Have you ever had the experience of placing Chinese coins on the water surface and keeping it afloat for a long time, even if their specific density is greater than the water? Have you ever noticed that water striders can walk on the water surface with their water-repellent legs? These phenomena arouse keen interest from biologists and chemists and it is difficult to be explained using our common knowledge of hydrophobicity and contact angle.

Researchers with Institute of Process Engineering, Chinese Academy of Sciences (IPE) found that floating coins can be well explained by mathematical modeling. In their work, the sharp corner was treated as a singular point in hydrostatic and thermodynamic analysis with respect to the floating of hydrophilic bodies. They proposed a hydrostatic analysis of a vertical cylinder with sharp corners floating on water surface. The sharp upper edge of a cylinder was modeled as revolutionary round surface on which the contact line anchors and moves as the weight of the cylinder was changed.

Foreign researcher modeled the edge effect by taking a sharp edge as a smoothed circular corner and interpreted successfully the resistance of sharp edges to spreading of liquids. IPE researchers followed this modeling method to explain physical mechanism for coins floating on the water. They conducted experiments to analyze the typical profile of surface anchoring at the corner, the effect of corner radius, cylinder radius, axisymmetry and hydrophilic solid cylinder. Results showed that the hydrostatic model for surface profile resulted in a third order ordinary differential equation. The numerical solution for practical conditions physically interpreted the floating coins and the edge effect. The numerical simulation was well agreed with the literature data.

Owing to their work the floating coins become an easily understood phenomenon, while the skating water strider still need more efforts. Researchers may improve the existing mathematical models by incorporating proper subscale or microscale structures and physical mechanisms to form multiscale models and enhance greatly their interpreting and predicting ability.

The paper was published in Journal of Colloid and Interface Science. Please see more results there.