Solar Radiation Reshapes the Deposition Patterns of Plasmonic Nanosuspension Droplets

Controlling deposition profiles from drying plasmonic nanofluid suspensions is crucial for applications ranging from surface-enhanced Raman spectroscopy to photothermal energy harvesting and desalination. In this study, we experimentally demonstrate that the simple switch-on of solar radiation reshapes the final morphology from drying plasmonic nanoparticle-laden droplets, transforming a "hill-shaped" pattern into a "basin-shaped" deposition structure. To demonstrate the underlying physics, we theoretically decompose the governing mechanisms and establish a comprehensive model of plasmonic nanofluid droplet evaporation exposed to solar radiation, and the results are in excellent agreement with the experimental findings. We reveal that solar radiation induces a nonmonotonic interfacial temperature distribution, characterized by a warm droplet apex and three-phase contact line (TPCL) relative to an intermediate cold droplet surface. The resulting surface tension gradients, as a result of competing plasmonic heating, evaporative cooling, and liquid-phase thermal resistance, drive thermal Marangoni cells in reverse directions from the TPCL and the apex toward the intermediate surface. The competing flow restricts nanoparticles within a stagnation region near the edge, enriches particles as the contact line recedes, and finally forms the "basin-shaped" deposition in solar-on cases, in contrast to the unidirectional center-oriented Marangoni flow in the absence of solar illumination. The findings indicate exclusive physics of fluid dynamics with plasmonic nanofluids under solar radiation, providing new possibilities for photofluidics and light-controlled microfabrication.