We present a systematic study of the laser-driven Marangoni flow and curvature induced vortex formation in a copper sulfate pentahydrate solution, visualized by dispersed carbon nanotube (CNT) bundles. The experiments are carried out using different objectives of numerical aperture (NA) in the range of 0.1–0.6 to investigate the effect of focusing on the flow dynamics. The flow velocities measured (for 0.1 NA) are in the range of 2 mm/s–5 mm/s depending on the size of CNTs. Both primary and secondary vortices are observed in our experiment. In the primary vortex, with a sixfold increase in NA, a tenfold increase in the angular velocity of CNTs is measured. We also discuss the important role played by the curvature of the droplet in the vortex formation. The numerical simulations carried out for flow velocity are in agreement with the experimental values.

中文翻译：

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激光驱动的 Marangoni 流动和液滴中的涡流形成

我们对五水硫酸铜溶液中激光驱动的 Marangoni 流和曲率诱导涡旋形成进行了系统研究，通过分散的碳纳米管 (CNT) 束进行可视化。使用数值孔径 (NA) 在 0.1-0.6 范围内的不同物镜进行实验，以研究聚焦于流动动力学的影响。测得的流速（0.1 NA）在 2 mm/s–5 mm/s 范围内，具体取决于 CNT 的尺寸。在我们的实验中观察到初级和次级涡流。在初级涡流中，随着 NA 增加 6 倍，测量到 CNT 的角速度增加了 10 倍。我们还讨论了液滴曲率在涡流形成中的重要作用。