Scientists Have Described a Superhydrophobic Water Slide with Record Accuracy

superhydrophobic
Microsphere attached to the tip of an atomic force microscope moves toward a superhydrophobic surface. Source: Taras Molotilin

An international team of scientists led by Professor of the Physics Department of Moscow State University and head of the laboratory IPCE RAS Olga Vinogradova was the first to accurately describe the behavior of the fluid near the superhydrophobic surface of the check received analytical expressions in the experiment. A new study published in the journal Soft Matter.

Near certain surfaces of the fluid flow has the unusual property that its speed is not zero even in the wall directly adjacent to the layer (i.e., the liquid does not adhere, and slides along the surface). This effect is called the hydrodynamic slip, and was first described more than two hundred years ago, but has since attracted little attention, as did not have any significant effect on the overall flow of the fluid.

The situation changed significantly with the advent of superhydrophobic materials in which a chemical surface hydrophobicity combined with an unusual relief (for example, grooves, microcolonies). The grooves of such bubbles remained texture in which the liquid can slide practically without resistance, which significantly increased the length of sliding on these surfaces.

Sophisticated superhydrophobic surfaces require new hydrodynamic theories to describe them. New approaches have predicted not only the reduction of viscous drag, and the unusual behavior of liquids in the vicinity of some anisotropic (ie, having the properties which depend on the direction) surfaces. For example, near the wall coated extended grooves directed at an angle to the main flow, the fluid can be rotated to the side, causing an active mixing, separation or embedded therein sized particles.

A team of researchers of the Lomonosov Moscow State University and the IPCE in recent years, developed a theory of hydrodynamic plain near anisotropic superhydrophobic surfaces, but still in the experiment, these theories could verify only indirectly. In the new work by using an atomic force microscope, the scientists were able not only to accurately determine the length of the slip, but also to verify the analytical formulas describing the behavior of the fluid at different distances from the superhydrophobic surface.

Using a microscope, allowing a constant rate for lowering a spherical microparticle superhydrophobic surface is immersed in the liquid. In this case both the experimenter could accurately track the position of the sphere in the channel, and to measure the force exerted on it by the fluid. The authors obtained an exact theoretical solution for this process, on the basis of what has been measured slip length of the experimental dependence of the hydrodynamic forces on the heights above the superhydrophobic surface.

According to the authors, the results will serve as a starting point for the development of new systems superhydrophobic. Now, when the accuracy of the proposed theories no longer in doubt, the scientists were able to use many theoretical ideas previously obtained. Among them, the separation of the particles in the superhydrophobic channels system electroosmotic flow and more.