There is up to now no macroscopic brush-like device that is suitable for gentle particle removal. The reason is simple: small particles adsorb rather tightly to a flat surface due to strong adhesion forces. In modern semiconductor industry the production of microchips must be conducted in high-level class clean rooms in order to avoid the adsorption of dust (organic or inorganic micron- to submicron sized particles) on structured surfaces. In other words, in addition to providing a soap we implement selectivity: particles are mobilized and moved at the time of illumination, and only across the illuminated area. We propose the concept of light induced diffusioosmosis driving the flow, which can remove, gather or pattern a particle assembly at a solid-liquid interface. In this work we show that when a spatial gradient in the composition of trans- and cis- isomers is created near a solid-liquid interface, a substantial hydrodynamic flow can be initiated, the spatial extent of which can be set, e.g., by the shape of a laser spot. Such photo-soaps contain optical switches (azobenzene molecules), which upon illumination with light of appropriate wavelength undergo reversible trans-cis photo-isomerization resulting in a subsequent change of the physico-chemical molecular properties.
![malvern zetasizer nano zs zen1020 with sample malvern zetasizer nano zs zen1020 with sample](https://www.n8equipment.org.uk/img/equipment/zetasizer-nano-zs-16435.jpg)
With photosensitive azobenzene containing surfactants we overcome these limitations. The obvious idea of using detergents to ease these processes suffers from a lack of control: the action of any conventional surface-modifying agent is immediate and global.
![malvern zetasizer nano zs zen1020 with sample malvern zetasizer nano zs zen1020 with sample](https://www.cnfusers.cornell.edu/sites/default/files/styles/don_camera/public/tool-images/zetasizer.jpg)
The strong adhesion of sub-micron sized particles to surfaces is a nuisance, both for removing contaminating colloids from surfaces and for conscious manipulation of particles to create and test novel micro/nano-scale assemblies.