In this work, molecular dynamics simulations show that liquid in a nanopore can be at thermodynamically stable high pressure even when connected to conventional bulk liquid. Such high pressure is associated with strong surface − liquid interaction. Evaporation of liquid in the pore creates a flow from the low pressure (bulk) region to the high pressure (nanopore) region. Such a counterintuitive flow occurs due to pressure being reduced in the pore from its thermodynamically stable state. The transition from high pressures to negative pressures in thin liquid films is also studied. This work provides insight into a possible mechanism of passive liquid transport in tall trees such as redwoods.

Boiling and evaporation are two distinct phenomena of liquid- to-vapor phase change. In this work, boiling was coupled with nanoscale evaporation to enhance critical heat flux (CHF), which is the maximum heat flux that nucleate boiling can reach. The coupling was attainted by creating buried nanochannels underneath the surface. This work consists of (1) nano fabrication of buried nanochannels, (2) experiments of boiling, surface wickability characterization, and wicking in nanochannels, and (3) CFD simulations of wicking in nanochannels at different temperatures. The main achievements are listed as following.

• Enhanced boiling CHF by 105% by coupling of boiling with nanoscale evaporation.

• CHF enhancement dominated by Extra contact line created by wicking inside nanochannels.

• Differentiated the roles of contact line extension and wicking in predicting CHF.

Wicking is the spread of liquid in a porous medium primarily driven by surface tension, curvature, and the solid-liquid inter- molecular forces of attraction. There are two major advantages to explore the physics behind wicking using buried nanochannels: (1) accurate modeling of capillary pressure without predictingmeniscus shape is achieved as the area of channel side walls is at least an order of magnitude greater than area of meniscus; and (2) evaporation is hindered to allow wicking in a long distances. This work includes (1) nano fabrication of buried nanochannels, (2) experiments of wicking and image analysis, and (3) CFD simulations of wicking in nanochannels. The following findings has been published.

• Wicking radius in channels linearly related to drop base radius above the channels.

• Characterized wicking as wicking-dominated and evaporation-dominated regimes.

• Analytically modeled the wicking radius evolution in wicking-dominated regime.

A microlayer is a thin, multiscale liquid film present near the contact line, where liquid-vapor interface meets solid surface. Microlayer is present in most natural phenomena and engineering applications involving liquid-vapor phase change, such as boiling, heat pipes, evaporation from micropores and microchannels, etc. Due to its unsteady multiscale nature, the fundamentals of microlayer, such as its origin, evolution, movement, and effect of surface wettability on microlayer have not yet been studied in situ in a vapor bubble.

We were able to achieve such a fundamental study by using laser heating methodology. A vapor bubble was created by laser heating and stayed at steady state as the evaporation in microlayer region was balanced by the condensation in the upper part the bubble. A monochromatic light source was used for illumination, generating dark and bright interference fringes. Thus, this technique "paused" the bubble growth, allowing in situ investigation of the microlayer of a single bubble in boiling. The following are what we achieved:

• Discovered that the bubble formed with a completely wetted bubble base, i.e. microlayer covered the entire bubble base and the contact line was not present.

• Created and studied a steady state vapor bubble in boiling.

• Obtained permissible range of maximum heat transfer coefficient possible in boiling.

This work extends the fundamental knowledge of microlayer in boiling and can help design novel surfaces and mechanisms to enhance boiling heat transfer.

Boiling has been widely used in industry, from nuclear power plants to electronics cooling, due to its efficiency in heat transfer. The maximum heat flux that boiling can reach without irreversible damage is known as critical heat flux (CHF). CHF enhancement by micro/nano scale surface modifications has been extensively investigated during the past decade.

In our work, we discovered a new CHF enhancement mechanism based on the extended knowledge of microlayer: early evaporation of microlayer using microenginnered surface. Ridges with height of a few microns partition the microlayer and disconnect it from the bulk liquid, triggering the early evaporation of microlayer, thus increasing the bubble departure frequency, enhancing CHF. The following novel findings were published:

• Proposed and validated a new boiling CHF enhancement mechanism.

• Enhanced CHF by 120% with only 18% increase in surface area; this is the highest such enhancement reported so far.

• Identified critical height of micro/nano structures to enhance boiling heat transfer.

• Developed an analytical CHF model for micro/nano engineered surface.

This work opens up a new field of CHF enhancement approach and can potentially be coupled with other techniques to further push the limits of boiling heat transfer.

Bioethanol production from lignocellulosic biomass waste consists of three steps: pretreatment, hydrolysis of carbohydrates (cellulose) to reducing sugar, and fermentation of sugar to ethanol. The conventional pretreatment destroys the protective structure of hemicellulose and lignin, and reduces cellulose crystallinity to increase its accessible area for hydrolysis.

We developed a two-step microwave assisted pretreatment route based on the idea of components separation: hemicellulose was extracted as the first product in the first step; then lignin and cellulose were separated in the second step. All three components were ready for subsequent chemical processes for different applications. Here the cellulose can be hydrolyzed directly. Compared to pretreatment without microwave radiation, the hydrolysis rate of the pretreated corn stover was improved by 85% under the same condition. For 10 g corn stover, we obtained 2.48 g hemicellulose, 0.95 g lignin, and 3.55 g reducing sugar. The yield of hemicellulose was up to 30% more than the particular extraction of hemicellulose, with up to 10 times faster extraction rate.