Dion S. Antao

Real‐Time Manipulation with Magnetically Tunable Structures

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.

Surface Structure Enhanced Microchannel Flow Boiling

We investigated the role of surface microstructures in two-phase microchannels on suppressing flow instabilities and enhancing heat transfer. We designed and fabricated microchannels with well-defined silicon micropillar arrays on the bottom heated microchannel wall to promote capillary flow for thin film evaporation while facilitating nucleation only from the sidewalls. Our experimental results show significantly reduced temperature and pressure drop fluctuation especially at high heat fluxes. A critical heat flux (CHF) of 969 W/cm2 was achieved with a structured surface, a 57% enhancement compared to a smooth surface. We explain the experimental trends for the CHF enhancement with a liquid wicking model. The results suggest that capillary flow can be maximized to enhance heat transfer via optimizing the microstructure geometry for the development of high performance two-phase microchannel heat sinks.

Prediction and Characterization of Dry-out Heat Flux in Micropillar Wick Structures

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.

High amplitude nonlinear acoustic wave driven flow fields in cylindrical and conical resonators

A high fidelity computational fluid dynamic model is used to simulate the flow, pressure, and density fields generated in a cylindrical and a conical resonator by a vibrating end wall/piston producing high-amplitude standing waves. The waves in the conical resonator are found to be shock-less and can generate peak acoustic overpressures that exceed the initial undisturbed pressure by two to three times. A cylindrical (consonant) acoustic resonator has limitations to the output response observed at one end when the opposite end is acoustically excited. In the conical geometry (dissonant acoustic resonator) the linear acoustic input is converted to high energy un-shocked nonlinear acoustic output. The model is validated using past numerical results of standing waves in cylindrical resonators. The nonlinear nature of the harmonic response in the conical resonator system is further investigated for two different working fluids (carbon dioxide and argon) operating at various values of piston amplitude. The high amplitude nonlinear oscillations observed in the conical resonator can potentially enhance the performance of pulse tube thermoacoustic refrigerators and these conical resonators can be used as efficient mixers.

Dynamic Evolution of the Evaporating Liquid–Vapor Interface in Micropillar Arrays

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.

Design of Lubricant Infused Surfaces

Lubricant infused surfaces (LIS) are a recently developed and promising approach to fluid repellency for applications in biology, microfluidics, thermal management, lab-on-a-chip, and beyond. The design of LIS has been explored in past work in terms of surface energies, which need to be determined empirically for each interface in a given system. Here, we developed an approach that predicts a priori whether an arbitrary combination of solid and lubricant will repel a given impinging fluid. This model was validated with experiments performed in our work as well as in literature and was subsequently used to develop a new framework for LIS with distinct design guidelines. Furthermore, insights gained from the model led to the experimental demonstration of LIS using uncoated high-surface-energy solids, thereby eliminating the need for unreliable low-surface-energy coatings and resulting in LIS repelling the lowest surface tension impinging fluid (butane, γ ≈ 13 mN/m) reported to date.

Heat Transfer Enhancement During Water and Hydrocarbon Condensation on Lubricant Infused Surfaces

Vapor condensation is routinely used as an effective means of transferring heat or separating fluids. Dropwise condensation, where discrete droplets form on the condenser surface, offers a potential improvement in heat transfer of up to an order of magnitude compared to filmwise condensation, where a liquid film covers the surface. Low surface tension fluid condensates such as hydrocarbons pose a unique challenge since typical hydrophobic condenser coatings used to promote dropwise condensation of water often do not repel fluids with lower surface tensions. Recent work has shown that lubricant infused surfaces (LIS) can promote droplet formation of hydrocarbons. In this work, we confirm the effectiveness of LIS in promoting dropwise condensation by providing experimental measurements of heat transfer performance during hydrocarbon condensation on a LIS, which enhances heat transfer by ≈450% compared to an uncoated surface. We also explored improvement through removal of noncondensable gases and highlighted a failure mechanism whereby shedding droplets depleted the lubricant over time. Enhanced condensation heat transfer for low surface tension fluids on LIS presents the opportunity for significant energy savings in natural gas processing as well as improvements in thermal management, heating and cooling, and power generation.

Suppressing high-frequency temperature oscillations in microchannels with surface structures

Two-phase microchannel heat sinks are attractive for thermal management of high heat flux electronic devices, yet flow instability which can lead to thermal and mechanical fatigue remains a significant challenge. Much work has focused on long-timescale (∼seconds) flow oscillations which are usually related to the compressible volume in the loop. The rapid growth of vapor bubbles which can also cause flow reversal, however, occurs on a much shorter timescale (∼tens of milliseconds). While this high-frequency oscillation has often been visualized with high-speed imaging, its effect on the instantaneous temperature has not been fully investigated due to the typical low sampling rates of the sensors. Here, we investigate the temperature response as a result of the high-frequency flow oscillation in microchannels and the effect of surface microstructures on this temperature oscillation with a measurement data acquisition rate of 1000 Hz. For smooth surface microchannels, fluid flow oscillated between complete ...

Coexistence of Pinning and Moving on a Contact Line

Textured surfaces are instrumental in water repellency or fluid wicking applications, where the pinning and depinning of the liquid-gas interface plays an important role. Previous work showed that a contact line can exhibit nonuniform behavior due to heterogeneities in surface chemistry or roughness. We demonstrate that such nonuniformities can be achieved even without varying the local energy barrier. Around a cylindrical pillar, an interface can reside in an intermediate state where segments of the contact line are pinned to the pillar top while the rest of the contact line moves along the sidewall. This partially pinned mode is due to the global nonaxisymmetric pattern of the surface features and exists for all textured surfaces, especially when superhydrophobic surfaces are about to be flooded or when capillary wicks are close to dryout.

Reducing instability and enhancing critical heat flux using integrated micropillars in two-phase microchannel heat sinks

We present a novel design of two-phase microchannel heat sink with integrated micropillars on the bottom heated surface. The microchannel can achieve significantly reduced flow boiling instability, and an enhanced heat transfer coefficient (40%) and critical heat flux (17%) compared to that without micropillars. In this design, the liquid film on the heated surface is sustained due to the capillary force within the micropillars and thin film evaporation dominates the heat transfer. The experimental results indicate that the capillary pressure can be maximized without introducing large viscous drag when the microstructure geometry is optimized. The insights gained from this work guide the design of stable, high performance two-phase microchannel heat sinks.