Ultrasonic cavitation at solid surfaces

In spite of the great potential of applying high-intensity ultrasound, which enables high-temperature and high-pressure chemistry with a reactor near room temperature and ambient pressure, sonochemistry at solid surfaces is at a weak stage of understanding with regards to the development of new materials and composite nanostructures. The science towards a quantitative understanding is only now emerging. On the other hand, in many applications an ultrasonic bath is used without thinking of the mechanism. Often surfaces are exposed to ultrasound for cleaning. Since ultrasonic treatment is not an exotic process and applicable even on large scale in industrial manufacturing, controlling the process may lead to new applications making use of the specially designed surface. This review is intended to summarize recent progress in this field and to point out most promising directions of ultrasound application for the development of new materials with functional surfaces.     

Numerical investigation of the effect of properties of the liquid on cavitation at solid surfaces

Numerical calculations were carried out on a computer with Mercon's method to investigate the effect of properties of the liquid, namely, density, pressure of the saturated vapor (gas content in a bubble), surface tension, viscosity, and wave resistance, on the instantaneous radius, velocity, and shape of the surface of a compressed cavitation bubble for various distances between two parallel solid walls confining the liquid, times, and angular coordinates. The theoretical relations found agree with earlier experimental data. Recommendations are given for using the present results in practical development of technological sonication processes and in operation of ultrasonic equipment and hydrosystems.Institute of Applied Physics, Academy of Sciences of Belarus, Minsk, Belarus. University of Trieste, Physics Department, Cavitation Laboratory, Italy. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 66, No. 4, pp. 412–420, April, 1994.     

Controlled effect of ultrasonic cavitation on hydrophobic/hydrophilic surfaces

Controlling cavitation at the solid surface is of increasing interest, as it plays a major role in many physical and chemical processes related to the modification of solid surfaces and formation of multicomponent nanoparticles. Here, we show a selective control of ultrasonic cavitation on metal surfaces with different hydrophobicity. By applying a microcontact printing technique we successfully formed hydrophobic/hydrophilic alternating well-defined microstructures on aluminium surfaces. Fabrication of patterned surfaces provides the unique opportunity to verify a model of heterogeneous nucleation of cavitation bubbles near the solid/water interface by varying the wettability of the surface, temperature and ultrasonic power. At the initial stage of sonication (up to 30 min), microjets and shock waves resulting from the collapsing bubbles preferably impact the hydrophobic surface, whereas the hydrophilic areas of the patterned Al remain unchanged. Longer sonication periods affect both surfaces. These findings confirm the expectation that higher contact angle causes a lower energy barrier, thus cavitation dominates at the hydrophobic surfaces. Experimental results are in good agreement with expectations from nucleation theory. This paper illustrates a new approach to ultrasound induced modification of solid surfaces resulting in the formation of foam-structured metal surfaces.     

The effect of a solid wall on the closure of a spherical cavitation pocket

We examine the effect of a solid wall on the closure of a spherical cavitation cavity or pocket. It is demonstrated that asymmetric flow substantially reduces the induced pressure and rate of closure for the cavity containing the gas. while in the case of a vapor cavity asymmetry of flow leads to conclusions qualitatively different from those which follow from the classical Rayleigh solution for an infinite fluid.     

超声空化效应对溶液电导率的影响

大功率超声作用于溶液时会产生空化效应,并影响溶液的物理化学性质,如电导率、液体粘度及液体表面张力等。文章对不同功率的超声作用于弱电解质溶液时其电导率的变化进行了实验研究,发现溶液电导率与超声波强度有关。当强度增大到有空化产生时,电导率开始减小;继续增大强度,溶液电导率减小到一定程度后有小幅回升现象出现。且液体内空化效应的出现将导致液体的电导率减小,空化达到一定强度时,空化效应引起的局部瞬态高温高压、冲击波和微射流导致的新导电粒子,会使电导率小幅回升。     

Molecular self-assembly at solid surfaces

Self-assembly, the process by which objects initially distributed at random arrange into well-defined patterns exclusively due to their local mutual interactions without external intervention, is generally accepted to be the most promising method for large-scale fabrication of functional nanostructures. In particular, the ordering of molecular building-blocks deposited at solid surfaces is relevant for the performance of many organic electronic and optoelectronic devices, such as organic field-effect transistors (OFETs), organic light-emitting diodes (OLEDs) or photovoltaic solar cells. However, the fundamental knowledge on the nature and strength of the intermolecular and molecule-substrate interactions that govern the ordering of molecular adsorbates is, in many cases, rather scarce. In most cases, the structure and morphology of the organic-metal interface is not known and it is just assumed to be the same as in the bulk, thereby implicitly neglecting the role of the surface on the assembly. However, this approximation is usually not correct, and the evidence gathered over the last decades points towards an active role of the surface in the assembly, leading to self-assembled structures that only in a few occasions can be understood by considering just intermolecular interactions in solid or gas phases. In this work we review several examples from our recent research demonstrating the apparently endless variety of ways in which the surface might affect the assembly of organic adsorbates.     

材料表面抗空蚀涂层的研究进展

空蚀现象广泛存在于海洋平台、船舶机械和能源发电等领域.这种腐蚀现象不仅造成了巨大的经济损失,也成为相关从业人员的安全隐患.本文概述了抗空蚀涂层材料技术的研究进展,重点介绍了抗空蚀金属涂层技术和抗空蚀聚合物涂层技术.最后对目前抗空蚀材料存在的问题及未来发展进行了展望.     

Luggin's capillary in studying the effect of electrochemical reaction on mechanical properties of solid surfaces

In order to study the mechanism of the electrochemical polarization and active components reduction influence on the mechanical properties of surfaces, a new method has been developed: microscratching of the sample surface under polarization conditions in an electrochemical cell, with continuous monitoring potential in the area adjacent close to the moving indentor. Essential changes in this potential are observed during indentor movement, corresponding with the high activity of the newly formed surface. The very edge of the surface with a groove, where the hard stressed state is combined with accessibility to the medium state, appears to be the most sensitive to this action. This allows one to observe effects of hydrogen embrittlement and active metal cation reduction in the contact zone.     

固体表面润湿性机理及模型

润湿性是固体表面的重要特征之一。人工控制和制备固体表面润湿性已成为研究的热点，而且逐渐被应用于国防、工农业生产和人们日常生活等不同领域。论述了固体表面润湿性的有关理论基础，包括表面张力、表面自由能、润湿过程及其条件。分析了固体表面接触角的Young模型、Wenzel模型和Cassie模型等几种理论模型，以及解释了接触角滞后现象。     

A perturbation analysis on solid polymer surfaces

A linear stability model was formulated to analyze the perturbation of solid polymer surfaces. Surface energy and thermal stress were considered as the main variables. The surface tends to more unstable as the temperature increase. This is interpreted as the dominancy of the lattice vacancy diffusion over surface mass diffusion and the increase in thermal stress.     

Activation of diatomic molecules at solid surfaces

The interaction of a diatomic molecule with a well-defined single crystal surface represents the prototype of the elementary processes involved in heterogeneous catalysis, and it can be studied down to atomic length and extremely short (fs) time-scales. Our present understanding is illustrated by examples concerning the activation of hydrogen (including also the breakdown of thermal equilibrium during rapid laser-induced associative desorption), the role of steps as 'active sites' in the dissociative adsorption of NO or N2, and the mechanism and kinetics of the catalytic oxidation of CO.     

Nucleation of supercooled water on solid surfaces

Heterogeneous nucleation of water was investigated using molecular dynamics simulation. Solid with fcc (111) surface was placed at the bottom of a cell consisting of 864 water molecules. ST2 model with NPT ensemble was used. The pressure and temperature were set to be 0.1 MPa and 275 K, respectively. The interaction between water and the solid was based on the equations proposed by Spohr. Exception was made on the lattice constant which was slightly modified to fit with that for ice structure. The shape of the solid surface was considered. It was found that the only one layer of water molecules was adsorbed in a case of a flat surface, whereas ice nucleation occurred by removing some of the atoms from the surface. Spohr's interaction was also modified so that the dipole moment of water became anti-ferroelectric. It was found that the modification increased the ice growth, further. The effect of lattice constant of solid on nucleation was also investigated. It was found that the variation on lattice constant with a few percent from that of ice was acceptable for nucleation, especially on shrinking side. On expanding side, however, it gave some gaps for water molecules to fit in other than that for ice structure, and it prevented the growth of ice. Hence, a guideline for the selection of ice nucleus material was obtained.     

孔板流量计内空化现象的数值模拟

以计算流体动力学CFD软件为工具,通过引入Schnerr-Sauer空化模型,并结合多相流Mixture模型与RNG k-ε湍流模型,对两通道非标准孔板流量计在空化发生条件下的内部流场进行数值模拟。考查等效直径比β=0.7时,流场中蒸汽体积分数和压力分布的变化规律,入口压力对空化数的影响以及雷诺数对流出系数和压力损失的影响。结果表明:入口压力增加到一定值时,在环隙边缘处首先发生空化现象,并且随着压力的增大,空化发生的区域增大,空化程度加剧;空化数随入口压力的增大而减小;空化效应对流量计的流出系数的影响较大,而对压力损失的影响较小。研究结果对孔板流量计的测量误差原因分析和提高测量精度有参考价值。     

Roton growth resistance of helium-4 solid surfaces

Experimental data available on roton contribution to the growth resistance of helium-4 surfaces from the work of Amrit, Legros and Poitrenaud (ref. 4) are analysed using the quantum kind model of Edwards, Mukherjee and Pettersen (ref. 5). The results are in strong favour of dissipation being due to kink-roton interactions. Further, they also support the idea that the angular anisotropy of the roton growth resistance of rough crystral surfaces is related to the density of steps.     

Physical mechanism of the wetting of solid surfaces

This article examines physical principles in the theory of wetting. Wetting angles are calculated by using isotherms of the disjoining pressure of wetting liquid films on solid substrates.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 53, No. 5, pp. 795–802, November, 1987.