Flow focusing is an effective method to form thin jets. It can be characterized by the formation of a steady cone-jet configuration in the core of a focusing high-speed fluid stream, as the focused fluid is continuously supplied through a capillary needle. The jet issued from the vertex of the cone passes through an orifice, and eventually breaks up into monodisperse droplets due to flow instability. First proposed in 1998, the flow focusing principle has been adopted to develop a series of capillary flow techniques such as single flow focusing, electro-flow focusing, co-flow focusing and microfluidic flow focusing. These techniques are steady, controllable and gentle in producing monodisperse droplets, particles and capsules down to micrometer scale and below. Therefore they have great significance in science, technology and engineering applications. In flow focusing, the formation of the stable cone is the prerequisite condition to form the stable jet; the process parameters influence the perturbations deposited on the jet interface; and the growth of perturbations results in the breakup of the jet. This is a complex problem in fluid mechanics due to its multi-scale, multi-interface and multi-coupling characteristics. Jet instability analysis is the most useful tool for exploring the mechanisms of jet breakup. In this paper, we review the progress of flow focusing with different geometrical structures during recent two decades, and summarize the key mechanics problems of flow focusing including process control, flow modes, scaling laws and instability analyses. The methods and achievements in the study of jet instability are also briefly described. Finally, some future research topics and opportunities for applications are provided.

As China accelerates the implementation of the marine power strategy, the demand for advanced underwater sound-absorbing materials has become increasingly urgent. Unlike air absorption, the high hydrostatic pressure and complex marine environment impose more stringent requirements regarding underwater sound-absorbing materials. The essence of the sound absorption problem is how to efficiently transform elastic energy into heat or other forms of energy. This paper reviews the traditional underwater sound absorbing materials based on both the intramolecular friction and the energy dissipation mechanisms. The main problem of traditional underwater sound-absorbing material is attributable to its poor sound absorption performance under low frequency and high hydrostatic pressure. On the one hand, this is because the underwater sound-absorbing material is of limited thickness. Besides, due to the limitation of the mass density law, it cannot effectively absorb the low-frequency sound waves from the water. On the other hand, elastic material, such as polymer, becomes hard under high hydrostatic pressure, thus the conversion efficiency of acoustic elastic energy is greatly reduced. With the development of local resonance theory and the concept of metamaterials, a series of new underwater sound absorbing materials have been produced, which provide new ways to solve the problems encountered in developing underwater sound absorbing materials. The local resonance theory states that a small-scale structure can control the spread of long sound waves. Therefore, it can solve the problem of sound absorption at low-frequencies. This paper focuses on the theory of local resonance, the development of new sonar wood and phonon glass, and other novel underwater sound absorbing materials. Based on the local resonance theory, the phonon glass material can improve the compression performance by introducing the porous metal skeleton structure, and solve the problem of poor sound absorption performance under high hydrostatic pressure. At the end of this paper, the future development of underwater sound-absorbing materials is explored.

固体材料在冲击拉伸载荷作用下常常会断裂成多个碎片 (碎片化), 固 体材料碎片化的物理机制是多点损伤同时在固体中成核和发展, 导致固体多 处破坏. 自 Mott 对固体的动态碎裂问题进行了开创性研究后, 几十年来, 对 固体动态碎裂机制的研究一直是应用物理学、力学、航天和兵器工程等领域 共同关心的重要课题. 本文介绍了在冲击拉伸载荷作用下固体的动态碎裂研 究的发展历史, 给出相关的理论分析、实验研究和数值模拟的研究进展, 特别 针对现有的各种关于碎片尺度、碎片分布、以及碎片化物理机制的理论模型 进行了较详尽的阐述和讨论, 最后指出现有实验和理论研究中仍然存在的关 键科学问题及进一步的研究展望. Materials usually break into many pieces (fragments) under high strain-rate tension. Understanding the fragmentation properties of solids is important to the researchers in fields of physics, mechanics, aerospace and defense engineering. In this paper, we reviewed the researches on tensile fragmentation undertaken since 1940s. Recent advances in the fragmentation studies from theoretical analysis, experimental investigation, and numerical simulation are addressed. Some suggestions were provided for the future development in this field.

The propagation of a shock wave depends not only on the conditions that it is generated, but also on the media where it propagates. Variations in driver condition, boundary condition and/or physical/chemical properties of the media may alter the behav-iors of the shock wave propagation, while the flowfield also varies along with the shock wave motion. Although shock wave propagation and interactions exist widely in the nature and human activities, there is still a long way to go for a better understanding of its complex mechanism, for theoretical descriptions of the phenomena, as well as for enrichment of the application. In this paper, we review the shock wave interactions and related analyses, and further discuss several hot topics, including shock/shock interaction, shock/boundary layer interaction, shock/turbulence interaction, shock focusing/ignition and instability of shocked interface.

Pentamode materials, made of conventional solids through microstructure de-sign, may have acoustic property of complex fluid. Its superior capacity for underwater acoustic wave manipulation stimulates recently an intense research activity. In this review, pentamode materials and their recent progress are explained from di?erent angles: basic concept, microstructure design, acoustic applications and fabrication technology, in order to provide an overall view on this kind of special material.

Functionally graded carbon nanotube reinforced composite (FG-CNTRC) is a new generation of advanced composite materials, where carbon nanotubes (CNTs) are used as the reinforcements in a functionally graded pattern. The mechanical behavior of FG-CNTRC has emerged as one of the recent hot research topics in materials science and engineering. This paper presents a review of the developments in the modeling and analysis of FG-CNTRC structures. The emphasis are put on the linear and nonlinear bending, buckling and postbuckling, and free and forced vibration of FG-CNTRC beams, plates, shells and shell panels under various loading, boundary and environmental conditions. The presented progresses lay foundation for future studies in the area of FG-CNTRC structures. Some critical aspects for further explorations are highlighted.

Computed tomography (CT) and magnetic resonance imaging (MRI) are widely used in clinical practice for diagnose of vascular diseases, such as artery stenosis, aneurysm and vascular malformation. Besides static images, the newly developed phase-contrast magnetic resonance imaging (PCMRI) is able to capture hemodynamic parameters from timesequenced scanning; these parameters include velocity magnitude and directions. However, long time scanning is usually required for data collection. Alternatively, hemodynamic parameters can be obtained by image based computational fluid dynamics (CFD), this only demands normal scan of CT or MRI images for 3-D construction. This report will compare both methods, present examples of hemodynamic analysis in clinical application, and discuss the future directions of hemodynamic research.

Moving contact line (MCL) is the triple-phase region (TPR) formed by two impermeable fluids moving on a solid surface. TPR covers multiple scales, where the interactions among phases influence the dynamic behaviors of the entire fluid field. Owing to its significant applications and rapid development in the fields of energy, aerospace, biology, etc., new challenges emerge in MCL problems. Scaling analysis is an important tool to characterize self-similar expansion of the MCL. Focusing on the scaling relations of MCLs, we review the progresses of physical mechanics investigations under mechano-electro-thermalchemical multifield coupled conditions for MCL on rigid/flexible solid surfaces with complex geometries, including hydraulic interior corner, micro-pillar-arrayed surface, dissolvable surface, lag zone in hydraulic fracturing, etc. Through a combined study of multiscale experiments, large-scale molecular dynamics simulations, molecular kinetic theory and hydrodynamics, new phenomena were discovered, such as solid-like precursor film, single-file water-molecular precursor chain, and zigzag MCL. From the interface structure at atomic level to the flow characteristics at continuum level, we discuss the scaling laws of self-similar expansion, and the physical mechanisms and dynamic rules, such as driving source, energy dissipation, boundary conditions, etc. We explore the answers to the Huh-Scriven paradox under multifield circumstance, and outlook the prospects and applications of MCL.

With the vigorous development of the momentous projects in aerospace en-gineering field, new mechanical problems emerge continuously. Therefore, the research on engineering mechanics plays a prominent role in the developments of the space technol-ogy. The launch and operation environment for spacecraft is becoming more stringent as the spacecraft nowadays is developing on the track of super high speeds, deep space explo-ration and multifunction. Large-area and large flexible structure equipped on the spacecraft has proposed new challenges on the dynamics of deployment in orbit, modal identification and rigid-flexible coupling dynamics. Meanwhile, high-precision and high-resolution earth-observing request leads to new problems of micro-vibration control and thermally induced vibration control in orbit. All these problems urge more accurate methods for ground sim-ulations and experiments. In this paper, we outline the new mechanical problems arising from the launch of spacecraft, in orbit operation, ground simulations and experiments, etc. We focus on coupled dynamics, aerodynamics, multi-body dynamics, structure dynamics and experimental mechanics. The new problems and development directions of engineering mechanics are discussed in the end.

Microstructured superhydrophobic surfaces have broad applications such as anti-fouling and drag reduction.The performance of such surfaces strongly depends on the stability of liquid-gas interfaces, which affects physical processes including wetting transition, restoration and bubble evolution.Various physical factors including pressurization and gas diffusion may destabilize the liquid-air interfaces, and lead to evolution in different manners.In this paper, we first summarize the three types of interfacial stability problems for liquid-gas interfaces.Relying on external stimulations, the liquid-air interface may evolve into different stages and exhibit different morphologies.The recent progress of research on the stability and control of liquid-air interfaces in both droplet systems and submersion circumstances has been reviewed.Based on this review, remaining challenges for future research have been given.