A theory for designing capillary systems of constant interaction

Abstract The attraction or repulsion force between two solid objects at a liquid-gas interface (i.e. the lateral capillary interaction force) is generally considered to be highly dependent on both the shapes of the solid surfaces and the relative positions. To challenge this, we propose an exotic capillary system in which the stationary exotic plate piercing the interface can produce a constant capillary interaction force on a partly immersed cylinder at different horizontal positions. This constancy is owing to the counterbalancing of variations in the horizontal components of the surface tension force and the hydrostatic pressure force on the cylinder. Through analysing the meniscus profiles and the capillary interaction force in the proposed exotic capillary system, five types of exotic plate shapes are identified. More importantly, a further analysis indicates that the lateral capillary interaction force in the proposed exotic capillary system remains constant even for different vertical positions of the cylinder. To verify the practicality of our theory, a three-dimensional numerical investigation is conducted, and the results show that the lateral capillary interaction force can remain nearly constant as expected. The proposed capillary system provides new insight into controlling the capillary interaction force. The constant-force regimes suggest passive routes for interfacial transport and micromanipulation, force regulation and suppression of undesired capillary aggregation.