An atomic force microscopy study of the adhesion of a silica sphere to a silica surface - Effects of surface cleaning

An atomic force microscope (AFM) has been used to quantify directly the adhesive interactions between a silica sphere and a planar silica surface. Electrostatic double-layer interactions have also been quantified through analysis of approach curves. The surfaces of the sphere and planar surface were treated prior to measurements either by ethanol washing or by plasma treatment. Adhesion forces were then measured in 0.01 M NaCl solutions at pH 3 and 8. The adhesion force did not vary greatly with pH for a given cleaning procedure. However, the magnitudes of the adhesion forces were substantially less for the plasma treated surfaces. The adhesion forces did not vary systematically with the loading force. Agreement of the adhesion measurements with theory (DLVO, using a non-retarded Hamaker constant based on the latest interpretation of spectroscopic data for water) was good for the ethanol treated surface at pH 3 - conditions where double layer interactions are negligible. However, the plasma treated surface at pH 3 showed adhesion an order of magnitude lower than calculated. In contrast, adhesion at pH 8 was in both cases greater than theoretical expectations, though the lower adhesion for the plasma treated surface was in quantitative agreement with the increased electrostatic double-layer interactions induced by plasma treatment. The results show that the adhesion of such surfaces is a complex phenomenon and that non-DLVO interactions probably play a substantial overall role.