging of a 3D elevation temperature field due to laser beam excitation in a microfluidic chip component. The complete methodology, theoretical thermo-optical model and signal processing (reconstruction and compressive sensing) will be exposed. This work shows the recent advances made for measurement in microfluidic systems and for semi-transparent media in general, despite the fact that the proposed method is limited optically to a purely absorbing medium (without scattering) and to the observation of relative temperature elevation.
References
[1] R. Chen, B. Shi, K. Song, M.M.Z.G.C. Lok, M. Jiang, S. An, P. Tao, B. Fu, C. Song, J. Wang, T. Deng, W. Shang, Advances in Three-Dimensional Temperature Sensing: From Materials to Applications, Advanced Materials. (2025).
[2] C. Bourgès, J. Maire, S. Chevalier, S. Dilhaire, Surface and average volume temperature measurements in semitransparent media based on multispectral thermotransmittance, Int J Heat Mass Transf. 234 (2024) 126087.
[3] J. Letessier, A. Netter, J. Maire, S. Chevalier, Infrared Photothermal Heterodyne Imaging in Thermally Thick Medium for Thermo-optic Property Characterizations, J Phys Chem Lett. (2025) 11987–11995.
[4] J. Gregor, T. Benson, Computational Analysis and Improvement of SIRT, IEEE Trans Med Imaging. 27 (2008) 918–924.
