液滴撞击过程中气体捕获及微纳气泡生成的机理研究

Additive manufacturing based on inkjet printing has been widely employed in coating deposition on solid surface, precision component manufacturing and other production process. Since droplet is the basic unit of ink-jet printing, the control of its impact behavior is directly related to the level of production process in related advanced manufacturing industry. It is therefore of great theoretical significance and application value for the advanced manufacturing industry in China to improve the production process, by realizing the optimized control of drop impact behavior through the study on the gas entrapment dynamics and the mechanism of micro/nano bubble formation during drop impact. This project investigates the dynamics of drop impact onto liquid pool or solid surface by theoretical, experimental and numerical methods. A systematic study is executed to reveal the effects of liquid properties (viscosity, density and surface tension), geometric characteristics (drop size and initial shape), kinetic conditions (drop vibration and impact velocity) and surface properties (liquid pool or solid surface), on the gas entrapment and micro/nano bubble formation. We investigate the evolution of characteristic length of the entrapped gas layer, and establish the scaling law which determines the transition between different regimes of micro/nano bubble formation. We then propose qualitative and quantitative theories of the mechanism of gas entrapment and micro/nano bubble formation, which can provide theoretical guidance and method to eliminate micro/nano bubbles and improve the level of production process of the additive manufacturing based on inkjet printing.

固体表面涂层沉积、精密元器件制造等过程中广泛应用到喷墨打印增材制造方法。液滴作为喷墨打印的基本单元，对其撞击行为的调控直接关系到相关先进制造业的生产工艺水平。因此，研究液滴撞击过程中的气体捕获行为、微纳气泡演化机理，实现液滴撞击行为的优化调控，对提高我国先进制造业的生产工艺水平具有重要理论意义和应用价值。本项目结合使用理论分析、数值模拟和实验方法研究液滴对液池或固体表面的撞击过程，系统性研究液体物性(粘度、密度、表面张力等)、液滴几何特征(大小、初始形状等)、动力学特征(液滴振动、撞击速度等)和(液池或固体)表面性质等因素对撞击过程中气体捕获行为和微纳气泡演化模式的影响机制，澄清被捕获空气层尺度的变化规律，表征微纳气泡演化模式的无量纲标度律，构建气体捕获行为及微纳气泡演化模式的机理解释及其定量化理论，为消除微纳气泡、提高喷墨打印增材制造工艺水平提供理论指导和方法。

基于喷墨打印的增材制造方法广泛应用于特种涂层制备、精密元器件制造等先进制造领域。液滴撞击过程作为喷墨打印基本过程，调控液滴撞击行为，消除液滴撞击所致气体捕获现象及微纳气泡生成，直接关系到相关先进制造生产工艺水平。本项目通过理论分析、数值模拟和实验方法相结合，系统研究了液滴撞击过程中气体捕获现象及微纳气泡生成的机理。揭示了液体物性、初始几何和动力学条件等特征参数对空气层收缩、微纳气泡生成及其形态演化的影响规律，揭示了单一气泡、环形气泡、竖直分裂等空气层收缩机理，构建了基于Re数和We数的气泡形态演化空间，建立了基于局部Ohnesorge数的气泡形态间演化无量纲标度律。通过分析被捕获空气层特征尺度（如最大直径、质心、边缘坐标等）及涡量场分布，发现了涡脱落现象在气泡形态演化中的重要作用，揭示了涡脱落现象对空气层边缘振动及气泡竖直分裂的影响机制，提出了破坏竖直分裂的粘性机理与涡脱落机理。针对被捕获空气层收缩这一关键过程，对二维无限长空气层、轴对称空气柱等不同初始几何形状空气层收缩过程进行了数值模拟和理论分析，发现了涡脱落、平滑收缩、粘性收缩等收缩机制和轴对称空气柱收缩过程的末端断裂机制，揭示了收缩速度先增大后减小的演化规律以及收缩速度在小Ohnesorge数时遵循惯性标度律并随Oh数增大而逐步向粘性标度律转变的规律，建立了收缩速度相对于时间的无量纲标度律，阐明了涡脱落现象与空气层边缘振动现象的相互作用机理，构建了基于局部韦伯数We的空气层边缘振动临界条件。本项目研究成果为消除微纳气泡、提高喷墨打印增材制造等先进工艺水平提供了理论指导和方法。

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