Humidity influences the adhesion between the surfaces of hydrophilic particles. One reason is capillary condensation. Under ambient conditions, water condenses into the gaps between neighboring particles. Such a water meniscus causes an attrac¬tive force, called capillary force. Several aspects of capillary forces are analyzed experimentally and theoretically:

1. Capillary forces of hydrophilic particles sensitively depend on the surface structure on the 0.5 nm scale. As a result, surface roughness has a drastic effect. A simple, approximate formalism was developed to take surface roughness and heterogeneity into account when calculating capillary forces analytically.
   • Butt, H.-J.: Capillary forces: Influence of roughness and heterogeneity. Langmuir 2008, 24, 4715-4721.
   • Butt, H.-J. & M. Kappl: Normal capillary forces. Adv. Colloid Interface Sci. 2009, 146, 48-60.
2. Measurement of capillary forces by colloid probe experiments allows to characterize certain properties of thin liquid films, such as the film thickness, the viscosity and the surface tension.
   • Ally, J., E. Vittorias, A. Amirfazli, M. Kappl, E. Bonaccurso, C.E. McNamee & H.-J. Butt: Interaction of a microsphere with a solid supported liquid film. Langmuir 2010, 26, 11797-11803.
3. Capillary forces between soft elastic surface are stronger than between relatively stiff surfaces. The capillary force was calculated and a criterion is suggested to distinguish between soft and stiff.
   • Butt, H.-J., W.J.P. Barnes, A. Del Campo-Becares, M. Kappl & F. Schoenfeld: Capillary forces between soft, elastic spheres. Soft Matter 2010, 23, 5930-5936.
4. Strong electric fields enhance capillary condensation. A generalized Kelvin equation has been derived to quantify this phenomenon. It is relevant for scanning tunneling microscopy and various AFM based nanolithographies.
   • Butt, H.-J., M. Untch, A. Golriz, S.A. Pihan & R. Berger: Electric field-induced condensation: An extension of the Kelvin equation. Phys. Rev. E 2011, 83, 061604.