The effects of laser machining parameters on roughness and contact angle for boron carbide ceramic surface

Hydrophobicity or Hydrophilicity properties of boron carbide ceramic surface can be controlled by manipulating the roughness of the surface and thus the contact angle. In this study, the wettability of the boron carbide surface was manipulated by laser with different parameters. The contact angle was measured at 9^th seconds after the droplet was dropped onto the laser-treated surface. Also, the change in contact angle in the first 9 seconds after the droplet was dropped was examined. For a more hydrophobic surface, it was aimed to have the largest contact angle at 9^th seconds after the droplet was dropped and the smallest change with time in contact angle. The effects of laser scan speed, power, frequency and overlap rate on the contact angle of the treated surface were investigated using the Taguchi method. It was calculated to what extent each examined parameter affected the result. The effectiveness rates of the two most effective parameters which were scan speed and overlap rate were obtained. Since the results of the verification tests performed to confirm the results obtained are quite compatible with the results predicted by the Taguchi method, the Taguchi method can be regarded as a reliable method in optimizing laser parameters.     

Effect of Laser Heating on Nonlinear Surface Deformation of Acoustically Levitated Droplet

Under the microgravity environment, new and high quality materials with a homogeneous crystal structure are expected to be manufactured by undercooling solidification, since the material manufacturing under the microgravity environment has no effect of gravity. However, the temperature change on the interface of the material in space is expected to affect on the material processing due to the changing of physical property corresponding temperature. The purpose of the present study is to investigate effect of the laser heating on surface deformation of large levitated droplet. A water droplet levitated in the acoustic standing wave is heated by YAG laser. In order to increase the water droplet temperature, rhodamine 6G is solved in it to achieve high absorbance of the laser. Droplet from 2.5 to 5.5 mm in diameter were levitated and heated. The deformation of the droplet interface has been observed by high speed video camera. We used the radiation thermometer for the measurement of the temperature of droplet surface. It is noticed that the heated droplet deforms with its resonance frequencies. The experimental result of shape oscillation makes it possible to simulate the oscillation.     

Capillary‐Assisted Fabrication of Biconcave Polymeric Microlenses from Microfluidic Ternary Emulsion Droplets

In this study, a simple capillary‐based approach for producing biconcave polymeric microlenses with uniform size and shape from ternary emulsion droplets is presented. Monodisperse ternary emulsion droplets (0.6–4.0 nL) are produced which contain a photocurable segment of an acrylate monomer and two non‐curable segments of silicone oil (SO) by using a microfluidic sheath‐flowing droplet generator on a glass chip. The curvature radius of the interfaces separating the droplet segments, as well as the droplet size, and production rate can be flexibly varied by changing the flow conditions of the organic and aqueous phases. Subsequently, off‐chip suspension photopolymerization yields non‐spherical polymeric microparticles with two spherical concave surfaces templated by two SO segments at random positions. By ultraviolet light irradiation of ternary droplets with two SO segments trapped by the interior wall of a cylindrical microcapillary (internal diameter: 130 μm), biconcave microlenses can be produced with two spherical concave surfaces with a common lens axis. The produced lenses are suitable for use as optical diverging lenses.     

Axisymmetric oscillations in a slowly rotating cyclindrical container filled with viscous liquid

For a slowly rotating cylindrical container filled with an incompressible and viscous liquid the axisymmetric damped natural frequencies are obtained for a slipping and an anchored contact line of the equilibrium free liquid surface at the container wall. It was found that there exists in contrast to frictionless liquid a range where only aperiodic motion of the liquid is possible. The influence of an increase of the speed of spin was found to be an increase of the oscillation frequency at nearly constant decay magnitude.     

激光接力测量方法和误差分析

为了解决工程实际应用中遇到的被测物体长度大于作为基准的激光束长度的问题,提出了一个接力测量方法.该方法将被测物体分段测量,相邻两段之间有一定长度的重合部分,利用重合部分在两段测量中的测量结果,用最小二乘直线拟合找出相邻两坐标系间的关系,从而将分段测量的数据表达在一个统一的坐标系中.以直线型被测物体为例,详细介绍了激光接力测量方法.分析了方法的误差并给出了依据测量误差要求选取测量参数的方法.结合应用实例,讨论了影响测量精度的因素.结果应用到客运专线轨道平顺度检测中,取得了满意的效果.     

Rapid Solidification Process of a Water Droplet Due to Depressurization

This paper reports an experimental and numerical study of rapid solidification of a water droplet due to depressurization. During the experiment, a distilled water droplet was suspended on a thermocouple, which was also used to measure the droplet temperature, and the droplet surface temperature was captured by an infrared thermograph. The experimentally measured data indicates that freezing occurs from the droplet surface when the droplet temperature reaches a certain subcooling. A mathematical model was constructed to simulate the temperature transition and the temperature distribution within the sphere. The model considers the pressure reduction in the test vessel, the kinetic condition for undercooled solidification, and the heat transfers due to convection and sublimation at the ice surface. A coordinate transformation method was used to capture the two moving boundaries within the droplet, which are internal solidification interface and surface sublimation interface. The model-predictions agree well with the measured temperature data, demonstrating the soundness of the present model. The results show that the rapid solidification of a water droplet due to depressurization is a typical non-equilibrium phase transition, with a lower ambient pressure, the solidification speed will be faster, and the duration time for droplet center temperature keeps constant will be shorter.     

Droplet size and morphology analyses of dry liquid

Dry water is a free-flowing powder consisting of numerous solid particle-stabilized water droplets with typical sizes and volumes of 10^?6–10^?4 m and 10^?3–10³ pL, respectively. We describe the first characterization of dry water stabilized with hydrophobic silica nanoparticles, by using laser diffraction droplet size distribution analysis. The water droplet dimensions were measured to be a few tens of micrometers in air, by using the laser diffraction method. These dimensions correspond well with measurements by both laser diffraction and optical microscopy methods for a Pickering-type water-in-n-dodecane emulsion prepared by dispersing dry water in n-dodecane. Optical microscopy confirmed that the dry water consisted of flocs of non-spherical water droplets, and the flocs ranged in size from a few tens of micrometers to a few millimeters in air. On the basis of these results, the flocs of water droplets were proposed to dissociate into individual water droplets under the air blast during droplet size measurement by the laser diffraction method. It was also confirmed that pressurizing the dry water between two glass slides led to encapsulated water leaking from the silica nanoparticle shells. This on-demand pressure-sensitive water leak phenomenon shows a possible usage of the dry waters as a material delivery carrier.     

线聚焦光学相干层析术的研究

提出了一种高速光学相干层析(OCT)成像技术方案。利用柱面镜的成像特性将传统OCT的点聚焦成像模式改变为线聚焦成像模式,从而降低二维OCT图像的扫描维数,达到提高成像速度的目的。利用ZEMAX光学软件对系统进行光线追迹获得光束经过柱面镜后的聚焦情况。随后采用635nm的激光光源和柱面镜构建了实验系统,实验结果很好地验证了光线追迹仿真结果。     

Observations of the Effects of Oxide Skins on the Oscillations of Electromagnetically Levitated Metal Droplets

The surface tensions of liquid metals can be derived from measurements of the natural oscillation frequencies of levitated drops through the Rayleigh relation, In general, during terrestrial measurements, a spectrum consisting of three to five dominant oscillation frequencies (in the range of 30 to 60 Hz) are found, rather than the single one predicted by Rayleigh, due to deformation of the drop shape by gravity and the supporting magnetic field. Cummings and Blackburn have derived a correction factor to align the measured frequencies with the Rayleigh frequency, which has been shown to hold through micro-gravity experiments by Egry et al., for the majority of metals that have a liquid surface. Work at the NPL on more complex, commercial alloys has found that, in some cases, oscillation frequencies may be split into more complex spectra exhibiting seven to nine oscillation frequencies. This has been attributed to formations of oxide from the metal collecting on the surface of the droplet. Observations of the frequency spectra and high speed video images of the levitated drops are discussed.     

Ti6Al4V表面纹理制备及其润湿性

采用激光加工技术在Ti6Al4V表面分别加工直线、网格和具有规则点阵状结构的表面纹理,采用自组装技术制备自组装分子膜。采用扫描电镜、形貌分析仪和接触角测量仪对成膜后的钛合金表面进行形貌和接触角的表征与测量。结果表明,通过激光加工和沉积自组装分子膜,可显著增大Ti6Al4V的水接触角。其中直线纹理的试样表面水接触角可达124.8°,网格纹理的试样表面接触角可达126.1°,点阵状纹理的试样表面接触角可达151.6°。表面接触角与表面粗糙度相关,随着表面粗糙度值的增大,接触角呈增大趋势,当表面粗糙度>4μm时,接触角均>150°,形成超疏水表面。     

圆柱类金属构件表面裂纹的激光超声识别方法研究

为了利用激光超声技术有效地识别圆柱表面裂纹,提出利用圆柱表面波信号增强和小波包-奇异值分解(WPT-SVD)方法识别圆柱类金属构件表面裂纹的位置和深度。建立了圆柱的激光超声显式有限元模型,分析了圆柱表面裂纹对表面波的模式转化作用。利用圆柱表面裂纹在激发源位置附近时激光超声扫描信号增强的现象,识别圆柱表面裂纹的位置。在已识别圆柱表面裂纹位置的基础上,通过分析圆柱表面裂纹检测信号的时频特点,利用WPT-SVD提取圆柱表面信号的时频特征,定义参数k r表征裂纹深度的变化,识别圆柱表面裂纹深度。搭建了激光超声圆柱表面裂纹检测实验系统,开展了实验研究,实验结果表明所提出的圆柱表面信号增强和WPT-SVD方法可以识别出圆柱表面裂纹的位置和深度。     

Mode matching for a passive resonant ring laser gyroscope

An analytical method of matching the mode of an input laser to the lowest-order mode of a passive resonant ring laser gyro is described, as are the steps in determining the location and focal length of cylindrical mode matching lenses. Results were obtained with no mode matching, with a compromise spherical lens, with horizontal mode matching only, and with the proper cylindrical mode matching lenses. Compared with no mode matching, the latter case shows that the amplitude of the lowest-order mode is increased approximately 2.5 times. In addition, the number and intensity of higher-order modes are reduced to near zero, and the relative intensity of the lowest-order mode to the higher-order mode increased from approximately 5 to approximately 60 times greater.     

Diode laser bending of tongues from slotted steel tubes

A high power diode laser was used to bend slotted tubes. Two stainless steel tubes, different in diameter but with the same thickness, were cut to generate a rectangular tongue on their cylindrical surface. Subsequently these tongues were formed by diode laser in similar process conditions (laser power, rim speed and focalization condition). Two finite element models were built to predict the laser forming processes for the two different tubes and a good agreement between numerical and experimental results was obtained. Even if the tubes behave in a similar way under laser exposure, a larger deformation of the tongue is present for the biggest tube. Moreover, the deformation history during the laser scan differs. However, in both cases, the single tongue deformed in analogy with a flat sheet metal, despite of the cylindrical surface.     

Effect of Temperature Change on Interfacial Behavior of an Acoustically Levitated Droplet

Under the microgravity environment, new and high quality materials with a homogeneous crystal structure are expected to be manufactured by undercooling solidification, since the material manufacturing under the microgravity environment is more static than that under the normal gravity. However, the temperature change on the interface of the material in space can affect on the material processing. The purpose of the present study is to investigate effect of the temperature change of interface on the large levitated droplet interface. A water droplet levitated by the acoustic standing wave is heated by YAG laser. In order to heat the water droplet by the laser heating, rhodamine 6G is solved in it to achieve high absorbance of the laser. The droplet diameter is from 4 to 5.5 mm. The deformation of the droplet interface is observed by high speed video camera. The temperature of droplet is measured by the radiation thermometer. It is noticed that the larger droplet under the higher sound pressure tends to oscillate remarkably by the laser heating.     

Fabrication and evaluation of variable focus X-ray lenses

We report on the fabrication, preliminary metrology, and X-ray transmission results of variable-focus cylindrical beryllium lenses. Each lens consists of a number of 1-mm-diameter lenslets made by precision computer numerical control (CNC) drilling into a beryllium substrate. The substrate is then cut into five parts, each having a different number of lenslets, ranging from 10 to 50. Each lens is then cut diagonally, using an electron discharge machine, to provide it with focusing ability. Unlike the traditional lenses having a fixed focal distance for a given energy, the present lenses provide for a wide range of photon energies and focal distances. Additionally, X-ray transmission through the lens is enhanced by reducing lenslet wall thickness to about 50 μm, the thinnest reported to date.     

Photoacoustic Excitation of the Interior Surface of an Acoustic Resonator for Speed of Sound Measurement

A remote optical technique is proposed as a valid alternative to conventional electroacoustic excitation of sound inside an acoustic resonator. Efficient photoacoustic conversion of an amplitude-modulated light beam, impinging on the interior solid surface of the resonator wall, into a stationary acoustic field is achieved. The reported results were obtained in a cylindrical stainless steel resonator with a volume of about 572 cm³; the radiation source is the 514 nm line of an Ar⁺ laser. The laser beam can be focused on different points of the resonator internal surface. The signal frequency dependence is well interpreted in terms of gas-microphone detection theory. The precision obtained in the measurement of resonance frequency f _N and halfwidth g _N of the cavity modes is on the order of 10^-6 of f _N and is likely to be further improved by minor refinements of the experimental apparatus.     

用特殊辅助面解画法几何交线问题

提出利用特殊位置辅助面截交线投影的特点(45°投影面垂直面、底角45°圆锥面及其它特殊位置垂直面)求解画法几何中的截交线与相贯线(平面立体截交线、圆柱与圆锥的截交线;圆柱与圆柱,圆柱与圆锥的相贯线)以及直线的落影、回转曲面立面阴影的作图方法。     

Experimental Studies of Liquid Droplet Coalescence on the Gradient Surface

Visual experiments were conducted to study the behaviors of liquid droplet coalescence on the surface with gradient surface energy. The microscopic contour of the gradient energy surface, which was fabricated on the base of a silicon chip by diffusion-controlled silanization of alkyltrichlorosilanes, was characterized by atom force microscopy. The effects of droplets on the three-phase contact line and the contact angle were obtained. It was observed that the process of droplet coalescence accelerates the droplet speed on the gradient surface. On the high-energy surface, the triple-phase contact line of the coalescing droplet is prone to pinning. The interfacial energy released from coalescence is the main source for promoting the motion. Furthermore, the nonequilibrium capillary pressure gradient inside coalescence droplets is considered to aid in accelerating the coalescence.     

Mechanically tunable aspheric lenses via additive manufacture of hanging elastomeric droplets for microscopic applications

Abstract

Mechanically deformable lenses with dynamically tunable focal lengths have been developed in this work. The fabricated five types of aspheric polydimethylsiloxane (PDMS) lenses presented here have an initial focal length of 7.0, 7.8, 9.0, 10.0 and 10.2 mm. Incorporating two modes of operation in biconvex and concave–convex configurations, the focal lengths can be tuned dynamically as 5.2–10.2, 5.5–9.9, 6.6–11.9, 6.1–13.5 and 6.6–13.5 mm respectively. Additive manufacturing was utilized to fabricate these five types of aspheric lenses (APLs) via sequential layering of PDMS materials. Complex structures with three-dimensional features and shorter focal lengths can be successfully produced by repeatedly depositing, inverting and curing controlled PDMS volume onto previously cured PDMS droplets. From our experiments, we empirically found a direct dependence of the focal length of the lenses with the amount (volume) of deposited PDMS droplets. This new mouldless, low-cost, and flexible lens fabrication method is able to transform an ordinary commercial smartphone camera into a low-cost portable microscope. A few microscopic features can be readily visualized, such as wrinkles of ladybird pupa and printed circuit board. The fabrication technique by successively applying hanging droplet and facile mechanical focal-length-tuning set-up can be easily adopted in the development of high-performance optical lenses.     

PZT和磁致伸缩孤立波传感器的性能研究与分析

孤立波方法被证实可用于测量均匀与非均匀材料的杨氏模量,而用于采集信号的孤立波传感器的性能直接决定测量结果的精度。介绍了两种基于不同原理的孤立波传感器,分别为采用压电材料制成的圆柱传感器,与基于维拉里与法拉第效应的磁致伸缩传感器。利用两种传感器分别测量了同一批水灰比分别为0.42、0.45与0.50的混凝土样本,将两者100次的测量结果均值与标准参考值相对比,从而对比两种传感器测量的精度。同时,分别分析了两者测量值的标准偏差,对比两者的可重复性强度。实验结果表明,三种水灰比样本的测量值相对于标准参考值,圆柱压力传感器的偏差率分别为6.48%、5.04%与1.85%,磁致伸缩传感器的偏差率分别为0.78%、3.08%与4.94%,因此两种传感器的误差率均低于10%,均可用与实际测量中,但后者的精度略高于前者,同时后者的标准偏差低于前者,因此具有更高的可重复性。