Research progress on bulk nanobubbles

There are still fundamental problems lying in the basic research of bulk nanobubbles. Are the bulk nanobubbles reported in the literature nano scale bubbles or contaminants in fact? At present, there is not yet sufficient experimental evidence to show that the bulk nanoparticles are only gas bubbles but not other nano scale contaminants. If they are indeed nanobubbles, what causes the bulk nanobubbles observed in the literature to be much more stable than being predicted by the Epstein-Plesset theory?This paper firstly discusses the contradiction between the traditional theory prediction and the observed lifetime of the bulk nanobubbles, and then discusses whether the so-called nanobubbles are gas aggregates. We review the existing typical models, and the influence of different conditions on the stability of bulk nanobubbles, for paving the road to a clear understanding of the stability mechanism of bulk nanobubbles. In addition, the representative production methods and characterization methods of bulk nanobubbles are discussed in order to offer some guidance to their wide range of commercial applications.     

Enrichment of surface charge contributes to the stability of surface nanobubbles

Surface nanobubbles are spontaneously formed at the interface between hydrophobic surfaces and aqueous solutions, which show extraordinarily longer lifetime than that was predicted by the classical thermodynamics model. In the present work, by using a surface plasmon resonance microscopy (SPRM) to quantitatively measure the dissolution kinetics of individual surface nanobubbles in real time, we explored the effects of ionic strength and pH value on the dissolution rates (lifetime) of nanobubbles. The results revealed that nanobubbles could exist stably for a long time in low-concentration electrolyte solutions or high-concentration non-electrolyte solutions, while they dissolved quickly in high-concentration electrolyte solutions. With the increase of ionic strength, the dissolution rates were accelerated by 2–3 orders of magnitude, and thus the lifespan of these surface nanobubbles was significantly shortened. In addition to ionic strength, it was further found that, with the increase of acidity or alkalinity of the solution, the dissolution rates of the surface nanobubbles were faster than that in neutral solution. These results demonstrated that the interfacial charge enrichment significantly contributed to the extraordinary stability of the surface nanobubbles.     

Concentration of nitrogen molecules needed by nitrogen nanobubbles existing in bulk water

This paper investigates the stability of nitrogen nanobubbles under dif~ ferent concentrations of nitrogen molecules by molecular dynamics simulations. It is found that the stability of nanobubbles is very sensitive to the concentration of nitrogen molecules in water. A sharp transition between disperse states and assemble states of nitrogen molecules is observed when the concentration of nitrogen molecules is changed. The relevant critical concentration of nitrogen molecules needed by the existing nitrogen nanobubbles is analyzed.     

Theory of sorption hysteresis in nanoporous solids: Part II Molecular condensation

Motivated by the puzzle of sorption hysteresis in Portland cement concrete or cement paste, we develop in Part II of this study a general theory of vapor sorption and desorption from nanoporous solids, which attributes hysteresis to hindered molecular condensation with attractive lateral interactions. The classical mean-field theory of van der Waals is applied to predict the dependence of hysteresis on temperature and pore size, using the regular solution model and gradient energy of Cahn and Hilliard. A simple “hierarchical wetting” model for thin nanopores is developed to describe the case of strong wetting by the first monolayer, followed by condensation of nanodroplets and nanobubbles in the bulk. The model predicts a larger hysteresis critical temperature and enhanced hysteresis for molecular condensation across nanopores at high vapor pressure than within monolayers at low vapor pressure. For heterogeneous pores, the theory predicts sorption/desorption sequences similar to those seen in molecular dynamics simulations, where the interfacial energy (or gradient penalty) at nanopore junctions acts as a free energy barrier for snap-through instabilities. The model helps to quantitatively understand recent experimental data for concrete or cement paste wetting and drying cycles and suggests new experiments at different temperatures and humidity sweep rates.     

Strength certificate and the constraint equation relating stress and strain invariants for inhomogeneous rocks in bulk stress state

Mechanical models of rocks subjected to various bulk stress states are considered. The models contain the Nadai parameter, which takes into account various types of the bulk stress state, the potential energy of bulk variation, and the potential energy of variations in the shape of rock elements in the equilibrium stress-strain state. These variables permit determining the conditions under which rock massifs stay in equilibrium and the conditions under which rock can cease to be in equilibrium owing to the bulk stress state action; they also allow one to justify the physical parameters of the rock strength certificate under the bulk stress state action and determine the constraint equation relating the stress and strain invariants for inhomogeneous rocks.     

铁基大块非晶合金的摩擦磨损性能研究

采用电弧熔炼、铜模吸铸法制备Fe基大块非晶合金,利用球-盘式摩擦磨损试验机进行干摩擦磨损试验,研究了铸态Fe基大块非晶合金的摩擦磨损行为及热处理对其耐磨性的影响,利用扫描电子显微镜观察合金磨损表面形貌,分析了Fe基大块非晶合金以及相同成分晶态合金的磨损机理.结果表明:在本试验条件下,铸态Fe基大块非晶合金的耐磨性高于相同成分的晶态合金,热处理可以有效提高铸态Fe基大块非晶合金的耐磨性,在保持完全非晶状态的前提下,退火态非晶合金的磨损率较铸态非晶合金减小约40%;材料的结构和性能对合金的摩擦系数影响不大,当进入稳定阶段后Fe基大块非晶合金的摩擦系数稳定在0.58左右;不同处理状态的Fe基大块非晶合金和相同成分晶态合金的磨损机制不同,非晶合金的磨损机制以疲劳磨损为主兼有磨粒磨损,相同成分晶态合金的磨损机制为粘着磨损和磨粒磨损共同作用的结果.     

A review of microstructure in concentrated suspensions and its implications for rheology and bulk flow

An overview of present understanding of microstructure in flowing suspensions is provided. An emphasis is placed on how the microstructure leads to observable bulk flow phenomena unique to mixtures. The bridge between the particle and bulk scales is provided by the mixture rheology; one focus of the review is on work that addresses the connection between microstructure and rheology. The non-Newtonian rheology of suspensions includes the well-known rate dependences of shear thinning and thickening, which have influence on bulk processing of suspensions. Shear-induced normal stresses are also measured in concentrated suspensions and include normal stress differences, and the isotropic particle pressure. Normal stresses have been associated with shear-induced migration, and thus have influence on the ultimate spatial distribution of solids, as well as the flow rate during processing; a second focus is on these uniquely two-phase behaviors and how they can be described in terms of the bulk rheology. An important bulk fluid mechanical consequence of normal stresses is their role in driving secondary flows.     

ZrCuNiAlAg块体非晶合金JH-2模型研究

为了更好的进行ZrCuNiAlAg块体非晶合金药型罩的爆炸成形及侵彻仿真研究,首要就是建立其材料模型。本文结合ZrCuNiAlAg块体非晶合金力学性能试验结果计算得到了材料的JH-2模型参数,研究确定了ZrCuNiAlAg块体非晶合金JH-2模型。为了验证ZrCuNiAlAg块体非晶合金JH-2模型的准确性,采用Autodyn建立了平板撞击试验有限元模型,模拟了ZrCuNiAlAg块体非晶合金材料在高压、高应变率等环境条件下的变形过程,仿真计算得到的靶板背面自由面粒子速度与试验结果相比,速度平均偏差均在3%以内,表明ZrCuNiAlAg块体非晶合金JH-2模型能很好的描述该材料在大变形、高应变率、高压等环境条件下的力学行为,验证了ZrCuNiAlAg块体非晶合金JH-2模型准确性。     

Free-surface ductility in bulk forming processes

Free-surface ductility for bulk forming processes is investigated by finite element (FE) simulations of micromechanical representative volume elements (RVEs) subjected to typical deformation histories. Evidence is provided that failure in bulk forming might be explained by stability considerations rather than by void growth criteria. A readily applicable condition for stability is derived; the strain path curvature is shown to delay significantly the onset of instability. The criterion is used to obtain bulk forming limit diagrams both in principal strain and in principal stress space.     

水中爆炸表面空穴的理论研究

针对水中装药爆炸直达冲击波在自由面反射形成表面空穴的问题,应用水中爆炸冲击波声学近似理论,认为冲击波不能穿过空穴传播,得出一维水中爆炸表面空穴范围、水质点运动速度、水层抛射速度、空穴闭合时间以及空穴闭合产生的压力峰值与持续时间等,并将结论扩展到二维空间.通过具体算例,对比本方法与F.A.Costanzo等采用Arons...     

铝箔力学性能的实验研究

铝箔已被广泛应用于电子工业,现又被用作锂电池正极集流体,因而对于铝箔的力学性能要求也在不断提高。通过表征和研究铝箔的力学性能(弹性模量、屈服强度、断裂强度等),能够为铝箔相关技术的可靠性研究提供必要的数据支持和理论指导,从而使铝箔得到合理和可靠的使用。本文运用微拉伸、纳米压痕和动态力学分析(DMA)实验,分别研究了不同厚度的H18态和O态铝箔的力学性能。结果表明两者的弹性模量均约为30GPa,仅为块材的一半;H18态铝箔材料的断裂强度要明显强于块材,而O态铝箔材料的断裂强度则明显小于块材;此外,H18态铝箔材料的屈服强度明显大于块材,O态铝箔材料的屈服强度与块材相仿。并且,随着厚度的增加,H18态铝箔材料的延伸率显著增大,但是仍远小于块材。通过扫描电子显微镜(SEM)对铝箔材料断裂形貌进行微观分析,发现铝箔的拉伸断裂方式为脆性断裂。     

裂隙挤喷流对孔隙介质排水体积模量的影响

实际岩石比如沉积形成的岩石往往是裂隙和孔隙并存的孔隙介质. 由于扁状的裂隙与近似球形或圆管形的孔隙具有不同的可压缩性,当孔隙介质受压时,液体会从易压缩的裂隙中挤出流入不易压缩的孔隙中,这种挤喷流会引起弹性模量的频散和能量的耗散. 着重研究了裂隙挤喷流和液体可压缩性对孔道变形的影响,推导出了动载荷作用下排水体积模量的表达式. 与挤喷流相关的裂隙附加柔度会引起排水体积模量随频率变化,使得孔隙介质呈现黏弹性. 频率越高,模量的实部越大,岩石抵抗变形的能力越强. 而模量的虚部体现了挤喷流对能量的耗散. 裂隙密度主要决定模量频散的幅度以及能量耗散的强度,且裂隙密度越大,模量频散幅度越大,能量耗散也越强. 裂隙的纵横比主要决定模量频散速率最快或能量耗散最强时对应的特征频率. 若孔隙介质中不含有裂隙,即裂隙密度是0时,排水体积模量退化为Biot理论中的排水体积模量.     

散粒体-贮仓结构-地基的动力特性分析

以筒仓模型结构为例，考虑到散粒体-结构-地基的相互作用，对弹性地基上筒仓内的散粒体的不同计算模型进行了多种工况、系统的有限元动力分析计算。通过与筒仓模型动力实验结果进行了比较，得出结论：在对地基施以水平激励时，弹性地基上筒仓的动力响应大于刚性地基上筒仓的动力响应，散粒体与仓壁的相对运动对筒仓结构有减振作用。     

Hf基非晶合金夹芯结构长杆弹的侵彻行为

为研究Hf基非晶合金的变形行为及高速侵彻性能,分别开展了Hf基非晶合金材料静动态力学性能和Hf基非晶合金夹芯结构长杆弹高速侵彻45钢靶体试验研究,并与45钢夹芯长杆弹侵彻结果进行对比。研究发现:Hf基非晶合金具有较高的断裂强度,断裂时伴随有能量释放现象;Hf基非晶合金夹芯长杆弹侵彻钢靶过程可分为3个阶段:开坑、夹芯结构侵彻和剩余弹体侵彻。Hf非晶合金在侵彻过程中发生了明显的释能反应,显著地增强了弹体毁伤效应,扩大了侵彻弹孔直径,增加了弹体侵彻深度和弹孔体积。在高速冲击下,Hf基非晶合金夹芯长杆弹表现出优异的侵彻性能,可以为非晶合金材料在高效毁伤领域的应用提供新思路。     

DEM speedup: Stiffness effects on behavior of bulk material

A number of techniques exist for minimizing the computational cost of discrete element simulations (DEMs). One such method is a reduction of particle stiffness, which allows for bigger time steps and therefore fewer iterations in a simulation. However, the limits and drawbacks of this approach are still unclear, and may lead to invalid results. This paper investigates the effect of a stiffness reduction on bulk behavior by examining three case studies. Two cases demonstrate that particle stiffness can be reduced without affecting the bulk material behavior, whereas the third test shows that a stiffness reduction influences the bulk behavior.     

DEM speedup： Stiffness effects on behavior of bulk material

A number of techniques exist for minimizing the computational cost of discrete element simulations （DEMs）. One such method is a reduction of particle stiffness, which allows for bigger time steps and therefore fewer iterations in a simulation. However, the limits and drawbacks of this approach are still unclear, and may lead to invalid results. This paper investigates the effect of a stiffness reduction on bulk behavior by examining three case studies. Two cases demonstrate that particle stiffness can be reduced without affecting the bulk material behavior, whereas the third test shows that a stiffness reduction influences the bulk behavior.     

Bulk Cavitation and the Possibility of Localized Interface Deformation due to Surface Layer Swelling

We consider the uniform swelling of a compressible hyperelastic surface layer with finite thickness that is attached to an underlying bulk material composed of a non-swelling incompressible hyperelastic material. In addition to classically smooth solutions, two additional phenomena may occur for sufficiently large swelling. One is the formation of cavities in the interior of the underlying bulk material. The other is the disappearance of smooth solutions in the surface layer while the underlying bulk material remains intact. It is conjectured that the latter may be associated with the concentration of deformation at the swelling interface. Both phenomena are investigated by the consideration of solutions to a boundary value problem for a sphere involving radial deformation with a prescribed swelling field that acts as an effective loading device. Specific material models for both the compressible swollen surface layer and the non-swollen incompressible bulk are invoked so as to permit an analytical treatment. Swelling thresholds are obtained that depend on the thickness of the surface layer for the onset of these separate phenomena.     

Strain measurements on hydrostatically stressed materials using differential-transformer transducers

An apparatus for directly measuring orthogonal strains of hydrostatically stressed polymeric materials is described. The compliance of the principal dimensions of regular-shaped specimens permits calculation of bulk compliance, bulk modulus, and anisotropy that may exist in the material. The application of differential transformers to hydrostatic strain measurements is discussed.