The Ephemeral Jelly

Though this jet-like internal flow pattern inside of a water droplet resembles a mushroom, it swims like a jellyfish. This microscopic phenomenon was observed when two micro-droplets coalesce. Internal droplet flow has remained a challenge to visualize for over a century due to the small length and time scales associated with the flows. In order to study droplet coalescence dynamics, we developed a droplet dispensing and visualization system. To track the internal flow front, and ethanol (20 wt%) water mixture was used as the working fluid. Due to the unequal evaporation rates between water and ethanol, a density gradient is developed at the liquid-gas interface of the droplets, resulting in a lower refractive index at the interface compared to that in the bulk liquid. Jet-like internal flow and vortex rings create a jellyfish-like structure inside the merging droplet. The chronophotograph demonstrates the previously unidentified jet-like flow that occurs during the coalescence of droplets having different sizes.Ope

Cleansing for Life

Dr. Nenad MiljkovicIf you have been seeing small flies or gnats in your kitchen, they're probably fruit flies. Fruit flies are primarily nuisance pests and have the potential to contaminate food with bacteria and other disease-producing organisms. However, fruit flies itself have extreme self-cleaning and bactericidal behavior; good news for the surface science! Their body parts, especially the wings have a unique combination of micro/nano-architecture and surface chemistry that cause extreme water repellency (superhydrophobicity), which prevents wings contamination with debris and microorganisms by enabling good self-cleaning activity in dusty and humid environment. In order to study this phenomena, we developed a dual dust-droplet dispensing and visualization system. A coal particle (~500µm length) resembling a dust on the wing surface, jumps off the surface by converting the surface free energy into kinetic energy when coalesced with a microscopic water droplet (~140µm radius). The chronophotograph demonstrates the previously unidentified single droplet jumping that occurs during the coalescence of a water droplet with a coal/dust particle

Polydimethylsiloxane-silane synergy enables dropwise condensation of low surface tension liquids

Despite decades of research on promoting the dropwise condensation of steam, achieving dropwise condensation of low surface tension liquids remains a challenge. The few coatings reported to promote dropwise condensation of low surface tension liquids either require complex fabrication methods, are substrate dependent or have poor scalability. Here, the rational development of a coating, which is applicable to all conventionally used condenser metals, is presented by combining a low contact angle hysteresis polydimethylsiloxane with a low surface energy silane using atmospheric vapor phase deposition. The siloxane-silane coating enables the dropwise condensation of fluids with surface tensions as low as 15 mN m−1 in pure vapor conditions. This siloxane-silane coating enables a 274%, 347%, and 636% heat transfer enhancement during ethanol, hexane, and pentane condensation, respectively, when compared to filmwise condensation on the same un-coated surfaces. Furthermore, this coating exhibits 15 days of steady dropwise condensation with no apparent signs of coating degradation. This study not only demonstrates the possibility of achieving stable dropwise condensation of low surface tension fluids on scalable, structure-less surfaces, it also develops design principles for creating facile, substrate-independent, durable, and scalable omniphobic coatings for a plethora of applications.The authors gratefully acknowledge funding support from the Office of Naval Research (ONR) under Grants No. N00014-16-1-2625 and No. N00014-18-S-F004. The authors also gratefully acknowledge funding support from the National Science Foundation under Award No. 1554249 and the Air Conditioning and Refrigeration Center. N.M. gratefully acknowledges funding support from the International Institute for Carbon Neutral Energy Research (WPI-I2CNER), sponsored by the Japanese Ministry of Education, Culture, Sports, Science, and Technology

Modular Heat Sinks for Enhanced Thermal Management of Electronics

Power electronics are vital for the generation, conversion, transmission, and distribution of electrical energy. Improving the efficiency, power density, and reliability of power electronics is an important challenge that can be addressed with electrothermal codesign and optimization. Current thermal management approaches utilize metallic heat sinks (HSs), resulting in parasitic load generation due to different potentials between electronic components on the printed circuit board (PCB). To enable electrical isolation, a thermal interface material (TIM) or gap pad is placed between the PCB and HS, resulting in poor heat transfer. Here, we develop an approach to eliminate TIMs and gap pads through modularization of metallic HSs. The use of smaller modular heat sinks (MHSs) strategically placed on high power dissipation areas of the PCB enables elimination of electrical potential difference, and removal of electrical isolation materials, resulting in better cooling performance due to direct contact between devices and the HS. By studying a gallium nitride (GaN) 2 kW DC-DC power converter as a test platform for electrothermal codesign using the modular approach, and benchmarking performance with a commercial off-the-shelf HS design, we showed identical power dissipation rates with a 54% reduction in HS volume and a 8 degrees C reduction in maximum GaN device temperature. In addition to thermal performance improvement, the MHS design showed a 73% increase in specific power density with a 22% increase in volumetric power density

VISION-iT: A Framework for Digitizing Bubbles and Droplets

Quantifying the nucleation processes involved in liquid-vapor phase-change phenomena, while dauntingly challenging, is central in designing energy conversion and thermal management systems. Recent technological advances in the deep learning and computer vision field offer the potential for quantifying such complex two-phase nucleation processes at unprecedented levels. By leveraging these new technologies, a multiple object tracking framework called “vision inspired online nuclei tracker (VISION-iT)” has been proposed to extract large-scale, physical features residing within boiling and condensation videos. However, extracting high-quality features that can be integrated with domain knowledge requires detailed discussions that may be field- or case-specific problems. In this regard, we present a demonstration and discussion of the detailed construction, algorithms, and optimization of individual modules to enable adaptation of the framework to custom datasets. The concepts and procedures outlined in this study are transferable and can benefit broader audiences dealing with similar problems

Ultra-resilient multi-layer fluorinated diamond like carbon hydrophobic surfaces

Abstract Seventy percent of global electricity is generated by steam-cycle power plants. A hydrophobic condenser surface within these plants could boost overall cycle efficiency by 2%. In 2022, this enhancement equates to an additional electrical power generation of 1000 TWh annually, or 83% of the global solar electricity production. Furthermore, this efficiency increase reduces CO2 emissions by 460 million tons /year with a decreased use of 2 trillion gallons of cooling water per year. However, the main challenge with hydrophobic surfaces is their poor durability. Here, we show that solid microscale-thick fluorinated diamond-like carbon (F-DLC) possesses mechanical and thermal properties that ensure durability in moist, abrasive, and thermally harsh conditions. The F-DLC coating achieves this without relying on atmospheric interactions, infused lubricants, self-healing strategies, or sacrificial surface designs. Through tailored substrate adhesion and multilayer deposition, we develop a pinhole-free F-DLC coating with low surface energy and comparable Young’s modulus to metals. In a three-year steam condensation experiment, the F-DLC coating maintains hydrophobicity, resulting in sustained and improved dropwise condensation on multiple metallic substrates. Our findings provide a promising solution to hydrophobic material fragility and can enhance the sustainability of renewable and non-renewable energy sources