ABSTRACT
Nano to micro-scale patterning of the surface of a soft elastomeric layer is important for variety of practical and technological applications, e.g., for creating superhydrophobic surfaces [1, 2]; generating structural colour [3]; biological [4] and chemical sensors [5]; scaffolds for tissue engineering [6]; micro-electromechanical systems [7]; patterned adhesives [8]; and so on. Many of these applications demand not just patterning of the material surface but hierarchical patterning and in many cases it is not just physical texturing of the material but spatial modulation of one or more physical properties [9] also becomes important. These applications demand novel fabrication methods which can generate patterns with controlled geometric
lengthscales yet economical to implement over a large area. Beyond traditional lithographic routes, several bottom-up approaches have been devised which have several advantages [10-16]: they do not require any mask or any sophisticated and expensive equipment, are not limited by the optical wavelength of light used for developing the photoresist, involve fewer steps and do not require expensive and often toxic chemicals to be handled. Here we present a novel technique in which surface undulations can be created by embedding microchannels in the bulk of an elastic film and filling in these channels with a liquid that wets its surface. Such a liquid forms concave meniscus inside the channel so that pressure in the liquid remains sub-atmospheric. We show that this pressure alters the stress field in the elastic wall around the channel which finally results in buckling of the thin skin above the channel. The liquid pressure alters also the shape of the channel and its cross-sectional area. In fact, the wetted curved surface of the channel increases, releasing interfacial energy which can increase the elastic energy of the layer. The buckling phenomenon leads also to modulation of the shear modulus of the layer which does not remain isotropic but varies along different directions. As a result, when a hemi-spherical indenter is brought in contact with this film the contact area does not remain circular but depending on the orientation of the channel, a near-elliptic contact area appears whose major axis lies along the length of the channel. For films with very thin skin, contact area with sharp corners appears, the height to width ratio (hc/wc) of which decreases with increase in the contact load. In fact, hc/wc ratio varies differently during loading and unloading signifying different stress profiles in the film in the vicinity of the channel. This simple method can be useful for generating complex patterns in a controlled manner.
