Yin Zongjun, Zhang Chengbin
School of Energy and Environment, Southeast University, Nanjing, Jiangsu 210096, P. R. China.
Langmuir. 2025 Jul 8;41(26):16786-16798. doi: 10.1021/acs.langmuir.5c00566. Epub 2025 Jun 27.
The difference in diameters of unequal-sized water nanodroplets can significantly alter the collision regimes, and a quantitative model for predicting nanodroplet breakup remains unclear. This study employs molecular dynamics simulations to investigate the head-on collisions of unequal-sized nanodroplets. The interactions during head-on collisions are analyzed over a broad spectrum of Weber numbers and size ratios, allowing for a comprehensive characterization of the collision regime diagram. Binary nanodroplet collisions are characterized by coalescence and breakup regimes. Further exploration reveals that the coalescence regime in topological changes involves three distinct modes: coalescence following regular deformation (CRD), coalescence following reverse encapsulation (CRE), and coalescence after experiencing holes (CEH), and the breakup regime can be categorized into three distinct modes: rim-ring fragmentation (RF), partial breakup (PB), and divergent splattering (DS). The average film thickness at the minimum flattened state is found to scale as ∼ during the regular deformation regime of colliding equal-sized water nanodroplets; however, this scaling does not hold in the breakup regime due to the influence of thermal fluctuations, which induce fluctuations and perforations in the spreading sheet. The mechanism of spreading sheet puncture is clarified as a short-wavelength instability amplified by thermal fluctuations. Based on the dispersion relation characterizing the instability of a nanoscale liquid film, the critical liquid film thickness for sheet puncture follows a scaling law, ∼ α(λ + 2), where α = 1.5 is a fitting parameter, λ is the nanoscale critical wavelength associated with the short-wavelength instability, and is the characteristic length determined by the competition between thermal fluctuations and surface tension. Furthermore, an analysis model based on energy balance is proposed to determine the binary water nanodroplet breakup by utilizing the Reynolds and Ohnesorge numbers as fundamental parameters. This model expression accurately quantifies the transition between coalescence and breakup regimes, hence providing valuable guidance for nanodroplet interaction applications.
大小不等的水纳米液滴直径差异会显著改变碰撞状态，而预测纳米液滴破碎的定量模型仍不明确。本研究采用分子动力学模拟来研究大小不等的纳米液滴的正面碰撞。在广泛的韦伯数和尺寸比范围内分析正面碰撞过程中的相互作用，从而全面表征碰撞状态图。二元纳米液滴碰撞的特征是聚并和破碎状态。进一步探索发现，拓扑变化中的聚并状态涉及三种不同模式：常规变形后聚并（CRD）、反向包裹后聚并（CRE）和出现孔洞后聚并（CEH），破碎状态可分为三种不同模式：边缘环破碎（RF）、部分破碎（PB）和发散飞溅（DS）。发现在碰撞的等尺寸水纳米液滴的常规变形状态下，最小扁平化状态下的平均膜厚度按 ∼ 缩放；然而，由于热涨落的影响，这种缩放关系在破碎状态下不成立，热涨落在扩展薄片中引起涨落和穿孔。扩展薄片穿孔的机制被阐明为热涨落放大的短波长不稳定性。基于表征纳米级液膜不稳定性的色散关系，薄片穿孔的临界液膜厚度遵循缩放定律 ∼ α(λ + 2)，其中α = 1.5 是拟合参数，λ 是与短波长不稳定性相关的纳米级临界波长， 是由热涨落和表面张力之间的竞争决定的特征长度。此外，提出了一种基于能量平衡的分析模型，以雷诺数和奥内佐格数作为基本参数来确定二元水纳米液滴的破碎。该模型表达式准确地量化了聚并和破碎状态之间的转变，从而为纳米液滴相互作用应用提供了有价值的指导。
