Large-scale Generation of Patterned Bubble Arrays on Printed Bi-functional Boiling Surfaces.
ABSTRACT: Bubble nucleation control, growth and departure dynamics is important in understanding boiling phenomena and enhancing nucleate boiling heat transfer performance. We report a novel bi-functional heterogeneous surface structure that is capable of tuning bubble nucleation, growth and departure dynamics. For the fabrication of the surface, hydrophobic polymer dot arrays are first printed on a substrate, followed by hydrophilic ZnO nanostructure deposition via microreactor-assisted nanomaterial deposition (MAND) processing. Wettability contrast between the hydrophobic polymer dot arrays and aqueous ZnO solution allows for the fabrication of heterogeneous surfaces with distinct wettability regions. Heterogeneous surfaces with various configurations were fabricated and their bubble dynamics were examined at elevated heat flux, revealing various nucleate boiling phenomena. In particular, aligned and patterned bubbles with a tunable departure frequency and diameter were demonstrated in a boiling experiment for the first time. Taking advantage of our fabrication method, a 6 inch wafer size heterogeneous surface was prepared. Pool boiling experiments were also performed to demonstrate a heat flux enhancement up to 3X at the same surface superheat using bi-functional surfaces, compared to a bare stainless steel surface.
Project description:Functionalized interfaces enhancing phase-change processes have immense applicability in thermal management. Here, a methodology for fabrication of surfaces enabling extreme boiling heat transfer performance is demonstrated, combining direct nanosecond laser texturing and chemical vapor deposition of a hydrophobic fluorinated silane. Multiple strategies of laser texturing are explored on aluminum with subsequent nanoscale hydrophobization. Both superhydrophilic and superhydrophobic surfaces with laser-engineered microcavities exhibit significant enhancement of the pool boiling heat transfer. Surfaces with superhydrophobic microcavities allow for enhancements of a heat transfer coefficient of over 500%. Larger microcavities with a mean diameter of 4.2 ?m, achieved using equidistant laser scanning separation, induce an early transition into the favorable nucleate boiling regime, while smaller microcavities with a mean diameter of 2.8 ?m, achieved using variable separation, provide superior performance at high heat fluxes. The enhanced boiling performance confirms that the Wenzel wetting regime is possible during boiling on apparently superhydrophobic surfaces. A notable critical heat flux enhancement is demonstrated on superhydrophobic surfaces with an engineered microstructure showing definitively the importance and concomitant effect of both the surface wettability and topography for enhanced boiling. The fast, low-cost, and repeatable fabrication process has great potential for advanced thermal management applications.
Project description:For phase-change cooling schemes for electronics, quick activation of nucleate boiling helps safeguard the electronics components from thermal shocks associated with undesired surface superheating at boiling incipience, which is of great importance to the long-term system stability and reliability. Previous experimental studies show that bubble nucleation can occur surprisingly early on mixed-wettability surfaces. In this paper, we report unambiguous evidence that such unusual bubble generation at extremely low temperatures-even below the boiling point-is induced by a significant presence of incondensable gas retained by the hydrophobic surface, which exhibits exceptional stability even surviving extensive boiling deaeration. By means of high-speed imaging, it is revealed that the consequently gassy boiling leads to unique bubble behaviour that stands in sharp contrast with that of pure vapour bubbles. Such findings agree qualitatively well with numerical simulations based on a diffuse-interface method. Moreover, the simulations further demonstrate strong thermocapillary flows accompanying growing bubbles with considerable gas contents, which is associated with heat transfer enhancement on the biphilic surface in the low-superheat region.
Project description:Boiling--a process that has powered industries since the steam age--is governed by bubble formation. State-of-the-art boiling surfaces often increase bubble nucleation via roughness and/or wettability modification to increase performance. However, without active in situ control of bubbles, temperature or steam generation cannot be adjusted for a given heat input. Here we report the ability to turn bubbles 'on and off' independent of heat input during boiling both temporally and spatially via molecular manipulation of the boiling surface. As a result, we can rapidly and reversibly alter heat transfer performance up to an order of magnitude. Our experiments show that this active control is achieved by electrostatically adsorbing and desorbing charged surfactants to alter the wettability of the surface, thereby affecting nucleation. This approach can improve performance and flexibility in existing boiling technologies as well as enable emerging or unprecedented energy applications.
Project description:We demonstrate that the contact line (CL) motion on energetically heterogeneous solid surfaces occurs in a coupled fashion as against the traditional staggered stick-slip motion. Introducing chemical inhomogeneities at nanoscale induces a local change in dynamic contact angles which manifests as a smooth and continuous motion of the CL. Nanoscale chemically inhomogeneous surfaces comprising of gold, palladium and nickel were generated on copper substrates to demonstrate the underlying CL dynamics. The spatial variations of chemical constituents were mapped using elemental display scanning electron microscope images. Further, the coupled and stick-slip motion was confirmed for a sliding water droplet on these surfaces, and then used in studying the pool boiling bubble dynamics of a single bubble from nucleation to departure. The coupled motion was seen to increase the CL velocity thereby increasing the contribution from transient conduction heat transfer. Consequently, a ~2X increase in the boiling critical heat flux (CHF) was observed. Enhancing the pool boiling performance by introducing nanoscale surface features is an attractive approach in many applications and this work provides a framework and understanding of the CL motion induced through the chemical inhomogeneity effects.
Project description:Due to its high heat removal capability and exploitation of latent heat, boiling is considered to be one of the most effective cooling methods in industry. Surface structure and wettability are two factors imposing boiling phenomena. Here, we propose an effective and facile method for surface enhancement via crenarchaeon Sulfolobus Solfataricus P2 bio-coatings. The positive effects of such surfaces of bio-coatings were assessed, and enhancements in heat transfer and cooling were obtained. Visualization was also performed, and bubble dynamics of generated bubbles and vapor columns from the tested surfaces with bio-coatings are here presented. Superior performance in terms of boiling heat transfer and cooling was reached with the use of crenarchaeon Sulfolobus Solfataricus P2 coated surfaces. Thus, this study clearly demonstrates the potential of futuristic surfaces with bio-coatings to achieve substantial energy saving and efficiency.
Project description:Boiling is an efficient heat-transfer mechanism because of the utilization of latent heat of vaporization and has the potential to be used for cooling high-power electronic devices. Surface enhancement is one of the widely used techniques for heat-transfer augmentation in boiling systems. Here, an experimental investigation was conducted on chemical vapor deposition-grown three-dimensional (3D) foamlike graphene-coated silicon surfaces to investigate the effect of pore structures on pool boiling heat transfer and corresponding heat-transfer enhancement mechanisms. 3D graphene-coated samples with four graphene thicknesses were utilized along with a plain surface to investigate boiling heat-transfer characteristics and enhancement mechanisms. A high-speed camera was used to provide a deeper understanding of the bubble dynamics upon departure of emerging bubbles and visualize vapor columns in different boiling regimes. On the basis of the obtained results, in addition to interfacial evaporation, mechanical resonance of the 3D structure had also a considerable effect on vapor column formation. The results indicated that there is an optimum thickness, which exhibits the best performance in terms of boiling heat transfer.
Project description:Boiling, a dynamic and multiscale process, has been studied for several decades; however, a comprehensive understanding of the process is still lacking. The bubble ebullition cycle, which occurs over millisecond time-span, makes it extremely challenging to study near-surface interfacial characteristics of a single bubble. Here, we create a steady-state vapor bubble that can remain stable for hours in a pool of sub-cooled water using a femtosecond laser source. The stability of the bubble allows us to measure the contact-angle and perform in-situ imaging of the contact-line region and the microlayer, on hydrophilic and hydrophobic surfaces and in both degassed and regular (with dissolved air) water. The early growth stage of vapor bubble in degassed water shows a completely wetted bubble base with the microlayer, and the bubble does not depart from the surface due to reduced liquid pressure in the microlayer. Using experimental data and numerical simulations, we obtain permissible range of maximum heat transfer coefficient possible in nucleate boiling and the width of the evaporating layer in the contact-line region. This technique of creating and measuring fundamental characteristics of a stable vapor bubble will facilitate rational design of nanostructures for boiling enhancement and advance thermal management in electronics.
Project description:Today's trends for enhancing boiling heat transfer in terrestrial and space applications focus on removal of bubbles to prevent formation of a vapor layer over the surface at high overheat. In contrast, this paper presents a new boiling regime that employs a vapor-air bubble residing on a small heater for minutes and driving cold water over the surface to provide high heat flux. Single-bubble boiling of water was investigated under normal gravity and low gravity in parabolic flights. Experiments demonstrated a negligible effect of gravity level on the rate of heat transfer from the heater. Due to self-adjustment of the bubble size, the heat flux provided by boiling rose linearly up with increasing heater temperature and was not affected by a gradually rising water temperature. The fast response and stable operation of single-bubble boiling over a broad range of temperatures pave the way for development of new devices to control heat transfer by forming surface domains with distinct thermal properties and wettability. The bubble lifetime can be adjusted by changing the water temperature. The ability of heating water on millimeter scales far above 100?°C without an autoclave or a powerful laser provides a new approach for processing of biomaterials and chemical reactions.
Project description:Surface wettability is recognized as playing an important role in pool boiling and the corresponding heat transfer curve. In this work, a systematic study of pool boiling heat transfer on smooth surfaces of varying wettability (contact angle range of 5° - 180°) has been conducted and reported. Based on numerical simulations, boiling curves are calculated and boiling dynamics in each regime are studied using a volume-of-fluid method with contact angle model. The calculated trends in critical heat flux and Leidenfrost point as functions of surface wettability are obtained and compared with prior experimental and theoretical predictions, giving good agreement. For the first time, the effect of contact angle on the complete boiling curve is shown. It is demonstrated that the simulation methodology can be used for studying pool boiling and related dynamics and providing more physical insights.
Project description:Here, we demonstrate that heat removed in pool boiling from a heater mimicking high-power microelectronics could be used to facilitate a swing-like motion of the heater before being finally dissipated. This swing-like motion could be beneficial for shedding a large vapor bubble that encapsulates high-power heaters in microgravity where buoyancy force is unavailable for vapor bubble removal. The swing-like motion is propelled by vapor bubble recoil, the force which exists irrespective of gravity and buoyancy. We also demonstrate that this force could be significantly enhanced by depositing on the heater surface supersonically blown polymer nanofibers with cross-sectional diameters below 100?nm. These nanofibers provide additional nucleation sites, resulting in much more frequent bubble nucleation and departure, and thus a higher overall vapor recoil force propelling the heater motion. Such nanofibers strongly adhere to the heater surface and withstand prolonged harsh pool boiling. The measured velocity of the model swing-like heater in Novec 7300 fluid is about 1?cm/s.