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

• Applicant: Professor Dr. Steffen Hardt
• Subject Area: Fluid Mechanics
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 525057618

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.

• DFG Programme: Research Grants