On the Physics of Fluid Transport and Phase Change in Nanoconfinements

Understanding the underlying physics of fluid behavior at nanoconfined geometries is essential to address many common challenges existing in science and engineering applications such as nanomedicine, energy conversion and storage, water purification, membrane science and electronics/photonics cooling. As the confinement dimensions shrink to nanoscale, the role of fluid/wall interactions as well as surface forces become more significant in the transport phenomena. These interactions result in the properties and behavior of the nanoconfined fluid to deviate considerably from those of the bulk, so that the classical theories no longer hold. In this dissertation, our focus is to study fluid behavior in nanoconfinements in the context of fluid transport and phase change. To this end, we developed nanofluidic devices, which consist of 2-D planner nanochannels with a height ranging from 180 nm to 10 nm, on silicon ship through the MEMS fabrication techniques. The simple and deterministic structure of our developed devices allow us to investigate the validity of classical equation and hydrodynamic properties at the nanoscale, to recognize the source of deviation, and to explore atypical phenomena (physics) emerging at this scale, such as extremely high evaporative heat flux, formation of interfacial viscous layer, breakdown of capillary wicking and the concept of surface tension nanogates. To explore phase-change at nanoconfinements, we studied thin-film evaporation in nanochannels under absolute negative pressure in both transient and steady-state conditions. We demonstrated that thin-film evaporation in nanochannels can be a bubble- free process even at temperatures higher than boiling temperature, providing high reliability in thermal management systems. To achieve this bubble-free characteristic, the dimension of nanochannels should be smaller than the critical nucleolus dimension. In transient evaporative conditions, there is a plateau in the velocity of liquid in the nanochannels, which limits the evaporative heat flux. This limit is imposed by liquid viscous dissipation in the moving evaporative meniscus. In contrast, in steady-state condition, unprecedented average interfacial heat flux of 11 ± 2 kW cm−2 is achieved in the nanochannels, which corresponds to liquid velocity of 0.204 m s-1. This ultrahigh heat flux is demonstrated for a long period of time. The vapor outward transport from the interface is both advective and diffusion controlled. The momentum transport of liquid to the interface is the limiting physics of evaporation at steady state. The developed concept and platform provide a rational route to design thermal management technologies for high- performance electronic systems. To investigate liquid transport at nanoconfinements, we studied capillary driven flow in nanochannels, and demonstrated the role of interfacial viscosity in capillary motion slowdown in nanochannels through a combination of experimental, analytical and molecular dynamics techniques. We showed that the slower liquid flow is due to the formation of a thin liquid layer adjacent to the channel walls with a viscosity substantially greater than the bulk liquid. By incorporating the effect of the interfacial layer, we presented a theoretical model that accurately predicts the capillarity kinetics in nanochannels of different heights. Non-equilibrium molecular dynamic simulation confirmed the obtained interfacial viscosities. The viscosities of isopropanol and ethanol within the interfacial layer were 9.048 mPa.s and 4.405 mPa.s, respectively (i.e. 279% and 276% greater than their bulk values). We also demonstrated that the interfacial layers are 6.4 nm and 5.3 nm-thick for isopropanol and ethanol, respectively. To examine the governing mass transport mechanism at sub-10 nm scale, both optical and electrical metrologies were utilized to identify the nature of fluid. we demonstrate that capillary wicking breaks down at a sub-10 nm scale for some fluids, changing governing physics of the mass transport and leading to a quasi-static liquid-vapor interface experiencing dynamic process of wetting and liquid fracture in a cyclic manner. The scale of capillary breakdown is a function of interfacial tension of the liquid and could be tuned based on the system requirements. The capillary breakdown results in surface tension nanogates that are turned on/off via external stimuli such as minimal temperature actuation or applied voltage. These nanogates are highly effective and tunable for ion transport playing a critical role in functionality of biological systems. The surface tension nanogates promise platforms to govern nano-scale functionality of wide spectrum of systems and foresee application in drug delivery systems, energy conversion, power generation, sea water desalination and ionic separation.