Institute Of Mechanics,Chinese Academy of Sciences
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Study on interfacial dynamics of dissolutive optimization at micro-/nano-scale
No full text溶解优化问题是指针对特定的固液界面体系，在外场作用下，采用溶解的方式使界面形貌发生演化，体系势能降低，特定的界面性能得到优化的问题。溶解优化问题涉及到多个过程，包括溶解、界面动力学、性能优化过程。阐明不同溶解条件下固体形貌的演化机制，有助于加深对以上过程的认识，解决能源开采、生物医药、微纳制造等多个领域中的瓶颈难题。
本文主要针对溶解优化的三个关键科学问题：溶解导致的形貌演化、溶解过程的尺寸效应、溶解形状的优化性能展开研究。溶解优化是一个跨尺度问题，溶剂化与扩散过程起源于微观尺度，形状与性能的优化存在于微观与宏观尺度。因此采用跨尺度研究方法，通过分子动力学模拟研究溶解优化过程，采集固液界面的形貌、溶解速度、液体压强分布、固体应力分布等物理量随时间的变化，从原子尺度理解溶解优化的机理。并通过微米尺度实验进行对照，发现溶解优化过程中的尺寸效应，确定溶解优化过程在实际应用中的可行性。为了深刻理解影响溶解界面动力学的微观机制，综合考虑了初始形状、几何尺寸、外在驱动、材料属性等因素下的溶解优化过程。此外，通过分子动力学模拟测试了不同溶解形状的优化性能，提出了优化形状的潜在应用。本文聚焦于溶解优化问题中逐渐深入的三个典型模型：针尖、通道、阵列基底，主要研究内容和成果如下：
纳米针尖的溶解优化。模拟了纳米针尖的溶解优化过程，阐明了针尖轮廓演化的机理，制定了纳米针尖的优化准则，提出了衡量针尖尖锐度的无量纲参数，厘清了固体溶解能力与初始尺寸对针尖尖锐度的影响机制，结合理论和模拟考察了纳米针尖的毛细力。研究结果揭示了弯液面附着下纳米针尖的溶解机理，为优化纳米针尖的制造提供了理论支持。
纳米流体通道的溶解优化。模拟了纳米流体通道的溶解优化过程，阐明了端口效应在纳米流体通道溶解中的作用机理，厘清了溶解能力、流体速度和通道长度对溶解优化过程的影响，并利用实验进行了对照，通过模拟考察了纳米流体通道的输运性能和储能性能。研究结果阐明了纳米流体通道的溶解界面动力学，并为设计具有优化性能的纳米流体通道提供了理论支持。
纳米阵列基底的溶解优化。模拟了圆柱阵列下基底的溶解优化过程，阐明了圆柱阵列阻碍下基底的形状演化机制，厘清了圆柱间距、驱动力、圆柱-基底相互作用对溶解过程的影响，表征了优化纳米阵列基底的输运性能和方向性。研究结果有助于加深对纳米阵列基底界面动力学的理解，并为提高纳米阵列基底的输运性能提供了可行的手段。</p
High Efficiency Production of Functional Small Extracellular Vesicles through Cellular Self-Motivation
No full textIn stem cell therapies, small extracellular vesicles (sEVs) are extremely limited in application due to their limited production. Here, we propose a new concept of "cellular self-stimulation" and develop a cost-effective method for the preparation of sEVs, which enables the conversion of cellular traction to self-generated stimulation through piezoionic hydrogels and enhances the ability of cells to secrete sEVs by more than an order of magnitude. The traction of the adherent cells leads to deformation of the piezoionic substrate, which in turn translates into a millivolt-level electrical signal acting on the cell itself, stimulating the cell to produce more sEVs. These sEVs remain biologically intact and have shown excellent efficacy in in vitro and in vivo assays, confirming the superior therapeutic potential of high concentrations of sEVs. This provides a strong impetus for the development and dissemination of stem cell therapies
Experimental Investigation of Transverse Jet Interaction in Low-Density Hypersonic Crossflows
No full textTransverse jets have been effectively employed for attitude/orbit control within the aerospace applications. However, there is a paucity of experimental data on the flow characteristics of transverse jet interaction with hypersonic crossflows at high flight attitudes, particularly those associated with low-density and high-temperature conditions. This study aims to experimentally investigate the flow characteristics of transverse jets in a hypersonic crossflow using a flat-plate model. Schlieren imaging, nitric oxide planar laser-induced fluorescence (NO-PLIF) imaging, and pressure transducer techniques are employed to visualize flow structures and conduct parametric measurements. Numerical simulations are conducted to shed light on the flow structures and elucidate the underlying mechanisms as well. The results indicate that NO-PLIF imaging can effectively capture the spatially resolved structures surrounding the jet exit. Both the Mach disk height and barrel shock width are positively correlated with the jet momentum ratio J and angle of attack alpha. Furthermore, the pressure plateau and peak distribution upstream and downstream of the sonic orifice are also significantly influenced by J and alpha. Specifically, peak pressure increases with an increase in J, while it drops for a larger alpha. These results enhance our understanding of flow characteristics of transverse jet interactions in low-density, high-temperature hypersonic crossflows and provide validations for subsequent numerical simulations
Efficient condensation on spiked surfaces with superhydrophobic and superhydrophilic coatings
No full textSteam condensation on the surface of a solid is a widely observed mode of energy transfer in nature and various industrial applications. The condensation efficiency is closely related to the material properties and geometric morphology of the solid surface, as well as the method of liquid removal. Despite many surface modification strategies having been employed to enhance condensation, most experiments use gravity to remove condensate, thereby limiting the application scenarios. Here, we report a method without relying on gravity that uses superhydrophilic and superhydrophobic coatings as well as spiked surfaces to achieve efficient steam condensation and rapid removal of the liquid. We reveal that hydrophobic spiked surfaces facilitate the dropwise condensation, while hydrophilic bottom grooves promote liquid spreading, and the suction in capillary gaps can promptly remove the condensate liquid. Thus, a high heat flux density of 920.7 kW/m2 and a high condensation efficiency 94% are obtained in an open environment. Additionally, we demonstrate that the pipe with a spiked inner surface has a significantly higher condensation efficiency compared to the smooth pipe. This finding provides a new approach for efficient steam condensation technology in phase-change heat transfer applications
Melting Enhancement of Phase Change Material with Fins and Graphene Nanoplatelets Under Micro/Low Gravity for Thermal Management in Space Detector
No full textThermal energy storage is an efficient way for thermal control of near-earth and deep space detectors, but the melting rate is restricted by low heat transfer performance of phase change material (PCM) and disappearance or suppression of natural convection under micro/low gravity. To accelerate melting of PCM under micro/low gravity, graphene nanoplatelet (GNP)-enhanced PCM with fins are proposed. The effects of fin shape, GNP concentration and gravity level on dynamic melting characteristics considering thermocapillary convection are investigated. The results show that the improvement effect of rectangular fins on melting rate is higher than that of triangular fins under microgravity; with the decrease of gravity level, the melting rate is reduced. The presence of GNP significantly promotes the melting under micro/low gravity. At GNP concentration of 0.03 vol%, the reduction of melting time can reach 58.2%, 49.4% and 51.6% at microgravity, moon's gravity of 1.625 m s-2 and Mars' gravity of 3.711 m s-2, respectively
Shock resistance of a bio-inspired double corrugated sandwich panel impacted by a graded cellular projectile
No full textSandwich structures with a thin-walled core layer exhibit remarkable shock resistance, but most of them suffer from high initial peak stress, limiting their load mitigation ability. Inspired by the S-shaped corrugated wall of the cuttlefish bone and the herringbone corrugation of Odontodactylus syllabus dactyl, a sandwich panel with a bio-inspired double corrugated (BDC) core is proposed to enhance the shock resistance. Impact simulations and experiments using graded cellular projectiles were conducted to analyze the effects of core layer configuration on the shock resistance performance of the sandwich panels and to validate the necessity of well-designed graded cellular projectiles in simulating blast loads. It is found that compared to hexagonal honeycomb and bio-inspired single corrugated sandwich panels of the same density, the BDC sandwich panels exhibit superior shock resistance performance, with a reduction of 97.9% and 40.7% in maximum transmission stress and maximum deformation, and an increase of 38.0% in crushing force efficiency. The maximum transmission stress of the BDC sandwich panel is mitigated by the herringbone corrugations, and higher plateau stress is achieved. The underlying mechanism is that herringbone corrugations change the deformation mode, causing less plastic deformation at impact onset to attenuate peak stress, and later generating more wrinkles to increase plateau stress. A stable plateau stress and deformation during impact are guaranteed by the non-hermetic corrugated walls because they permit air to escape, avoiding strain hardening. The present findings provide a new inspiration and method for novel protective structure design and testing
Optical signal characteristics analysis of atmospheric disturbance density fields generated by high-speed aircraft
No full textAircraft disturbs t he adjacent atmospheric environment in flight, forming spatial distribution features of atmospheric density that differ from the natural background, which may potentially be utilized as tracer characteristics to introduce new technologies for indirectly sensing the presence of aircraft. In this p spheric disturbance density fie scattering light of an atmosp bution of the disturbance de aper, the concept of a long-range aircraft detection based on the atmold is proposed, and the detection mode of tomographic imaging of the heric disturbance flow field is designed. By modeling the spatial distrinsity field, the scattered echo signal images of active light towards the disturbance field at long distance are simulated. On this basis, the characteristics of the disturbance optical signal at the optimal detection resolution are analyzed. The results show that the atmospheric disturbance flow field of the supersonic aircraft presents circular in the light-scattering echo images. The disturbance signal can be further highlighted by differential processing of the adjacent scattering images. As the dis bance signal increases, and the signal intensity and contrast with the background decrease. Under the ground-based observation conditions of the aircraft at a height of 10000 m, a Mach number of 1.6, and a detection distance of 100 km, the contrast between the disturbance signal and the backtance behind the aircraft increases, the diffusion range of the distu
A data-driven approach to identify the optimal sub-laminates for homogeneity design under the concept of double-double composites
No full textThis paper presents the design of the sub-laminate under the concept of the double-double composite using the data-driven method. As the key advantage of the double-double composites is the reduced repeat number of sub-laminates necessary to achieve homogeneity, it is therefore crucial to determine the best pattern of sub-laminates which ensures the optimal lightweight design with the minimum thickness. In the data-driven framework, first, a generative neural network model was built for generating the sub-laminates fitting in the scope of the homogeneous criterion. Then, a symbolic regression model was built for quantitatively finding the hidden layup patterns in the dataset of these sub-laminates. It is found that the form of double-double and triple-double stands out in the vast design space of all the possible layup sequences. The 4-layer sub-laminate of [B/ - /i//i/ - B] and 6-layer sub-laminate of [B/ - /i/ - y/ + y/ + /i/ - B] are most recommended as they meet the homogeneous criterion with less thickness and offer larger design space of mechanical properties. The established data-driven framework can be extended to other scenarios especially in finding the common design rules of laminates
Minimum scale and spatial resolution requirement for direct numerical simulations of compressible turbulence
No full textIn the direct numerical simulation (DNS) of compressible turbulence using Navier-Stokes equations, due to the incomplete resolution of shocklets, the classical grid resolution criterion based on the usual Kolmogorov length scale appears insufficient for high-order statistics. The present study discusses the minimum scale of compressible turbulence under the continuum assumption, and establishes new spatial resolution requirements for DNS. We first define the minimum shock scale for one-dimensional Burgers turbulence, and derive a spatial resolution criterion essential for fully resolving the second-and third-order velocity gradient moments. We demonstrate that this shock scale definition is also applicable to one-dimensional Navier-Stokes turbulence, and validate the spatial resolution requirement through numerical simulations of the Shu-Osher problem. The analysis is then extended to multi-dimensional turbulence. Through theoretical analysis and numerical studies, we conclude that the minimum local Kolmogorov scale, nmin, describes the smallest structure in turbulence and is determined by the strongest shocklet. Furthermore, we establish a requirement of nmin/Delta x >= 1.5 for compressible turbulence, and validate it through DNSs of two-dimensional compressible turbulence with different grid resolutions. The present study provides a reference on spatial resolution requirement for DNS of compressible turbulence