Nanofluids generically represent a new class of fluids engineered by dispersing controlled nanometer-sized materials (nanoparticles, nanofibers, nanotubes, nanowires, nanorods, nanosheet, or droplets) in bare fluids. Importantly, nanofluids (NFs) have been found to possess enhanced thermophysical properties such as thermal conductivity, thermal diffusivity, viscosity, and convective heat transfer coefficients compared to those of bare fluids like organic solvents or water. Then, since the first study was published in 19951, the use of NFs has been devised for different applications such as coolants in automobile transmission systems, electronic cooling applications, solar water heating devices, nuclear reactors, radiators, or low-cost spectrally selective optical filters.
In these applications, the thermophysical properties of the NF, including its heat transfer characteristics (thermal conductivity, heat capacity) and hydrodynamic properties (surface tension, viscosity), play crucial roles for their performance. Therefore, the precise characterizations of surface and bulk thermophysical properties of a NF are indispensable to calibrate them and predict their capabilities2. Nonetheless, while many well-established methods confidently provide these characterizations at large scale, the situation becomes much more complex when at least one dimension reaches the micrometer world, as for lubrication film in engine for instance, because most methods utilize contact and cannot anymore be implemented in situ. This prompted the emergence of contactless electrical, acoustical or optical techniques for milli- and micro-characterizations.
Nonetheless, the majority of complex fluids, such as NFs and biofluid (human saliva) for instance, absorbs some light almost all over the visible spectrum so that light has often been considered as a serious drawback; conversely, we show that eventually light absorption may become an appealing forcing method to probe thermophysical properties at small scale. Indeed, due to the Gaussian shape of laser beams, optical absorption will produce locally an inhomogeneous heating which may trigger Marangoni interfacial stresses and finally a thermocapillary deformation on the NF free surface, even if the heating is weak in amplitude, as the driving mechanism is related to the thermal gradient. We show that this thermal signature may be fully used to characterize the thermophysical physical properties of the fluid under investigation.
Finally, when driven by direct laser fluid interaction, interface dynamic and deformation3 of NFs presents two outstanding challenges which up to now limited its practical application in absorbing liquids. On the one hand, when exciting NFs and bio-fluids4 (human saliva), other mechanical effects of light may also be at work so that a complex interplay between optical radiation pressure, thermocapillary stresses, and scattering density forces5 forms which requires very sensitive and precise measurements to properly disentangle the different mechanisms and avoid inaccurate determination of thermophysical properties. On the other hand, laser heating can be dramatically detrimental when the absorption of pump laser becomes huge enough to damage the fluid under investigation. This is particularly true for soft biological fluids or systems where the fluid is confined to a closed surface, such as heat pipes.
In our manuscript “A versatile interferometric technique for probing the thermophysical properties of complex fluids” (https://rdcu.be/cMqM4), we address these two concerns in details, by proposing a versatile optical technique based on pump excitation-probe interferometry to characterize the thermophysical properties of both nanofluids and biological fluids in a contactless way. Three very different configurations are illustrated. At first, the NF is heated from the bottom through an opaque substrate. This methodology provides the first scale-scale measurements of the thermophysical properties (viscosity, surface tension coefficient, and diffusivity) of complex NF and bio-fluid without damaging and competing forces. A second approach consists in illuminating the fluid from its free surface (exposure from the top for deposited drops). In this configuration, we show a precise characterization of NF by quantitively isolating the competing forces, taking advantage of the different time scales of these forces. The third configuration consists in investigating thermophysical properties of NFs when confined in a metal-cavity. In this case, the transient thermoelastic deformation of metal surface provides the properties of NF as well as thermo-mechanical properties of the metal. Considering this versatility, our technique works for nearly all liquids and can thus be applied to a wide range of application scenarios for precise in-situ characterization of the thermophysical properties of complex fluids at a small scale.
An important consequence of heating the substrate from the bottom and NF inside the metal cavity is that it works for nearly all kinds of fluids as it eliminates the effects of scattering and radiation forces, as well as the potential damage induced by the laser. Additionally, measuring the surface deformation beyond the thermal limit without the use of any electrical modulation distinguishes our work in terms of sensitivity as compared to past approaches. The second capability is the ability to measure weakly absorbing NFs thermal properties for a thick substrate by isolating the competing effects of thermal, scattering, and momentum transfer. The fluid dynamics can be obtained by analyzing different time scales in the surface deformation signal. We measured the thermophysical properties of human saliva by merely changing the temperature of the sample by just a few µK and less than a second. This analysis can also be applicable to structured surfaces, e.g., skin, and cloth, on which direct laser shinning is not possible. The third capability is that the transient thermo-elastic signal is very useful for NFs in confined environments, where heat coupling between the solid walls and NFs provides thermophysical properties of the nanofluid only if a solid reference sample is used.
Consequently, our results show how laser actuation on the one hand and interferometric methods on the other hand, offer the possibility to characterize in-situ, in a contactless way and in very short time, the thermophysical properties of NFs, hybrid-NFs, and biological fluids at small scale, when other methods mainly fail.
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