Dynamics of laser-induced cavitation bubble during expansion over sharp-edge geometry submerged in liquid – an inside view by diffuse illumination

Laser ablation in liquids is growing in popularity for various applications including nanoparticle production, breakdown spectroscopy, and surface functionalization. When laser pulse ablates the solid target submerged in liquid, a cavitation bubble develops. In case of “finite” geometries of ablated solids, liquid dynamical phenomena can occur inside the bubble when the bubble overflows the surface edge. To observe this dynamics, we use diffuse illumination of a flashlamp in combination with a high-speed videography by exposure times down to 250 ns. The developed theoretical modelling and its comparison with the experimental observations clearly prove that this approach widens the observable area inside the bubble. We thereby use it to study the dynamics of laser-induced cavitation bubble during its expansion over a sharp-edge (“cliff-like” 90°) geometry submerged in water, ethanol, and polyethylene glycol 300. The samples are 17 mm wide stainless steel plates with thickness in the range of 0.025–2 mm. Bubbles are induced on the samples by 1064-nm laser pulses with pulse durations of 7–60 ns and pulse energies of 10–55 mJ. We observe formation of a fixed-type secondary cavity behind the edge where low-pressure area develops due to bubble-driven flow of the liquid. This occurs when the velocity of liquid overflow exceeds ~20 m s⁻¹. A re-entrant liquid injection with up to ~40 m s⁻¹ velocity may occur inside the bubble when the bubble overflows the edge of the sample. Formation and characteristics of the jet evidently depend on the relation between the breakdown-edge offset and the bubble energy, as well as the properties of the surrounding liquid. Higher viscosity of the liquid prevents the generation of the jet.     

空间波函数的对称性对氦原子双光子双电离过程中电子关联的影响

通过数值求解含时薛定谔方程，研究了氦原子具有对称空间波函数的1s2s 1S态和具有反对称空间波函数的1s2s 3S态分别作为初态的双光子双电离过程. 结果表明，对于初态为单重态1s2s 1S的双光子双电离过程，两个电离电子的能量分布随激光脉冲持续时间的增加呈现由单峰到双峰的变化，这里的单峰和双峰分别意味着两个电离电子主要携带相等和不等的能量；然而对于初态为三重态1s2s 3S的双光子双电离过程，两个电离电子的能量分布随激光脉冲持续时间的增加总是保持双峰结构. 这些结果表明当原子的初态处于反对称空间波函数时，两电子的空间密度分布具有较少的重叠，从而导致电子在超短激光脉冲中电离时电子关联能无法平均分配.     

多片导模法蓝宝石晶体的缺陷研究

使用导模法(EFG)生长了多片a面蓝宝石晶体。显微拉曼光谱结合电感耦合等离子体发射光谱(ICP-AES)测试得出晶体的气泡中可能存在含S化合物。晶体表面明显的生长条纹主要与温度、生长速度的波动以及模具的加工精度有关。化学腐蚀分析表明晶体位错密度在4.2×10⁴ cm^-2,未存在小角度晶界缺陷,双晶摇摆曲线半峰宽(FWHM)为70.63″。由于采用石墨保温材料,晶体中存在F心与F⁺色心。晶体在400~3 000 nm波段透过率高于80%,空气中退火后可减弱色心吸收。本文研究结果可为蓝宝石晶体缺陷形成理论研究提供参考,也可为导模法蓝宝石工业生产技术改进提供借鉴。     

Simulating bubble dynamics in a buoyant system

Multiphase flows are critical components of many physical systems; however, numerical models of multiphase flows with large parameter gradients can be challenging. Here, two different numerical methods, volume of fluid (VOF) and smoothed particle hydrodynamics (SPH), are used to model the buoyant rise of isolated gas bubbles through quiescent fluids for a range of Bond and Reynolds numbers. The VOF is an Eulerian grid–based method, whereas the SPH is Lagrangian and mesh free. Each method has unique strengths and weaknesses, and a comparison of the two approaches as applied to multiphase phenomena has not previously been performed. The VOF and SPH simulations are compared, verified, and validated. Results using two-dimensional VOF and SPH simulations are similar to each other and are able to reproduce numerical benchmarks and experimental results for sufficiently large Morton and Reynolds numbers. It is also shown that at low Reynolds numbers, the two methods, SPH and VOF, diverge in the transient regime of the bubble rise. Regimes that require simulations capable of representing three-dimensional drag are identified as well as regimes in which results from VOF and SPH diverge.     

最大泡压法测表面张力等温线的数据处理方法比较*

对乙酸、丙酸、正丁酸、2-甲基丁酸、3-甲基丁酸、正戊酸、正己酸、乙醇、丙醇、异丙醇、正丁醇、异丁醇、1,2-丙二醇等13种常用试剂在25 ℃下的表面张力数据拟合方法进行了比较,分析了图解法处理数据及多项式、一阶指数函数、幂函数、Shishkovsky经验公式拟合数据的优缺点。指出Shishkovsky经验公式拟合结果准确,受数据样本量影响小,是理想的表面张力数据拟合方法。     

Dynamic Visualization of Free Radicals at Single Oxygen Bubbles using Chemiluminescence

The generation of free radicals is a key process in the formation and the collapse of the bubbles in water, however, the direct and dynamic observation of the radicals in this process at single bubbles has never been achieved. Here, the hydroxyl (OH^.) and oxygen (O₂^.−) radicals at single oxygen bubbles are continuously traced using chemiluminescence (CL), in which these radicals at the bubble react with the surrounding luminol in the solution emitting the light. Varied increase trends of luminescence are observed in the generation of a bubble, floating, short parking at the water/air interface and the final explosion, revealing the complexity in the distribution of radicals at the bubble unprecedentedly. Despite more radicals are observed at the bubble generated at a deep position under the water for the stabilization, almost the same amount of radicals are included in the bubbles that is independent on the water pressure during the production of the bubble. This rich information collected from the dynamic study of bubbles illustrates the complicated generation and distribution process of radicals at the bubbles, and will facilitate the understanding of the function about the bubbles.     

缓变主流中三维气泡的非线性振动

空化现象和水下噪声机制与液体中气泡的动力学行为密切相关．在无粘势流的假定下，采用多参数摄动分析，研究了缓变主流中三维气泡的非线性体积模态振动．推导了关于缓变泡形展开的各阶扰动方程，获得了一阶振动的演化方程和一些特殊情况下的解析解；并采用高阶有限元离散的边界积分方程方法，对平面固壁和自由面附近三维气泡的固有频率进行了数值计算     

Motion of inviscid fluid tongues and bubbles in a Hele-Shaw cell occupied by a viscoelastic fluid

Problems of steady-state motion of an inviscid fluid tongue or bubble in a Hele-Shaw cell occupied by a viscoelastic fluid are considered. The problems generalize the well-known Saffman-Taylor problems [1, 2] in the approximation in which the pressure jump on the interface between the two fluids depends on the normal velocity of the interface [4]. In the zero approximation the results obtained by means of an iteration technique are in good agreement with the analytic solutions obtained by Saffman and Taylor [1, 2].__________Translated from Izvestiya Rossiiskoi Academii Nauk, Mekhanika Zhidkosti i Gaza, No. 1, 2005, pp. 117–123. Original Russian Text Copyright © 2005 by Entov, Kolganov, and Kolganova.     

Explosive vaporization in microenclosures

The explosive vaporization of a liquid above planar microheaters induces a fast increase of pressure that is exploited in many thermally driven actuators in MEMS components such as ink jet printer cartridges, pumps, valves and optical switches. Some of these components need to enclose the working fluid as it is the case of valves in which the heated liquid is separated from the flow that it regulates by a flexible membrane. To achieve a better insight into the thermodynamic processes involved, the present work investigates experimentally an enclosed microsystem designed and fabricated for this purpose, composed of a small liquid volume (8 nL) heated by a electric pulse for 2 μs supplied to a planar microfabricated heater. During the heating, the temperature-induced change in resistance can be determined by imposing a defined current and measuring the voltage drop over the heater. While the chip is based on a silicon substrate with integrated platinum heaters and sensors, the structure enclosing the fluid (cavity and fluidic access to it) is made of a silicone elastomer, poly(dimethylsiloxane) (PDMS). This transparent material is widely used in microfluidics and allows for flexible and transparent walls that can be deflected by increasing the pressure inside the cavity. To seal the system the inlet and the outlet were closed by blocking them with a metallic stab. In the present work we visualize vaporization of isopropanol in contact with a suddenly heated planar resistor for two different cavity heights, 150 μm and 16 μm. The rate of temperature rise of the thin liquid layer in contact with the heater is of the order of 10⁷ K s⁻¹ for a pulse duration of 2 μs. We compare bubble growth and collapse for the open and closed systems. Compared to the open system, the bubble growth in the closed system is considerably damped.