Yangying Zhu

Unified Model for Contact Angle Hysteresis on Heterogeneous and Superhydrophobic Surfaces

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.

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.

A Particle Resuspension Model in Ventilation Ducts

A turbulent burst model is used and evaluated for particle resuspension in ventilation ducts in this study. The model is a prolongation of the turbulent burst model developed by Cleaver and Yates with an approach to the critical jump-start air velocity for particle resuspension. This critical jump-start air velocity is introduced to estimate the fraction of particles resuspended under the turbulent burst. The model results were compared with experimental data that is available in literature. Both the model results and experiments show that resuspension rate increased with the increase of particle diameter and air speed in ducts. However, the model results did not show a significant decay of particle resuspension rate over time, which was shown in the experiments. Limitations of the model are discussed to explain the discrepancy between the model and the experimental results. Copyright 2012 American Association for Aerosol Research

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.

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.

Electrowetting-on-dielectric actuation of a vertical translation and angular manipulation stage

Adhesion and friction during physical contact of solid components in microelectromechanical systems (MEMS) often lead to device failure. Translational stages that are fabricated with traditional silicon MEMS typically face these tribological concerns. This work addresses these concerns by developing a MEMS vertical translation, or focusing, stage that uses electrowetting-on-dielectric (EWOD) as the actuating mechanism. EWOD has the potential to eliminate solid-solid contact by actuating through deformation of liquid droplets placed between the stage and base to achieve stage displacement. Our EWOD stage is capable of linear spatial manipulation with resolution of 10 μm over a maximum range of 130 μm and angular deflection of approximately ±1°, comparable to piezoelectric actuators. We also developed a model that suggests a higher intrinsic contact angle on the EWOD surface can further improve the translational range, which was validated experimentally by comparing different surface coatings. The capabilit...

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.