This paper's focus is on understanding how surface heterogeneity alters surface forces. An atomic force microscope was used to measure interactions between a 20 μm hydrophobic (−CH3) sphere and a patterned self-assembled monolayer surface of alkanethiols that consists of hydrophobic (−CH3) and hydrophilic (−COOH) stripes of controlled width from 0.27 to 2.9 μm. Of significance to this study is the ratio of the particle radius to the stripe size. This ratio varied from 3.5 to 37, as a heterogeneous patterning on a length scale much larger than the sphere cannot affect the force. The results demonstrate that hydrophobic and hydrophilic interactions are not additive and the net interaction between these kind of surfaces depends on the relative size of hydrophobic and hydrophilic sites on the surface. Independent of whether the hydrophobic sphere is positioned on top of a hydrophobic or hydrophilic area, we measure repulsions, with the same strength and decay length of the electrostatic interaction. The magnitude of the heterogeneous energy is greatest when the patch size is smallest. Our results indicate that interactions, jump into contact distances and adhesion data, between a 20 μm methyl terminated sphere and a heterogeneous surface that has equal methyl and carboxylic acid areas will average. In all other cases, when the surface has unequal amounts of hydrophobic and hydrophilic areas, the hydrophobic sphere will clearly differentiate between the different patches and will accurately map high and low adhesion areas on the surface.
- Kokkoli, E., and Zukoski, C.F. "Surface Pattern Recognition by a Colloidal Particle", Langmuir, 2001, 17 (2): 369-376.
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