Directional Droplet Propulsion on Gradient Boron Nitride Nanosheet Grid Surface Lubricated with a Vapor Film below the Leidenfrost Temperature

Delayed Leidenfrost Effect of a Cutting Droplet on a Microgrooved Tool Surface

Regulation over the generation of the Leidenfrost phenomenon in liquids is vitally important in a cutting fluid/tool system, with benefits ranging from optimizing the heat transfer efficiency to improving the machining performance. However, realizing the influence mechanism of liquid boiling at various temperatures still faces enormous challenges. Herein, we report a kind of microgrooved tool surface by laser ablation, which could obviously increase both the static and dynamic Leidenfrost point of cutting fluid by adjusting the surface roughness (Sa). The physical mechanism that delays the Leidenfrost effect is primarily due to the ability of the designed microgroove surface to store and release vapor during droplet boiling so that the heated surface requires higher temperatures to generate sufficient vapor to suspend the droplet. We also find six typical impact regimes of cutting fluid under various contact temperatures; it is worth noting that Sa has a great influence on the transform threshold among six impact regimes, and the likelihood that a droplet will enter the Leidenfrost regime decreases with increasing Sa. In addition, the synergistic effect of Sa and tool temperature on the droplet kinetics of cutting droplets is investigated, and the relationship between the maximum rebound height and the dynamic Leidenfrost point is correlated for the first time. Significantly, cooling experiments on the heated microgrooved surface are performed and demonstrate that it is effective to improve the heat dissipation ability of cutting fluid by delaying the Leidenfrost effect on the microgrooved heated surface.

Reducing Contact Time of Droplets Impacting Superheated Hydrophobic Surfaces

Reducing the contact time (t_c ) of a droplet impacting a solid surface is crucial in various fields. Superhydrophobic (SHB) surfaces are used to reduce t_c at room temperature. However, at high temperatures, SHB surfaces cannot achieve t_c reduction because of the failure of the coating materials or the Leidenfrost (LF) effect. Therefore, a surface that can suppress the LF effect and reduce t_c at high temperatures is required. To create such a surface, a double-reentrant groove (DRG) array surface with an overhanging structure on top of the microgrooves is developed. The overhanging structure renders the surface hydrophobic (HB). Despite its HB nature, the DRG surface's LF point (LFP) is observed at ≈530 °C, which is higher than the LFP on other HB surfaces. Moreover, a t_c smaller than the inertia-capillary limit on the DRG surface is observed at between 400 and 500 °C. Accordingly, the DRG surface is currently the only HB surface for t_c reduction at high temperatures. The DRG surface avoids the limitation of low LFPs observed on HB surfaces. Due to its HB properties, the DRG surface is determined to exhibit self-cleaning characteristics and can be used in various applications at high temperatures.

Explosive Pancake Bouncing on Hot Superhydrophilic Surfaces

The rapid detachment of liquid droplets from engineered surfaces in the form of complete rebound, pancake bouncing, or trampolining has been extensively studied over the past decade and is of practical importance in many industrial processes such as self-cleaning, anti-icing, energy conversion, and so on. The spontaneous trampolining of droplets needs an additional low-pressure environment and the manifestation of pancake bouncing on superhydrophobic surfaces requires meticulous control of macrotextures and impacting velocity. In this work, we report that the rapid pancake-like levitation of impinging droplets can be achieved on superhydrophilic surfaces through the application of heating. In particular, we discovered explosive pancake bouncing on hot superhydrophilic surfaces made of hierarchically non-interconnected honeycombs, which is in striking contrast to the partial levitation of droplets on the surface consisting of interconnected microposts. This enhanced droplet bouncing phenomenon, characterized by a significant reduction in contact time and increase in the bouncing height, is ascribed to the production and spatial confinement of pressurized vapor in non-interconnected structures. The manifestation of pancake bouncing on the superhydrophilic surface rendered by a bottom-to-up boiling process may find promising applications such as the removal of trapped solid particles.

Integrated Smart Janus Textile Bands for Self-Pumping Sweat Sampling and Analysis

Wearable sweat sensors have spearheaded the thrust toward personalized health monitoring with continuous, real-time, and molecular-level insight in a noninvasive manner. However, effective sweat sampling still remains a huge challenge. Here, we introduce an intelligent Janus textile band that bridges the gap between self-pumping sweat collection, comfortable epidemic microclimate, and sensitive electrochemical biosensing via an integrated wearable platform. The dominant sweat sampling configuration is a textile with Janus wettability, which is fabricated by electrospinning a hydrophobic polyurethane (PU) nanofiber array onto superhydrophilic gauze. Based on a contact-pumping model, the Janus textile can unidirectionally and thoroughly transport sweat from skin (hydrophobic side) to embedded electrode surface (hydrophilic side) with epidemic comfort. On-body experimentation reveals that the sensitive detection of multiple biomarkers including glucose, lactate, K⁺, and Na⁺ is achieved in the pumped sweat. Such smart Janus textile bands can effectively drain epidermal sweat to targeted assay sites via interface modifications, representing a reinforced and controlled biofluids analysis pathway with physiological comfort.

Smart Superhydrophobic Shape Memory Adhesive Surface toward Selective Capture/Release of Microdroplets

Controllable manipulation of microdroplets is significant for the microfluidics, biomedical areas, microreactors, and so on; however, until now, reports about no-loss and selective capture/release of different microdroplets are still rare. Herein, we report a new superhydrophobic shape memory adhesive surface that can solve this problem. The surface is prepared by sticking a pillar-structured superhydrophobic polyurethane layer onto a shape memory polyurethane-cellulose nanofiber (PU-CNF) layer. Because of the good shape memory performance of the PU-CNF layer, the obtained surface can memorize and display various microstructure arrangements during the stretching/releasing process. Meanwhile, multiple superhydrophobic adhesive states from low-adhesive rolling performance to high-adhesive pinning performance can be observed on the surface, and all these adhesive states can be reversibly controlled between each other. Based on the smart shape memory ability in surface adhesion, not only traditional in situ capture/release of one microdroplet but also selective capture and release of different microdroplets can be realized. This work reports a new superhydrophobic shape memory adhesive surface; it is envisioned that this smart surface would be a powerful platform for microfluidics systems, complex droplet transportation, biological analysis, and so on.