In this work, wicking is studied in ∼728 nm height crossconnected nanochannels buried under a SiO 2 surface. Pores of diameter ∼2 μm, present at each intersect of nanochannels, allow a water droplet placed on the surface to wick into the channels. Experiments are conducted for two different channel widths/spacings and five different water droplet volumes. Wicking characteristics show the occurrence of wicking-dominant and evaporationdominant regimes, with each further divided into two subregimes. All experimental data in wicking-dominant regime are in good agreement with two analytical models which can be used to predict the wicking distance evolution in such nanochannels. The analysis shows that wicking is initially governed by surface tension and viscous forces as there is unhindered supply of liquid from the droplet. After this initial phase, hydrodynamic dissipation within the droplet sitting on the top surface dictates wicking inside the channels.

Pool boiling is explicitly coupled with nanoscale evaporation by using buried nanochannels of height ∼728 nm and ∼100 nm to enhance critical heat flux (CHF) by ∼105%. Additional menisci and contact line formation in nanochannels are found to be the dominant factors of CHF enhancement. Wicking assists in creating the additional contact line but does not serve as the primary measurable factor in predicting such enhancement based on CFD simulations and wicking experiments. This work provides clarity on the roles of contact line and wicking in boiling heat transfer.

Droplet wicking and evaporation in porous nanochannels is experimentally studied on a heated surface at temperatures ranging from 35 o C to 90 o C. The fabricated geometry consists of cross-connected nanochannels of height 728 nm with micropores of diameter 2 µm present at every channel intersection; the pores allow water from a droplet placed on the top surface to wick into the channels. Droplet volume is also varied and a total of 16 experimental cases are conducted. Wicking characteristics such as wicked distance, capillary pressure, viscous resistance and propagation coefficient are obtained at the high surface temperatures. Evaporation flux from the nanochannels/micropores is estimated from the droplet experiments, but is also independent confirmed via a new set of experiments where water is continuously fed to the sample through a microtube such that it matches the evaporation rate. High heat flux as high as ~294 W/cm 2 is achieved from channels and pores. The experimental findings are applied to evaluate the use of porous nanochannels geometry in spray cooling application, and is found to be capable of dissipating high heat fluxes upto ~77 W/cm 2 at temperatures below nucleation, thus highlighting the thermal management potential of fabricated geometry.

Cross-connected buried nanochannels of height ∼728 nm, with micropores of ∼2 μm diameter present at each intersection, are used in this work to numerically and experimentally study droplet-coupled evaporation dynamics at room temperature. The uniformly structured channels/pores, along with their well-defined porosity, allow for computational fluid dynamics simulations and experiments to be performed on the same geometry of samples. A water droplet is placed on top of the sample causing water to wick into the nanochannels through the micropores. After advancing, the meniscus front stabilizes when evaporation flux is balanced with the wicking flux, and it recedes once the water droplet is completely wicked in. Evaporation flux at the meniscus interface of channels/pores is estimated over time, while the flux at the water droplet interface is found to be negligible. When the meniscus recedes in the channels, local contact line regions are found to form underneath the pores, thus rapidly enhancing evaporation flux as a power-law function of time. Temporal variation of wicking flux velocity and pressure gradient in the nanochannels is also independently computed, from which the viscous resistance variation is estimated and compared to the theoretical prediction.

Experiments of water wicking in 1D silicon-dioxide nanochannels of heights 59 nm, 87 nm, 124 nm and 1015 nm are used to estimate the disjoining pressure of water which was found to be as high as ~1.5 MPa while exponentially decreasing with increasing channel height. Such a relation resulting from curve fitting of experimentally-derived data was implemented and validated in computational fluid dynamics. This methodology integrates experimental nanoscale physics into continuum simulations thus enabling the numerical study of various phenomena where disjoining pressure plays an important role. Main TextA nanoscale thin liquid film on a surface can have significantly different properties than its bulk form [1]. At such short distances, intermolecular interactions with surface atoms can dominate and define new equilibrium positions/velocities of liquid atoms; as these fundamental parameters are statistically averaged to estimate thermodynamic properties [2], substantial changes in density, pressure, surface tension, viscosity, etc. can occur. Distances at which a surface can affect liquid properties depends on the atomic composition: if either atom is non-polar, the presence of only weak and short-range van der Waal's force limits such changes to <5 nm [3,4]; however, if both atoms are polar, strong and long-range electrostatic forces can alter properties up to tens to hundreds of nanometers from the surface [5][6][7]. The latter scenario often occurs in practical situations involving water on various surfaces.