Flightless-I Controls Fat Storage in Drosophila

Triglyceride homeostasis is a key process of normal development and is essential for the maintenance of energy metabolism. Dysregulation of this process leads to metabolic disorders such as obesity and hyperlipidemia. Here, we report a novel function of the Drosophila flightless-I (fliI) gene in lipid metabolism. Drosophila fliI mutants were resistant to starvation and showed increased levels of triglycerides in the fat body and intestine, whereas fliI overexpression decreased triglyceride levels. These flies suffered from metabolic stress indicated by increased levels of trehalose in hemolymph and enhanced phosphorylation of eukaryotic initiation factor 2 alpha (eIF2??). Moreover, upregulation of triglycerides via a knockdown of fliI was reversed by a knockdown of desat1 in the fat body of flies. These results indicate that fliI suppresses the expression of desat1, thereby inhibiting the development of obesity; fliI may, thus, serve as a novel therapeutic target in obesity and metabolic diseases

Mechanistic Understanding and Enhancing Pool Boiling Heat Transfer via Surface Property and Structure Design

Boiling is a vital process used to transfer heat effectively via harnessing the large latent heat of vaporization for a variety of energy and thermal management applications. The boiling heat transfer performance is described mainly by critical heat flux (CHF) and heat transfer coefficient (HTC), which quantifies the operational heat flux limit and the efficiency of boiling heat transfer, respectively. The goal of this thesis is two-fold: fundamental understanding on the mechanisms associated with CHF and significantly enhancing pool boiling heat transfer. First, we addressed the large discrepancy of experimental CHF values on flat surfaces reported in the literature by accounting for hydrocarbon adsorption and oxidation of metallic surfaces during boiling. Accordingly, we developed an experimental protocol based on this understanding on the causes of spread in CHF values and used the protocol throughout this thesis for consistent experimental measurements. We subsequently investigated the effects of surface structures on enhanced CHF during pool boiling of hemi-wicking surfaces. We systematically designed micropillar surfaces with controlled roughness and wickability, and combined the results with scaling analysis to obtain a unified descriptor for CHF. This unified descriptor represents the combined effects of the extended contact line length and volumetric wicking rate, which shows a reasonable correlation with CHF values with our experiments and literature data. Next, we engineered boiling surfaces to achieve simultaneous CHF and HTC enhancements. We developed a microtube structure, where a cavity is defined at the center of a pillar, to enhance the heat transfer characteristics in controllable manner. In addition to uniform microtube arrays, we designed a surface with microtube clusters interspersed with micropillars, referred to as tube-clusters in pillars (TIP), to mitigate the earlier boiling crisis of uniform microtube arrays due to the extensive bubble coalescence. While uniform microtube arrays and TIP surfaces showed significant enhancement of both CHF and HTC compared to a flat surface, there was an intrinsic trade-off between CHF and HTC associated with the nucleation site density. Accordingly, we proposed hierarchical TIP (h-TIP) surfaces to control vapor nucleation with multi-scale structures while providing capillary wicking. These surfaces showed CHF and HTC enhancements up to 138 and 389%, respectively, compared to a flat surface. Finally, we investigated the use of sandblasting as a scalable surface engineering technique for enhanced pool boiling heat transfer for industry-scale applications. Pool boiling results along with surface characterizations on silicon surfaces showed that surface roughness and volumetric wicking rates increased with the sandblasting abrasive size. As a result, CHF and HTC values enhanced up to 192.6 and 433.6% compared to a flat surface, respectively. This thesis provides important insights to understand the role of surface properties and structures on pool boiling heat transfer, thereby providing guidelines for the systematic design of surface structures for enhanced pool boiling heat transfer.Ph.D

Criteria for antibubble formation from drop pairs impinging on a free surface

© 2020 American Physical Society. Antibubbles are fluid entities with the inverse phase of regular bubbles. While the structure and stability of antibubbles have been studied, a fundamental understanding of antibubble formation remains limited. We report a theoretical and experimental study of antibubble formation. In the experiment, pairs of surfactant-laden water drops impinged successively on the surface of the same liquid reservoir to create antibubbles. We propose four criteria for antibubble formation from a scaling analysis. Two dimensionless groups prescribe the likelihood of antibubble formation, the summative Weber number and the ratio of timescales between the capillarity driven pinch-off and the viscous drainage of air

Criteria for antibubble formation from drop pairs impinging on a free surface

Antibubbles are fluid entities with the inverse phase of regular bubbles. While the structure and stability of antibubbles have been studied, a fundamental understanding of antibubble formation remains limited. We report a theoretical and experimental study of antibubble formation. In the experiment, pairs of surfactant-laden water drops impinged successively on the surface of the same liquid reservoir to create antibubbles. We propose four criteria for antibubble formation from a scaling analysis. Two dimensionless groups prescribe the likelihood of antibubble formation, the summative Weber number and the ratio of timescales between the capillarity driven pinch-off and the viscous drainage of air

Microtube Surfaces for the Simultaneous Enhancement of Efficiency and Critical Heat Flux during Pool Boiling

Boiling is an essential process in numerous applications including power plants, thermal management, water purification, and steam generation. Previous studies have shown that surfaces with microcavities or biphilic wettability can enhance the efficiency of boiling heat transfer, that is, the heat transfer coefficient (HTC). Surfaces with permeable structures such as micropillar arrays, in contrast, have shown significant enhancement of the critical heat flux (CHF). In this work, we investigated microtube structures, where a cavity is defined at the center of a pillar, as structural building blocks to enhance HTC and CHF simultaneously in a controllable manner. We demonstrated simultaneous CHF and HTC enhancements of up to 62 and 244%, respectively, compared to those of a smooth surface. The experimental data along with high-speed images elucidate the mechanism for simultaneous enhancement where bubble nucleation occurs in the microtube cavities for increased HTC and microlayer evaporation occurs around microtube sidewalls for increased CHF. Furthermore, we combined micropillars and microtubes to create surfaces that further increased CHF by achieving a path to separate nucleating bubbles and rewetting liquids. This work provides guidelines for the systematic surface design for boiling heat transfer enhancement and has important implications for understanding boiling heat transfer mechanisms

Enhanced Laplace Pressures for Functional Surfaces: Wicking, Switchability, and Selectivity

Wetting functionalities of rough surfaces are largely determined by the Laplace pressure generated across liquid–gas interfaces formed within surface structures. Typically, rough wetting surfaces create negative Laplace pressures, enabling capillary wicking, while rough non-wetting surfaces create positive Laplace pressures, exhibiting fluid repellency. Here, with microfabricated reentrant structures, it is shown that the same surface can exhibit either a negative or positive Laplace pressure, regardless of its intrinsic wettability. This material-independent Laplace pressure duality enables or enhances a range of wetting functionalities including wicking, switchability, and selectivity. On the same surface, capillary rise, capillary dip, and the combination of the two which leads to further enhancement of the total sustainable capillary height and Laplace pressure, the driving force for wicking is demonstrated. Further, active switching of wetting states between the hemiwicking and the repellent Cassie state on reentrant structures is shown. Moreover, with a water-hexane mixture system, selective wetting of reentrant structures are demonstrated, that is, water can be selectively wicked or repelled in the presence of hexane, and vice versa. These functionalities are achieved, which would typically require complex chemical coatings, solely using surface structures, thus largely expanding the design space for a wide range of thermofluidic applications

Enhanced Laplace Pressures for Functional Surfaces: Wicking, Switchability, and Selectivity

Abstract Wetting functionalities of rough surfaces are largely determined by the Laplace pressure generated across liquid–gas interfaces formed within surface structures. Typically, rough wetting surfaces create negative Laplace pressures, enabling capillary wicking, while rough non‐wetting surfaces create positive Laplace pressures, exhibiting fluid repellency. Here, with microfabricated reentrant structures, it is shown that the same surface can exhibit either a negative or positive Laplace pressure, regardless of its intrinsic wettability. This material‐independent Laplace pressure duality enables or enhances a range of wetting functionalities including wicking, switchability, and selectivity. On the same surface, capillary rise, capillary dip, and the combination of the two which leads to further enhancement of the total sustainable capillary height and Laplace pressure, the driving force for wicking is demonstrated. Further, active switching of wetting states between the hemiwicking and the repellent Cassie state on reentrant structures is shown. Moreover, with a water‐hexane mixture system, selective wetting of reentrant structures are demonstrated, that is, water can be selectively wicked or repelled in the presence of hexane, and vice versa. These functionalities are achieved, which would typically require complex chemical coatings, solely using surface structures, thus largely expanding the design space for a wide range of thermofluidic applications