Self-aligned hemispherical formation of microlenses from colloidal droplets on heterogeneous surfaces

Abstract

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.