Stimuli‐Responsive Bioinspired Materials for Controllable Liquid Manipulation: Principles,Fabrication, and Applications

Many emerging interfacial technologies, such as self‐cleaning surfaces, oil/water separation, water collection, and microfluidics, are essentially liquid manipulation processes. In this regard, micro‐nanostructures of the living organisms are highly preferable, by virtue of the evolutionary pressure and the adaptation to the specific environments, to inspire the optimization of man‐made interfaces. With the increasing demands of modern life, research, and industry, intelligent materials with stimuli‐responsive liquid manipulation functions have gained substantial attention from interfacial scientists. This review introduces the recent progress in the development of stimuli‐responsive liquid‐manipulating materials with bioinspired structures and surface chemistry according to two classified manipulation modes: (i) smart manipulation of liquid wetting behaviors, including lyophobic/lyophilic and superlyophobic/superlyophilic, and (ii) smart manipulation of liquid motion behaviors, including coalescence, transportation, rolling/adhesion, and sliding/pinning. At the beginning of the presentation of each classification, the theoretical basis and the sources of inspiration are introduced comprehensively to ensure a better understanding. This review mainly focuses on the mechanisms, fabrication, and applications of the state‐of‐the‐art works related to smart and biomimetic liquid‐manipulating materials. Finally, conclusions and future prospects are provided, and the remaining problems and promising breakthroughs in fabricating large‐scale, cost‐effective, and efficient smart liquid‐manipulating materials are outlined.     

Nematic Liquid Crystals Embedded in Cubic Microlattices: Memory Effects and Bistable Pixels

The confinement of liquid crystals in geometries with frustrating boundary conditions gives rise to nontrivial effects such as bistability and memory. It is shown that large memory effects arise when nematic liquid crystals are embedded in cubic micrometer‐sized scaffolds made by two‐photon polymerization. The electric field alignment of the liquid crystals inside the porous medium is maintained when the applied field is above a threshold (approximately 2 V per micrometer of cell thickness). The onset of the memory is an on/off type process for each individual pore of the scaffold, and the memory typically starts emerging in one region of the structure and then propagates. The global memory effects in porous structures with controlled geometry are enhanced with respect to the case of random porous structures. This work is a proof of the “memory from topology” principle, which was previously suggested by computer simulations. These new materials can pave the way to new types of bistable displays.     

Deterministic Manipulation of Multi-State Polarization Switching in Multiferroic Thin Films

Deterministically controllable multi-state polarizations in ferroelectric materials are promising for the application of next-generation non-volatile multi-state memory devices. However, the achievement of multi-state polarizations has been inhibited by the challenge of selective control of switching pathways. Herein, an approach to selectively control 71° ferroelastic and 180° ferroelectric switching paths by combining the out-of-plane electric field and in-plane trailing field in multiferroic BiFeO₃ thin films with periodically ordered 71° domain wall is reported. Four-state polarization states can be deterministically achieved and reversibly controlled through precisely selecting different switching paths. These studies reveal the ability to obtain multiple polarization states for the realization of multi-state memories and magnetoelectric coupling-based devices.     

Magnetic Tilting in Nematic Liquid Crystals Driven by Self-Assembly

Self-assembly is one of the crucial mechanisms allowing the design multifunctional materials. Soft hybrid materials contain components of different natures and exhibit competitive interactions which drive self-organization into structures of a particular function. Here a novel type of a magnetic hybrid material where the molecular tilt can be manipulated through a delicate balance between the topologically-assisted colloidal self-assembly of magnetic nanoparticles and the anisotropic molecular interactions in a liquid crystal matrix is demonstrated.     

Carboxylates versus Fluorines: Boosting the Emission Properties of Commercial BODIPYs in Liquid and Solid Media

A new and facile strategy for the development of photonic materials is presented that fufills the conditions of being efficient, stable, and tunable laser emitters over the visible region of spectrum, with the possibility of being easily processable and cost‐effective. This approach uses poly(methyl methacrylate) (PMMA) as a host for new dyes with improved efficiency and photostability synthesized. Using a simple protocol, fluorine atoms in the commercial (4,4‐difluoro‐4‐bora‐3a,4a‐diaza‐s‐indacene) (F‐BODIPY) by carboxylate groups. The new O‐BODIPYs exhibit enhanced optical properties and laser behavior both in the liquid and solid phases compared to their commercial analogues. Lasing efficiencies up to 2.6 times higher than those recorded for the commercial dyes are registered with high photostabilities since the laser output remain at 80% of the initial value after 100 000 pump pulses in the same position of the sample at a repetition rate of 30 Hz; the corresponding commercial dye entirely loses its laser action after only 12 000 pump pulses. Distributed feedback laser emission is demonstrated with organic films incorporating new O‐BODIPYs deposited onto quartz substrates engraved with appropriated periodical structures. These dyes exhibit laser thresholds up to two times lower than those of the corresponding parent dyes with lasing intensities up to one order of magnitude higher.     

Unidirectional Liquid Manipulation Via an Integrated Mesh with Orthogonal Anisotropic Slippery Tracks

The rational manipulation of fluid behavior by functional interfaces plays an indispensable role in the development of advanced materials and devices involving liquid/solid interactions. Previous examples of the liquid “diode” that allows fluid penetration in only one direction rely mainly on the remarkable wettability gradient/contrast. Inspired by the wetting phenomena of the rice leaf and the Pitcher plant, an integrated mesh with orthogonal anisotropic slippery tracks (IMOAS) is presented here that can realize similar unidirectional droplet penetration using a distinct mechanism. The unidirectional droplet penetration can be conveniently switched via the 90° rotation of the IMOAS, showing a highly controllable liquid manipulation. The droplet tends to slip on the surface, which can maximize the contact area between the liquid and the tracks, and complies with the principle of the lowest surface energy. Based on this unique liquid controlling strategy, droplet manipulation of the IMOAS during fog harvesting and droplet self‐regulation has been conducted to illustrate its potential applications. The current design could aid the understanding of liquid unidirectional penetration and unlock additional possibilities for the optimization of fluid‐related systems.     

Lightweight Liquid Metal Entity

Liquid metals are of great importance in developing wearable devices and soft robotics owing to its high conductivity and flexibility. However, the high density of such metals turned out to be big concern for many practical situations. With generalized purpose, a new conceptual material as lightweight liquid metal entity, which can be as light as water, is proposed here. For illustration, an unconventionally ultralight material composed of eutectic galliumindium alloys (eGaIn) and glass bubbles is demonstrated, whose density can be reduced below 2.010 even 0.448 g cm^‐3, even lighter than water, but still maintains excellent conformability, electric conductivity, and stiffness variety under temperature regulation. Such material is further adopted to build various complicated structures through origami or force regulation, representing various application scenarios and can be reused for eight times without evident loss in function. Based on these tests, buoyancy component for water‐related devices is designed, which offers the functions of a switch and loading element. The lightweight liquid metal entities are promising for making diverse advanced soft robotics and underwater devices in the near future.     

Phototunable Azobenzene Cholesteric Liquid Crystals with 2000 nm Range

Phototuning of more than 2000 nm is demonstrated in an azobenzene‐based cholesteric liquid crystal (azo‐CLC) consisting of a high‐helical‐twisting‐power, axially chiral bis(azo) molecule (QL76). Phototuning range and rate are compared as a function of chiral dopant concentration, light intensity, and thickness. CLCs composed of QL76 maintain the CLC phase regardless of intensity or duration of exposure. The time necessary for the complete restoration of the original spectral properties (position, bandwidth, baseline transmission, and reflectivity) of QL76‐based CLC is dramatically reduced from days to a few minutes by polymer stabilization of the CLC helix.     

Magnetic,Multilayered Nanotubes of Low Aspect Ratios for Liquid Suspensions

This work presents a synthesis route for low‐aspect‐ratio nanotubes consisting of a layer of magnetite (Fe₃O₄) sandwiched between SiO₂ layers. In this template‐based strategy, self‐ordered porous alumina membranes are combined with the atomic layer deposition of SiO₂ and Fe₂O₃. An optimized electrochemical setup yields nanoporous Al₂O₃ membranes on 4‐inch Al substrates, which serve as templates for the large‐scale fabrication of nanotubes. A selective chemical etching step releases the magnetic tubes for suspension in a carrier fluid and permits recycling of the underlying aluminum foils for the fabrication of subsequent nanotube batches. The nanotubes consisting of an iron oxide layer protected by a silica shell are magnetically characterized in suspensions as well as in dried form on a substrate. High‐resolution transmission electron imaging reveals a polycrystalline, magnetite spinel structure of iron oxide, with the proper stoichiometry proven by the presence of the Verwey transition. Furthermore, field‐dependent viscosity measurements show an enhancement of the magnetoviscosity, thus demonstrating the technological potential of nanotube suspensions as a new class of ferrofluidic solutions. Owing to the tubular shape being closed at one end, these nanoparticles might additionally function as magnetic containers for targeted drug‐delivery or as chemical nanoreactors.     

Bio‐Inspired Photonic Materials: Prototypes and Structural Effect Designs for Applications in Solar Energy Manipulation

Natural creatures have evolved elaborate photonic nanostructures on multiple scales and dimensions in a hierarchical, organized way to realize controllable absorption, reflection, or transmitting the desired wavelength of the solar spectrum. A bio‐inspired strategy is a powerful and promising way for solar energy manipulation. This feature article presents the state‐of‐the‐art progress on bio‐inspired photonic materials on this particular application. The article first briefly recalls the physical origins of natural photonic effects and catalogues the typical natural photonic prototypes including light harvesting, broadband reflection, selective reflection, and UV/IR response. Next, typical applications are categorized into two primary areas: solar energy utilization and reflection. Recent advances including solar‐to‐electricity, solar‐to‐fuels, solar‐thermal (e.g., photothermal converters, infrared detectors, thermoelectric materials, smart windows, and solar steam generation) are highlighted in the first part. Meanwhile, solar energy reflection involving infrared stealth, radiative cooling, and micromirrors are also addressed. In particular, this article focuses on bioinspired design principles, structural effects on functions, and future trends. Finally, the main challenges and prospects for the next generation of bioinspired photonic materials are discussed, including new design concepts, emerging ideas, and possible strategies.