Quick self-assembly of bio-inspired multi-dimensional well-ordered structures induced by ultrasonic wave energy

Abstract

Bottom-up self-assembly of components, inspired by hierarchically self-regulating aggregation of small subunits observed in nature, provides a strategy for constructing two- or three-dimensional intriguing biomimetic materials via the spontaneous combination of discrete building blocks. Herein, we report the methods of ultrasonic wave energy-assisted, fast, two- and three-dimensional mesoscale well-ordered self-assembly of microfabricated building blocks (100 μm in size). Mechanical vibration energy-driven self-assembly of microplatelets at the water-air interface of inverted water droplets is demonstrated, and the real-time formation process of the patterned structure is dynamically explored. 40 kHz ultrasonic wave is transferred into microplatelets suspended in a water environment to drive the self-assembly of predesigned well-ordered structures. Two-dimensional self-assembly of microplatelets inside the water phase with a large patterned area is achieved. Stable three-dimensional multi-layered self-assembled structures are quickly formed at the air-water interface. These demonstrations aim to open distinctive and effective ways for new two-dimensional surface coating technology with autonomous organization strategy, and three-dimensional complex hierarchical architectures built by the bottom-up method and commonly found in nature (such as nacre, bone or enamel, etc.).

Fig 1. Fabrication and releasing of the hexagonal microplatelets made of polysilicon as the building blocks for 100-μm-size mesoscale self-assembly (scale bars are 100 μm).

Fig 2. Optical images of self-assembled three-dimensional “soccer balls” by hexagonal polysilicon microplatelets at the water surface in the glass beaker.

(a) An individual “soccer ball” showing the organized pattern of microplatelets (scale bar is 500 μm). (b) A cluster of “soccer balls” randomly generated bubbles by ultrasonic waves and the schematic illustration of the “soccer ball” (inset; scale bar is 400 μm). (c) The “soccer balls” varying in size associated with the size of the bubbles with smaller “soccer ball” with the diameter about 700 μm (inset; scale bar is 200 μm) and larger “soccer ball” with diameter about 4 mm (scale bar is 1 mm).

Fig 3. Monolayer mesoscale self-assembly of hexagonal microplatelets at water-air interface of droplets and real-time dynamical observation of the self-assembling process driven by mechanical agitation.

(a) Experimental setup used for self-assembly, including an inverted plastic Petri dish and an optical microscope with inverted lens. (b) Three droplets of microplatelets suspension on the inner surface of a plastic Petri dish. (c) Schematic illustration of self-assembly of microplatelets at water-air interface of the droplets hanging at the bottom surface of the plastic Petri dish for inverted lens to observe. (d) Optical image frames of self-assembling process, showing the microplatelets gradually formed an integrated and well-ordered pattern (scale bar is 200 μm).

Fig 4. Highly ordered two-dimensional self-assembly of mesoscale hexagonal microplatelets in water.

(a) Schematic illustration of the self-assembling process by using ultrasonic waves. Microplatelets self-assembled with top or bottom face adhering to the surface of the glass tube. (b-c) Photographs of (b) manipulation of self-assembly happening inside the glass tube by using ultrasonic waves, and (c) observation of organized pattern after self-assembly was finished by using stereomicroscope. (d) Optical image of the large area two-dimensional self-assembled aggregation at the bottom concave surface of the glass tube. (e-f) Zoom-in optical images of (e) area of dense and well-packed two-dimensional pattern formed by self-assembly (scale bar is 400 μm), and (f) defective area where self-assembled microplatelets still demonstrated precise organization as predesigned (scale bar is 400 μm).

Fig 5. Formation of three-dimensional well-ordered self-assembly of microplatelets at the water-air interface of the glass tube.

(a) Schematic illustration showing the self-assembled structures appeared at the water-air interface of glass tube in comparison with two-dimensional self-assembly that appeared at the bottom concave. (b-d) Optical images of (b) organized and stable three-dimensional multi-layered self-assembly that interconnected each other forming aggregates with size of several millimeters (scale bar is 500 μm); (c) crystalline structure which consisted of multi-layered stacks that extended in the hexagonal plane and in the direction perpendicular to it (scale bar is 100 μm); (d) long bar was well-ordered as a stacked structure without misalignment (scale bar is 100 μm). (e) SEM image of the nacre from mollusk shells (inset; scale bar is 1 cm) made of microscopic tablets of calcium carbonate (scale bar is 5 μm). (f) Optical image of biomimetic artificial nacre that was self-assembled by using fish-scale shape microplatelets (inset) as building blocks (scale bar is 100 μm).