The scientists have used so-called Janus particles – in this case polystyrene particles with a diameter of approx. 1 µm and half-coated with gold. The particles were dispersed in water between two glass slides and illuminated from below with green laser light. As a result, the particles started to move – propelled by the asymmetrical local heating.
Using laser microscopic techniques the researchers could show that the surface properties of the glass slides has a significant influence on the velocity of the particles. Above a bare negatively charged glass surface a temperature gradient induced by thermal radiation from the gold-coated hemisphere leads to a concentration gradient of ions within the electric double layer. The consequence: Further local micro flows make the particle tilt toward the substrate. Thus, the laterally resulting velocity is decreasing and the particles become slower overall.
Enabling targeted drug delivery
However, if the glass slide is functionalized with a structure of uncharged polymer brushes, the ion concentration at the brush-water interface is lower – resulting in less tilting local micro flows and thus a higher particle velocity. Additionally, the researchers have shown how this velocity is influenced by various polymer chain lengths.
The results are relevant for targeted drug delivery in biological systems where these swimming particles would often make contact with complex surfaces. The findings also lay the foundation for future experimental studies of the collective behaviour of micro-swimmer systems in complex environments – a research area which the group Theory of Soft Matter (Benno Liebchen) focuses on. Appropriate experiments could serve as a bridge between experimentally controllable artificial systems and functional biological matter.
The article was published as “supplementary cover” in the journal Langmuir and shows an example where research objectives of two profile areas (Thermo-Fluids & Interfaces and From Materials to Product Innovations) interlink.
Sebastian Keuth/Regine von Klitzing/mho
M. Heidari, A. Bregulla, S. Muinos Landin, F. Cichos, R. von Klitzing: Self-propulsion of Janus particles near a brush-functionalized substrate, Langmuir 2020, https://doi.org/10.1021/acs.langmuir.0c00461