Comment on biomimetic ultrathin whitening by capillary-force-induced random clustering of hydrogel micropillar arrays

Nanoscale Elastocapillary Effect Induced by Thin-Liquid-Film Instability

Dense arrays of high-aspect-ratio (HAR) vertical nanostructures are essential elements of microelectronic components, photovoltaics, nanoelectromechanical, and energy storage devices. One of the critical challenges in manufacturing the HAR nanostructures is to prevent their capillary-induced aggregation during solution-based nanofabrication processes. Despite the importance of controlling capillary effects, the detailed mechanisms of how a solution interacts with nanostructures are not well understood. Using in situ liquid cell transmission electron microscopy (TEM), we track the dynamics of nanoscale drying process of HAR silicon (Si) nanopillars in real-time and identify a new mechanism responsible for pattern collapse and nanostructure aggregation. During drying, deflection and aggregation of nanopillars are driven by thin-liquid-film instability, which results in much stronger capillary interactions between the nanopillars than the commonly proposed lateral meniscus interaction forces. The importance of thin-film instability in dewetting has been overlooked in prevalent theories on elastocapillary aggregation. The new dynamic mechanism revealed by in situ visualization is essential for the development of robust nanofabrication processes.

Complete wetting of elastically responsive substrates

We analyze theoretically complete wetting of a substrate supporting an array of parallel, vertical plates which can tilt elastically. The adsorbed liquid tilts the plates, inducing clustering, and thus modifies the substrate geometry. In turn, this change in geometry alters the wetting properties of the substrate and, consequently, the adsorption of liquid. This geometry-wetting feedback loop leads to stepped adsorption isotherms with each step corresponding to an abrupt change in the substrate geometry. We discuss how this can be used for constructing substrates with tunable wetting and adsorption properties.

Control of shape and size of nanopillar assembly by adhesion-mediated elastocapillary interaction

Control of self-organization of nanofibers into regular clusters upon evaporation-induced assembly is receiving increasing attention due to the potential importance of this process in a range of applications including particle trapping, adhesives, and structural color. Here we present a comprehensive study of this phenomenon using a periodic array of polymeric nanopillars with tunable parameters as a model system to study how geometry, mechanical properties, as well as surface properties influence capillary-induced self-organization. In particular, we show that varying the parameters of the building blocks of self-assembly provides us with a simple means of controlling the size, chirality, and anisotropy of complex structures. We observe that chiral assemblies can be generated within a narrow window for each parameter even in the absence of chiral building blocks or a chiral environment. Furthermore, introducing anisotropy in the building blocks provides a way to control both the chirality and the size of the assembly. While capillary-induced self-assembly has been studied and modeled as a quasi-static process involving the competition between only capillary and elastic forces, our results unequivocally show that both adhesion and kinetics are equally important in determining the final assembly. Our findings provide insight into how multiple parameters work together in capillary-induced self-assembly and provide us with a diverse set of options for fabricating a variety of nanostructures by self-assembly.

Stability of high-aspect-ratio micropillar arrays against adhesive and capillary forces

High aspect-ratio (HAR) micropillar arrays have many interesting and technologically important applications. Their properties, such as large mechanical compliance, large surface area, and a topography that is well-separated from the underlying substrate, have allowed researchers to design and explore biomimetic dry adhesives, superhydrophobic, superoleophobic, and tunable wetting surfaces, mechanical sensors and actuators, and substrates for cell mechanics studies. However, the mechanical compliance and large surface area of the micropillars also make these structures susceptible to deformation by adhesive and capillary surface forces. As a result such micropillars, particularly those made from soft polymers, can prove challenging to fabricate and to use in various applications. Systematic understanding of these forces is thus critical both to assemble stable micropillar arrays and to harness these surface forces toward controlled actuation for various applications. In this Account, we review our recent studies on the stability of HAR polymeric micropillar arrays against adhesive and capillary forces. Using the replica molding method, we have successfully fabricated HAR epoxy micropillar arrays with aspect ratios up to 18. The stability of these arrays against adhesive forces is in agreement with theoretical predictions. We have also developed a new two-step replica molding method to fabricate HAR (up to 12) hydrogel micropillar arrays using monomers or monomer mixtures. By varying the monomer composition in the fabrication process, we have fabricated a series of hydrogel micropillar arrays whose elastic moduli in wet state range from less than a megapascal to more than a gigapascal, and we have used these micropillar arrays to study capillary force induced clustering behavior as a function of the modulus. As a result, we have shown that as liquid evaporates off the micropillar arrays, the pillars bend and cluster together because of a much smaller capillary meniscus interaction force while the micropillar structures are surrounded by a continuous liquid body. Previously, researchers had often attributed this clustering effect to a Laplace pressure difference because of isolated capillary bridges. Our theoretical analysis of stability against capillary force and micropillar cluster size as a function of pillar elastic modulus agrees well with our experimental observations. The fabrication approaches presented here are quite general and will enable the fabrication of tall, stable micropillar arrays in a variety of soft, responsive materials. Therefore, researchers can use these materials for various applications: sensors, responsive wetting, and biological studies. The new insights into the capillary force induced clustering of micropillar arrays could improve rational design and fabrication of micropillar arrays that are stable against capillary force. In addition, these results could help researchers better manipulate capillary force to control the assembly of micropillar arrays and actuate these structures within novel devices.