Wetting of liquid droplets on two parallel filaments

Droplet wetting on two parallel filaments may assume a barrel-shaped morphology or a liquid bridge depending upon the filament diameter and spacing, droplet volume, and contact angle. This paper is aimed to examine the dependency of droplet wetting length upon the above parameters. In the process, morphology of either a barrel-shaped droplet or a liquid bridge sitting on two parallel filaments is determined numerically by using surface finite element method (SFEM). Variation of wetting length with contact angle is examined at varying droplet volume, filament spacing, and droplet morphology. It is found that the droplet wetting length increases with decreasing filament spacing ratio as well as contact angle while it also increases with the growth of droplet volume. The dependency of wetting length upon contact angle behaves sensitive to filament spacing in the case of stable liquid bridges, while it exhibits nearly constant sensitivity to the contact angle in the case of barrel-shaped droplets. The quantitative relations yielded in this study can be considered as characteristic curves applicable for a variety of droplet-on-filament systems, particularly useful to wetting property characterization of filaments, micro liquid delivery, biological cell manipulation, etc.     

非晶态合金表面的水润湿动力学

采用分子动力学模拟方法研究了改进的Simple pointcharge模型SPC/E水滴在Cu_(50)Zr_(50)非晶薄膜上的润湿行为和铺展过程.通过与CuZr(110)和(100)晶面对比研究发现,水滴在Cu_(50)Zr_(50)非晶薄膜表面上表现出较高的铺展速度.水滴在非晶合金表面的铺展过程中形成了明显的吸附层;而在晶态表面,水滴铺展前沿呈脚状形态.分析结果表明非晶表面的水分子在吸附层内呈现完全无序的单层排列方式,而在晶态表面,特别是(100)晶面,吸附层水分子呈双层有序排列.这种吸附层结构的差异导致了吸附层内水分子方向的差异:非晶表面吸附层内水分子方向倾向平行于表面,而晶态基底上吸附层内的水分子倾向于指向液滴内部.由此造成了非晶表面吸附层中的水分子与液滴内部以相对较弱的氢键相互作用,这使得上层水分子能够较容易扩散至吸附层前沿,促进液滴铺展.     

水滴撞击结冰过程的分子动力学模拟

利用三维分子动力学模拟方法,研究了纳米尺度水滴撞击冷壁面的结冰过程.数值模拟中,统计系统采用微正则系综,势能函数选用TIP4P/ice模型,温度校正使用速度定标法,牛顿运动方程的求解采用文莱特算法,水滴内部结冰过程则通过统计垂直方向水分子温度分布来判定.研究发现,当冷壁面温度降低时,水滴完全结冰的时间减小,但水滴降至壁面温度的时间却增大;同时随着壁面亲水性降低,水滴内部热传递速度减慢(尤其是冷壁面与水滴底端分子层间),水滴内部温度趋于均匀,但水滴完全结冰时间延长.     

固液黏着功的Berthelot平均规则的推广及应用

以固液黏着功的Berthelot几何平均规则及其推广为基础的Zisman方程、Fowkes方程和Owens-Wendt方程是固体表面张力测定的基础.对Berthelot几何平均规则进行了进一步的推广,并以此为基础,对Zisman方程中的参数给出了推广的表示式,并对Fowkes方程和Owens-Wendt方程进行了进一步的推广. 关键词： 接触角 Berthelot规则 Fowkes方程 Owens-Wendt方程     

分子动力学模拟压水反应堆中联氨对水的影响

本文采用分子动力学方法模拟在常温常压下(1 atm,298 K)和在压水堆环境下(155 atm,626 K),水分子数为256,联氨(N₂H₄)分子数为0,25,50,75等不同数目时,水和联氨粒子系统的动力性质和微观结构.同时探讨了联氨分子的引入对水中溶解氧的影响.从模拟结果可知,在常温常压下,当联氨的分子数为0,25,50,75时,粒子系统的均方位移会随联氨分子数的增加而增加;联氨分子数为0与为25,50,75比较时会少一个数量级;压水堆环境下,联氨分子数 关键词： 分子动力学 压水堆 联氨     

Dynamic hydrophobicity of water droplets on the line-patterned hydrophobic surfaces

Static and dynamic hydrophobicities of water droplet on a patterned surface prepared using fluoroalkylsilanes with different molecular chain lengths were investigated. Contact angles on the patterned surfaces well agreed with values predicted using Cassie’s theory. On the same line width ratio, total retention force was governed by the fluoroalkylsilane with slow-sliding acceleration. The total retention force decreased with the decreasing width ratio of silane with slow-sliding acceleration on the surface. These results imply that the sliding acceleration of water droplets on a hydrophobic surface depends both on chemical composition and patterning structure.     

Exciton luminescence and many-body effect of monolayer WS2 at room temperature

Monolayer transition metal dichalcogenides favor the formation of a variety of excitonic quasiparticles, and can serve as an ideal material for exploring room-temperature many-body effects in two-dimensional systems. Here, using mechanically exfoliated monolayer WS₂ and photoluminescence (PL) spectroscopy, exciton emission peaks are confirmed through temperature-dependent and electric-field-tuned PL spectroscopy. The dependence of exciton concentration on the excitation power density at room temperature is quantitatively analyzed. Exciton concentrations covering four orders of magnitude are divided into three stages. Within the low carrier concentration stage, the system is dominated by excitons, with a small fraction of trions and localized excitons. At the high carrier concentration stage, the localized exciton emission from defects coincides with the emission peak position of trions, resulting in broad spectral characteristics at room temperature.     

纳米铜团簇在常温和升温过程中能量特征的分子动力学研究

采用分子动力学方法和F-S多体势函数，模拟研究纳米铜团簇常温下能量特征及其在升温直到熔化过程中的变化，确定了常温下纳米铜团簇的表面原子厚度和表面能，给出在不同温度下纳米铜团簇能量大小分布比例和能量的概率密度，细致描述了团簇升温过程团簇内部原子和表面原子之间不同的变化特征. 关键词： 铜团簇 分子动力学 能量特征 温度     

分子动力学模拟压水反应堆中氢气对水的影响

通过分子动力学方法模拟了在常温常压下(1 atm, 298 K)和在压水堆环境下(155 atm, 626 K), 水分子数为256, 氢分子数为0, 25, 50, 75和100等不同数目时, 粒子系统的动力学性质和微观结构, 分析了不同氢气对水中溶解氧的影响. 从模拟结果可知, 在常温常压和压水堆环境下, 当氢粒子数分别为0, 25, 50, 75和100时, 粒子系统的均方位移会随氢分子数增加而增加, 并且常温常压下的增长幅度远小于压水堆环境下的增长幅度, 如压水堆环境下氢分子数为75时系统的均方位移约是常温常压下氢分子数为75时系统的均方位移的6.02倍, 比压水堆环境下氢分子数0时系统的均方位移增加了131.88%. 此外, 粒子系统的微观结构, 从径向分布函数看, 在常温常压下随着氢分子数目的增加而小幅度增加, 这与常温常压下因氢气溶解在水中增大了氧离子周围的粒子密度相符合. 而在压水堆环境下, 氢分子数为75, 50, 25与为0时的水比较, 其径向分布均不会有太大的变化, 而分子数为100时会出现明显增加, 与为0时的水比较其径向分布增加了22.00%. 模拟结果表明, 往压水堆中的水加入氢气能明显地抑制水中的溶解氧.     

Sliding behavior of oil droplets on nanosphere stacking layers with different surface textures

Two facile coating techniques, gravitational sediment and spin coating, were applied for the creation of silica sphere stacking layers with different textures onto glass substrates that display various sliding abilities toward liquid drops with different surface tensions, ranged from 25.6 to 72.3 mN/m. The resulting silica surface exhibits oil repellency, long-period durability > 30 days, and oil sliding capability. The two-tier texture offers a better roll-off ability toward liquid drops with a wide range of γ_L, ranged from 30.2 to 72.3 mN/m, i.e., when the sliding angle (SA) < 15°, the oil droplet start to roll off the surface. This improvement of sliding ability can be ascribed to the fact that the two-tier texture allows for air pockets (i.e., referred to as the Cassie state), thus favoring the self-cleaning ability. Taking Young-Duprè equation into account, a linearity relationship between sine SA and work of adhesion (W_ad) appears to describe the sliding behavior within the W_ad region: 2.20-3.03 mN/m. The smaller W_ad, the easier drop sliding (i.e., the smaller SA value) takes place on the surfaces. The W_ad value ∼3.03 mN/m shows a critical kinetic barrier for drop sliding on the silica surfaces from stationary to movement states. This work proposes a mathematical model to simulate the sliding behavior of oil drops on a nanosphere stacking layer, confirming the anti-oil contamination capability.     

碳纳米胶囊中水分子的分子动力学研究

采用分子动力学方法研究了不同温度下碳纳米胶囊中水分子及其氢键的聚集密度分布,讨论了水分子内部键角及其取向规律.计算结果表明,由于碳纳米胶囊的束缚作用,水分子主要聚集在与胶囊形状相似的三个薄层中,随着温度的升高,聚集密度峰均会展宽并向管壁移动.氢键的分布规律与水分子聚集密度类似并对其取向角分布有明显影响.与通常情况不同,在1000K高温时仍存在相当数量的氢键.在3100 K附近,碳纳米胶囊发生破裂,溢出少量水分子后自动愈合.     

The apparent state of droplets on a rough surface

The factors influencing the state and wetting transition of droplets on a rough surface are both complex and obscure. The change in wetting is directly reflected by changes under the contact condition of the droplets with the surface. The recent study about the wettability of the superhydrophobic surface under the condensing condition arouses the new understanding about the apparent state of droplets on a rough surface. In this work, to validate the existence of droplets in an intermediate state, a microscale pillar topological polydimethylsiloxane (PDMS) surface was manufactured and its wettability under various conditions was studied. According to the experimental data, it is proposed that the wetting state of a rough surface may be embodied using the contact area ratio of a solid/liquid/gas droplet with the projective plane. A general calculation model for the apparent contact angle of droplets is given and expressed diagrammatically. It is found that the measured apparent contact angles of droplets at different states on the surface falls within the range predicted by our proposed equation. Supported by the National Natural Science Foundation of China (Grant No. 50606025)     

微矩形凹槽表面液滴各向异性浸润行为的研究

受自然界启发,仿生微结构被广泛用于调控固-液界面的性质.研究显示,液滴在微结构表面的各向异性浸润行为可用于实现微流动方向和速度的控制,且其各向异性浸润与微结构的尺寸和分布等密切相关.本文研究了微矩形凹槽尺寸对液滴各向异性浸润行为的影响规律.结果显示,液滴沿平行沟槽的方向具有较小的运动阻力、易铺展,因此具有较小接触角;而垂直于沟槽方向,由于沟槽的阻隔作用具有较大运动阻力,因而具有较大接触角,并且在垂直方向液滴的浸润过程是三相线一系列钉扎和跳跃行为.在微矩形凹槽表面,液滴沿平行方向接触角θ//与肋板宽度R和凹槽宽度G密切相关,其值与表面固体面积比成反比;而垂直于沟槽方向的接触角θ⊥随肋板宽度R和凹槽宽度G变化基本保持不变.同时各向异性液滴的变形比L/W、特征方向接触角比值θ⊥/θ//与表面固体面积比成正比.研究结果有助于加深理解微结构表面浸润行为的机制,并为微矩形凹槽在微流动控制方向的应用提供技术支持.     

Studies on electrowetting of room temperature ionic liquids

This paper reports experimental investigations on the electrowetting behavior of ionic liquids in comparison with aqueous electrolytes, which is one of the important research topics in optofluidics. The effect of applied voltage on the contact angle is reported in detail. In addition, the liquid–solid material interfacial tension and the thickness of insulating layer are estimated under certain conditions. Related conclusions are valuable in the field of future electrowetting applications.     

Rapid growth of nanocrystalline CuInS2 thin films in alkaline medium at room temperature

Layer-by-layer (LbL) deposition of CuInS₂ (CIS) thin films at room temperature (25 °C) from alkaline CuSO₄ + In₂(SO₄)₃ and Na₂S precursor solutions was reported. The method allowed self-limited growth of CIS films with nanocrystalline structure and composed of densely packed nanometer-sized grains. The as-deposited CIS film was 250 nm thick and composed of closely packed particles of 20-30 nm in diameter. The alkaline cationic precursor solution was obtained by dissolving CuSO₄ and InSO₄ in deionized water with a appropriate amount of hydrazine monohydrate (H-H) and 2,2′,2″-nitrilotriethanol (TEA). CIS films were annealed at 200 °C for 2 h and effect of annealing on structural, optical, and surface morphological properties was thoroughly investigated by means of X-ray diffraction, scanning electron microscopy, transmission electron microscopy, UV-vis spectrometer, C-V, and water contact angle techniques, respectively.     

X-ray diffraction study of KOCN at room temperature

The room temperature structure of KOCN has been successfully refined in space group I4/mcm. The OCN anion is disordered through 180° head-tail flipping and the positional coordinates and displacement parameters could not be separated for the N and O end atoms. The displacement parameters are compared for isomorphous KOCN, KN₃ and KSCN.     

壁面材质和温度场对熔融硅润湿角的影响

本文建立了毛细模型, 采用微流动两相流水平集法计算了熔融态硅液与壁面的润湿角, 以人造金刚石作为壁面材料的计算结果与实验结果进行比较, 验证了该模型和计算方法的正确性. 在此基础上, 分别选用碳化硅、石墨和人造金刚石作为壁面材料, 探讨了不同壁面材料表面张力和壁面黏附力对润湿角的影响规律. 结果发现, 相同温度下的毛细力作用使得熔融硅液出现起伏上升现象; 润湿角均有不同程度的减小然后增大, 最终趋于稳定; 初始阶段, 由于气/熔融硅液表面张力与气/壁面表面张力之差变化较大, 液面起伏波动较大; 随后趋于稳定上升. 同时发现石墨作为壁面材料时, 以上变化更易趋于稳定. 该研究为熔体中生长晶体硅获得更稳定的生长环境提供了理论依据.     

Relationship between sliding acceleration of water droplets and dynamic contact angles on hydrophobic surfaces

This study measured sliding acceleration of water droplets on hydrophobic solid surfaces and used expanding and contracting method to compare that value with dynamic contact angles. Sliding action of the droplet was classified into three motion categories: constant accelerated motion, constant velocity and stasis. Differences exist in the dependencies of contact radius and the injection-suction rate in dynamic contact angle hysteresis according to these categories. This method is an effective indicator of water droplets’ sliding acceleration.     

Performance study of the PTFE-THGEM at room and low temperature

GPM based on THGEM has shown its competitive strength compared to the conventional PMT, espe- cially in the low background research such as dark matter detection. A kind of THGEM made from PTFE, named PTFE-THGEM, is developed for the GPM to be used in CDEX. The PTFE has a lot of advantages especially its low level radioactivity. The PTFE-THGEM was tested at room and cryogenic temperature. It has a high gain in different gases and shows good stability at room temperature. The gain of a single PTFE-THGEM reached 112 at 117 K. The penning effect is also discussed in this paper to explain the ＂abnormal＂ phenomena of the gain in different gases.