Microstructures formation by deposition of toluene drops on polystyrene surface

Here we develop a new approach for producing diverse microstructures by deposition of nano-litre solvent droplets onto polymer surface, which is based on a syringe system coupling with an adjustable substrate stage. Two basic procedures, contact mode and non-contact mode, are used for providing either the sessile drop or the pendent drop. In the contact mode, the influences of process parameters and intrinsic parameters are extensively investigated. By varying the process parameters such as the substrate-approaching and retraction speeds and the delay between these two movements, the microstructures can be tuned from concave to convex, whereas by varying intrinsic parameters such as the initial drop volume,V-shaped and U-shaped concave structures can typically be generated. The drying of these microstructures can partially remove entrapped solvent from the swollen polymer material, and the removed solvent volume is found to correspond to the width of microstructures. However, the shapes of these microstructures show no changes before and after drying. Ripples perpendicular to the stretch direction appear in the center of the microstructures for the stretched polymer substrate and become much more regular with increasing stretch ratio. As a sessile solvent drop is replaced by a pendent solvent drop, a non-contact mode results. Without direct contact between the solvent and the polymer substrate, solvent vapor diffuses from the droplet surface into the polymer matrix and eventually provides concave structures. The shape of the concave structures is dependent on the exposure time of the solvent vapor.