The gravity effect on the shape of a sea-water drops on a superhydrophobic surface during evaporation

Abstract

Nowadays there are a lot of discussions about the validity of the classical Young equation, which determines the angle of contact between the surface of a solid and a liquid, since it does not take into consideration the effect of gravity. There is an analysis of the size of a sea-water drop on a superhydrophobic surface obtained on a sample of A40S shipbuilding steel, taking into account the ratio of the surface tension and gravity force, made in the paper.

In the given experiment, the superhydrophobic layer was created by aerosol spraying of the two-component chemical solution of the NeverWet Base Coat system. Optical observations and photography were carried out on a specially designed unit for further graphic analysis of the geometry of the drop.

The balance of vertical force projections was controlled by determining the Bond number when measuring the diameter of the perimeter of wetting by drops of a solid surface. The deformation of the shape was estimated through the ratio of the horizontal and vertical diameters in the middle section of the drop. During the evaporation, a drop of dilute sea water changed its volume in the interval $V=10-0,5$~$\mu l$, at the end of the test impurity concentration corresponding to the natural sea water of the Kerch Strait was reached.

It is shown that there is a noticeable variation in the value of the contact angle for drops with a volume of more than $1\mu l$, which at various stages of evaporation can reach 2--10$^\circ$. With a decrease in $V=0,5$~$\mu l$, such a divergence was absent, and the contact angle assumed its maximum value $\theta_{C}=165^\circ$.

Such studies contribute to a realistic assessment of the behavior of sea water droplets on the superhydrophobic surfaces of materials used in marine technology.