Droplet最新文献

Front Cover: The cover image is based on the Research Article Highly efficient spray cooling enabled by acoustic microdroplet atomizer by Chen and Li.

Cover description: An acoustic microdroplet atomizer is reported by Wenming Li to achieve superior spray cooling performance. This acoustic atomizer, composed of a Lead Zirconate Titanate (PZT) transducer and silicon inverted pyramid nozzles, is designed to precisely control the droplet generation, overcoming the limitations of traditional spray methods such as pressure-driven, injector-based, and piezoelectric spray. (DOI: 10.1002/dro2.70002)

Back Cover: The cover image is based on the Research Article Reconsideration on the maximum deformation of droplets impacting on solid surfaces by Hu et al.

Cover description: The maximum spreading of impact droplets on surfaces, reflecting energy exchange between liquid and solid matters, plays a crucial role in droplet-related applications. We identify and highlight the often-overlooked yet important distinction between maximum droplet width and maximum contact length, arising from the geometric configuration of protruding rim influenced by the surface contact angle. (DOI: 10.1002/dro2.163)

Inside Back Cover: The cover image is based on the Research Article Fog collection with hairy wires by Feng et al.

Cover description: This cover image illustrates the enhanced fog collection performance of a hairy wire compared to a conventional smooth cylindrical metal wire of the same size. The unique structure of the hairy wire promotes efficient fog deposition and drainage, significantly improving water capture efficiency. This innovative and practical design offers a simple, affordable solution to mitigate water scarcity challenges. (DOI: 10.1002/dro2.166)

Symmetry typically characterizes the impact of a liquid droplet on a solid surface, where uniform spreading is followed by radial retraction. Breaking this symmetry traditionally relies on engineering surface properties. Here, we introduce an alternative approach to achieve asymmetric droplet impact by incorporating a pair of bubbles into the liquid droplet, resulting in the coexistence of spreading and retraction. The asymmetric dynamics originate from the anisotropic capillary effects that can be adjusted by varying the volume fraction of bubbles and the impact velocity. The early onset of retraction enhances upward liquid momentum, facilitating prompt droplet takeoff and significantly reducing both the contact area (up to 50%) and contact time (up to 60%). This reduction also diminishes heat exchange between the droplet and the surface. Our findings pave the way for applications that capitalize on reduced contact times through droplet engineering, eliminating the need for surface modifications.

Contactless, spatiotemporal droplet maneuvering plays a critical role in a wide array of applications, including drug delivery, microfluidics, and water harvesting. Despite considerable advancements, challenges persist in the precise transportation, splitting, controlled steering, and functional adaptability of droplets when manipulated by electrical means. Here, we propose the use of orbital electrowetting (OEW) on slippery surfaces to enable versatile droplet maneuvering under a variety of conditions. The asymmetric electrowetting force that is generated allows highly efficient droplet manipulation on these surfaces. Our results demonstrate that droplets can be split, merged, and steered with exceptional flexibility, precision, and high velocity, even against gravity. Additionally, the OEW technique facilitates the manipulation of droplets across different compositions, volumes, and arrays in complex environments, leaving no residue. This novel droplet maneuvering mechanism and control strategy are poised to impact a range of applications, from chemical reactions and self-cleaning to efficient condensation and water harvesting.

Liquid film cooling serves as a critical thermal protection mechanism in rocket thrusters. The interaction between oxidizer droplet, which is deposited from mainstream region of thrust chamber, and fuel film on the wall inevitably influences cooling efficiency, which is poorly understood in existing research. This study experimentally investigated hypergolic reaction between white fuming nitric acid droplet and ionic liquid fuel film at elevated wall temperature T_w using synchronized high-speed and infrared thermography. Results show that reaction progresses through inertia-dominant spreading, mixing, and culminates in intense liquid-phase explosion (micro-explosion). An elevated T_w intensifies micro-explosion, increasing the risk of wall exposure and leading to the decline of cooling efficiency. Paradoxically, the increase in local film temperature inversely correlates with T_w, which is related to reduced explosion delay time. These findings first provide thermal and hydrodynamic data essential for the design of future thermal protection measures for small hypergolic liquid rocket thrusters and offer theoretical basis for optimizing liquid film cooling systems in bipropellant propulsion architectures.

Emulsions are inherently thermodynamically unstable dispersions that are widely involved in food processing, cosmetic preparation, and drug delivery. The existing ultrasonic emulsification techniques mainly rely on the direct contact between the sonicator probe and liquids, which causes localized high temperature and pressure within the liquid and influences the final properties of the obtained emulsion. In this work, a containerless emulsification approach has been realized by using ultrasonic levitation. The emulsification of water‒oil system can be promoted by adjusting the emitter‒reflector distance to alter the acoustic radiation pressure on the surface of the levitated drop. The dynamic behaviors of the emulsification process were monitored by using a high-speed camera, and the sound field was analyzed via numerical simulation. The experimental results showed that atomization of droplets driven by sound field was the main driving force for emulsification. This method can be used to prepare emulsions in which the average diameter of the droplets was about 2–3 µm. The work provided a new method for containerless emulsification, thus shedding light on the preparation of contamination-free pharmaceuticals.

Achieving mobile liquid droplets on solid surfaces is crucial for various practical applications, such as self-cleaning and anti-fouling coatings. The last two decades have witnessed remarkable progress in designing functional surfaces, including super-repellent surfaces and lubricant-infused surfaces, which allow droplets to roll/slide on the surfaces. However, it remains a challenge to enable droplet motion on hydrophilic solid surfaces. In this work, we demonstrate mobile droplets containing ionic surfactants on smooth hydrophilic surfaces that are charged similarly to surfactant molecules. The ionic surfactant-laden droplets display ultra-low contact angle and ultra-low sliding angle simultaneously on the hydrophilic surfaces. The sliding of the droplet is enabled by the adsorbed surfactant ahead of three-phase contact line, which is regulated by the electrostatic interaction between ionic surfactant and charged solid surface. The droplet can maintain its motion even when the hydrophilic surface has defects. Furthermore, we demonstrate controlled manipulation of ionic surfactant-laden droplets on hydrophilic surfaces with different patterns. We envision that our simple technique for achieving mobile droplets on hydrophilic surfaces can pave the way to novel slippery surfaces for different applications.

Droplets are ubiquitous in nature and play an essential role in spray cooling, which is a highly efficient cooling approach for high-power-density miniaturized electronic devices. However, conventional pressure-driven spray faces significant challenges in controlling microdroplet characteristics, particularly the droplet size and spray direction, both of which critically impact cooling performance. Herein, to conquer these challenges, we designed an acoustic microdroplet atomizer composed of a lead zirconate titanate (PZT) transducer and silicon inverted pyramid nozzles. This design enables precise control of droplet generation, overcoming the limitations of traditional spray methods. The acoustic atomization technology minimizes excess liquid accumulation while significantly enhancing thin liquid film evaporation. Compared to the conventional droplet generation techniques such as pressure-driven, injector-based, and piezoelectric spray, our acoustic atomizer achieves superior cooling performance. Notably, we demonstrate a high heat flux of ∼220 W/cm² with a 3.6-fold enhancement at a low flow rate of 24 mL/min, achieving significantly improved cooling efficiency. Finally, our acoustic atomizer provides precise control over droplet size, velocity, and flow rate by adjusting the number of nozzles and the PZT transducer's resonant frequency, elevating spray cooling performance. This novel acoustic atomization cooling technology holds great promise for practical applications, particularly in the thermal management of compact electronic components.

Bubble growth kinetics has been attracting vast attention in water electrolysis and other gas evolution reactions, but mostly investigated under ambient pressure. For practical scenarios, bubble evolution is usually carried out under high pressure. To better understand the bubble growth kinetics, we monitored the hydrogen bubble evolution process at increased pressures during electrochemical hydrogen production. Unlike the common sense that high pressures could result in smaller bubble size, our results revealed that the increased pressure would increase the aerophilicity of electrode surface, with decreased bubble contact angle from 111° to 89° for 0.1‒2.0 MPa, increased detachment size from 233 to 1207 µm, and reduced growth coefficient from 230 to 10.9 for the high pressures from 0.1 to 3.0 MPa. The steady high-pressure bubble growth kinetics are basically governed by the as-formed supersaturation in bulk solution, which is the balance between the driving force (current density) and the enlarged solubility of bulk solution under high pressure. Insufficient driving force would induce the depletion of bulk supersaturation and stagnate the bubble growth. Further investigation on high-pressure bubble evolution behaviors should shed light on practical industrial electrode design with extended usage life.