•Surfaces with periodically changed wettability were produced by a ns marking laser.•Heat transfer was investigated on uniformly and non-uniformly wettable surfaces.•Microporous surfaces with ...non-uniform wettability enhance boiling heat transfer.•The most bubble nucleations were observed in the vicinity of the microcavities.•Results agree with the predictions of the nucleation criteria.
Microstructured uniformly and non-uniformly wettable surfaces were created on 25-μm-thin stainless steel foils by laser texturing using a marking nanosecond Nd:YAG laser (λ=1064nm) and utilizing various laser fluences and scan line separations. High-speed photography and high-speed IR thermography were used to investigate nucleate boiling heat transfer on the microstructured surfaces. The most pronounced results were obtained on a surface with non-uniform microstructure and non-uniform wettability. The obtained results show up to a 110% higher heat transfer coefficients and 20–40 times higher nucleation site densities compared to the untextured surface. We show that the number of active nucleation sites is significantly increased in the vicinity of microcavities that appeared in areas with the smallest (10μm) scan line separation. Furthermore, this confirms the predictions of nucleation criteria and proves that straightforward, cost-effective nanosecond laser texturing allows the production of cavities with diameters of up to a few micrometers and surfaces with non-uniform wettability. Additionally, this opens up important possibilities for a more deterministic control over the complex boiling process.
•Boiling on functionalized steel foils was investigated by high-speed IR imaging.•Wall-temperature distributions were calculated at various heat fluxes.•Distributions’ parameters revealed important ...differences in boiling performance.•Surface with microcavities provided the lowest standard deviation of wall superheat.•Compared to coatings, laser texturing didn’t increase heaters’ thermal resistance.
Nucleate boiling was examined on Joule heated stainless steel foils, functionalized by PDMS-silica coating and/or nanosecond-laser texturing. The nucleating bubbles and transient temperature fields were visualized through high-speed IR and video recordings. The differences in boiling performance were evaluated through wall-temperature distributions. Results confirmed that smooth surfaces require high activation temperatures and produce larger bubbles, while wall-temperature distributions display higher standard deviations, higher local superheats, and bimodal shapes. Similarly, relatively high activation temperatures were observed on the superhydrophilic surface, where the enhanced liquid replenishment on the active nucleation sites reduces the bubble departure diameters and prevents formation of local hotspots. Consequently, the analyzed temperature distributions have negative skewness and decreased standard deviation. The highest heat transfer coefficient was achieved on a laser textured surface with non-uniform wettability and multi-scale microcavities. Here, nucleation site density was as high as 200cm−2 at 300kW/m2, while wall-temperature distributions demonstrated by far the lowest standard deviation. The temperature distributions also proved that annealed PDMS-silica coating significantly increased the thermal resistance of the entire heater. On the contrary, laser textured surfaces provided an even better boiling performance compared to coated surfaces and did not increase heater’s thermal resistance. This additionally endorses the coating-free, direct laser texturing method as a cutting-edge technology in the development of surfaces capable of significantly enhanced boiling heat transfer.
•Practicable, robust, scalable binary surfaces were pursued for boiling enhancement.•The current boiling performance limits were pushed for PF-5060 on these surfaces.•Presence of a trapped ...non-boiling liquid in the binary surface enhances boiling.
A novel idea for the improvement of boiling heat transfer is that of a binary surface – where a non-boiling liquid coats sub-surface irregularities and aids in heat transfer to the primary working fluid. The threefold goal of this effort was to: (i) prepare durable, low-cost, scalable binary surfaces for boiling heat transfer enhancement, (ii) conduct pool boiling experiments on these surfaces, and (iii) derive the physical mechanisms perceived as responsible for the observed boiling enhancements. Accordingly, robust binary surfaces were prepared on copper using a facile, scalable bulk micro-manufacturing approach. These surfaces consist of numerous micro-/nano-cavities filled by a non-boiling liquid creating puddles around solid islands. Boiling experiments were carried out using a dielectric liquid, PF-5060, as the primary working fluid (the boiling liquid). It was observed that, compared to a smooth/plain copper surface, a binary copper surface, with water as the NBL, was able to simultaneously enhance the maximum heat flux limit by ∼2.2 times and the average heat transfer coefficient by ∼7.5 times. A maximum heat flux of 35.06W/cm2 was recorded, which is higher than the enhancements reported so far in literature for the pool boiling of PF-5060 on any enhanced surface. It was further found that decreasing the contact angle of the non-boiling liquid on the binary surface enhances both the heat transfer coefficient and the maximum heat flux limit.
The application of microtechnology to traditional mechanical industries is limited owing to the lack of suitable micropatterning technology for durable materials including metal. In this research, a ...glassy carbon (GC) micromold was applied for the direct metal forming (DMF) of a microstructure on an aluminum (Al) substrate. The GC mold with microdome cavities was prepared by carbonization of a furan precursor, which was replicated from the thermal reflow photoresist master pattern. A microdome array with a diameter of 8.4 μm, a height of ~0.74 μm, and a pitch of 9.9 μm was successfully fabricated on an Al substrate by using DMF at a forming temperature of 645 °C and an applied pressure of 2 MPa. As a practical application of the proposed DMF process, the enhanced boiling heat transfer characteristics of the DMF microdome Al substrate were analyzed. The DMF microdome Al substrate showed 20.4 ± 2.6% higher critical heat flux and 34.1 ± 5.3% higher heat transfer coefficient than those of a bare Al substrate.
Microtechnology has changed our world since the last century, when silicon microelectronics revolutionized sensor, control and communication areas, with applications extending from domotics to ...automotive, and from security to biomedicine. The present century, however, is also seeing an accelerating pace of innovation in glassy materials; as an example, glass-ceramics, which successfully combine the properties of an amorphous matrix with those of micro- or nano-crystals, offer a very high flexibility of design to chemists, physicists and engineers, who can conceive and implement advanced microdevices. In a very similar way, the synthesis of glassy polymers in a very wide range of chemical structures offers unprecedented potential of applications. The contemporary availability of microfabrication technologies, such as direct laser writing or 3D printing, which add to the most common processes (deposition, lithography and etching), facilitates the development of novel or advanced microdevices based on glassy materials. Biochemical and biomedical sensors, especially with the lab-on-a-chip target, are one of the most evident proofs of the success of this material platform. Other applications have also emerged in environment, food, and chemical industries.
The present Special Issue of Micromachines aims at reviewing the current state-of-the-art and presenting perspectives of further development. Contributions related to the technologies, glassy materials, design and fabrication processes, characterization, and, eventually, applications are welcome.
Wastewater with high ammonia nitrogen concentration is treated by air stripping in most cases as it is difficult to treat with biological processes. For energy saving, granular activated carbon (GAC) ...and microwave (MW) radiation was combined in this study, in a continuous flow-through system (GAC-MW), to remove ammonia nitrogen from experimental wastewater. The results showed that GAC-MW was effective and can achieve consistent removal of 94% of ammonia without additional aeration. The mechanism was investigated by observing the removal effects of GAC and MW independently. This demonstrated that the GAC temperature rise in the MW field, and the subsequent transfer of boiling heat to the fluid in steams was the key ammonia removal mechanism. We also varied the operating conditions; increased pH and MW power, decreased inflow rate and initial ammonia concentration, and improved the ammonia removal rate. The optimal GAC bed size for our specific flow-rate was determined by MW penetration depth into the GAC medium and the contact time between the GAC and the test fluid. The removal efficiency declined from 99% to 83% over 31-h exposure in extended operation. The changes in GAC properties included a decrease in specific surface area and increased GAC macropore volume, which maintained high removal efficiency throughout the extended operational period.
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