Laser printing hierarchical structures with the aid of controlled capillary-driven self-assembly

Abstract

Capillary force is often regarded as detrimental because it may cause undesired distortion or even destruction to micro/nanostructures during a fabrication process, and thus many efforts have been made to eliminate its negative effects. From a different perspective, capillary force can be artfully used to construct specific complex architectures. Here, we propose a laser printing capillaryassisted self-assembly strategy for fabricating regular periodic structures. Microscale pillars are first produced by localized femtosecond laser polymerization and are subsequently assembled into periodic hierarchical architectures with the assistance of controlled capillary forces in an evaporating liquid. Spatial arrangements, pillar heights, and evaporation processes are readily tuned to achieve designable ordered assemblies with various geometries. Reversibility of the assembly is also revealed by breaking the balance between the intermolecular force and the elastic standing force. We further demonstrate the functionality of the hierarchical structures as a nontrivial tool for the selective trapping and releasing of microparticles, opening up a potential for the development of in situ transportation systems for microobjects.laser printing | capillary force | self-assembly | hierarchical structures | microobject trapping P eople learn from daily life experience that individual filaments can coalesce, as with wet hairs or paintbrushes pulling out of paint. Analogs of this elastocapillary coalescence exist and are often amplified in micro/nanoelectromechanical system manufacturing processes because the capillary force tends to dominate over the standing force that needs to be overcome for coalescence when the scale is reduced (1, 2). When one aims to create slender structures, the dominant capillary force drives them to collapse, cluster, or be completely destroyed. Some efforts such as adopting a supercritical-point dryer (3, 4) have been made to eliminate these unwanted behaviors when fabricating high-aspect-ratio structures. However, from another point of view, the capillary-driven bottom-up self-assembly can be exploited as a valuable tool to construct complex architectures. Compared with other driving forces for self-assembly such as magnetism (5), electrostatic force (6), and gravity (7), capillary force self-assembly features the advantage of simplicity, low cost, scalability, and tunability.The desire for scalable and cost-effective self-assembly schemes comes from observations of the natural world, where self-assembled filamentous structures with mechanical compliance at the microscopic and mesoscopic scale are ubiquitous and include, to name a few, actin bundles (8), gecko feet hairs (9), and Salvinia leaf (10). These structures give rise to diverse functions ranging from mechanical strengthening (11) and directional adhesion (12, 13) to superhydrophobicity (14) and structural colors (15), thus inspiring researchers to devote their efforts to mimic these multifunctional structures for decades. Many top-down fabricat...

Self‐Sealed Bionic Long Microchannels with Thin Walls and Designable Nanoholes Prepared by Line‐Contact Capillary‐Force Assembly

“…Compared with our previous work, the capillary‐driven self‐assembly channel here keeps stable because of the increased L and h a . In our experiment, the channel does not open in liquid.…”

“…The fabrication process of microchannels based on the enhanced “line‐contact” Van der Waals' force is described schematically in Scheme . In this experiment, NOA61 (Norland, USA) is chosen as the photo‐polymerization material due to its excellent elasticity compared with the SZ2080 photoresist (IESL‐FORTH, Greece) used in our previous works . Microwalls, whose location and height can be accurately determined by the nanotranslation stage (PI, E545), are readily fabricated by focusing a femtosecond laser (Ti:sapphire, Chameleon vision‐S, Coherent, wavelength 800 nm, repetition rate 80 MHz and pulse duration 75 fs) beam into the photosensitive polymer with an objective lens (50×, numerical aperture (NA) = 0.8).…”

“…In these varied channels, F c , F s , and F v change because of changing parameters according to Equations –, which proves the flexibility of this fabrication strategy. It is worth noting that though the cross sections of microchannels fabricated here are mainly triangles, which is useful in some applications, for example, the separation of microparticles, our method can also realize curve cross section by controlling the mechanical strength of microwalls in printing.…”

Investigation and Prediction on Regulation of Hydrophobicity of Polymethyl Methacrylate (PMMA) Surface by Femtosecond Laser Irradiation

“…Chen et al [11] developed a rapid one-step electrodepositing process to fabricate superhydrophobic cathodic surface on copper plate in the electrolytic solution containing nickel chloride, myristic acid and ethanol. However, the applications of these methods are limited due to their high cost, complicated process and large time consumption [12]. At present, the above processing methods of superhydrophobic structures have been widely used, but they are not so effective in exploring the rule of the contact angle.…”

Rapid Fabrication of Hydrogel Microstructures Using UV-Induced Projection Printing

“…For example, femtosecond laser polymerization was applied to fabricate microscale pillar arrays which assembled into periodic hierarchical architectures can, through the aid of capillary-driven self-assembly, be designed to trap and release micro-objects, which is a useful feature for biomedical devices or chemical analysis [10]. The biocompatibility and excellent mechanical properties of hydrogels also make them well suited to bioengineering, with micropatterned poly(ethylene glycol methacrylate) (PEGDMA) brushes patterned with synthetic peptides using photomasks already being used to study cell migration [11].…”