Doctor of Philosophy, Massachusetts Institute of Technology (2017)
- Suppressing high-frequency temperature oscillations in microchannels with surface structures APPLIED PHYSICS LETTERS 2017; 110 (3)
- Electrowetting-on-dielectric actuation of a vertical translation and angular manipulation stage APPLIED PHYSICS LETTERS 2016; 109 (24)
- Surface Structure Enhanced Microchannel Flow Boiling JOURNAL OF HEAT TRANSFER-TRANSACTIONS OF THE ASME 2016; 138 (9)
Prediction and Characterization of Dry-out Heat Flux in Micropillar Wick Structures
2016; 32 (7): 1920-1927
Thin-film evaporation in wick structures for cooling high-performance electronic devices is attractive because it harnesses the latent heat of vaporization and does not require external pumping. However, optimizing the wick structures to increase the dry-out heat flux is challenging due to the complexities in modeling the liquid-vapor interface and the flow through the wick structures. In this work, we developed a model for thin-film evaporation from micropillar array wick structures and validated the model with experiments. The model numerically simulates liquid velocity, pressure, and meniscus curvature along the wicking direction by conservation of mass, momentum, and energy based on a finite volume approach. Specifically, the three-dimensional meniscus shape, which varies along the wicking direction with the local liquid pressure, is accurately captured by a force balance using the Young-Laplace equation. The dry-out condition is determined when the minimum contact angle on the pillar surface reaches the receding contact angle as the applied heat flux increases. With this model, we predict the dry-out heat flux on various micropillar structure geometries (diameter, pitch, and height) in the length scale range of 1-100 μm and discuss the optimal geometries to maximize the dry-out heat flux. We also performed detailed experiments to validate the model predictions, which show good agreement. This work provides insights into the role of surface structures in thin-film evaporation and offers important design guidelines for enhanced thermal management of high-performance electronic devices.
View details for DOI 10.1021/acs.langmuir.5b04502
View details for Web of Science ID 000370987200028
View details for PubMedID 26808963
Dynamic Evolution of the Evaporating Liquid Vapor Interface in Micropillar Arrays
2016; 32 (2): 519-526
Capillary assisted passively pumped thermal management devices have gained importance due to their simple design and reduction in energy consumption. The performance of these devices is strongly dependent on the shape of the curved interface between the liquid and vapor phases. We developed a transient laser interferometry technique to investigate the evolution of the shape of the liquid-vapor interface in micropillar arrays during evaporation heat transfer. Controlled cylindrical micropillar arrays were fabricated on the front side of a silicon wafer, while thin-film heaters were deposited on the reverse side to emulate a heat source. The shape of the meniscus was determined using the fringe patterns resulting from interference of a monochromatic beam incident on the thin liquid layer. We studied the evolution of the shape of the meniscus on these surfaces under various operating conditions including varying the micropillar geometry and the applied heating power. By monitoring the transient behavior of the evaporating liquid-vapor interface, we accurately measured the absolute location and shape of the meniscus and calculated the contact angle and the maximum capillary pressure. We demonstrated that the receding contact angle which determines the capillary pumping limit is independent of the microstructure geometry and the rate of evaporation (i.e., the applied heating power). The results of this study provide fundamental insights into the dynamic behavior of the liquid-vapor interface in wick structures during phase-change heat transfer.
View details for DOI 10.1021/acs.langmuir.5b03916
View details for Web of Science ID 000368563700016
View details for PubMedID 26684395
Nanoengineered materials for liquid–vapour phase-change heat transfer
Nature Reviews Materials
View details for DOI 10.1038/natrevmats.2016.92
Real-Time Manipulation with Magnetically Tunable Structures
2014; 26 (37): 6442-6446
Magnetically tunable micropillar arrays with uniform, continuous and extreme tilt angles for real-time manipulation are reported. We experimentally show uniform tilt angles ranging from 0° to 57°, and develop a model to accurately capture the behavior. Furthermore, we demonstrate that the flexible uniform responsive microstructures (μFUR) can dynamically manipulate liquid spreading directionality, control fluid drag, and tune optical transmittance over a large range.
View details for DOI 10.1002/adma.201401515
View details for Web of Science ID 000342843600010
View details for PubMedID 25047631
Unified Model for Contact Angle Hysteresis on Heterogeneous and Superhydrophobic Surfaces
2012; 28 (45): 15777-15788
Understanding the complexities associated with contact line dynamics on chemically heterogeneous and superhydrophobic surfaces is important for a wide variety of engineering problems. Despite significant efforts to capture the behavior of a droplet on these surfaces over the past few decades, modeling of the complex dynamics at the three-phase contact line is needed. In this work, we demonstrate that contact line distortion on heterogeneous and superhydrophobic surfaces is the key aspect that needs to be accounted for in the dynamic droplet models. Contact line distortions were visualized and modeled using a thermodynamic approach to develop a unified model for contact angle hysteresis on chemically heterogeneous and superhydrophobic surfaces. On a surface comprised of discrete wetting defects on an interconnected less wetting area, the advancing contact angle was determined to be independent of the defects, while the relative fraction of the distorted contact line with respect to the baseline surface was shown to govern the receding contact angle. This behavior reversed when the relative wettability of the discrete defects and interconnected area was inverted. The developed model showed good agreement with the experimental advancing and receding contact angles, both at low and high solid fractions. The thermodynamic model was further extended to demonstrate its capability to capture droplet shape evolution during liquid addition and removal in our experiments and those in literature. This study offers new insight extending the fundamental understanding of solid-liquid interactions required for design of advanced functional coatings for microfluidics, biological, manufacturing, and heat transfer applications.
View details for DOI 10.1021/la303070s
View details for Web of Science ID 000311191300005
View details for PubMedID 23057739
- A Particle Resuspension Model in Ventilation Ducts AEROSOL SCIENCE AND TECHNOLOGY 2012; 46 (2): 222-235