Y. Sungtaek Ju

Fabrication and Characterization of the Capillary Performance of Superhydrophilic Cu Micropost Arrays

We report the fabrication of dense arrays of super-hydrophilic Cu microposts at solid fractions as high as 58% and aspect ratios as high as four using electrochemical deposition and chemical oxidation techniques. Oxygen surface plasma treatments of photoresist molds and a precise control of the initial electrodeposition current are found to be critical in creating arrays of nearly defect-free Cu posts. The capillary performance of the micropost arrays is characterized using capillary rate of rise experiments and numerical simulations that account for the finite curvatures of liquid menisci. For the given wick morphology, the capillary performance generally decreases with increasing solid fraction and is enhanced by almost an order of magnitude when thin nanostructured copper oxide layers are formed on the post surface. The present work provides a useful starting point to achieve optimal balance between the capillary performance and the effective thermal conductivity of advanced wicks for micro heat pipes.

Phonon heat transport in silicon nanostructures

Phonon heat conduction is one of the critical research areas for nanoelectronics. The thermal conductivity of silicon nanostructures is studied to gain insight into heat conduction in silicon and related semiconductors. We experimentally show that phonon-boundary scattering results in a significant reduction in the thermal conductivity of crystalline silicon films of thickness below 100 nm at room temperature, which is consistent with the previously reported data for silicon nanowires. Analysis of the data suggests that phonon modes that dominate heat conduction in silicon are fully excited at temperatures substantially below the Debye temperature.

Bubble nucleation on hydrophobic islands provides evidence to anomalously high contact angles of nanobubbles

We observe stable steady-periodic vapor bubble nucleation on islands of nanoscopically smooth hydrophobic materials microfabricated on a silicon substrate. The minimum surface superheat required for the onset of bubble nucleation is very low (∼9 °C), which cannot be explained by the established models of heterogeneous bubble nucleation. A modified bubble nucleation model indicates that the observed minimum superheat can be explained when one assumes the existence of a nanoscale interfacial gas phase with anomalously high contact angles (>160°). Our data therefore provide independent evidence that supports previous atomic force microscopy and infrared spectroscopy studies of the topography of nanobubbles.

A comparative study of the morphology and wetting characteristics of micro/nanostructured Cu surfaces for phase change heat transfer applications

A comparative study of oxidation methods to create Cu surfaces with controlled wettability is reported. Micro/nanostructures of Cu oxides are formed on Cu substrates using different chemical and thermal oxidation methods. The morphology and wetting characteristics of the resulting surfaces are characterized using atomic force microscopy, scanning electron microscopy, X-ray diffraction, and contact angle measurements. Chemical oxidation in alkali solutions can form uniform copper oxide layers with high roughness factors without causing thermal stress problems that often hamper thermal oxidation. By combining chemical oxidation with a hydrophobic coating, a wide range of wettability control is demonstrated from superhydrophilic (  < 10°) to superhydrophobic (  > 170°). Superhydrophilic CuO layers uniformly formed on Cu powder and Cu micropost wick surfaces lead to significant improvement in the capillary and heat transfer performance compared with comparable unoxidized Cu wicks. The present work motivates further studies to exploit the benefits of nanostructured Cu surfaces in various phase change heat transfer applications.

Nanoscale Heat Conduction Across Metal-Dielectric Interfaces

We report a theoretical study of heat conduction across metal-dielectric interfaces in devices and structures of practical interest. At cryogenic temperatures, the thermal interface resistance between electrodes and a substrate is responsible for substantial reduction in the maximum permissible peak power in Josephson junctions. The thermal interface resistance is much smaller at elevated temperatures but it still plays a critical role in nanoscale devices and structures, especially nanolaminates that consist of alternating metal and dielectric layers. A theoretical model is developed to elucidate the impact of spatial nonequilibrium between electrons and phonons on heat conduction across nanolaminates. The diffuse mismatch model is found to provide reasonable estimates of the intrinsic thermal interface resistance near room temperature as well as at cryogenic temperatures.

Development and characterization of a microfluidic chamber incorporating fluid ports with active suction for localized chemical stimulation of brain slices

We report a novel microfluidic chamber incorporating fluid ports with active suction to achieve localized chemical stimulation of brain slices. A two-level soft-lithography process is used to fabricate fluid ports with integrated injection and suction holes that are connected to underlying microchannels. Fluorescence imaging, particle tracking velocimetry, and cell staining are used to characterize flows around a fluid port with or without active suction to validate effective localization of injected chemicals. To demonstrate biological applicability of the chamber, we show an induction of cortical spreading depression (CSD) waves in mouse brain slices through controlled focal delivery of potassium chloride solution.

Reversible thermal interfaces based on microscale dielectric liquid layers

We present a reversible thermal interface that can circumvent limitations of direct solid-solid contacts. A thin continuous layer of a dielectric liquid is formed between two solid substrates to provide a low-resistance heat conduction path. The liquid is initially confined in an array of discrete microchannels and undergoes reversible morphological transition into a continuous film as the loading pressure is increased. We theoretically and experimentally determine the relationship between loading pressure and liquid morphology. The interfaces can achieve thermal resistance comparable to that of solid-solid contacts but at loading pressures orders of magnitude smaller.

EWOD (electrowetting on dielectric) digital microfluidics powered by finger actuation

We report finger-actuated digital microfluidics (F-DMF) based on the manipulation of discrete droplets via the electrowetting on dielectric (EWOD) phenomenon. Instead of requiring an external power supply, our F-DMF uses piezoelectric elements to convert mechanical energy produced by human fingers to electric voltage pulses for droplet actuation. Voltage outputs of over 40 V are provided by single piezoelectric elements, which is necessary for oil-free EWOD devices with thin (typically <1 μm) dielectric layers. Higher actuation voltages can be provided using multiple piezoelectric elements connected in series when needed. Using this energy conversion scheme, we confirmed basic modes of EWOD droplet operation, such as droplet transport, splitting and merging. Using two piezoelectric elements in series, we also successfully demonstrated applications of F-DMF for glucose detection and immunoassay. Not requiring power sources, F-DMF offers intriguing paths for various portable and other microfluidic applications.

Cortical Sensory Plasticity in a Model of Migraine with Aura

The migraine attack is characterized by alterations in sensory perception, such as photophobia or allodynia, which have in common an uncomfortable amplification of the percept. It is not known how these changes arise. We evaluated the ability of cortical spreading depression (CSD), the proposed mechanism of the migraine aura, to shape the cortical activity that underlies sensory perception. We measured forepaw- and hindpaw-evoked sensory responses in rat, before and after CSD, using multielectrode array recordings and two-dimensional optical spectroscopy. CSD significantly altered cortical sensory processing on a timescale compatible with the duration of the migraine attack. Both electrophysiological and hemodynamic maps had a reduced surface area (were sharpened) after CSD. Electrophysiological responses were potentiated at the receptive field center but suppressed in surround regions. Finally, the normal adaptation of sensory-evoked responses was attenuated at the receptive field center. In summary, we show that CSD induces changes in the evoked cortical response that are consistent with known mechanisms of cortical plasticity. These mechanisms provide a novel neurobiological substrate to explain the sensory alterations of the migraine attack.

Impact of Nonequilibrium Between Electrons and Phonons on Heat Transfer in Metallic Nanoparticles Suspended in Dielectric Media

Controlled heating of nanoparticles is a key enabling technology for various nanomanufacturing and biomedical applications. A theoretical study of energy transport in nanoparticles is conducted to elucidate the role of electron-phonon spatial nonequilibrium in heat conduction across metal-dielectric interfaces. The continuum two-temperature heat conduction model is shown to capture the apparent size dependence of the thermal interface resistance of An nanoparticle suspensions. Consideration of coupling between electrons and atomic vibrations is important in understanding energy transport in nanoscale metallic structures suspended in a dielectric medium.