Effects of Capillary Forces on a Hydrogel Sphere Pressed against a Surface

A theoretical treatment is provided for effects of capillary forces on a hemispherically shaped hydrogel sample pressed against a solid hydrophilic surface. It is pointed out that the adhesion of a hydrogel to a surface resulting from capillary forces is different from that of a nonporous solid because of the porous nature of the hydrogel. Because of this, the Laplace pressure subtracts from the osmotic pressure inside the gel. For neutral gels, it can exceed the osmotic pressure, causing the gel to deswell. For charged gels, since the counterions inside the gel generally provide much higher osmotic pressure than that due to monomers alone (which is the only source of osmotic pressure in neutral hydrogels), the Laplace pressure is less likely to make the gel deswell. The Laplace pressure can, however, be large enough to deswell asperities (due to surface roughness) on the gel surface, increasing the contact area. This could result in an increase in the friction and ionic electrical conductivity between the gel and the surface (if the surface is an electrical conductor).