Exploring the space of light-induced flow patterns on liquid surfaces

当用合适波长的光照射液体表面时，用光可切换表面活性剂修饰液体表面允许产生流场。辐照引起光异构化，即表面活性剂的两种异构体状态之间的过渡。这两种不同的状态会产生不同的表面张力，这就是为什么光照射模式可以产生马兰戈尼流。在过去，我们已经证明，在此基础上，小颗粒可以利用由马兰戈尼流引起的水动力沿着液体表面拖动。大部分的初步工作是基于一个单一的激光光斑聚焦在液体表面。现在，我们想在更一般的背景下探索光诱导流模式。我们的目的是揭示光强的空间分布和由此产生的流动模式之间的关系。为此，我们利用空间光调制器来创建几乎任意的照射模式，并通过跟踪附着在液体表面的粒子来测量产生的流场。实验方法依赖于增加辐照模式的复杂性。这些实验将伴随着建模/模拟活动，旨在揭示流场产生的基本原理，如叠加原理。在更广泛的背景下，目标是在解决反问题方面取得重大进展，即确定产生给定流场所需的辐照模式。当大量的微或纳米颗粒附着在液体表面时，光诱导对这些微或纳米级物体的操纵和重排可以为快速发展的光学超表面领域开辟新的前景。具体来说，潜在的未来应用可能在于可重构光学超表面领域。

Decorating a liquid surface with photoswitchable surfactants allows creating a flow field when the surface is irradiated with light of a suitable wavelength. The irradiation induces a photoisomerization, i.e. a transition between two isomeric states of the surfactants. The two different states give rise to different surface tensions, which is why light irradiation patterns allow creating Marangoni flows. In the past, we have demonstrated that on this basis, small particles can be dragged along a liquid surface utilizing the hydrodynamic forces due to the Marangoni flow. Most of the preliminary work was based on a single laser spot focused on a liquid surface. Now, we would like to explore the photoinduced flow patterns in a more general context. We aim at uncovering the relationship between the spatial distribution of the light intensity and the resulting flow patterns. To this end, we utilize a spatial light modulator to create almost arbitrary irradiation patterns and measure the resulting flow field by tracking particles attached to the liquid surface. The experimental approach relies on ramping up the complexity of the irradiation patterns. The experiments will be accompanied by modeling/simulation activities that aim at uncovering principles underlying the generation of flow fields, such as the superposition principle. In a broader context, the goal is to make significant progress towards solving an inverse problem, which is to determine the irradiation pattern required to create a given flow field. In a scenario in which a multitude of micro- or nanoparticles are attached to a liquid surface, the light-induced manipulation and rearrangement of these micro- or nanoscale objects could open up new perspectives in the rapidly developing field of optical metasurfaces. Specifically, a potential future application could lie in the area of reconfigurable optical metasurfaces.