Direct observation of spreading precursor liquids in a corner.

Precursor liquid is a nanoscale liquid creeping ahead of the macroscopic edge of spreading liquids, whose behaviors tightly correlate with the three-phase reaction efficiency and patterning accuracy. However, the important spatial-temporal characteristic of the precursor liquid still remains obscure because its real-time spreading process has not been directly observed. Here, we report that the spreading ionic liquid precursors in a silicon corner can be directly captured on video using in situ scanning electron microscopy. In situ spreading videos show that the precursor liquid spreads linearly over time ([Formula: see text]) rather than obeying the classic Lucas-Washburn law ([Formula: see text]) and possesses a characteristic width of ∼250-310 nm. Theoretical analyses and molecular dynamics simulations demonstrate that the unique behaviors of precursor liquids originate from the competing effect of van der Waals force and surface energy. These findings provide avenues for directly observing liquid/solid interfacial phenomena on a microscopic level.

Ultrafast Impact Superspreading on Superamphiphilic Silicon Surfaces for Effective Thermal Management.

Controllable impact spreading behavior is critical for effective thermal management of spray cooling. However, splash and retraction are common problems on hydrophobic (HPB) and hydrophilic (HPL) surfaces. Herein, by regulation of surface wettability, we report a controllable ultrafast impact superspreading behavior (superspreading time of ∼3.0 ms) without splash and retraction on superamphiphilic (SAPL) silicon surfaces. Analysis of dynamic wetting processes combined with observation of lateral force microscopy images on SAPL surfaces reveals the existence of a precursor film at the spreading edge induced by heterogeneous surface wettability at nanoscale. Further study indicates that the inhibition of splash results from the high liquid flux in precursor film, which suppresses the interposition of air at the spreading edge. The reduction of Laplace forces owing to the presence of precursor film inhibits retraction at the spreading frontier. Taking advantage of this impact superspreading behavior on SAPL surfaces, effective heat dissipation is demonstrated, offering uniform and high heat flux for the spray cooling process.

Large-Scale Metal-Organic Framework Nanoparticle Monolayers with Controlled Orientation for Selective Transport of Rare-Earth Elements.

Long-range ordered membranes comprised of porous nanoparticles have been pursued in precise separations for a long time. Yet most of the fabrication methods suffer from limited substrates or lack of precise control over crystal orientation. Herein, large-scale metal-organic framework (MOF) monolayer membranes with controlled orientations are prepared through an interfacial self-assembly process confined by superlyophilic substrates. The superspreading of reactant microdroplets results in an ultrathin liquid layer under an immiscible oil as a confined reactor. The concomitant MOF (ZIF-8) particles spontaneously assemble into monolayers with controlled orientations, determined by the particles' contact angles at the liquid/liquid interface, which can be regulated by solvent compositions. Therein both gas-adsorption and ion-transport tests prove that the ⟨111⟩-oriented membrane exhibits a minimized mass-transfer resistance. The as-prepared membrane can selectively transport rare-earth elements (REEs), and a La3+/K+ selectivity of 14.3 is achieved. Molecular dynamics simulations reveal that the REEs-selectivity is associated with the distinct difference in ion-membrane binding energies, demonstrating the potential of ZIF-8 membranes for use in high-efficiency recovery of REEs from industrial wastes.

Liquid-Superspreading-Boosted High-Performance Jet-Flow Boiling for Enhancement of Phase-Change Cooling.

Enhanced boiling heat transfer via surface engineering is a topic of general interest for its great demand in industrial fields. However, as a dynamic interfacial phenomenon, a deep understanding of its process and mechanism, including liquid re-wetting and vapor departure, is still challenging. Herein, a micro-/nanostructured Cu surface containing a periodic microgroove/pyramid array with rich nanowrinkles is designed, where superspreading (<134.1 ms) of organic cooling agents highly boosts the liquid re-wetting process, causing a discontinuous solid-liquid-vapor three-phase contact line and ultralow under-liquid bubble adhesion force (≈1.3 µN). Therefore, a characteristic, ultrafast jet-flow boiling (bubbles rapidly ejected in multiple strips) is obtained on this surface, giving a priority to nucleation (superheat ≈ 1.5 °C) and simultaneously enhancing the critical heat flux and heat-transfer coefficient by up to 80% and 608%, respectively, compared with a flat surface. In situ observation and analysis of the nucleation, growth, and departure of micro-sized jet-flow bubbles reflects that microgrooves/pyramids with nanowrinkles promote the latent heat exchange process by superspreading-induced ultrafast liquid re-wetting and constant vapor film coalescing. Based on the designed structures, high-performance phase-change cooling for central processing unit heat management in supercomputer centers is accomplished with an ultralow power usage effectiveness (PUE < 1.04).

Bioinspired Superspreading Surface: From Essential Mechanism to Application.

The dynamic behavior of liquids on surfaces is ubiquitous in nature and has aroused wide attention from researchers. Among them, the superspreading surface has been extensively investigated and applied in areas ranging from film fabrication to antibiofouling, separation, etc. However, the traditional equilibrium contact angle (CA) at the thermodynamic steady-state cannot completely depict the dynamic spreading process of liquids, because the performance of these surfaces is controlled not only by the final steady superhydrophilicity (CA < 5°) but also by the superspreading speed of liquids with a CA of ∼0°. Moreover, as the most basic prerequisite for superspreading, the long-held intrinsic wetting threshold (IWT) of 90°, which divides hydrophobic and hydrophilic surfaces, is also controversial.In this Account, we summarize and condense the commonalities of our related research, further formally propose the concept of "superspreading", and recommend using "superspreading time (ST)" and "curve of superspreading radius versus spreading time (SRST)" to quantify its performance. Learning from nature is the most effective way to artificially fabricate superspreading surfaces. To begin, we first review some typical superspreading surfaces we found in nature and introduce the strategies adopted by the surfaces for surviving or realizing special functions. Then, we systematically review our recent understanding of the essential mechanism of superspreading surfaces across multiple length scales─from the molecular origin of the newly found IWT of ∼65° for water to the macroscopic respective functions of nanostructure and microstructure in superspreading. Armed with the in-depth fundamental mechanism, we propose the designing principle of high-performance superspreading surfaces. Following that, we summarize the commonly utilized methods, including modifying surface composition to give the surface intrinsic hydrophilicity and changing surface structure to improve the superspreading performance. Subsequently, we introduce the recently developed practical applications by virtue of the outstanding property of the superspreading surface, including the fabrication of a self-assembled film on the solid-gas surface and solid-liquid interface, a self-assembled water barrier for antibiofouling and oil repellency, high-efficiency separation and heat dissipation, etc. Finally, we discuss the remaining major challenges and the future development trends in the superspreading field. This Account serves to arouse wide attention and efforts in the superspreading field to strengthen mechanism research and promote practical large-area applications.

Microchannel and Nanofiber Array Morphology Enhanced Rapid Superspreading on Animals' Corneas.

The dynamic spreading phenomenon of liquids is vital for both understanding wetting mechanisms and visual reaction time-related applications. However, how to control and accelerate the spreading process is still an enormous challenge. Here, a unique microchannel and nanofiber array morphology enhanced rapid superspreading (RSS) effect on animals' corneas with a superspreading time (ST) of 830 ms is found, and the respective roles of the nanofiber array and the microchannel in the RSS effect are explicitly demonstrated. Specifically, the superspreading is induced by in-/out-of-plane nanocapillary forces among the nanofiber array; the microchannel is responsible for tremendously speeding up the superspreading process. Inspired by the RSS strategy, not only is an RSS surface fabricated with an ST of only 450 ms, which is, respectively, more than 26 and 1.8 times faster than conventional superamphiphilic surfaces and animal's corneas and can be applied as RSS surfaces on video monitors to record clear videos, but also it is demonstrated that the RSS effect has tremendous potential as advanced ophthalmic material surfaces to enhance its biocompatibility for clear vision.

Bioinspired Ultrafast-Responsive Nanofluidic System for Ion and Molecule Transport with Speed Control.

The design of an intelligent nanofluidic system for regulating the transport of substances such as ions and molecules is significant for applications in biological sensing, drug delivery, and energy harvesting. However, the existing nanofluidic system faces challenges in terms of an uncontrollable transport speed for molecules and ions and also a complex preparation processes, low durability, and slow response rate. Herein, we demonstrate the use of a bioinspired ferrofluid-based nanofluid that can facilitate multilevel ultrafast-responsive ion and molecule transport with speed control. Specifically, we reversibly deform bulk ferrofluids using a magnet and wet/dewet the outer surface of superhydrophilic nanochannels for building a smart transport system. By changing the direction and strength of the external magnetic field, a speed control, ultrafast-responsive molecular transport (<0.1 s), and controlled current gating ratio are achieved owing to the different pattern changes of ferrofluids on the outer surface of nanochannels. We also illustrate a practical application of this strategy for antibacterial devices to control the transport of drug molecules in a programmed manner. These results suggest that molecule transport can be further complexified and quantified through an intelligent nanofluidic system.

Magnetic Actuation Multifunctional Platform Combining Microdroplets Delivery and Stirring.

Multifunctional droplets manipulation devices are in urgent need for various laboratory operations such as chemical reaction and biological analysis. However, most current techniques that achieved a controllable droplet transport system mainly rely on passive diffusion for mixing, limiting their practical applications. Here, we develop a magnetic controlled dimple on slippery surface (MCDSS) that enables arbitrary direction or even uphill droplet transport through the synergy between gravitational force and asymmetrical droplet deformation. Further experiments demonstrate that our system could also be used for stirring microdroplets and accelerating the mixing speed by more than one hundred times. In addition, the microstir strategy could help to avoid locally uneven production of precipitation or gas in heterogeneous reactions. This combination of droplet delivery and agitation may have a promising future for application in various fields, for example, laboratory-on-a-chip platforms and microengines.

Micro-/nano-voids guided two-stage film cracking on bioinspired assemblies for high-performance electronics.

Current metal film-based electronics, while sensitive to external stretching, typically fail via uncontrolled cracking under a relatively small strain (~30%), which restricts their practical applications. To address this, here we report a design approach inspired by the stereocilia bundles of a cochlea that uses a hierarchical assembly of interfacial nanowires to retard penetrating cracking. This structured surface outperforms its flat counterparts in stretchability (130% versus 30% tolerable strain) and maintains high sensitivity (minimum detection of 0.005% strain) in response to external stimuli such as sounds and mechanical forces. The enlarged stretchability is attributed to the two-stage cracking process induced by the synergy of micro-voids and nano-voids. In-situ observation confirms that at low strains micro-voids between nanowire clusters guide the process of crack growth, whereas at large strains new cracks are randomly initiated from nano-voids among individual nanowires.

Bioinspired Self-Healing Liquid Films for Ultradurable Electronics.

Resistive strain sensors play a crucial role in the development of flexible and stretchable electronics because of their excellent sensitivity and conformability. However, such sensors suffer from poor durability because of the low adhesion strength between the solid conductive layer and polymer and the irreparable dry friction inside the conventional solid conductive layers. Here, inspired from the structures and excellent abrasion resistance of tear films on animal corneas, we demonstrate ultradurable strain sensors based on uniform self-healing wear-free liquid films formed on biomimetic microvilli made from modified polydimethylsiloxane (PDMS). Ethanol solutions containing ionic liquids (ILs) are added to PDMS microvilli, which are superlyophilic due to the surface chemistry and special structures. During evaporation, ILs are driven upward by Laplace pressure and join into continuous conductive films. As the sensing layer, when repeatedly stretched and released, the capillary-stabilized liquid film is lossless because of wet friction, and the cracks will recover completely after release due to the capillary-force-induced self-healing capability, allowing the strain sensors to exhibit high durability of over 22 500 loading-unloading cycles. This work presents an approach for the construction of ultradurable electronics.

A Magnetic Gated Nanofluidic Based on the Integration of a Superhydrophilic Nanochannels and a Reconfigurable Ferrofluid.

The design of intelligent gating in nanoscale is the subject of intense research motivated by a broad potential impact on science and technology. However, the existing designs require complex modification and are unstable, which restrict their practical applications. Here, a magnetic gated nanofluidic is reported based on the integration of superhydrophilic membranes and reconfigurable ferrofluid, which realizes the gating of the nanochannel by adjusting the steric configuration of the ferrofluid. This system could achieve ultrahigh gating ratio up to 10 000 and excellent stability up to 130 cycles without attenuation. Experiments and theoretical calculations demonstrate that the switch is controlled by the synergy of magnetic force and the interfacial tension. The introduction of ferrofluid and superhydrophilic nanochannels in this work presents an important paradigm for the nanofluidic systems and opens a new and promising avenue to various developments in the fields of materials science, which may be utilized in medical devices, nanoscale synthesis, and environmental analysis.

Foolproof Method for Fast and Reversible Switching of Water-Droplet Adhesion by Magnetic Gradients.

Reversible switching of water-droplet adhesion on solid surfaces is of great significance for smart devices, such as microfluidics. In this work, we designed a foolproof method for fast and reversible magnet-controlled switching of water-droplet adhesion surfaces by doping iron powders in soft poly(dimethylsiloxane). The water adhesion is adjusted by magnetic field-induced structure changes, avoiding complex chemical or physical surface design. The regulation process is so convenient that only tens of milliseconds are needed. The on-site responsive mechanism extends its use to unusual curved surfaces. Moreover, the excellent reversibility and stability make the film an ideal candidate for real-time applications.