Recently, a research titled “Giant and explosive plasmonic bubbles by delayed nucleation” was published online in Proceedings of the National Academy of Sciences of the United States of America (PNAS). It was a joint effort of Dr. Wang Yuliang, an associate professor of the School of Mechanical Engineering and Automation at Beihang University, Dr. Detlef Lohse, a professor of the University of Twente and a member of the Royal Netherlands Academy of Arts and Sciences, along with other researchers.

Illuminated by a laser, nanoparticles of noble metals (e.g., gold, silver and platinum) can generate a plasmonic effect, quickly converting light energy to thermal energy. In a liquid environment, the converted thermal energy can vaporize the water rapidly and produce micron-sized bubbles, which are called plasmonic bubbles. Being a research focus in the micro-nano field, these bubbles have great potential for applications in sunlight harvesting (Fig. 1A), cell therapy (Fig. 1B), enhanced medical imaging (Fig. 1C), micromanipulation (Fig. 1D) and so on.

Fig. 1 Applications of plasmonic bubbles in several fields

With the help of advanced micro-nano measurement technology, Dr. Wang has been researching on disease diagnosis at a cellular level and cell therapy based on plasmonic effect, and his team cooperates with Dr. Detlef Lohse, a celebrated expert in fluid mechanics, to study the growth mechanism of plasmonic bubbles.

Researches into plasmonic bubbles illuminated by a continuous laser have been conducted on the long-term (milliseconds to seconds) timescale, but Dr. Wang and his collaborators turned their attention to a shorter one. In their previous researches (published in ACS Nano), they revealed the dynamic growth mechanism of plasmonic bubbles. Combining this finding with the principle of plasmonic effect and the heat conduction characteristic of the material, the team speculated that more violent dynamics of plasmonic bubbles would be displayed at a shorter timescale.

To support the speculation, the team used the Brandaris 128 ultrafast imaging system featuring a frame rate of up to 25 Mfps and nanosecond resolution, and they became the first to observe the large plasmonic bubble (Fig. 2) quickly generated on an immersed gold nanoparticle (GNP)-decorated surface a short while after laser irradiation. Named the initial giant bubble, the bubble differs from the plasmonic bubbles that are normally observed in the millisecond time scale in terms of appearance and dynamics. It grows at 2,000 times the speed of the bubbles previously observed, and collapses in about tens of microseconds. The team further established a model based on thermal diffusion and liquid spinodal decomposition, revealing the growth mechanism of the initial giant bubble.

Fig. 2 Growth and collapse of an initial giant bubble

The research also illustrates the whole evolution process of plasmonic bubbles under continuous laser irradiation at the nanosecond to second timescales (Fig. 3). The super dynamic characteristic of the initial giant bubble found in this research lays the groundwork for the control and application of plasmonic bubbles.

Fig. 3 Four phases of the evolution of the plasmonic bubbles on the surface of the material under laser irradiation

The paper is available at http://www.pnas.org/content/early/2018/07/06/1805912115

Reported by Wen Lifang

Edited by Sun Yecheng

Translated by Li Mingzhu