Olga Vinogradova, Professor at the Faculty of Physics, the Lomonosov Moscow State University, Director of laboratory at the Frumkin Institute of Physical chemistry and Electrochemistry (the Russian Academy of Sciences), together with Salim Maduar, a junior researcher of her group, being a part of an international scientific team, have suggested a new method of manipulation of microparticles at solid-liquid interface in water. The research has been published in Scientific Reports -- a journal of Nature Publishing Group.
Scientists have suggested a method, based on addition of a photoresponsive surfactant into water. Illumination of this surfactant with laser light results in a formation of fast waterflows moving particles. This allows one to manipulate particles, for instance, clean surfaces from contaminations without any risk of damaging. On the contrary, it allows one also to gather or pattern a micro- and nanoparticle assembly of necessary configuration and size at a solid-liquid interface.
The key component of the proposed method is a photosensitive surfactant, which can change its conformation under illumination of light of appropriate wavelength. In one situation it looks like a rod, in another - like a tick. If you illuminate a solution of such a surfactant, you'll see that molecules inside a light spot will change conformation and the system will generate concentration gradients of "rods" and "ticks".
Theoretical physicists from Moscow have explained that concentration gradients near charged solid-liquid interface lead to an unusual phenomenon - a diffusio-osmotic flow, which allows one to manipulate particles at a solid-liquid interface. The authors have shown that if you select laser wavelength correctly, you could make particles move in the required direction - to remove them out from the light spot or, in contrast, gather towards its center. Scientists have succeded to describe the system theoretically, what has allowed, in its turn, to optimize conditions providing a highest liquid velocity. It was found that diffusio-osmotic flow is very sensitive to whether water is salty or pure. In the last case the velocity could increase in several times.
In spite of the fact that the initial aim of the new method was soft cleaning of surfaces, such as semiconducting crystals for microelectronics, scientists have also found several unusual applications. For instance, moving a laser spot, you could "draw" on the surface, as laser will leave a visible trace with enhanced or, oppositely, decreased concentration of microparticles. In the original article authors also show photos and videos, where the logo of the University of Potsdam, a "happy man" and a "heart" shaped pattern - all made of microparticles by lateral repositioning the laser spotacross the solid liquid interface.