Search for: All records

Abstract Aircraft icing, resulting from the freezing of supercooled water droplets on exposed surfaces, presents considerable hazards to flight safety by impairing aerodynamic performance and operating efficiency. This study empirically examines the interaction dynamics of supercooled water droplets and dielectric barrier discharge (DBD) plasma actuators, emphasizing electrical, thermal, and phase transition phenomena. Supercooled droplets were produced via sonic levitation in a freezer set at −10°C and subsequently deposited onto the plasma actuator surface at −5°C. Electrical diagnostics indicated a reduction in current intensity following droplet impact which inhibited plasma discharge activity. Thermal imaging detected localized heating at nucleation locations, indicating a temperature plateau during freezing caused by latent heat release. A study of spatial temperature along the droplet x-axis revealed a pronounced thermal gradient, with the most significant temperature rise occurring adjacent to the plasma-exposed area. High-speed imaging elucidated droplet dynamics, demonstrating spreading, descent towards the ground electrode, and subsequent retraction following stabilization. These discoveries improve the comprehension of plasma-droplet interactions, aiding in the improvement of plasma-based anti-icing technology. This research promotes the creation of effective and environmentally friendly solutions for aviation safety and other areas affected by icing hazards. 

Abstract In order to improve the quality of products during additive manufacturing, we developed a novel freezing sublimation-based method for inkjet-based three-dimensional (3D) printing technology, which can significantly improve the uniformity of material distribution in printed products. In our previous studies, we used a laboratory prototype with single droplets of inkjet solution containing colloidal particles to prove the concept of this study. However, understanding the interaction between droplets on the printing substrate surface is also crucial for determining the printing resolution and accuracy of this method, which cannot be fully investigated through single droplet-based experimental studies. To fill this knowledge gap, we conducted a series of experiments on colloidal droplet impingement, freezing, and sublimation on substrates using dual droplets. The experimental setup allowed the release of two droplets in quick succession from a modified nozzle with two needles. These droplets coalesced on the substrate surface due to spreading during their impingement processes. Observations revealed that the coalescence pattern of these two droplets varied depending on the time interval between their release. When the second droplet was released immediately after the first, their coalescence was governed by fluid dynamics. However, when the second droplet was released after the first droplet had frozen on the substrate, it spread above the ice surface of the first droplet in a relatively slower process. This observation provides new insights for the continued study and optimization of the proposed novel freezing sublimation-based 3D printing method. 

Abstract In this paper, an experimental study was conducted to characterize the dynamics and thermal evolutions of colloidal droplets impinging and freezing on solid surfaces with different wettabilities (i.e., superhydrophobic vs. hydrophilic) towards the development of a novel freezing-based inkjet additive manufacturing (AM) technology. The experiments were carried out in the Freeze Layering Inkjet Printing (FLIP) facility in the Mechanical Engineering Department of the City College of New York (CCNY). While the transient impinging/freezing dynamics of colloidal droplets were resolved by using a high-speed imaging system, the thermal evolutions of the freezing colloidal droplets were also characterized by using a high-speed high-resolution Infrared (IR) thermal imaging system. In addition, the deposition patterns formed under the conventional evaporation-based approach and the novel freezing-sublimation approach were also evaluated by using an automated high-accuracy freeze dryer facility. It was found that the time scale of the dynamic process (i.e., impact, spreading, receding, rebounding, and settling) was much faster than the thermal process (i.e., nucleation and solidification) for both surfaces. The dynamics of the colloidal droplets during the impinging process were coupled with the thermal processes that govern the freezing of the droplets. The surface wettability had a direct effect on both the freezing footprint and the freezing rate. 

Inkjet-based three-dimensional (3D) printing is widely used for fast and efficient non-contact manufacturing, yet it suffers from several drawbacks, such as coarse resolution, lack of adhesion, manufacturing inconsistency, and uncertain final part mechanical properties. These undesirable effects are related to complex flow phenomena in colloidal droplets in inkjet 3D printing, particularly the internal flows and droplet deformations during the deposition and drying processes. These challenges are due to the colloidal suspension droplets being kept in the liquid state during printing. To overcome these disadvantages, this paper presents a novel freezing-sublimation-based inkjet 3D printing concept that freezes the colloidal droplets upon impact followed by sublimation, eliminating the undesirable particle transport and fluid motions during deposition. A series of experiments were conducted to characterize the colloidal droplet behaviors during the impinging/freezing and sublimation processes and evaluate the effects of the freezing process on droplet impinging dynamics as well as the final deposition patterns through sublimation. It was demonstrated that the deposition patterns obtained from this new method are much more uniform than the conventional evaporation-based deposition method. Both qualitative and quantitative methods were applied to analyze the colloidal droplet profiles during the printing process (impinging, freezing, and sublimation), as well as the final deposition patterns. The study shows promising results of using this new method, providing a foundation for the development of the novel freezing-sublimation-based inkjet 3D printing technique. 

Binder jetting additive manufacturing (AM) is an innovative form of 3D printing that generates complex and advanced structures of various materials by jetting binder drops onto a powder bed. The drops on the bed cure the powder to form structures in a quick and efficient manner. However, the method suffers several flaws including manufacturing inconsistencies and coarse resolution of structures. These flaws may be explained by complex interactions between the binder drop and the powder during the printing process. Therefore, a better understanding of these interactions will be instrumental in the development of binder jetting for fabricating multipurpose, higher quality functional structures. In this study, these complex interactions are analyzed during the impact and subsequent processes. The impact dynamics of binder drops on a powder surface were examined by using a custom impingement rig under various test conditions (i.e., different impact velocities and binder viscosities). A high-speed imaging system was used to capture the transient details of the drop-powder interactions. This study concludes that an increase in drop impact velocity results in a greater range of particle ejection. A lower drop viscosity results in a higher dry spread of particles while a higher drop viscosity results in a greater number of binder-encapsulated particles. Across all cases, binder drops absorb particle granules at a rate inverse to their viscosity.