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The high-volume synthesis of two-dimensional (2D) materials in the form of platelets is desirable for various applications. While water is considered an ideal dispersion medium, due to its abundance and low cost, the hydrophobicity of platelet surfaces has prohibited its widespread use. Here we exfoliate 2D materials directly in pure water without using any chemicals or surfactants. In order to exfoliate and disperse the materials in water, we elevate the temperature of the sonication bath, and introduce energy via the dissipation of sonic waves. Storage stability greater than one month is achieved through the maintenance of high temperatures, and through atomic and molecular level simulations, we further discover that good solubility in water is maintained due to the presence of platelet surface charges as a result of edge functionalization or intrinsic polarity. Finally, we demonstrate inkjet printing on hard and flexible substrates as a potential application of water-dispersed 2D materials. The hydrophobic surfaces of many two-dimensional (2D) materials prevents direct exfoliation in water without the use of chemical, surfactant, or surface treatments. Here, the authors exfoliate and disperse 2D materials in pure water via simple control of the temperature of the sonication bath and storage.

The hydrogel of biomolecule-assisted metal/organic complex has the superior ability to form a uniform, continuous, and densely integrated structure, which is necessary for fine thin film fabrication. As a representative of nature-originated polymers with abundant reactive side chains, we select the gelatin molecule as an element for weaving the metal cations. Here, we demonstrate the interaction between the metal cation and gelatin molecules, and associate it with coating quality. We investigate the rheological property of gelatin solutions interacting with metal cation from the view of cross-linking and denaturing of gelatin molecules. Also, we quantitatively compare the corresponding interactions by monitoring the absorbance spectrum of the cation. The coated porous structure is systematically investigated from the infiltration of gelatin-mediated Gd 0.2 Ce 0.8 O 2−δ (GDC) precursor into Sm 0.5 Sr 0.5 CoO 3−δ (SSC) porous scaffold. By applying the actively interacting gelatin–GDC system, we achieve a thin film of GDC on SSC with excellent uniformity. Compare to the discrete coating from the typical infiltration process, the optimized thin film coated structure shows enhanced performance and stability.

For effective ocean energy harvesting, it is necessary to understand the coupled motion of the piezoelectric nanogenerator (PENG) and ocean currents. Herein, we experimentally investigate power performance of the PENG in the perspective of the fluid–structure interaction considering ocean conditions with the Reynolds number ( Re ) values ranging from 1 to 141,489. A piezoelectric polyvinylidene fluoride micromesh was constructed via electrohydrodynamic (EHD) jet printing technique to produce the β-phase dominantly that is desirable for powering performance. Water channel was set to generate water flow to vibrate the flexible PENG. By plotting the Re values as a function of nondimensional bending rigidity ( K B ) and the structure-to-fluid mass ratio ( M* ), we could find neutral curves dividing the stable and flapping regimes. Analyzing the flow velocities between the vortex and surroundings via a particle image velocimetry, the larger displacement of the PENG in the chaotic flapping regime than that in the flapping regime was attributed to the sharp pressure gradient. By correlating M *, Re , K B , and the PENG performance, we conclude that there is critical K B that generate chaotic flapping motion for effective powering. We believe this study contributes to the establishment of a design methodology for the flexible PENG harvesting of ocean currents.

Direct printing methods are widely recognized as efficient techniques for manufacturing printed electronics. However, several challenges arise when printing on nonplanar surfaces, especially using the drop‐on‐demand (DoD) approach. These challenges include ink flow due to gravity, precise ink deposition, and reproducibility. This study introduces an innovative method for highly accurate DoD material jetting on nonplanar 3D conductive surfaces, enabling precise production and trajectory control of charged droplets. The technique involves using a grounded 3D substrate as the target, where in‐flight droplets are subjected to an external electric field generated by gate electrode installed on a piezo activated droplet dispenser. Individual droplets are generated and controlled using a complex trigger system that relays variable‐voltage signals to the gate electrode. Moreover, a predictive model for droplet deposition, exhibiting an accuracy of 87%, is developed utilizing supervised machine learning (ML). This approach significantly improves the accuracy and repeatability of droplet deposition. Overall, this study presents an effective method of integrating piezoelectric and electrohydrodynamic printing technologies, complemented by ML. It addresses the challenges associated with printing on nonplanar surfaces using the DoD material jetting technique and shows considerable promise for enhancing efficiency, accuracy, and repeatability in the manufacturing of printed electronics. Direct printing methodologies encounter substantial difficulties on nonplanar substrates, including gravity‐induced ink flow distortion and deposition accuracy in drop‐on‐demand processes. To mitigate these issues, an advanced electrohydrodynamic jetting approach is developed, leveraging a grounded 3D substrate for precise droplet trajectory control. A machine learning‐based predictive model achieves 87% accuracy, significantly enhancing deposition precision, reproducibility, and efficiency in printed electronics manufacturing.

AbstractIn this study, the visualization of the flow inside a Taylor cone formed during an electrohydrodynamic (EHD) spraying is conducted to analyze its stability among five liquid candidates. A micro-PIV with a micro-nozzle is used for the visualization, and the physical properties as well as measured values are utilized in the analysis. First, in forming the Taylor cone, the electrohydrodynamic force is required to be sufficiently large in order to overcome the surface tension of the liquid. Thus, among the five liquids tested here, three, in this case IPA, EtOH, and MeOH, form a Taylor cone due to the relatively low surface tension levels as compared to the others. Once electrohydrodynamic jetting occurs, the average and maximum velocities become monotonically proportional to the average current. As the velocities are the smallest in using IPA, the circulation flow becomes superior to the extrusive flow, which yields the stable formation of a Taylor cone. Also, low fluctuation of the instantaneous currents supports the stable formation of IPA. Consequently, IPA shows the most stable formation of the Taylor cone in our condition due to the lowest average current and low-level surface tension. Eventually, micro-PIV would be a good tool in choosing an optimal fluid for stable EHD spraying.Graphic abstract

Biological tiny structures have been observed on many kinds of surfaces such as lotus leaves, which have an effect on the coloration of Morpho butterflies and enhance the hydrophobicity of natural surfaces. We investigated the micro-scale and nano-scale structures on the wing surfaces of insects and found that the hierarchical multiple roughness structures help in enhancing the hydrophobicity. After examining 10 orders and 24 species of flying Pterygotan insects, we found that micro-scale and nano-scale structures typically exist on both the upper and lower wing surfaces of flying insects. The tiny structures such as denticle or setae on the insect wings enhance the hydrophobicity, thereby enabling the wings to be cleaned more easily. And the hydrophobic insect wings undergo a transition from Cassie to Wenzel states at pitch/size ratio of about 20. In order to examine the wetting characteristics on a rough surface, a biomimetic surface with micro-scale pillars is fabricated on a silicon wafer, which exhibits the same behavior as the insect wing, with the Cassie-Wenzel transition occurring consistently around a pitch/width value of 20.

Precise and rapid detection of biomolecules is a fast-growing research theme in the field of biomedical engineering. Based on the surface-enhanced Raman scattering, micro/nano-scale structures composed of noble metals (e.g., gold and silver) play a critical role in plasmonics. However, it is still limited to structuring nanomaterials in a specific manner. Here, we investigated a novel surface-enhanced Raman spectroscopy (SERS) application using one-dimensional nanomaterials and micro-encapsulation methods. With the immiscible nature of fluids, the nanomaterials were properly captured inside a number of droplets for encapsulation, deforming to micro-ring nanostructures. To yield uniform sizes of the silver micro-ring structures, a microchannel system was designed to characterize particle sizes via microscopic approaches. We were able to obtain printable silver nanowire micro-ring ink, and investigated the SERS substrate effect of the silver micro-ring structure. This fabrication method can be used in many other SERS-based biomedical engineering applications in the near future.

We briefly summarized how to design and fabricate an insect-mimicking flapping-wing system and demonstrate how to implement inherent pitching stability for stable vertical takeoff. The effect of relative locations of the Center of Gravity (CG) and the mean Aerodynamic Center (AC) on vertical flight was theoretically examined through static force balance consideration. We conducted a series of vertical takeoff tests in which the location of the mean AC was determined using an unsteady Blade Element Theory (BET) previously developed by the authors. Sequential images were captured during the takeoff tests using a high-speed camera. The results demonstrated that inherent pitching stability for vertical takeoff can be achieved by controlling the relative position between the CG and the mean AC of the flapping system.

The dimple occurs by sudden pressure inversion at the droplet’s bottom interface when a droplet collides with the same liquid-phase or different solid-phase. The air film entrapped inside the dimple is a critical factor affecting the sequential dynamics after coalescence and causing defects like the pinhole. Meanwhile, in the coalescence dynamics of an electrified droplet, the droplet’s bottom interfaces change to a conical shape, and droplet contact the substrate directly without dimple formation. In this work, the mechanism for the dimple’s suppression (interfacial change to conical shape) was studied investigating the effect of electric pressure. The electric stress acting on a droplet interface shows the nonlinear electric pressure adding to the uniform droplet pressure. This electric stress locally deforms the droplet’s bottom interface to a conical shape and consequentially enables it to overcome the air pressure beneath the droplet. The electric pressure, calculated from numerical tracking for interface and electrostatic simulation, was at least 108 times bigger than the air pressure at the center of the coalescence. This work helps toward understanding the effect of electric stress on droplet coalescence and in the optimization of conditions in solution-based techniques like printing and coating.

When a droplet is placed on an inclined plane, it can slide down the plane by gravitational force. This phenomenon can occur in many industrial applications such as surface coating and printing, as well as in evaluating the characteristics of a surface. The slippery droplet can be analyzed from the balance between the gravitational force and the sum of the capillary and resistance forces. Researchers have suggested various equations of force balance with several hypotheses. To increase the reliability of the force balance correlation, it is important to obtain an accurate inside velocity profile and the three-dimensional (3D) shape of a droplet. In this paper, an analysis system of the slippery droplet was developed which includes optical correction based on a ray tracing method for velocity field measurement and an ellipse fitting method for droplet shape reconstruction. These correction methods were verified by a numerical simulation and applied to a slippery droplet on an inclined plate coated with Poly methyl methacrylate (PMMA). The wall shear force calculated from the velocity field and that calculated from the force balance were in good agreement.