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.

Inkjet Printing of Microcomponents: Theory, Design, Characteristics and Applications

“…For example, as demonstrated in Figure 3(b), pure water droplets spontaneously formed (self-assembled) the circular and stripe shapes on hydrophilic (glass) domains only of the structured surfaces, on which the glass were pre-patterned with hydrophobic (Teflon) domain by photolithography. Based on this concept of self-assembly on structured surfaces, stable solid formations of droplet deposition for striped channel (Chen et al, 2007) and circular shapes (Chen et al, 2009) were experimentally realized by inkjet printing of colloidal inks. …”

“…Some research efforts to avoid the non-uniform droplet deposit were recently reported (Chang et al, 2004;Chen et al, 2004;Weon & Je, 2010), since the deposit thickness is important for many applications such as biochips, LCD color filters, and light-emitting displays. Among them, special treatment on either homogeneous or heterogeneous surfaces plays a critical role on controlling the final deposit formations during evaporation, because of pinning or de-pinning condition as boundary constraints (Chen et al, 2009). …”

“…Based on the this kind of application, more persisting efforts were made within recent years about the improvement of ink formulas (Kim et al, 2009;Chang et al, 2011), surface treatments and characteristics (Koo et al, 2005;Chen et al, 2010), and large-area and active lighting (Bale et al, 2006). In a similar development way, more extensive studies for inkjet printing applications were also directed to other fields including biochips (Xu et al, 2005;Gutmann et al, 2005), optical microlenses (Nallani et al, 2006;Chen et al, 2009), microelectromechanical systems (Fuller et al, 2002;Cho et al, 2006;Alfeeli et al, 2008), and electronic components (Lee et al, 2005;Scandurra et al, 2010;Perelaer et al, 2010). Among them, many of the fundamental processes (e.g., surface treatments and droplet depositions) and components (e.g., inkjetprinted polymers and conductors) are in common with the present display applications, in particular the polymer light-emitting and liquid crystal displays (Katayama, 1999;Mentley, 2002;van der Vaart et al, 2005).…”

Inkjet Printing of High Aspect Ratio Superparamagnetic SU-8 Microstructures with Preferential Magnetic Directions

“…As a solution, drop-on-demand (DOD) IJP of polymers as a fast prototyping method allows maskless fabrication of various shapes [20], among which hemispherical structures with enhanced profile. Confining the ink either by adjusting the surface energy or topographically by edge confinement enables targeting specific aspect ratios and high thicknesses [21][22][23]. Technically, confining a liquid drop by pinning its contact line on a convex edge can be done with edges whose minimum heights are as low as the length range of the polymer chain [24,25].…”

Inkjet-Printed Polymeric Microstructures in $n$-Sided Regular Polygonal Cavities

“…In this case, the microfluidic scheme can be considered to be droplet vaporization deposition (DVD) at low temperature (less than 100 • C), significantly differing from the conventional hightemperature PVD and CVD processes, and also possesses additional precise-patterning ability to various shaping in single step without etching process involved [25], [26]. Based on a DVD process, the varying size hemispherical lenses of simplest circular footprints have been fabricated on either homogeneous [27] or heterogeneous [28] surfaces in our previous studies, demonstrating the self-formation and shaping controllability of droplets by surface tension. With small open cavities rather than flat surfaces with special treatment, the various microstructures in polygonal footprints [29] can be formed and released during droplet evaporation exhibiting drastic change of morphological surfaces from previous ones [27], [28].…”