Surface pattern recognition by a colloidal particle

This paper's focus is on understanding how surface heterogeneity alters sur face forces. An atomic force microscope was used to measure interactions be tween a 20 mum hydrophobic (-CH3) sphere and a patterned self-assembled mon olayer surface of alkanethiols that consists of hydrophobic (-CH3) and hydr ophilic (-COOH) stripes of controlled width from 0.27 to 2.9 mum. Of signif icance 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 leng th scale much larger than the sphere cannot affect the force. The results d emonstrate that hydrophobic and hydrophilic interactions are not additive a nd the net interaction between these kind of surfaces depends on the relati ve size of hydrophobic and hydrophilic sites on the surface. Independent of whether the hydrophobic sphere is positioned on top of a hydrophobic or hy drophilic area, we measure repulsions, with the same strength and decay len gth of the electrostatic interaction. The magnitude of the heterogeneous en ergy is greatest when the patch size is smallest. Our results indicate that interactions,jump into contact distances and adhesion data, between a 20 m um methyl terminated sphere and a heterogeneous surface that has equal meth yl and carboxylic acid areas will average. In all other cases, when the sur face has unequal amounts of hydrophobic and hydrophilic areas, the hydropho bic sphere will clearly differentiate between the different patches and wil l accurately map high and low adhesion areas on the surface.