Abstract
This paper proposes a method of micro-fabrication for the formation of complex non-planar shapes by depositing a colloid evaporative droplet onto topographically structured surfaces. The micro-droplet composed of polyurethane (PU) was self-driven by surface tension to adjust their three-dimensional (3D) shapes bound with the surface of the micro-well array. The micro-wells formed from poly-dimethysiloxane (PDMS) consisted of vertical sidewalls to constrain the fluids as boundary conditions in the Young-Laplace equation until drying. Two types of wetting regimes (fully and partially), corresponding to different droplet volume and allocation, were categorized to perform the shaping process in which evolving fluid contours were self-formed utilizing the general principle of minimal surface energy with certain features of the shape set by the wetting of the PDMS. Using the heterogeneous surfaces, slight-concave circular terraces and half-moon shapes with high curvature were fabricated with micrometer dimensions (well diameter of 900 νm). The formed structures were observed to release themselves from the hydrophobic wells by the de-wetting (de-pinned) process in the completion of evaporation. Moreover, the effects of the boundaries were further explored for half-moon shapes by giving three distinct footprints of the partially wetting droplets. In these cases, both experimental results and numerical calculations were performed and compared to illustrate the significant influence of the fluid contact angle (∼90°) and position with a curved boundary line on final formed shape, in particular for the change of curvature ( = 0.62-1.36). Compared to those traditional assays operated on two-dimensional (2D) flat surfaces, this structured-well one could greatly enhance the control of 3D topographic formation in terms of aspect ratio, thickness and curved degree. This novel operation of micro-fabrication is also appropriate for smaller and complex structures by using drop-on-demand inkjet for commercialized mass-production processes. © 2008 IOP Publishing Ltd.