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This paper proposes a drop-on-demand (DOD) theory and scheme for constructing hemispherical refractive microlenses onto underlying heterogeneous (laterally structured) surfaces that consist of hydrophilic s-domains and hydrophobic p-domains. In theory, the drops would self-align themselves into the s-domains by repelling the p-domains due to surface tension, precisely determining the placement though disobeying the Young–Laplace equation. Using a droplet generator (inkjet printhead), in our experiments, evaporative polyurethane (PU) drops well fitted their footprints (base radii) onto the s-domains with a radius of 100 µm surrounded by the p-domains of Teflon, where the photoresist AZ4620 was used for lifting off the corresponding domains of Teflon. As a result, plano-convex shapes with spherical curvatures were fabricated in an array (spacing L ∼ 100 µm) with base (footprint) radius (Rb) ∼ 95 µm and curvature radius (Rc) ∼ 122 µm. Thus, both the theoretical and experimental results agreed well in a hemispherical shape (deviation < 5%) with the contact angle θ ∼ 51°. Further, such an angle could be tunable with Δθ over 20° simply by varying drop volume. In addition, one interesting and significant finding, for two (and more) overflow drops (Rb > Rs + L/2 = 150 µm, in excess of volume), indicates that more complex shapes than spherical ones (such as dumbbell and cross-like) can also be constructed by bridging the drops. Compared to those previous methods using photolithographic techniques, the present method is potentially appropriate for the varying radius and complex placement of array patterns.

We demonstrate an inkjet-based microfluidic technique as droplet vaporization deposition (DVD) that can be applied in the incorporation of hydrophobic substrates with microcavities to generate individual polymeric structures. Using soft-lithography method, the SU-8 and polydimethysiloxane were patterned to form various polygon-shaped cavities (side length ∼200-700 μm and depth ∼25-110 μm) which acted as micromolds (templates) for forming the shapes of the droplets deposited. With aqueous polyurethane droplets generated (single volume ∼381 pL), the novel various microstructures in polygonal (tri-to hexagonal) forms were created through the deposition and evaporation processes and characterized with exotic performances in optics such as those of microlenses and micromirrors. Experimental and analytical results illustrated that the curvedsurface (sinusoidal) topographies of the structures self-formed and detached from the underlying substrates were dominated by capillary action of fluid during evaporation. The merits over the existing techniques such as photolithography includes the one-step, low-temperature (ambient condition, ∼1 atm at 25 • C), and cost-effective (no waste materials) processes of solid formations. The successful formation of these structures suggests a new mechanism for self-releasing/detaching from the molds. As a reliable technique for the fabrication of such microstructures, the present DVD method presents an alternative method for various applications particularly including optical microelectromechanical system devices and parts used for assembly.[2010-0238]

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