Shalabh C. Maroo

Wettability of Graphene

Graphene, an atomically thin two-dimensional material, has received significant attention due to its extraordinary electronic, optical, and mechanical properties. Studies focused on understanding the wettability of graphene for thermo-fluidic and surface-coating applications, however, have been sparse. Meanwhile, wettability results reported in literature via static contact angle measurement experiments have been contradictory and highlight the lack of clear understanding of the underlying physics that dictates wetting behavior. In this work, dynamic contact angle measurements and detailed graphene surface characterizations were performed to demonstrate that the defects present in CVD grown and transferred graphene coatings result in unusually high contact angle hysteresis (16-37°) on these otherwise smooth surfaces. Hence, understanding the effect of the underlying substrate based on static contact angle measurements as reported in literature is insufficient. The advancing contact angle measurements on mono-, bi-, and trilayer graphene sheets on copper, thermally grown silica (SiO2), and glass substrates were observed to be independent of the number of layers of graphene and in good agreement with corresponding molecular dynamics simulations and theoretical calculations. Irrespective of the number of graphene layers, the advancing contact angle values were also in good agreement with the advancing contact angle on highly ordered pyrolytic graphite (HOPG), reaffirming the negligible effect of the underlying substrate. These results suggest that the advancing contact angle is a true representation of a graphene-coated surface while the receding contact angle is significantly influenced by intrinsic defects introduced during the growth and transfer processes. These observations, where the underlying substrates do not affect the wettability of graphene coatings, is shown to be due to the large interlayer spacing resulting from the loose interlamellar coupling between the graphene sheet and the underlying substrate. The fundamental insights on graphene-water interactions reported in this study is an important step towards developing graphene-assisted surface coatings for heat transfer and microfluidics devices.

Critical height of micro/nano structures for pool boiling heat transfer enhancement

Critical heat flux (CHF) enhancement by surface modifications has been an extensively researched area in pool boiling heat transfer. Here we report a fundamental mechanism of CHF enhancement where nano/micro ridges are fabricated on surfaces to fragment and evaporate the metastable non-evaporating/adsorbed film present at the base of a bubble in the contact line region. CHF increase of ∼125% is obtained with only ∼40% increase in surface area. An analytical model is extended to explain the CHF enhancement and to determine the average non-evaporating film thickness, which serves as the critical height for nano/micro structures for pool boiling heat transfer enhancement.

NANO- AND MICROSTRUCTURES FOR THIN-FILM EVAPORATION-A REVIEW

Evaporation from thin films is a key feature of many processes, including energy conversion, microelectronics cooling, boiling, perspiration, and self-assembly operations. The phase change occurring in these systems is governed by transport processes at the contact line where liquid, vapor, and solid meet. Evidence suggests that altering the surface chemistry and surface topography on the micro- and the nanoscales can be used to dramatically enhance vaporization. The 2013 International Workshop on Micro- and Nanostructures for Phase-Change Heat Transfer brought together a group of experts to review the current state-of-the-art and discuss future research needs. This article is focused on the thin-film evaporation panel discussion and outlines some of the key principles and conclusions reached by that panel and the workshop attendees.

Negative pressures in nanoporous membranes for thin film evaporation

We present a nanoporous membrane-based approach, which decouples the capillary pressure from the viscous resistance, to achieve high driving pressures and efficient liquid delivery for thin film evaporation. By using alumina membranes with ≈150 nm pore diameters, absolute liquid pressures as low as −300 kPa were achieved using isopropyl alcohol, while dissipating maximum interfacial heat fluxes of ≈96 W/cm2. Design guidelines are provided to achieve higher interfacial heat fluxes with reduced membrane thicknesses. This work shows a promising approach to address thermal management needs for next generation electronic devices.

A Possible Role of Nanostructured Ridges on Boiling Heat Transfer Enhancement

Evaporation of a nanoscale meniscus on a nanostructured heater surface is simulated using molecular dynamics. The nanostructures, evenly spaced on the surface, are ridges with a width and height of 0.55 nm and 0.96 nm, respectively. The simulation results show that the film breaks during the early stages of evaporation due to the presence of nanostructures and no nonevaporating film forms (unlike a previous simulation performed in the absence of nanostructures where nonevaporating film forms on the smooth surface). High heat transfer and evaporation rates are obtained. We conclude that heat transfer rates can be significantly increased during bubble nucleation and growth by the presence of nanostructure ridges on the surface as it can break the formation of nonevaporating film. This causes additional chaos and allows the surrounding cooler liquid to come in contact with the surface providing heat transfer enhancements.

Nanoscale liquid-vapor phase-change physics in nonevaporating region at the three-phase contact line

Nanoscale liquid film evaporation is usually associated with super-high heat transport rates and can be found in natural processes and in many industrial and advanced technologies. In this paper, thin film evaporation is simulated in a nanochannel using molecular dynamics to study the effect of varying nanochannel height and film thickness. Three nanochannel heights (16.32, 25.5, and 35.7 nm; constant liquid film thickness=3 nm) and three liquid film thicknesses (2, 4, and 6 nm; constant nanochannel height=25.5 nm) are simulated to study six cases. A nonevaporating film is obtained for all six cases. Hamaker constant, vapor pressure, film thickness, and net evaporation and heat fluxes are evaluated. An additional simulation (case 7) is run with simultaneous evaporation-condensation; no nonevaporating film is obtained. Thus, the creation of a nonevaporating film, and its thickness (if the film forms), depends on the combination of three factors, namely, vapor pressure, substrate temperature, and solid-liqu...

Effect of Hydrophilic Defects on Water Transport in MFI Zeolites

The subnanometer pore structure of zeolites and other microporous materials has been proposed to act as a molecular sieve for various water separation technologies. However, due to the increased interaction between the solid and water in these nanoconfined spaces, it is unclear which type of interface, be it hydrophilic or hydrophobic, offers an advantageous medium for enhancing transport properties. In this work, we probe the role of hydrophilic defects on the transport of water inside the microporous hydrophobic MFI zeolite pore structure via combined sorption and high-pressure infiltration experiments. While the inclusion of defects was observed to increase the amount of water within the zeolite pore network by up to 7 times at the saturation pressure, the diffusivity of this infiltrated water was lowered by up to 2 orders of magnitude in comparison to that of water within the nearly defect-free hydrophobic MFI zeolite. Subsequently, the permeability of water within the more defective MFI zeolite was an order of magnitude lower than that of the nearly defect-free zeolite. The results from these experiments suggest that the intrinsic hydrophobic pore structure of MFI zeolites can facilitate faster water transport due to the decreased attraction between the water and the defect-free surface. While the strong attraction of water to the defects allows for water to infiltrate the porous network at lower pressures, the results suggest that this strong attraction decreases the mobility of the infiltrated water. The insights gained from this study can be utilized to improve the design of future membranes for water desalination and other separation techniques.

Materials, Fabrication, and Manufacturing of Micro/Nanostructured Surfaces for Phase-Change Heat Transfer Enhancement

This article describes the most prominent materials, fabrication methods, and manufacturing schemes for micro- and nanostructured surfaces that can be employed to enhance phase-change heat transfer phenomena. The numerous processes include traditional microfabrication techniques such as thin-film deposition, lithography, and etching, as well as template-assisted and template-free nanofabrication techniques. The creation of complex, hierarchical, and heterogeneous surface structures using advanced techniques is also reviewed. Additionally, research needs in the field and future directions necessary to translate these approaches from the laboratory to high-performance applications are identified. Particular focus is placed on the extension of these techniques to the design of micro/nanostructures for increased performance, manufacturability, and reliability. The current research needs and goals are detailed, and potential pathways forward are suggested.

Early Evaporation of Microlayer for Boiling Heat Transfer Enhancement

For over five decades, an enhancement in pool boiling heat transfer has been achieved by altering the surface wetting, wickability, roughness, nucleation site density, and providing separate liquid/vapor pathways. In this work, a new enhancement mechanism based on the early evaporation of the microlayer is discovered and validated. The microlayer is a thin liquid film present at the base of a vapor bubble. The presence of microridges on the silicon dioxide surface partitions the microlayer and disconnects it from the bulk liquid, causing it to evaporate sooner, thus leading to increase in the bubble growth rate, heat transfer, departure frequency, and critical heat flux (CHF). Compared to a plain surface, an ∼120% enhancement in CHF is obtained with only an ∼18% increase in surface area. A CHF enhancement map is developed on the basis of the ridge height and spacing, resulting in three regions of full, partial, and no enhancement. The new mechanism is validated by comparing the growth rate of a laser-created vapor bubble on a ridge-structured surface and a plain surface, and the corresponding prediction of the CHF enhancement is found to be in good agreement with the experimental boiling data. This discovery opens up a new field of CHF enhancement and can potentially be coupled with existing techniques to further push the limits of boiling heat transfer.

Steady State Vapor Bubble in Pool Boiling.

Boiling, a dynamic and multiscale process, has been studied for several decades; however, a comprehensive understanding of the process is still lacking. The bubble ebullition cycle, which occurs over millisecond time-span, makes it extremely challenging to study near-surface interfacial characteristics of a single bubble. Here, we create a steady-state vapor bubble that can remain stable for hours in a pool of sub-cooled water using a femtosecond laser source. The stability of the bubble allows us to measure the contact-angle and perform in-situ imaging of the contact-line region and the microlayer, on hydrophilic and hydrophobic surfaces and in both degassed and regular (with dissolved air) water. The early growth stage of vapor bubble in degassed water shows a completely wetted bubble base with the microlayer, and the bubble does not depart from the surface due to reduced liquid pressure in the microlayer. Using experimental data and numerical simulations, we obtain permissible range of maximum heat transfer coefficient possible in nucleate boiling and the width of the evaporating layer in the contact-line region. This technique of creating and measuring fundamental characteristics of a stable vapor bubble will facilitate rational design of nanostructures for boiling enhancement and advance thermal management in electronics.