How to remove dirt easily
It’s not all the same when it comes to dirt. Dust adheres only slightly to surfaces. But there is also dirt, such as dried paint, which adheres strongly. But how can the adhesive properties of a surface be specifically adjusted so that different types of dirt do not stick to it? This knowledge is essential to understand and minimize the contamination of surfaces by dirt particles. Scientists at the Max Planck Institute for Polymer Research (MPI-P) in Mainz have addressed this question.
All surfaces in our daily lives become dirty over time with particles such as dust, pollen or microorganisms. Therefore, surfaces that can be easily cleaned are desirable - i.e. surfaces where dirt particles are removed by rain, for example. Surfaces from which water drops simply roll off are promising candidates for this. Due to the low adhesion of water droplets and the resulting self-cleaning properties of the surface, they are called "super-hydrophobic" - i.e. super-water-repellent - surfaces. These surfaces are characterized by their micro-roughness, i.e. roughness in the range of a millionth of a meter, thus significantly reducing the contact area to water drops.
For a long time, however, it was not understood exactly how the effect of self-cleaning works on a microscopic level and how surfaces must be produced in order to function as effectively as possible. Scientists around Prof. Dr. Doris Vollmer and Dr. Rüdiger Berger (working group of Prof. H.-J. Butt) have now gained new insights into the self-cleaning process by microscopically imaging such a surface in the micrometer range. The special microscopy method, which uses a laser as the light source, allowed them to image how a drop of dirt particles rolls over the surface. This enabled them to show that a drop on a superhydrophobic surface contaminated with dirt particles is essentially only in contact with the dirt particles themselves - in other words, it hardly makes any contact with the surface. However, the size of the particles compared to typical length scales of surface roughness is essential for this. Depending on the surface, the particle size can vary between a few tens of nanometers and several micrometers.
"A surface works effectively when the length scale or pore size of the superhydrophobic surface is smaller than the dirt particle itself," says Doris Vollmer. "Then dirt is completely removed, for example from rain."
In a further step, the scientists have verified the obtained results using laser-based microscopy by means of force measurements. For this purpose, they used a highly sensitive measuring method developed at the MPI-P, which allows the friction of drops to be measured. They were thus able to show that the force required to move the drop results from the number of dirt particles and the adhesive force between the particles and the surface.
These very precise force measurements enabled the authors to make another important statement: Particles are only removed effectively if the adhesion between drop and particle is greater than the adhesive force between particle and surface. Then the dirt is carried away by the drop.
The rules they established in this way, which are intended to facilitate the design of a dirt-repellent surface, have been verified with particles of various sizes and types. Interestingly, particles such as dust behave similarly to carbonaceous substances such as soot.
Source: Chemie.de - Wie man Schmutz einfach entfernt
Original article: Science Advances - When and how self-cleaning of superhydrophobic surfaces works