Evaporation-Induced Cassie Droplets on Superhydrophilic Microstructured Surfaces

A droplet deposited on a rough, lyophilic surface satisfying the imbibition condition, results in complete wetting. However, in this work, we demonstrate that this behavior can be altered by superheating the substrate such that droplets can reside in a non-wetting Cassie state due to evaporation. Photolithography and deep reactive ion etching were used to fabricate a well-defined silicon micropillar array with diameter, height, and center-to-center spacings of 5.3, 21.7 and 27.5 μm, respectively. Water droplets placed on this microstructured surface at room temperature demonstrated superhydrophilic behavior with liquid filling the voids between pillars resulting in a vanishing contact angle. However, when the microstructured surface was superheated above a critical value, the superhydrophilicity was lost and non-wetting Cassie droplets were formed. The superheat required to deposit a Cassie droplet (>75°C) was found to be significantly higher than that required to sustain an already deposited Cassie droplet (<35°C). Interestingly, the superheat required to sustain a Cassie droplet after the initial deposition was found to decrease with the square of the droplet radius. These observations where an inherently superhydrophilic structured surface turns into superhydrophobic at nominal superheats has implications for phase change based heat transfer applications where the loss of contact between the substrate and the heat transfer fluid can be detrimental to the device performance. Topics: Drops, EvaporationUnited States. Office of Naval Researc

APPLICATION OF MACHINE LEARNING TO CHF MODELLING

Accurate prediction of CHF is still a challenging issue in the study of boiling heat transfer. Many factors contribute to the occurrence of CHF and the various trigger mechanisms are proposed to unravel physical phenomena behind CHF. However, those mechanisms cannot cover the multiple primary factors simultaneously and even some of them still remain controversially unresolved. In light of the complexity and difficulty of CHF modelling, hereby an ensemble-learning based framework is proposed to model and predict CHF based on the databank of CHF. Some prior trials have been done for three primary aspects of dominant factors, that is, surface morphology, geometrical dimension and operation condition. These three primary constituents are respectively analyzed though three different sub-models of the ensemble framework in Chapter 3, 4 and 5. In Chapter Three, relevant experiments about micro-pillar enhanced CHF are reviewed and the corresponding databank of microstructure enhanced CHF is compiled based on those CHF experiments from published papers. Although the impacts of micro-pillars on CHF are still not clear, through qualitative analyses, the parametrical trends of CHF with respect to geometrical parameters of pillar array can be roughly foreseen. Meanwhile, this study also evaluates performance of prediction accuracy among four current physical models of microstructure-enhanced CHF. Comparative results show that two capillary wicking models have higher prediction accuracy. Particularly, a special terminology, zero-infinity convergence, is introduced to discuss the parametrical trends of CHF and qualitatively assess veracity of two capillary wicking models. Given the drawbacks of current physical models, the DBN is proposed to more accurately predict CHF and study parametric trends of CHF based on the microstructure enhanced CHF databank. Different from the training process of other regression modelling problems, constrained CHF points, which are artificially derived from the training data datasets, are required to be coupled with the raw training datasets for achieving the zero-infinity convergence of the DBN based CHF model, exhibiting accurate parametric trends of CHF and improving the prediction accuracy. This new training technique provides a new reliable solution to the similar constrained machine learning problems. Numerical results demonstrate that DBN can achieve the best performance of CHF prediction in terms of prediction accuracy. Through studying parametrical trends of CHF reveals that micro pillar arrays with the same parameters on heat transfer substrates with different dimensional sizes presents different CHF enhancement profiles. The presented methodology provides new insights for CHF modelling in pool boiling enhanced by other surface modification techniques, including porous layer coating, nanoparticle deposition, textured roughen, and nanowire fabrication. The effects of dimensions and materials of boiling surfaces on CHF are correlated and studied through the GRNN modelling in Chapter Four. Instead of inputting all parameters that indicate the thermal properties of materials into the trained model, the aggregated parameters from the primitive parameters of thermal properties, thermal activity and thermal diffusivity, are utilized as the input parameters of the trained model. This technique not only could capture the effects of thermal properties of materials on CHF effectively but also helps reduce the computational loads. The trained model shows the similar parametric trends of CHF to that of the traditional empirical correlation with respect to the thermal activity. If the thermal activity of heat transfer substrate is beyond a certain value, the corresponding effect of thermal activity will be absent, which somehow implies that the thickness of heat transfer substrate will not impact CHF after the asymptomatic thickness is reached. On the other hand, thermal diffusivity still affects CHF occurrence even if the effect of thermal activity is negligible. When coming to the effect of dimension size on CHF, it was found that when the side length of square heat transfer substrate is 5 times greater than the capillary length of working fluid, the CHF will be independent on the side length. Otherwise, CHF will be affected by the side length, and the influence of side length on CHF reaches ultimate if the side length of square boiling surface is exactly equal to the Raleigh-Plateau instability wavelength. This instability wavelength is only dependent on the thermal properties of working fluids, meaning that the optimal side length for CHF optimization is only related to the thermal properties of working fluid, namely, the surface tension, and the liquid and vapor densities of working fluid. In Chapter Five of this study, n-support vector machine is adopted to explore and study experimental strategies for the data-driven approaches of CHF look-up table construction, on the basis of sparingly-distributed experimental CHF data points. In the virtue of the CHF look-up table of Groeneveld et al (2007), those CHF data was used as the reference data of this research. In this data collection, CHF data of the subcooled flow boiling (Xe \u3c 0) is chosen to concentrate on the PWR steady-state condition because the in the normal operation of PWR, the system is under the subcooled flow boiling. The numerical results have demonstrated that ν-SVM trained by well sparsely-distributed training data in the parameter region of interest (pressure and mass flux) can yield a fairly acceptable degree of CHF prediction accuracy. Procuring training data points that can imply the parametric behaviors of CHF with respect to pressure and mass flux for support vector machine is the essential key of machine learning to achieving a high level of CHF prediction accuracy. For capturing the pressure-variant CHF behavior, training data that are in the proximity of the CHF inflection point significantly contribute to the improvement of prediction accuracy. Hence, training data preparation physics-informed with knowledge of CHF inflection points definitely augments the prediction accuracy of CHF. How the parametrical trends of CHF with respect to pressure and mass flux are close to the linear trends determines the level of prediction accuracy when lacking of a good spread of training data points. Besides, it is found that CHF extrapolation to a higher pressure with many data points collected at different low pressures can be effectively achieved by SVM if a few CHF data points are available under the high pressure, especially for PWR pressure of 15.5 MPa. This announces a possibility of strategic integration experiments between high pressure and low pressure, reducing experimental costs associated with the high pressure testing in terms of efforts and money. The proposed methodologies provides engineers and experimentalists with useful strategies to construct the look-up table tabulation of advanced cladding materials of ATFs. It is found out that there are multiple sub-problems that could be divided for CHF prediction and each sub-problem has its individual suitable machine learning model. Those prior work done by this study proves that the data-driven CHF modelling by sub-models can provide accurate CHF prediction under various scenarios and correct parametrical trends with respect to separate variables. Last but not least, another contribution of this thesis to the field of boiling heat transfer is that two databanks of experimental CHF data are compiled for the CHF enhancement by microstructures. The compiled databanks provide useful information and guidelines to the future design of surface structures that will possibly be applied to heat exchanger and nuclear fuel rod

Oxygen Reduction Mechanisms in Nanostructured La0.8Sr0.2MnO3 Cathodes for Solid Oxide Fuel Cells

In this work we outline the mechanisms contributing to the oxygen reduction reaction in nanostructured cathodes of LaSrMnO (LSM) for Solid Oxide Fuel Cells (SOFC). These cathodes, developed from LSM nanostructured tubes, can be used at lower temperatures compared to microstructured ones, and this is a crucial fact to avoid the degradation of the fuel cell components. This reduction of the operating temperatures stems mainly from two factors: (i) the appearance of significant oxide ion diffusion through the cathode material in which the nanostructure plays a key role and (ii) an optimized gas phase diffusion of oxygen through the porous structure of the cathode, which becomes negligible. A detailed analysis of our Electrochemical Impedance Spectroscopy supported by first-principles calculations point toward an improved overall cathodic performance driven by a fast transport of oxide ions through the cathode surface

Micro/Nano-engineered techniques for enhanced pool boiling heat transfer

Environmental aspects such as water treatment as well as military applications and thermal management emphasize on the need for next generation cooling technologies based on boiling heat transfer. Micro/nano enhanced surfaces have shown a great potential for the performance enhancement in the systems involving boiling phenomena. The lack of fully understanding the mechanisms responsible for the enhancement on these surfaces and scalability of these technologies for large and complex geometries over the wide range of materials are two main issues. The goals of this dissertation are to provide an understanding about the fundamentals of pool boiling heat transfer (BHT) and critical heat flux (CHF) mechanisms on engineered surfaces, to develop new techniques for surface alteration for BHT and CHF enhancement, and to propose novel, facile and scalable surfaces modification techniques for related industries. Surfaces with artificial cavities, surfaces with different wettability, and surfaces with different porosities were fabricated and tested to shed light into the fundamentals of surface/boiling interaction. In addition, 3-D foam-liked graphene and crenarchaeon Sulfolobus solfataricus P2 bio-coating surface modification techniques were proposed for BHT and CHF enhancement. For artificial cavities it was shown that CHF occurrence on the hydrophilic surfaces is mainly due to hydrodynamic instability, while dry-out is the dominant CHF mechanism on the hydrophobic surfaces. The obtained results imply that although the increase in hole diameter enhances CHF for all the fabricated samples, the effect of pitch size depends on surface wettability such that CHF increases and decreases with pitch size on the hydrophobic and hydrophilic surfaces, respectively. For biphilic surfaces, a novel and facile process flow for the fabrication of biphilic surfaces was proposed. It was shown that boiling heat transfer coefficient and CHF increased with A*=AHydrophobic/ATotal up to 38.46%. Surfaces with A*>38.46% demonstrated a decreasing trend in CHF and heat transfer coefficient enhancement, which is caused by earlier interaction of nucleated bubbles, thereby triggering the generation of vapor blanket at lower wall superheat temperatures. This ratio could serve as a valuable design guideline in the design and development of new generation thermal systems. Pool boiling on pHEMA coated surfaces with thicknesses of 50, 100 and 200 nm were used to study the effect of surface porosity and inclination angle on heat transfer and bubble departure process. According to obtained results, combination of the effects of the interaction between active nucleation sites, the increase in bubble generation frequency, and the increase in bubble interactions were presented as the reasons behind the enhancement in heat transfer on coated surfaces. It was observed that under an optimum condition for the inclination angle, the porous coating provides a suitable escape path for vapor phase, which results in space to be filled by the liquid phase thereby enabling liquid replenishment. Pool boiling experiments conducted on 3D foam-like graphene coated surfaces to show the effect of graphene coating thickness on the pool boiling heat transfer performance. According to the obtained results, 3D structure of the coating has a significant effect on pool boiling heat transfer mechanism. Factors such as pore shape and mechanical resonance of the 3D structure could be possible reasons for bubbling behavior in developed nucleate boiling. Furthermore it was found that there exists an optimum thickness of 3D graphene coatings, where the maximum heat transfer coefficient were achieved. This is mainly due to the trapped bubbles inside the porous medium, which affects the bubble dynamics involving bubble departure diameter and frequency. A novel coating, crenarchaeon Sulfolobus solfataricus P2 biocoatings, were proposed for the performance enhancement of heating and cooling devices, thermofluidic systems, batteries, and micro- and nanofluidic devices. These biocoatings have the potential for addressing high heat removal requirements in many applications involving heat and fluid flows. Pool boiling experiments were performed on biocoated surfaces with thicknesses of 1 and 2μm. The obtained results indicated that biocoated surfaces enhance boiling heat transfer by providing numerous nucleation site densities and by increasing bubble interaction on the superheated surface. Interconnected channels inside the porous coating, and capillary pumping enhance liquid transportation and reduce the liquid-vapor counter flow resistance, thereby delating CHF condition. There is a strong potential economic value of research performed in the framework of this thesis. Refrigeration, automotive/aerospace engineering, thermal management companies will benefit from the commercial development of the performed researc

Micro and nanostructured surfaces for enhanced phase change heat transfer

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2013.Cataloged from PDF version of thesis.Includes bibliographical references (p. 61-65).Two-phase microchannel heat sinks are of significant interest for thermal management applications, where the latent heat of vaporization offers an efficient method to dissipate large heat fluxes in a compact device. However, a significant challenge for the implementation of microchannel heat sinks is associated with flow instabilities due to insufficient bubble removal, leading to liquid dry-out which severely limits the heat removal efficiency. To address this challenge, we propose to incorporate micro/nanostructures to stabilize and enhance two-phase microchannel flows. Towards this goal, this thesis focuses on fundamental understanding of micro/nanostructures to manipulate liquid and vapor bubble dynamics, and to improve overall microchannel heat transfer performance. We first investigated the role of micro/nanostructure geometry on liquid transport behavior. We designed and fabricated asymmetric nanostructured surfaces where nanopillars are deflected with angles ranging from 7 -52'. Uni-directional liquid spreading was demonstrated where the liquid propagates in a single preferred direction and pins in all others. Through experiments and modeling, we determined that the spreading characteristic is dependent on the degree of nanostructure asymmetry, height-to-spacing ratio of the nanostructures, and intrinsic contact angle. The theory, based on an energy argument, provides excellent agreement with experimental data. This work shows a promising method to manipulate liquid spreading with structured surfaces, which potentially can also be used to manipulate vapor bubble dynamics. We subsequently investigated the effect of micro/nanostructured surface design on vapor bubble dynamics and pool boiling heat transfer. We fabricated micro-, nano-, and hierarchically-structured surfaces with a wide range of well-defined surface roughness factors and measured the heat transfer characteristics. The maximum critical heat flux (CHF) was ~250 W/cm2 with a roughness factor of~-13.3. We also developed a force-balance based model, which shows excellent agreement with the experiments. The results demonstrate the significant effect of surface roughness at capillary length scales on enhancing CHF. This work is an important step towards demonstrating the promising role of surface design for enhanced two-phase heat transfer. Finally, we investigated the heat transfer performance of microstructured surfaces incorporated in microchannel devices with integrated heaters and temperature sensors. We fabricated silicon micropillars with heights of 25 [mu]m, diameters of 5-10 [mu]m and spacings of 5- 10 [mu]m in microchannels of 500 [mu]m x 500 [mu]m. We characterized the performance of the microchannels with a custom closed loop test setup. This thesis provides improved fundamental understanding of the role of micro/nanostructures on liquid spreading and bubble dynamics as well as the practical implementation of such structures in microchannels for enhanced heat transfer. This work serves as an important step towards realizing high flux two-phase microchannel heat sinks for various thermal management applications.by Kuang-Han Chu.Ph.D

TRANSPORT PHENOMENA ASSOCIATED WITH LIQUID METAL FLOW OVER TOPOGRAPHICALLY MODIFIED SURFACES

Brazing and soldering, as advanced manufacturing processes, are of significant importance to industrial applications. It is widely accepted that joining by brazing or soldering is possible if a liquid metal wets the solids to be joined. Wetting, hence spreading and capillary action of liquid metal (often called filler) is of significant importance. Good wetting is required to distribute liquid metal over/between the substrate materials for a successful bonding. Topographically altered surfaces have been used to exploit novel wetting phenomena and associated capillary actions, such as imbibitions (a penetration of a liquid front over/through a rough, patterned surface). Modification of surface roughness may be considered as a venue to tune and control the spreading behavior of the liquids. Modeling of spreading of liquids on rough surface, in particular liquid metals is to a large extent unexplored and constitutes a cutting edge research topic. In this dissertation the imbibitions of liquid metal has been considered as pertained to the metal bonding processes involving brazing and soldering fillers. First, a detailed review of fundamentals and the recent progress in studies of non-reactive and reactive wetting/capillary phenomena has been provided. An imbibition phenomenon has been experimentally achieved for organic liquids and molten metals during spreading over topographically modified intermetallic surfaces. It is demonstrated that the kinetics of such an imbibition over rough surfaces follows the Washburn-type law during the main spreading stage. The Washburn-type theoretical modeling framework has been established for both isotropic and anisotropic non-reactive imbibition of liquid systems over rough surfaces. The rough surface domain is considered as a porous-like medium and the associated surface topographical features have been characterized either theoretically or experimentally through corresponding permeability, porosity and tortuosity. Phenomenological records and empirical data have been utilized to verify the constructed model. The agreement between predictions and empirical evidence appears to be good. Moreover, a reactive wetting in a high temperature brazing process has been studied for both polished and rough surfaces. A linear relation between the propagating triple line and the time has been established, with spreading dominated by a strong chemical reaction

Biomimetic adhesion for transfer printing via microstructured surfaces

Demand for robust engineering techniques on the micro and nano scales has been steadily growing in the age of modern technology, not only because of the driving force to fit electronics into smaller form factors, but also for a variety of other applications, from devices with microfluidic functions to components whose interfacial behaviors are key features. In our research we attempted to develop a tool that facilitates assembly of a wide variety of devices on both conventional and novel surfaces in the hopes of both improving modern capabilities of technological fabrication, as well as opening up possibilities for new classes of devices that can be easily assembled on surfaces and in form factors that were not previously possible. In summary, primary benefit of this technology is the potential ability to fabricate a variety of electronic devices on any surface – thus expanding the versatility and ability to integrate different classes of technology in way that is not possible using modern, competing fabrication methods for micro and nano-scale chemical/electronic/mechanical devices. In the first two chapters, I will discuss background information relating to the basis and motivation for this technology, beginning with a summary of adhesion – how different types of adhesion occur and what their applicability is, with a focus on dispersive, or van der Waals adhesion – followed by a discussion of the field of biomimetics and how the study of naturally occurring dry adhesion techniques employed by animals such as geckos and insects has inspired a field of research into the use of dispersive intermolecular forces as an engineering solution for limitations of nanofabrication and assembly. In the following chapters I will describe our own group’s design, fabrication, and iii testing of a variety of microstructured surfaces intended to control adhesive strength by increasing it and decreasing it, as needed. Finally, I will present the results of our experiments and draw conclusions about the effectiveness and future potential of transfer printing via kinetically controlled microstructured stamps

Evaporation-induced non-wetting droplets on superhydrophilic surfaces

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2012."September 2012." Cataloged from PDF version of thesis.Includes bibliographical references (p. 66-68).A droplet deposited on a rough, lyophilic surface satisfying the imbibition condition, results in spontaneous spreading and hence complete wetting. However, in this thesis, we demonstrate that this wetting behavior can be altered by superheating the substrate such that droplets can reside in a non-wetting state due to evaporation. Photolithography and deep reactive ion etching were used to fabricate well-defined silicon micropillar arrays with a square pattern with varying pillar diameter, height, and center-to-center spacing. Water droplets placed on these microstructured surfaces at room temperature demonstrated superhydrophilic behavior with liquid filling the voids between pillars resulting in very low contact angle, and hence complete wetting. However, when the microstructured surface was superheated above a critical superheat, the superhydrophilicity was lost and non-wetting droplets were formed on the top surface of the micropillar array structure. The superheat required to deposit a non-wetting droplet (> 75°C) was found to be significantly higher than that required to sustain an already deposited non-wetting droplet (< 35°C). Moreover, the superheat required to sustain a non-wetting droplet after the initial deposition was observed to decrease with the square of the droplet radius. A 1-D lubrication type model based on a force balance between the wetting and non-wetting forces is developed which explains the mechanism by which non-wetting droplets can reside on superhydrophilic surfaces at superheated conditions due to induced evaporation. Moreover, the model predicts a square relationship between the superheat required to form non-wetting droplets and the droplet radius which is in agreement with experimental observation. These observations where non-wetting droplets reside on superhydrophilic surfaces at superheated conditions have implications for phase-change based heat transfer applications where the loss of contact between the substrate and the heat transfer fluid could be detrimental to the device performance.by Solomon Adera.S.M