Image of a PDMS surface and definition of the coordinate system.

(a) Topographic Atomic Force Microscope (AFM) image of a PDL coated PDMS patterned surface (top), and example of an AFM line scans obtained across the surface (bottom). (b) Coordinate system and the definition of the angular coordinate θ used in this paper. The x axis is defined as the axis perpendicular to the direction of the PDMS patterns. The directions corresponding to θ = 0, π/2, π, and 3π/2, and the pattern spatial period d (defined as the distance between two neighboring ridges) are also shown in (a). The line scan in (a) demonstrates that the patterns are periodic in the x direction, and have a constant depth of approximately 0.5 μm. The pattern spatial period is d = 3 μm.