Plasmonic microbubbles: nucleation, growth and collapse
Mikhail Zaytsev is a PhD student in the research group Physics of Fluids (POF). His supervisor is prof.dr. D. Lohse from the Faculty of Science and Technology (TNW).
Noble metal nanoparticles immersed in a liquid and irradiated by resonant light can rapidly heat up to high temperatures, due to their peculiar absorption ability, and transfer the heat to the surrounding liquid, leading to the formation of bubbles, which have broad application potential in many plasmonic-enhanced processes. For further implementation of these systems, many intriguing challenges should be solved. How exactly do these bubbles form? What is their composition? Which parameters are crucial in bubble dynamics? In this thesis, bubble formation due to the collective effect from nanoparticles immersed in liquid under low-intensity cw-laser irradiation has been extensively studied. All phases of the bubble life cycle have been investigated: from nucleation to growth and subsequent collapse with a broad set of parameters.
First, the nucleation and early life phase of these bubbles have been discussed. After some delay time after the beginning of the illumination, a bubble explosively grows and collapses again, typically within 10 microseconds (bubble life phase 1). The maximal bubble volume remarkably increases with decreasing laser power. The same holds for the delay time, which drastically increases with decreasing laser power, leading to higher energy provided to the system. This dumped energy shows a linear scaling relation with maximal volume, irrespectively of the gas concentration of the surrounding water, which confirms that the initial giant bubble is a pure vapor bubble. In contrast, delay time does depend on the gas concentration, as gas pockets and molecules clusters facilitate bubble formation, which leads to shorter delay times and lower nucleation temperatures. Right after the collapse of the initial bubble, much smaller unstable oscillating bubbles appear (bubble life phase 2) and maintain continuous cycles typically up to several milliseconds.
Right after the bubble stabilization, the steady growth takes over. The dynamics of this phase can be divided into two regimes: bubble growth controlled by water evaporation (bubble life phase 3) and a subsequent diffusive growth phase (bubble life phase 4). The rapid growth is similar for saturated and undersaturated water, which implies that during this regime, the water evaporation dominates. The steady growth in the second regime is distinctly different for air-equilibrated and degassed water, which is attributed to the influx of dissolved gas expelled from the water around the hot nanoparticles.
The behavior of plasmonic bubbles in organic liquids, namely n-alkanes, has also been investigated. Two different phases in the evolution of the bubbles can be distinguished. The first phase is similar to the one in water, when, after a short delay explosive microbubble forms. Despite the significant difference in boiling points, the exact size of this explosive microbubble barely depends on the carbon chain length of the alkane, but only on the laser power. In the second phase, right after the collapse of the explosive microbubble, a new bubble forms and starts steadily growing due to the vaporization of the surrounding liquid (contrary to water), which is highly saturated with gas. The bubble size in the second phase strongly depends on the alkane chain length and increases with a decreasing number of carbon atoms.
Finally, the shrinkage dynamics of plasmonic bubbles have been discussed. We show that a plasmonic bubble during its shrinkage undergoes two different phases, first – a rapid partial bubble shrinkage due to vapor condensation and second – a slow diffusion-controlled bubble dissolution. The history of bubble formation plays an essential role in shrinkage dynamics during the first phase, as it determines the initial bubble composition. Higher laser powers lead to more vaporous bubbles, while longer pulses and higher dissolved gas concentrations lead to more gaseous bubbles. The dynamics of the second phase barely depends on the history of bubble formation, but strongly on the dissolved gas concentration, which defines the gradient at the bubble interface. We observe a gradual transition from one scaling law to another with decreasing dissolved gas concentration and theoretically explain this transition.