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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 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. 

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. 

Individuals often aim to reverse undesired outcomes in interactions with automated systems, like loan denials, by either implementing system-recommended actions (recourse), or manipulating their features. While providing recourse benefits users and enhances system utility, it also provides information about the decision process that can be used for more effective strategic manipulation, especially when the individuals collectively share such information with each other. We show that this tension leads rational utility-maximizing systems to frequently withhold recourse, resulting in decreased population utility, particularly impacting sensitive groups. To mitigate these effects, we explore the role of recourse subsidies, finding them effective in increasing the provision of recourse actions by rational systems, as well as lowering the potential social cost and mitigating unfairness caused by recourse withholding.