Chemically heterogeneous surface, contact

C Cassie contact angle on a chemically heterogeneous surface... 

A treatment similar to Wenzel s has been given by Cassie and Baxter [69] for chemically heterogeneous surfaces. For the case of a surface consisting of two types of patches, for which the intrinsic contact angles are 0] and 02, respectively, minimization of the system free energy leads to the Cassie-Baxter equation ... 

Gao and McCarthy showed experimentally that the contact angle of a drop is defined by the triple line and does not depend upon the roughness under the bulk of the drop. A similar result for chemically heterogeneous surfaces was obtained by Extrand. " ... 

Fig. 3.12 (a) Configuration and materials for the two chemically heterogeneous surfaces 1 and 2, (b) water contact angles at t=0 and after the contact lines advance across the material boundary... 

Fig. 3.13 Schematics of the chemically heterogeneous surfaces fabricated by Gao and McCarthy, Surface a comprises a partially hydrolyzed hydrophilic spot (white) on a hydrophobic silicon wafer (gray) Surface b comprises a superhydrophobic (textured) spot on the hydrophobic wafer (white) d is the diameter for the heterogeneous spot and D is the diameter of the water droplet used in contact angle measurements (Reproduced with permission from [36], Copyright 2007 The American Chemical Society)...

When considering relea.se mechanisms, the physical and chemical heterogeneity of the adhesive/release interface cannot be ignored. At its most basic level, roughness of the release and PSA surface, the stiffness of the PSA and the method in which the PSA and release surface are brought together define the contact area of the interface. The area of contact between the PSA and release material defines not only the area over which chemical interactions are possible, but al.so potential mechanical obstacles to release. In practice, a differential liner for a transfer adhesive can be made to depend in part on the substrate roughness for the differences in release properties [21],... 

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