Gas–Vapor Interplay in Plasmonic Bubble Shrinkage

The understanding of the shrinkage dynamics of plasmonic bubbles formed around metallic nanoparticles immersed in liquid and irradiated by a resonant light source is crucial for the usage of these bubbles in numerous applications. In this paper, we experimentally show and theoretically explain that a plasmonic bubble during its shrinkage undergoes two different phases: first, a rapid partial bubble shrinkage governed by vapor condensation and, second, a slow diffusion-controlled bubble dissolution. The history of the bubble formation plays an important role in the shrinkage dynamics during the first phase as it determines the gas–vapor ratio in the bubble composition. Higher laser powers lead to more vaporous bubbles, while longer pulses and higher dissolved air concentrations lead to more gaseous bubbles. The dynamics of the second phase barely depends on the history of bubble formation, that is, laser power and pulse duration, but strongly on the dissolved air concentration, which defines the concentration gradient at the bubble interface. Finally, for the bubble dissolution in the second phase, with decreasing dissolved air concentration, we observe a gradual transition from a R(t) ∝ (t0 – t)1/3 scaling law to a R(t) ∝ (t0 – t)1/2 scaling law where t0 is the lifetime of the bubble and theoretically explain this transition.对于浸没于液体中、经共振光源辐照的金属纳米颗粒周围形成的等离激元气泡（plasmonic bubble），明晰其收缩动力学机制，是该类气泡在诸多应用中得以落地的核心前提。本研究通过实验验证并从理论层面阐释：等离激元气泡在收缩过程中会经历两个截然不同的阶段：其一为以蒸汽冷凝主导的快速部分收缩阶段，其二为受扩散控制的缓慢溶解阶段。气泡形成的历史过程对第一阶段的收缩动力学具有重要影响，因其直接决定了气泡内组分的气-蒸比例。更高的激光功率会生成富蒸汽气泡，而更长的脉冲宽度与更高的溶解空气浓度则会生成富气体气泡。第二阶段的动力学过程几乎不受气泡形成历史（即激光功率与脉冲时长）的影响，但却与溶解空气浓度密切相关——该浓度决定了气泡界面处的浓度梯度。最后，针对第二阶段的气泡溶解过程，我们观察到：随着溶解空气浓度降低，气泡半径随时间的变化会从R(t) ∝ (t₀ - t)^(1/3)的缩放规律，逐渐过渡到R(t) ∝ (t₀ - t)^(1/2)的缩放规律（其中t₀为气泡的寿命），并从理论上阐释了这一过渡现象。