## 1 Introduction  
[PERSON] and [PERSON] [1] first proposed that nanometer-sized solid particles could be suspended in conventional heat-transfer fluids to enhance the thermal conductivity. Thereafter, many scholars from different research areas recognized the potential of nanofluids, which could provide benefits not only for heat transfer [2], but also in other fields (e.g., biology, medicine, optics, electronics and energy). Nanofluids have renewed the research about solar energy applications [3, 4], especially in concentrated solar power (CSP) by direct absorption solar collectors (DASCs). The working principle of DASCs, which were firstly proposed by [PERSON] and [PERSON] [5], is the same as conventional parabolic trough collectors (PTCs). These systems converge the sunlight irradiance towards a central absorber tube, where a working fluid is heated and then used for power or heat generation. Conventional PTCs feature a dark coating on the absorber tube, which allows it to capture the light and carry the energy to the inner fluid, which is usually transparent. On the other side, DASCs are equipped with a transparent absorber tube, where a dark fluid flows. This provides a twofold advantage, because the conductive thermal resistance of the coating is removed, and the emittance losses towards the external environment are decreased. Indeed, whereas the external coating of the absorber tube represents its hottest region in the case of conventional PTCs, the temperature distribution of DASCs is reversed, so that the boundary of the absorber are colder. In the mid-1970s, when DASC systems were first tested, a black fluid based on a dispersion of micro-sized carbonaceous particles with organic stabilizers, called Indian Ink, was typically used. However, it was very unstable at high temperatures, resulting in sedimentation and serious clogging issues within a short operating time [6]. Nanofluids, properly produced and employed, could overcome these drawbacks. These dispersions of nano-sized solid particles in a base fluid represent a stable, high conductive and adaptable working fluid for DASC systems. Direct solar steam generation is another interesting field of application, which could benefit from the use of nanofluids. Steam has a wide spectrum of uses, like electricity generation, energy storage, biomass processing or water desalination and purification. To reduce the environmental impact linked to the steam production, different solutions based on renewable energies have been proposed through the years, and solar energy is not an exception. Solar towers or parabolic trough collectors are the architectures traditionally used for solar steam generation [7; 8]. The concept underlying these systems, concentrating the light to heat a bulk fluid over its boiling temperature, is not so efficient because the high temperature levels involve heavy losses due to heat convection and re-irradiation from the hot container. Instead, nanofluids could provide a new way to achieve more favorable steam generation. Many studies report the formation of vapor bubbles inside nanofluids, despite the average temperature of the bulk fluid being below the boiling point [9; 10; 11; 12]. Although the mechanisms governing this phenomenon are still not fully understood, the solid nanoparticles could act as nucleating centers triggering complex heat-transfer phenomena at the solid-liquid interface and eventually leading to the growth of vapor bubbles [13]. This can suggest that vapor could be generated locally, without the need for heating the whole bulk fluid. Remarkable efforts have been made to understand which nanofluid could better fit the requirements of the desired application [6; 10; 12; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23]. Concerning DASC systems, it is a hard task to find a nanofluid that provides suitable chemical, thermophysical, fluid-dynamics and optical properties at the same time, as it must work both as a heat carrier and as a light absorber.