Bioinspired Patterned Bubbles for Broad and Low-Frequency Acoustic Blocking

Patterned Surface Energy for Modulating Solid-Liquid Interfacial Properties

Surface energy, as an intrinsic property of solids, plays a crucial role in modulating the characteristics of solid surfaces, especially of the solid-liquid interface. Due to inevitable processes such as surface adsorption or contamination, the surface energy of practical solids is usually nonuniform. However, if this nonuniformity is rationally designed and effectively utilized, it is capable of endowing great potential for liquid manipulation. With the rapid development of microfabrication and surface modification techniques, a variety of artificial patterned surface energy surfaces (PSESs) have been fabricated, which extend the diversity, tunability, and precision of liquid-based applications. In this review, we discuss the regulation of solid-liquid interface properties with PSESs from a relatively macroscopic perspective, particularly focusing on how to control matter and energy through rational design. First, we provide a brief introduction about the definition and significance of PSESs. Then, matter selective adhesion by PSESs is summarized, including liquid dynamics regulation, crystallization inducement, and biosample self-distribution. In the following, we discuss how PSESs regulate physical fields, including the thermal field, electric field, and acoustic field, with an explanation centered on discontinuous solid-liquid contact on PSESs. Finally, associated challenges of surface energy regulation for liquid-based scenarios are included.

Generating Stable Nitrogen Bubble Layers on Poly(methyl methacrylate) Films by Photolysis of 2-Azidoanthracene

2-Azidoanthracene (2N3-AN) can act as a photochemical source of N2 gas when dissolved in an optically transparent polymer such as poly(methyl methacrylate) (PMMA). Irradiation at 365 or 405 nm of a 150 μm-thick polymer film submerged in water causes the rapid appearance of a surface layer of bubbles. The rapid appearance of surface bubbles cannot be explained by normal diffusion of N2 through the polymer and likely results from internal gas pressure buildup during the reaction. For an azide concentration of 0.1 M and a light intensity of 140 mW/cm2, the yield of gas bubbles is calculated to be approximately 40%. The dynamics of bubble growth depend on the surface morphology, light intensity, and 2N3-AN concentration. A combination of nanoscale surface roughness, high azide concentration, and high light intensity is required to attain the threshold N2 gas density necessary for rapid, high-yield bubble formation. The N2 bubbles adhered to the PMMA surface and survived for days under water. The ability to generate stable gas bubbles "on demand" using light permits the demonstration of photoinduced flotation and patterned bubble arrays.

3D Printed Structures for Ultrasound Attenuation in Underwater Environment

In this work, open or closed air cavity (air bubble) inclusion structures are 3D printed via direct ink writing and fused deposition modeling methods utilizing materials of polydimethylsiloxane silicone or thermoplastic polyurethane, respectively, and these structures are examined for their attenuation capacity concerning ultrasonic waves in underwater environment. It is found that several factors, such as interstitial fencing layer, air cavity fraction, material interface interaction, and material property, are fundamental elements governing the overall attenuation performance. Hence, via 3D printing technique, which could conveniently manipulate structure's cavity volume fraction, such as via filament size and filament density on surface, structures with tunable attenuation could be designed. In addition, considering directions where ultrasound would encounter interfaces, that is, if the geometry could induce more interface interactions, such as triangular shape compared with simple square, it is possible to obtain immense attenuation enhancement, which does pave an additional approach for attenuation optimization via convoluted structural interface design that is exclusively tailored by additive manufacturing.

Acoustic Streaming-Induced Multimodal Locomotion of Bubble-Based Microrobots

Acoustically-driven bubbles at the micron scale can generate strong microstreaming flows in its surrounding fluidic medium. The tunable acoustic streaming strength of oscillating microbubbles and the diversity of the generated flow patterns enable the design of fast-moving microrobots with multimodal locomotion suitable for biomedical applications. The acoustic microrobots holding two coupled microbubbles inside a rigid body are presented; trapped bubbles inside the L-shaped structure with different orifices generate various streaming flows, thus allowing multiple degrees of freedom in locomotion. The streaming pattern and mean streaming speed depend on the intensity and frequency of the acoustic wave, which can trigger four dominant locomotion modes in the microrobot, denoted as translational and rotational, spinning, rotational, and translational modes. Next, the effect of various geometrical and actuation parameters on the control and navigation of the microrobot is investigated. Furthermore, the surface-slipping multimodal locomotion, flow mixing, particle manipulation capabilities, the effective interaction of high flow rates with cells, and subsequent cancerous cell lysing abilities of the proposed microrobot are demonstrated. Overall, these results introduce a design toolbox for the next generation of acoustic microrobots with higher degrees of freedom with multimodal locomotion in biomedical applications.

Recoverable underwater superhydrophobicity from a fully wetted state via dynamic air spreading

Maintaining the superhydrophobicity underwater offers drag resistance reduction, antifouling, anti-corrosion, noise reduction, and gas collection for boat hulls and submarine vehicles. However, superhydrophobicity typically does not last long underwater since the Cassie state is metastable. Here, we report a reversible and localized recovery of superhydrophobicity from the fully wetted state via air bubble spreading. Composed of sparse fluorinated chained nanoparticles, the submerged surface shows super-low energy barrier for bubble attachment. Especially the recovered plastron exhibits excellent longevity. Based on a simplified, truncated nanocone model, the dynamic spreading of bubbles is analyzed considering two basic parameters, i.e., surface geometric structure and surface energy (which appeared as intrinsic water contact angle). Numerical simulation results via COMSOL confirms the effect of geometric structure on bubble spreading. This investigation will not only offer new insights for the design of robust recoverable superhydrophobic surfaces but also broaden the applications of superhydrophobic coatings.

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Superhydrophobicity is recovered from fully wetted state in submerged system

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The dynamic spreading of bubbles is theoretically analyzed

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The geometric criteria provide direction in designing superhydrophobic surfaces

Soft-Contact Acoustic Microgripper Based on a Controllable Gas-Liquid Interface for Biomicromanipulations

The manipulation of microscale bioentities is desired in many biological and biomedical applications. However, the potential unobservable damage to bioparticles due to rigid contact has always been a source of concern. Herein, a soft-contact acoustic microgripper to handle microparticles to improve the interaction safety is introduced. The system takes advantage of the acoustic-enhanced adhesion of flexible gas-liquid interfaces to capture-release, transport, and rotate the target, such as microbeads (20-65 µm) and zebrafish embryos (from 950 µm to 1.4 mm). The gas-liquid interface generated at the tip of a microcapillary can be precisely controlled by a pneumatic pressure source. The gas-liquid interface oscillation excited by acoustic energy imposes coupled radiation force and drag force on the microparticles, enabling multidimensional movements. Experiments with the microbeads are conducted to evaluate the claimed function and quantify the key parameters that influence the manipulation result. Additionally, 250 zebrafish embryos are captured, transported, and rotated. The hatching rate of the 250 manipulated embryos is approximately 98% similar to that of the nonmanipulated group, which proves the noninvasiveness of the method. The derived theories and experimental data indicate that the developed soft-contact microgripper is functional and beneficial for biological and medical applications.

Tunable Fluid-Type Metasurface for Wide-Angle and Multifrequency Water-Air Acoustic Transmission

Efficient acoustic communication across the water-air interface remains a great challenge owing to the extreme acoustic impedance mismatch. Few present acoustic metamaterials can be constructed on the free air-water interface for enhancing the acoustic transmission because of the interface instability. Previous strategies overcoming this difficulty were limited in practical usage, as well as the wide-angle and multifrequency acoustic transmission. Here, we report a simple and practical way to obtain the wide-angle and multifrequency water-air acoustic transmission with a tunable fluid-type acoustic metasurface (FAM). The FAM has a transmission enhancement of acoustic energy over 200 times, with a thickness less than the wavelength in water by three orders of magnitude. The FAM can work at an almost arbitrary water-to-air incident angle, and the operating frequencies can be flexibly adjusted. Multifrequency transmissions can be obtained with multilayer FAMs. In experiments, the FAM is demonstrated to be stable enough for practical applications and has the transmission enhancement of over 20 dB for wide frequencies. The transmission enhancement of music signal across the water-air interface was performed to demonstrate the applications in acoustic communications. The FAM will benefit various applications in hydroacoustics and oceanography.

Magnetic-actuated "capillary container" for versatile three-dimensional fluid interface manipulation

Fluid interfaces are omnipresent in nature. Engineering the fluid interface is essential to study interfacial processes for basic research and industrial applications. However, it remains challenging to precisely control the fluid interface because of its fluidity and instability. Here, we proposed a magnetic-actuated "capillary container" to realize three-dimensional (3D) fluid interface creation and programmable dynamic manipulation. By wettability modification, 3D fluid interfaces with predesigned sizes and geometries can be constructed in air, water, and oils. Multiple motion modes were realized by adjusting the container's structure and magnetic field. Besides, we demonstrated its feasibility in various fluids by performing selective fluid collection and chemical reaction manipulations. The container can also be encapsulated with an interfacial gelation reaction. Using this process, diverse free-standing 3D membranes were produced, and the dynamic release of riboflavin (vitamin B₂) was studied. This versatile capillary container will provide a promising platform for open microfluidics, interfacial chemistry, and biomedical engineering.

Phononic metastructures with ultrawide low frequency three-dimensional bandgaps as broadband low frequency filter

Vibration and noise control are among the classical engineering problems that still draw extensive research interest today. Multiple active and passive control techniques to resolve these problems have been reported, however, the challenges remain substantial. The recent surge of research activities on acoustic metamaterials for vibration and noise control are testimony to the fact that acoustic metamaterial is no longer limited to pure theoretical concepts. For vibration and noise control over an ultrawide frequency region, 3-D metastructures emerge as a novel solution tool to resolve this problem. In that context, the present study reports a novel proposal for 3-D monolithic phononic metastructures with the capability to induce low frequency ultrawide three-dimensional bandgaps with relative bandwidth enhancements of 157.6% and 160.1%. The proposed monolithic metastructure designs consist of elastic frame assembly that is connected with the rigid cylindrical masses. Such structural configuration mimics monoatomic mass-spring chain where an elastic spring is connected with a rigid mass. We develop an analytical model based on monoatomic mass-spring chain to determine the acoustic mode frequency responsible for opening the bandgap. The wave dispersion study reveals the presence of ultrawide bandgaps for both types of metastructures. The modal analysis shows distribution of vibration energy in the bandgap opening (global resonant mode) and closing (local resonant mode) bounding edges. We further analyze the band structures and discuss the physical concepts that govern such ultrawide bandgap. Vibration attenuation inside the bandgap frequency range is demonstrated by frequency response studies conducted by two different finite element models. Thanks to additive manufacturing technology, 3-D prototypes are prepared and low amplitude vibration test is performed to validate the numerical findings. Experimental results show the presence of an ultrawide vibration attenuation zone that spreads over a broadband frequency spectrum. The bandgaps reported by the proposed metastructures are scale and material independent. The research methodology, modelling and design strategy presented here may pave the way for the development of novel meta-devices to control vibration and noises over a broadband frequency range.

Plane and Surface Acoustic Waves Manipulation by Three-Dimensional Composite Phononic Pillars with 3D Bandgap and Defect Analysis

The current century witnessed an overwhelming research interest in phononic crystals (PnCs) and acoustic metamaterials (AMs) research owing to their fantastic properties in manipulating acoustic and elastic waves that are inconceivable from naturally occurring materials. Extensive research literature about the dynamical and mechanical properties of acoustic metamaterials currently exists, and this maturing research field is now finding possible industrial and infrastructural applications. The present study proposes a novel 3D composite multilayered phononic pillars capable of inducing two-dimensional and three-dimensional complete bandgaps (BGs). A phononic structure that consisted of silicon and tungsten layers was subjected to both plane and surface acoustic waves in three-dimensional and two-dimensional periodic systems, respectively. By frequency response study, the wave attenuation, trapping/localization, transmission, and defect analysis was carried out for both plane and surface acoustic waves. In the bandgap, the localized defect state was studied for both plane and surface acoustic waves separately. At the defect state, the localization of both plane and surface acoustic waves was observed. By varying the defect size, the localized frequency can be made tailorable. The study is based on a numerical technique, and it is validated by comparison with a reported theoretical work. The findings may provide a new perspective and insight for the designs and applications of three-dimensional phononic crystals for surface acoustic wave and plane wave manipulation, particularly for energy harvesting, sensing, focusing and waves isolation/attenuation purposes.

Harnessing bubble behaviors for developing new analytical strategies

Gas bubbles are easily accessible and offer many unique characteristic properties of a gas/liquid two-phase system for developing new analytical methods. In this minireview, we discuss the newly developed analytical strategies that harness the behaviors of bubbles. Recent advancements include the utilization of the gas/liquid interfacial activity of bubbles for detection and preconcentration of surface-active compounds; the employment of the gas phase properties of bubbles for acoustic imaging and detection, microfluidic analysis, electrochemical sensing, and emission spectroscopy; and the application of the mass transport behaviors at the gas/liquid interface in gas sensing, biosensing, and nanofluidics. These studies have demonstrated the versatility of gas bubbles as a platform for developing new analytical strategies.