Sidy Ndao

Extraordinary shifts of the Leidenfrost temperature from multiscale micro/nanostructured surfaces.

In the present work, the effects of surface chemistry and micro/nanostructuring on the Leidenfrost temperature are experimentally investigated. The functional surfaces were fabricated on a 304 stainless steel surface via femtosecond laser surface processing (FLSP). The droplet lifetime experimental method was employed to determine the Leidenfrost temperature for both machine-polished and textured surfaces. A precision dropper was used to control the droplet size to 4.2 μL and surface temperatures were measured by means of an embedded thermocouple. Extraordinary shifts in the Leidenfrost temperatures, as high as 175 °C relative to the polished surface, were observed with the laser-processed surfaces. These extraordinary shifts were attributed to nanoporosity, reduction in contact angle, intermittent liquid/solid contacts, and capillary wicking actions resulting from the presence of self-assembled nanoparticles formed on the surfaces. In addition to the shift in the Leidenfrost temperature, significant enhancement of the heat transfer in the film boiling regime was also observed for the laser-processed surfaces; water droplet evaporation times were reduced by up to 33% for a surface temperature of 500 °C.

Enhanced pool-boiling heat transfer and critical heat flux on femtosecond laser processed stainless steel surfaces

In this paper, we present an experimental investigation of pool boiling heat transfer on multiscale (micro/nano) functionalized metallic surfaces. Heat transfer enhancement in metallic surfaces is very important for large scale high heat flux applications like in the nuclear power industry. The multiscale structures were fabricated via a femtosecond laser surface process (FLSP) technique, which forms self-organized mound-like microstructures covered by layers of nanoparticles. Using a pool boiling experimental setup with deionized water as the working fluid, both the heat transfer coefficients and critical heat flux were investigated. A polished reference sample was found to have a critical heat flux of 91 W/cm2 at 40 °C of superheat and a maximum heat transfer coefficient of 23,000 W/m2 K. The processed samples were found to have a maximum critical heat flux of 142 W/cm2 at 29 °C and a maximum heat transfer coefficient of 67,400 W/m2 K. It was found that the enhancement of the critical heat flux was directly related to the wetting and wicking ability of the surface which acts to replenish the evaporating liquid and delay critical heat flux. The heat transfer coefficients were also found to increase when the surface area ratio was increased as well as the microstructure peak-to-valley height. Enhanced nucleate boiling is the main heat transfer mechanism, and is attributed to an increase in surface area and nucleation site density.

Flow Boiling of R134a in Circular Microtubes—Part I: Study of Heat Transfer Characteristics

An experimental study of two-phase heat transfer coefficients was carried out using R134a in uniformly heated horizontal circular microtubes with diameters from 0.50 mm to 1.60 mm over a range of mass fluxes, heat fluxes, saturation pressures, and vapor qualities. Heat transfer coefficients increased with increasing heat flux and saturation pressure but were independent of mass flux. The effects of vapor quality on heat transfer coefficients were less pronounced and varied depending on the quality. The data were compared with seven flow boiling correlations. None of the correlations predicted the experimental data very well, although they generally predicted the correct trends within limits of experimental error. A correlation was developed, which predicted the heat transfer coefficients with a mean average error of 29%. 80% of the data points were within the ±30% error limit.

Near-field NanoThermoMechanical memory

In this letter, we introduce the concept of NanoThermoMechanical Memory. Unlike electronic memory, a NanoThermoMechanical memory device uses heat instead of electricity to record, store, and recover data. Memory function is achieved through the coupling of near-field thermal radiation and thermal expansion resulting in negative differential thermal resistance and thermal latching. Here, we demonstrate theoretically via numerical modeling the concept of near-field thermal radiation enabled negative differential thermal resistance that achieves bistable states. Design and implementation of a practical silicon based NanoThermoMechanical memory device are proposed along with a study of its dynamic response under write/read cycles. With more than 50% of the worlds energy losses being in the form of heat along with the ever increasing need to develop computer technologies which can operate in harsh environments (e.g., very high temperatures), NanoThermoMechanical memory and logic devices may hold the answer.

Self-propelled droplets on heated surfaces with angled self-assembled micro/nanostructures

Directional and ratchet-like functionalized surfaces can induce liquid transport without the use of an external force. In this paper, we investigate the motion of liquid droplets near the Leidenfrost temperature on functionalized self-assembled asymmetric microstructured surfaces. The surfaces, which have angled microstructures, display unidirectional properties. The surfaces are fabricated on stainless steel through the use of a femtosecond laser-assisted process. Through this process, mound-like microstructures are formed through a combination of material ablation, fluid flow, and material redeposition. In order to achieve the asymmetry of the microstructures, the femtosecond laser is directed at an angle with respect to the sample surface. Two surfaces with microstructures angled at 45° and 10° with respect to the surface normal were fabricated. Droplet experiments were carried out with deionized water and a leveled hot plate to characterize the directional and self-propelling properties of the surfaces. It was found that the droplet motion direction is opposite of that for a surface with conventional ratchet microstructures reported in the literature. The new finding could not be explained by the widely accepted mechanism of asymmetric vapor flow. A new mechanism for a self-propelled droplet on asymmetric three-dimensional self-assembled microstructured surfaces is proposed.

High Temperature Near-Field NanoThermoMechanical Rectification

Limited performance and reliability of electronic devices at extreme temperatures, intensive electromagnetic fields, and radiation found in space exploration missions (i.e., Venus & Jupiter planetary exploration, and heliophysics missions) and earth-based applications requires the development of alternative computing technologies. In the pursuit of alternative technologies, research efforts have looked into developing thermal memory and logic devices that use heat instead of electricity to perform computations. However, most of the proposed technologies operate at room or cryogenic temperatures, due to their dependence on material’s temperature-dependent properties. Here in this research, we show experimentally—for the first time—the use of near-field thermal radiation (NFTR) to achieve thermal rectification at high temperatures, which can be used to build high-temperature thermal diodes for performing logic operations in harsh environments. We achieved rectification through the coupling between NFTR and the size of a micro/nano gap separating two terminals, engineered to be a function of heat flow direction. We fabricated and tested a proof-of-concept NanoThermoMechanical device that has shown a maximum rectification of 10.9% at terminals’ temperatures of 375 and 530 K. Experimentally, we operated the microdevice in temperatures as high as about 600 K, demonstrating this technology’s suitability to operate at high temperatures.

NanoThermoMechanical memory: Near-field heat transfer enabled negative differential thermal resistance and thermal latching

The three-dimensional design and modeling of a near-field NanoThermoMechanical memory is presented. Contrary to electronic memories, a NanoThermoMechanical memory utilizes heat solely to record, store, and recover data. Therefore it is immune to high temperature, intensive electric and magnetic fields, and can be powered with waste heat. Memory function of the NanoThermoMechanical memory is achieved through the existence of a bistable thermal system (i.e., has two stable equilibrium states under the same boundary conditions). The bistable thermal system occurs as a result of conjugate effects between a nonlinear heat input (resulting from negative differential thermal resistance) and heat losses from the NanoThermoMechanical memory device. Negative differential thermal resistance is specifically achieved through coupling between near-field thermal radiation and thermal expansion of the microbeam which the NanoThermoMechanical memory is made out of. Steady state analysis shows two stable states at the low and high temperatures of 1080 K and 1297 K, respectively, corresponding to the two memory states of ZERO and ONE. Complete read write cycle of the NanoThermoMechanical memory is also demonstrated using dynamic analysis. Using standard MEMS microfabrication techniques, the fabrication of the proposed NanoThermoMechanical memory devices can be easily accomplished.

Enhancing vapor generation at a liquid-solid interface using micro/nanoscale surface structures fabricated by femtosecond laser surface processing

Femtosecond Laser Surface Processing (FLSP) is a versatile technique for the fabrication of a wide variety of micro/nanostructured surfaces with tailored physical and chemical properties. Through control over processing conditions such as laser fluence, incident pulse count, polarization, and incident angle, the size and density of both micrometer and nanometer-scale surface features can be tailored. Furthermore, the composition and pressure of the environment both during and after laser processing have a substantial impact on the final surface chemistry of the target material. FLSP is therefore a powerful tool for optimizing interfacial phenomena such as wetting, wicking, and phasetransitions associated with a vapor/liquid/solid interface. In the present study, we utilize a series of multiscale FLSPgenerated surfaces to improve the efficiency of vapor generation on a structured surface. Specifically, we demonstrate that FLSP of stainless steel 316 electrode surfaces in an alkaline electrolysis cell results in increased efficiency of the water-splitting reaction used to generate hydrogen. The electrodes are fabricated to be superhydrophilic (the contact angle of a water droplet on the surface is less than 5 degrees). The overpotential of the hydrogen evolution reaction (HER) is measured using a 3-electrode configuration with a structured electrode as the working electrode. The enhancement is attributed to several factors including increased surface area, increased wettability, and the impact of micro/nanostructures on the bubble formation and release. Special emphasis is placed on identifying and isolating the relative impacts of the various contributions.

Thermal Stability of Rare Earth Oxide Coated Superhydrophobic Microstructured Metallic Surfaces

In this paper, we present a method of generating nearly superhydrophobic surfaces from Femtosecond Laser Surface Processed (FLSP) metallic substrates and the study of their thermal stability at high temperatures. Using an FLSP process, hierarchical micro/nano structures were fabricated on stainless steel 316 after which a 200 nm Cerium Oxide (CeO2) film was sputtered onto the surface. Before CeO2 deposition, the contact angle of sample was measured. Post CeO2 deposition, the contact angles were measured again. As a result of the cerium oxide deposition, the contact angle of the originally hydrophilic FLSP surface turned near superhydrophobic with an equilibrium contact angle of approximately 140°. Subsequently, the coated surfaces were annealed in air. The surface maintained its high contact angle from room temperature to about 160°C, after which it lost its hydrophobicity due to hydrocarbon burn off. For each annealing temperature, we monitored the chemical composition for the cerium oxide-coated FLSP surface using energy dispersive x-ray spectroscopy (EDS) and X-ray diffraction (XRD). Under a nitrogen rich annealing environment, the nearly superhydrophobic FLSP metallic surface maintained its high contact angle up to temperatures as high as 350°C. To further understand the physics behind the observed phenomenon, we investigated two additional samples of polished stainless steel 310 again coated with 200 nm of CeO2.Copyright

Enhanced pool-boiling heat transfer and critical heat flux using femtosecond laser surface processing

In this paper, we present the experimental investigation of pool boiling heat transfer on multiscale (micro/nano) functionalized metallic surfaces. The multiscale structures were fabricated via a femtosecond laser surface process (FLSP) technique which forms mound-like microstructures covered by layers of nanoparticles. Using a pool boiling experimental setup with deionized water as the working fluid, both the heat transfer coefficient and critical heat flux were investigated. The polished reference sample was found to have a critical heat flux of 91 W/cm2 at 40 °C of superheat and a maximum heat transfer coefficient of 23,000 W/m2-K. The processed sample was found to have a critical heat flux of 122 W/cm2 at 18 °C superheat and a maximum heat transfer coefficient of 67,400 W/m2-K. Flow visualization revealed nucleate boiling to be the main two-phase heat transfer mechanism. The overall heat transfer performance of the metallic multiscale structured surface has been attributed to both augmented heat transfer surface area and enhanced nucleate boiling regime. On the other hand, increase in the critical heat flux can be attributed to the superhydrophilic nature of the laser processed surface and the presence of nanoparticle layers.