Droplet-based mechanical transducers modulated by the symmetry of wettability patterns.

Asymmetric mechanical transducers have important applications in energy harvesting, signal transmission, and micro-mechanics. To achieve asymmetric transformation of mechanical motion or energy, active robotic metamaterials, as well as materials with asymmetric microstructures or internal orientation, are usually employed. However, these strategies usually require continuous energy supplement and laborious fabrication, and limited transformation modes are achieved. Herein, utilizing wettability patterned surfaces for precise control of the droplet contact line and inner flow, we demonstrate a droplet-based mechanical transducer system, and achieve multimodal responses to specific vibrations. By virtue of the synergistic effect of surface tension and solid-liquid adhesion on the liquid dynamics, the droplet on the patterned substrate can exhibit symmetric/asymmetric vibration transformation when the substrate vibrates horizontally. Based on this, we construct arrayed patterns with distinct arrangements on the substrate, and employ the swarm effect of the arrayed droplets to achieve three-dimensional and multimodal actuation of the target plate under a fixed input vibration. Further, we demonstrate the utilization of the mechanical transducers for vibration management, object transport, and laser modulation. These findings provide a simple yet efficient strategy to realize a multimodal mechanical transducer, which shows significant potential for aseismic design, optical molding, as well as micro-electromechanical systems (MEMS).

Heterogeneous Self-Assembly of a Single Type of Nanoparticle Modulated by Skin Formation.

Self-assembly of colloidal nanoparticles has generated tremendous interest due to its widespread applications in structural colorations, sensors, and optoelectronics. Despite numerous strategies being developed to fabricate sophisticated structures, the heterogeneous self-assembly of a single type of nanoparticle in one step remains challenging. Here, facilitated by spatial confinement induced by a skin layer in a drying droplet, we achieve the heterogeneous self-assembly of a single type of nanoparticle by quickly evaporating a colloid-poly (ethylene glycol) (PEG) droplet. During the drying process, a skin layer forms at the droplet surface. The resultant spatial confinement assembles nanoparticles into face-centered-cubic (FCC) lattices with (111) and (100) plane orientations, generating binary bandgaps and two structural colors. The self-assembly of nanoparticles can be regulated by varying the PEG concentration so that FCC lattices with homo- or heterogeneous orientation planes can be prepared on demand. Besides, the approach is applicable for diverse droplet shapes, various substrates, and different nanoparticles. The one-pot general strategy breaks the requirements for multiple types of building blocks and predesigned substrates, extending the fundamental understanding underlying colloidal self-assembly.

Tailoring vapor film beneath a Leidenfrost drop.

For a drop on a very hot solid surface, a vapor film will form beneath the drop, which has been discovered by Leidenfrost in 1756. The vapor escaping from the Leidenfrost film causes uncontrollable flows, and actuates the drop to move around. Recently, although numerous strategies have been used to regulate the Leidenfrost vapor, the understanding of surface chemistry for modulating the phase-change vapor dynamics remains incomplete. Here, we report how to rectify vapor by "cutting" the Leidenfrost film using chemically heterogeneous surfaces. We demonstrate that the segmented film cut by a Z-shaped pattern can spin a drop, since the superhydrophilic region directly contacts the drop and vaporizes the water, while a vapor film is formed on the superhydrophobic surrounding to jet vapor and reduce heat transfer. Furthermore, we reveal the general principle between the pattern symmetry design and the drop dynamics. This finding provides new insights into the Leidenfrost dynamics modulation, and opens a promising avenue for vapor-driven miniature devices.

Ink-Drop Dynamics on Chemically Modified Surfaces.

Inkjet printing provides an efficient routine for distributing functional materials into locations with well-designed arrangements. As one of the most critical factors in determining the printing quality, the impacting and depositing behaviors of ink drops largely depend on the wettability of the target surface. In addition to printing on solids with intrinsic wettability, various ink-drop impact dynamics and deposition morphologies have been reported through modifying the surface wettability including both homogeneous and heterogeneous, which opens up possibilities for applications such as advanced optic/electric device fabrication and highly sensitive detection. In this Perspective, we summarize recent progress in the modification methods of solid surface wettability and their capability in modulating the ink-drop impacting and depositing dynamics. The challenges facing ink-drop regulation by chemical modification methodologies are also envisaged at the end of the Perspective.

3D Optical Heterostructure Patterning by Spatially Allocating Nanoblocks on a Printed Matrix.

Heterostructures have attracted enormous interest due to the properties arising from the coupling and synergizing between multiscale structures and the promising applications in electronics, mechanics, and optics. However, it is challenging for current technologies to precisely integrate cross-scale micro/nanomaterials in three dimensions (3D). Herein, we realize the precise spatial allocation of nanoblocks on micromatrices and programmable 3D optical heterostructure patterning via printing-assisted self-assembly. This bottom-up approach fully exploits the advantages of printing in on-demand patterning, low cost, and mass production, as well as the merits of solution-based colloidal assembly for simple structuring and high-precision regulating, which facilitates the patterned integration of multiscale materials. Importantly, the luminescent nanoparticle assembly can be accurately coupled to the dye-doped polymer matrix by regulating the interface wettability, enabling facile multicolor tuning in a single heterostructure. Thus, the heterostructure can be specially encoded for anticounterfeiting and encryption applications due to the morphology-dependent and interface-coupling-induced luminescence. Moreover, with the capability to achieve single-nanoparticle resolution, these findings have great potential for designing photonic superstructures and advanced optical devices.

One-Pot Self-Assembly of Dual-Color Domes Using Mono-Sized Silica Nanoparticles.

Spots with dual structural colors on the skin of some organisms in nature are of tremendous interest due to the unique function of their dye-free colors. However, imitation of them requires complicated manufacturing processes, expensive equipment, and multiple predesigned building blocks. In this work, a one-pot strategy based on the phase-separation-assisted nonuniform self-assembly of monosized silica nanoparticles is developed to construct domes with dual structural colors. In drying poly(ethylene glycol)-dextran-based (PEG-DEX) droplets, monosized nanoparticles distribute nonuniformly in two compartments due to the droplet inner flow and different nanoparticle compatibility with the two phases. The dome colors are derived from the self-assembled nanoparticles and are programmable by regulating the assembly conditions. The one-pot strategy enables the preparation of multicolor using only one type of building block. With the dual-color domes, encrypted patterns with a high volume of contents are designed, showing promising applications in information delivery.

Programming Hydrogels with Complex Transient Behaviors via Autocatalytic Cascade Reactions.

It is challenging to design complex synthetic life-like systems that can show both autoevolution and fuel-driven transient behaviors. Here, we report a new class of chemical reaction networks (CRNs) to construct life-like polymer hydrogels. The CRNs are constituted of autocatalytic cascade reactions and fuel-driven reaction networks. The reactions start with only two compounds, that is, thiol of 4-arm-PEG-SH and thiuram disulfides, and undergo thiol oxidation (k1), disulfide metathesis (k2), and thionate hydrolysis-coupling reactions (k3) subsequently, leading to a four-state autonomous transition of sol(I) → soft gel → sol(II) → stiff gel. Moreover, thiuram disulfides can be applied as a fuel to drive the repeated occurrence of metathesis and hydrolysis-coupling reactions, generating dissipative stiff gel → sol(II) → stiff gel cycles. Systematic kinetics studies reveal that the event and lifetime of every transient state could be delicately tailored-up by varying the thiuram disulfide concentration, pH of the system, and thiuram structures. Since the consecutive transient behaviors are precisely predictable, we envision the strategy's potential in guiding the molecular designs of autonomous and adaptive materials for many fields.

Non-Hookean Droplet Spring for Enhancing Hydropower Harvest.

Nonlinear elastic materials are significant for engineering and micromechanics. Droplets with the merits of easy-accessibility, diversity, and energy-absorption capability exhibit a variety of non-Hookean elastic behaviors. Herein, benefiting from the confinement of heterogeneous-wettable parallel plates, the non-Hookean mechanics of the droplet-based spring are systematically investigated. Experimental results and theoretical analysis reveal that the force generated by the spring varies nonlinearly with its deformation, and a force model is accordingly built to depict the mechanics of springs with different sized/numbered droplets and confined by different wettability patterns. Importantly, for the droplet-based spring, the droplet-plate contact area expands nonlinearly with the pressing force, which is employed to optimize the output performance of the droplet-based triboelectric nanogenerator to 226% compared with the control test. This finding deepens the understanding of the non-Hookean behavior of droplet-based springs, and sheds light on applications in energy harvesting, micromechanics, and miniature optic/electric devices.

Adjustable object floating states based on three-segment three-phase contact line evolution.

SignificanceAdjusting the floating states when objects float on water shows great potential for assembly, mineral flotation, nanostructured construction, and floating robot design, but the real-time regulation of floating states is challenging. Inspired by the different floating states of a falling fruit, we propose a facile strategy to transform the object between different floating states based on a three-segment three-phase contact line evolution. In addition, the potential of floating state transformation in solar-powered water evaporation, interface catalysis, and drug delivery is demonstrated. These findings provide insights into floating regulation and show great potential for floating-related applications.

Reconfigurable Magnetic Liquid Metal Robot for High-Performance Droplet Manipulation.

Droplet manipulation is crucial for diverse applications ranging from bioassay to medical diagnosis. Current magnetic-field-driven manipulation strategies are mainly based on fixed or partially tunable structures, which limits their flexibility and versatility. Here, a reconfigurable magnetic liquid metal robot (MLMR) is proposed to address these challenges. Diverse droplet manipulation behaviors including steady transport, oscillatory transport, and release can be achieved by the MLMR, and their underlying physical mechanisms are revealed. Moreover, benefiting from the magnetic-field-induced active deformability and temperature-induced phase transition characteristics, its droplet-loading capacity and shape-locking/unlocking switching can be flexibly adjusted. Because of the fluidity-based adaptive deformability, MLMR can manipulate droplets in challenging confined environments. Significantly, MLMR can accomplish cooperative manipulation of multiple droplets efficiently through on-demand self-splitting and merging. The high-performance droplet manipulation using the reconfigurable and multifunctional MLMR unfolds new potential in microfluidics, biochemistry, and other interdisciplinary fields.

Enhanced Flexibility of the Segmented Honey Bee Tongue with Hydrophobic Tongue Hairs.

Fibrous surfaces in nature have already exhibited excellent functions that are normally ascribed to the synergistic effects of special structures and material properties. The honey bee tongue, foraging liquid food in nature, has a unique segmented surface covered with dense hairs. Since honey bees are capable of using their tongue to adapt to possibly the broadest range of feeding environments to exploit every possible source of liquids, the surface properties of the tongue, especially the covering hairs, would likely represent an evolutionary optimization. In this paper, we show that their tongue hairs are stiff and hydrophobic, the latter of which is highly unexpected as the structure is designed for liquid capturing. We found that such hydrophobicity can prevent those stiff hairs from being adhered to the soft tongue surface, which could significantly enhance the deformability of the tongue when honey bees feed at various surfaces and promote their adaptability to different environments. These findings bridge the relationship between surface wettability and structural characteristics, which may shed new light on designing flexible microstructured fiber systems to transport viscous liquids.

Breaking the symmetry to suppress the Plateau-Rayleigh instability and optimize hydropower utilization.

Droplet impact on solid surfaces is essential for natural and industrial processes. Particularly, controlling the instability after droplet impact, and avoiding the satellite drops generation, have aroused great interest for its significance in inkjet printing, pesticide spraying, and hydroelectric power collection. Herein, we found that breaking the symmetry of the droplet impact dynamics using patterned-wettability surfaces can suppress the Plateau-Rayleigh instability during the droplet rebounding and improve the energy collection efficiency. Systematic experimental investigation, together with mechanical modeling and numerical simulation, revealed that the asymmetric wettability patterns can regulate the internal liquid flow and reduce the vertical velocity gradient inside the droplet, thus suppressing the instability during droplet rebounding and eliminating the satellite drops. Accordingly, the droplet energy utilization was promoted, as demonstrated by the improved hydroelectric power generation efficiency by 36.5%. These findings deepen the understanding of the wettability-induced asymmetrical droplet dynamics during the liquid-solid interactions, and facilitate related applications such as hydroelectric power generation and materials transportation.

Lotus Metasurface for Wide-Angle Intermediate-Frequency Water-Air Acoustic Transmission.

Only 0.1% of the acoustic energy can transmit across the water-air interface because of the huge acoustic impedance mismatch. Enhancing acoustic transmission across the water-air interface is of great significance for sonar communications and sensing. However, due to the interface instability and subwavelength characteristics of acoustic metamaterials, wide-angle intermediate-frequency (10 kHz-100 kHz) water-air acoustic transmission remains a great challenge. Here, we demonstrate that the lotus leaf is a natural low-cost acoustic transmission metasurface, namely, the lotus acoustic metasurface (LAM). Experiments demonstrate the LAM can enhance the acoustic transmission across the water-air interface, with an energy transmission coefficient of about 40% at 28 kHz. Furthermore, by fabricating artificial LAMs, the operating frequencies can be flexibly adjusted. Also, the LAM allows a wide-angle water-to-air acoustic transmission. It will enable various promising applications, such as detecting and imaging underwater objects from the air, communicating between ocean and atmosphere, reducing ocean noises, etc.

Facile full-color printing with a single transparent ink.

Structural colors are promising candidates for their antifading and eco-friendly characteristics. However, high cost and complicated processing inevitably hinder their development. Here, we propose a facile full-color structural-color inkjet printing strategy with a single transparent ink from the common polymer materials. This structural color arisen from total internal reflections is prepared by digitally printing the dome-shaped microstructure (microdome) with well-controlled morphology. By controlling the ink volume and substrate wettability, the microdome color can be continuously regulated across whole visible regions. The gamut, saturation, and lightness of the printed structural-color image are precisely adjusted via the programmable arrangement of different microdomes. With the advantages of simple manufacturing and widely available inks, this color printing approach presents great potential in imaging, decoration, sensing, and biocompatible photonics.

Designable structural coloration by colloidal particle assembly: from nature to artificial manufacturing.

Structural color attracts considerable scientific interests and industrial explorations in various fields for the eco-friendly, fade-resistant, and dynamic advantages. After the long-period evolution, nature has achieved the optimized color structures at various length scales, which has inspired people to learn and replicate them to improve the artificial structure color. In this review, we focus on the design of artificial structural colors based on colloidal particle assembly and summarize the functional bioinspired structure colors. We demonstrate the design principles of biomimetic structural colors via the precise structure engineering and typical bottom-up methods. Some main applications are outlined in the following chapter. Finally, we propose the existing challenges and promising prospects. This review is expected to introduce the recent design strategies about the artificial structure colors and provide the insights for its future development.

Vapor-Induced Liquid Collection and Microfluidics on Superlyophilic Substrates.

Liquid manipulation on solid surfaces has attracted a lot of attention for liquid collection and droplet-based microfluidics. However, manipulation strategies mainly depend on chemical modification and artificial structures. Here, we demonstrate a feasible and general strategy based on the self-shrinkage of the droplet induced via specific vapors to efficiently collect liquids and flexibly carry out droplet-based reactions. The vapor-induced self-shrinkage is driven by Marangoni flow originating from molecular adsorption and diffusion. Under a specific vapor environment, the self-shrinking droplet exhibits unique features including reversible responsiveness, high mobility, and autocoalescence. Accordingly, by building a specific vapor environment, the thin liquid films and random liquid films on superlyophilic substrates can be recovered with a collection rate of more than 95%. Moreover, the vapor system can be used to construct a high-efficiency chemical reaction device. The findings and profound understandings are significant for the development of the liquid collection and droplet-based microfluidics.

Droplet Precise Self-Splitting on Patterned Adhesive Surfaces for Simultaneous Multidetection.

Precise separation and localization of microdroplets are fundamental for various fields, such as high-throughput screening, combinatorial chemistry, and the recognition of complex analytes. We have developed a droplet self-splitting strategy to divide an impacting droplet into predictable microdroplets and deposit them at preset spots for simultaneous multidetection. No matter exchange was observed between these microdroplets, so they could be manipulated independently. Droplet self-splitting was attributed to anisotropic liquid recoiling on the patterned adhesive surface, as influenced by the droplet Weber number and the width of the low-adhesive stripe. A quantitative criterion was also developed to judge the droplet self-splitting capability. The precise separation and distribution of microdroplets enabled simultaneous arrayed reactions and multiple analyte detection using one droplet of sample.

Programmable droplet manipulation by a magnetic-actuated robot.

Droplet manipulations are fundamental to numerous applications, such as water collection, medical diagnostics, and drug delivery. Structure-based liquid operations have been widely used both in nature and in artificial materials. However, current strategies depend mainly on fixed structures to realize unidirectional water movement, while multiple manipulation of droplets is still challenging. Here, we propose a magnetic-actuated robot with adjustable structures to achieve programmable multiple manipulations of droplets. The adjustable structure redistributes the resisting forces from the front and rear ends of the droplets, which determine the droplet behaviors. We can transport, split, release, and rotate the droplets using the robot. This robot is universally applicable for manipulation of various fluids in rough environments. These findings offer an efficient strategy for automated manipulation of droplets.

A Butterfly-Inspired Hierarchical Light-Trapping Structure towards a High-Performance Polarization-Sensitive Perovskite Photodetector.

Extensive applications for photodetectors have led to demand for high-responsivity polarization-sensitive light detection. Inspired by the elaborate architecture of butterfly Papilio paris, a 1D nanograting bonded porous 2D photonic crystal perovskite photodetector (G-PC-PD) using a commercial DVD master and 2D crystalline colloidal arrays template was fabricated. The coupling effect from grating diffraction and reflection of the PC stopband renders the enhanced light harvesting of G-PC-PD. The porous scaffold and nanoimprinting process afford a highly crystalline perovskite film. White light responsivity and detectivity of G-PC-PD are up to 12.67 A W-1 and 3.22×1013  Jones (6∼7 times that of a pristine perovskite photodetector). The highly ordered nanograting arrays of G-PC-PD enable polarization-sensitive light detection with a rate of -0.72 nA deg-1 . This hierarchical perovskite integrated nanograting and 2D PC architecture opens a new avenue to high-performance optoelectronic devices.