(4a) AFM Measurements of Adhesion Force on Hydrophobic Surfaces

Hydrophobic interactions between particles as well as between particles and surfaces in liquid phase are an emerging field, which gives an additional degree of freedom to tailor functional particle systems or to improve the particle based processes. The origin and the contributions of hydrophobic long and short range forces are still in discussion. The structuring of water at the solid surface, macroscopic capillary forces and nano-bubbles are used to explain the interactions. Using colloidal-probe AFM (cp-AFM) fundamental contact mode measurements have been carried out. The data of both, approach and retrieval of the cp-probe, have been exploited to characterize and to classify the type of interaction force. The focus of these investigations has been on attractive forces, forces distributions which derive from application properties like surface roughness respectively.

The experiments have been conducted using alumina surfaces. To modify the surface organic silane chemistry has been used, the maximum contact angle to pure water amounts to 104°. The properties of the pore liquid has been adjusted via the mixing ration of water to alcohol (ethanol / methanol). The use of liquid mixtures also shows selective adsorption on the hydrophobic surface – the organic compound acts as a surfactant.