NanoTherm Research Group

Nano-scale liquid-vapor transformation

In conversion form liquid to vapor phase, the conversion phenomenon occurs at the interface with the thickness of few nanometer. This thickness is equivalent to few molecular lengths. In this interface region, the mean molecular density is changing by few order of magnitude. This phenomenon belong to realm of irreversible thermodynamics which considers the evolution of a system between two equilibrium states. The governing equations for this evolution are simply called hydrodynamics equations. However, the solutions of these equations are still limited to simple multiphase systems. Specially as the implementation of phase-change phenomenon is quickly emerging in nano-scale system, understanding of the physics and development of new advanced application of liquid-vapor transformation at these scales are in a great demand. Our group cocurrently focuses on three aspects to address this challenge.

Physics of liquid-vapor transformation at nano-scale: Development of new models to shed light on the physics of the liquid-vapor transformation at the nano-scale. Specially, at these scales,the role of interfaces is dominant compared to the bulk phases as we have shown in our previous publications.
Boost in the conversion efficiency of this transformation: The energy required to drive the transformation should only be supplied to this interface. In most of the current implementation of the phase-change phenomenon, the entire bulk liquid is heated to derive the liquid-vapor phase change. A portion of absorbed heat by the bulk material is transferred to the liquid-vapor interface for phase transformation and the rest is dissipated to the surrounding environment. The dissipated energy leads to drop in the energy conversion efficiency. Heat localization is an approach in which the thermal energy is only supplied to the interface for phase transformation and dissipated energy is minimized consequently boosting the energy conversion efficiency. Heat localization can be achieved by different approaches such as plasmonic nanoparticles and hybrid structures.
New implementation of liquid-vapor transformation at small scales: The applications span in a wide range of energy harvesting, photo-thermal therapy (Cancer therapy, drug delivery), biosensors and actuators.

Highly efficient solar-thermal energy harvesting

Solar energy is one of the renewable energy resources with hourly incident solar flux on earth surface greater than the global energy consumption in a year. Photo-thermal applications for harvesting solar energy currently suffer from low efficiency and require high concentration of sunlight, which adds complexity and cost to the solar energy harvesting systems. In solar-thermal systems, high conversion efficiency is in a significant demand. Localization of heat is a promising approach to achieve these high efficiencies. The localization of heat leads to locally elevated temperature while minimizing the dissipated energy. This localization could be incorporated in all forms of phase-change processes (evaporation, boiling, condensation and freezing). In early research in this field, scientists utilized Plasmonic Nanoparticles (NPs) to reach locally elevated temperatures for a broad range of applications. There is a significant unexplored gap in fundamental understanding of this localization. The key challenge for increasing the efficiency is the design of the material structures for the energy conversion. Through the idea of heat localization, we have developed a material structure leading to solar steam generation at low optical concentration with the efficiency of 85%. This high efficiency is reached by a combination of properties in the developed material structures and promises a new approach for highly efficient solar-assited phase-change phenomena.

Tuning solid-liquid interaction

Surface thermodynamics and interfacial phenomena, i.e., studies of phenomena associated with liquid-fluid and solid-fluid interfaces, are important in many areas of applied science and engineering. Examples of these phenomena are wetting and adhesion, drug delivery, and micro-scale thermal fluidic systems. Classical theories of fluid mechanics and heat transfer are not applicable to the interfaces. Recently, new research findings have revealed substantial contributions of interfaces in the transport of energy during evaporation. These contributions were less considered in literature. Furthermore, new studies have reported differences in the thermodynamics properties of interfaces from those of bulk phases. For examples, density of fluid at the interface is found to be different than the one for the bulk phase; in-plane thermal conductivity of water-Au(111) or water-Quartz interface can be several times higher than that of bulk water. These findings have pioneered a new multidisciplinary area of research with significant emerging potential at the micro- and nano-scales.