Multimaterial surfaces featuring hierarchical structures hold significant promise across various domains, including self-cleaning mechanisms, droplet manipulation, microfluidics, and biomedicine. These applications arise due to the diverse functionalities resulting from both structural and material combinations.

In this context, we introduce an innovative and sustainable manufacturing technique called acoustic assembly photopolymerization (AAP) to create such functional structures. The AAP method utilizes an external acoustic field to arrange microparticles into microsized patterns while integrating photocuring to yield multilevel hierarchical features, spanning nanometer to micrometer scales. We further investigated the mechanics of this novel multimaterial manufacturing approach and established a model that links process parameters with resulting surface structures.

Our investigation involves the impact of material distribution and hierarchical topology on static contact angles (θ: varying from 76.72° to 160.54°), anisotropic wetting (Δθ: ranging from 0° to 87°), and interfacial bonding between particle-polymer composites (55.31 MPa to 98.06 MPa). To showcase potential applications, we developed three microreactors devices and demonstrated automated droplet manipulation. Notably, droplets on our proposed surfaces undergo continuous directional transport, fostering efficient passive mixing without external manipulation. Furthermore, we extend our surface fabrication technique to fog harvesting capability, showing enhanced capacity from 178 to 315 mg compared to flat surfaces.

In conclusion, the AAP process offers a fresh avenue to fashion multimaterial hierarchical surfaces endowed with predetermined functionalities capable of autonomous tasks without relying on external power or complex wiring systems. The AAP technology holds promise for diverse applications spanning microfluidics, tissue engineering, and potentially mechanical and battery systems, among other fields.

Multimaterial microstructures hold significant promise across diverse applications, particularly in miniature droplet control devices, owing to their versatile and distinctive hydrodynamic property. Despite these advantages, present-day microfabrication techniques face several challenges as they rely on surface modifications or a combination of two or more techniques, with limited control over the precision and accuracy of multimaterial designs. 

This project introduces a novel microfabrication approach named acoustic streaming-assisted two-photon polymerization (AS-TPP) to produce miniature devices with spatially varied material distribution. Here, we utilized acoustic streaming to locally control the hydrophilic nanoparticles with micro/nanoscale precision within a TPP-printed hydrophobic polymer matrix. The research evaluates the geometric accuracy and material composition of AS-TPP-fabricated components via scanning electron microscope (SEM) imaging and energy-dispersive X-ray spectroscopy (EDS) analysis.

To showcase the potential applications of this novel manufacturing process, a multimaterial surface is fabricated by mimicking the topography of rice leaves and the material composition pattern observed in beetle backs. Measurement of the anisotropic wetting characteristics and water collection efficiency of the fabricated multimaterial surface reveals enhancements of 4-fold and 3-fold, respectively, when compared to an isotropic single-material flat surface. Furthermore, the AS-TPP technique is used to manufacture a microfluidic device with a multimaterial channel design that demonstrates rapid and directional liquid transport within 2 seconds. 

The AS-TPP methodology showcases the capacity to engineer surface-wetting properties, facilitating precise and intelligent liquid management. The experimental results validate the high-resolution, multimaterial manufacturing proficiency inherent in the pioneering AS-TPP approach, showcasing its considerable potential for various applications, including microfluidics, surface engineering, and biomedical engineering.