Lateral vibration of a water drop and its motion on a vibrating surface

The resonant modes of sessile water drops on a hydrophobic substrate subjected to a small-amplitude lateral vibration are investigated using computational fluid dynamic (CFD) modeling. As the substrate is vibrated laterally, its momentum diffuses within the Stokes layer of the drop. Above the Stokes layer, the competition between the inertial and Laplace forces causes the formation of capillary waves on the surface of the drop. In the first part of this paper, the resonant states of water drops are illustrated by investigating the velocity profile and the hydrostatic force using a 3d simulation of the Navier-Stokes equation. The simulation also allows an estimation of the contact angle variation on both sides of the drop. In the second part of the paper, we investigate the effect of vibration on a water drop in contact with a vertical plate. Here, as the plate vibrates parallel to gravity, the contact line oscillates. Each oscillation is, however, rectified by hysteresis, thus inducing a ratcheting motion to the water droplet vertically downward. Maximum rectification occurs at the resonant states of the drop. A comparison between the frequency-dependent motion of these drops and the variation of contact angles on their both sides is made. The paper ends with a discussion on the movements of the drops on a horizontal hydrophobic surface subjected to an asymmetric vibration.     

Theoretical Study on the Capillary Force between an Atomic Force Microscope Tip and a Nanoparticle

Considering that capillary force is one of the most important forces between nanoparticles and atomic force microscope （AFM） tips in ambient atmosphere, we develop an analytic approach on the capillary force between an AFM tip and a nanoparticle. The results show that the capillary forces are considerably affected by the geometry of the AFM tip, the humidity of the environment, the vertical distance between the AFM tip and the nanoparticle, as well as the contact angles of the meniscus with an AFM tip and a nanoparticle. It is found that the sharper the AFM tip, the smaller the capillary force. The analyses and results are expected to be helpful for the quantitative imaging and manipulating of nanoparticles by AFMs.     

Crystallographic symmetry effect on the nucleation in non-equilibrium aggregation pattern

Some non-equilibrium patterns formed in different systems are composed by crystallites with correlations in their crystallographic orientation. This reflects the fact that crystallographic symmetry controls the successive nucleation in the crystallite aggregating process. Based on the crystallographic symmetry, a new classification to describe the different kinds of anisotropy for non-equilibrium aggregation pattern has been proposed: parallel aggregation, twin aggregation and radial (random) aggregation. This new classification can clarify the different anisotropy related to the crystal point group, which the previous classifications cannot. The nucleation mechanisms for these three kinds of patterns are also discussed based on the symmetric relationship among the crystallites. It is shown that the crystallographic symmetry plays an important role not only on the crystal polyhedral shape formed by growth, but also on the aggregation pattern formed by nucleation.     

Self-Assembling Process of Colloidal Particles into Two-Dimensional Arrays Induced by Capillary Immersion Force: A Simulation Study With Discrete Element Method

This paper presents a simulation study for self-assembling process of colloidal particles into two-dimensional arrays due to capillary immersion force. Discrete element method is used to simulate the dynamics of colloidal particles trapped in a thin liquid film. The previous model is improved in the following two points: a modification of the screening effect of capillary immersion force and introduction of periodic boundary condition. Snapshots provided by the simulations agree well with experimental images taken by atomic force microscopy. The self-assembling process is quantified with pair correlation function and coordination number. At lower coverage, colloidal particles rapidly form small clusters that consist of several particles in the early stage. Subsequently, chain-like structures with some branches are mainly generated. On the other hand, at higher coverage, large domains of hexagonal close-packed (HCP) structures are gradually generated. The rate of the growth of HCP domains is much slower than that of the generation of the small clusters and the chain-like structures.     

基于扫描电子显微镜的碳纳米管拾取操作方法研究

碳纳米管场效应管是未来纳米器件的发展方向,而制造纳米器件的前提是拾取碳纳米管,基于扫描电子显微镜(SEM)的微纳机器人操作系统能够实现碳纳米管拾取操作.本文建立拾取操作中碳纳米管与原子力显微镜(AFM)探针间范德瓦耳斯力力学模型,不同接触状态下范德瓦耳斯力越大越有利于拾取碳纳米管.在SEM视觉反馈图像中建立相对坐标系,首先提出倾角变值方法检测碳纳米管与AFM探针的接触状态,然后运用动态差值方法识别碳纳米管与AFM探针空间位姿并校正碳纳米管位姿,最后自下而上拾取碳纳米管.实验结果表明:拟合直线倾角变值较大时碳纳米管与AFM探针发生接触,动态差值变化为零时碳纳米管与AFM探针为空间线接触,在完全线接触模型下选择合适的接触角度、接触长度和拾取速度能够成功拾取碳纳米管.     

液滴在不同润湿性表面上蒸发时的动力学特性

基于润滑理论,采用滑移边界条件建立了二维液滴厚度的演化模型和移动接触线动力学模型,利用数值计算方法模拟了均匀加热基底上固着液滴蒸发时的动力学特性,分析了液-气、固-气和液-固界面张力温度敏感性对壁面润湿性和液滴动态特性的影响.结果表明,液滴的运动过程受毛细力、重力、热毛细力和蒸发的影响,重力对液滴铺展起促进作用,而毛细力、热毛细力则起抑制作用;通过改变界面张力温度敏感性系数,可使液滴蒸发过程中的接触线呈现处于钉扎或部分钉扎模式,且接触线钉扎模式下的液滴存续时间低于部分钉扎模式;提高液-气与液-固界面张力温度敏感系数均可改善壁面润湿性能,加快液滴铺展速率;而增大固-气界面张力温度敏感系数则导致壁面润湿性能恶化、延缓液滴铺展过程;通过改变固-气界面张力温度敏感系数更有利于调控处于蒸发状态下的液滴运动.     

Deformation of an elastic substrate by a three-phase contact line

Young's classic analysis of the equilibrium of a three-phase contact line ignores the out-of-plane component of the liquid-vapor surface tension. While it is expected that this unresolved force is balanced by the elastic response of the solid, a definitive analysis has remained elusive because of an apparent divergence of stress at the contact line. While a number of theories have been presented to cut off the divergence, none of them have provided reasonable agreement with experimental data. We measure surface and bulk deformation of a thin elastic film near a three-phase contact line using fluorescence confocal microscopy. The out-of-plane deformation is well fit by a linear elastic theory incorporating an out-of-plane restoring force due to the surface tension of the solid substrate. This theory predicts that the deformation profile near the contact line is scale-free and independent of the substrate elastic modulus.     

II. Recirculation flow induced by a bubble stream rising in a viscous liquid

The recirculation flow induced by the rising motion of a bubble stream in a viscous fluid within an open-top rectangular enclosure is studied. The three-dimensional volume averaged conservation equations are solved by a control-volume method using a hybrid finite differencing scheme to describe the liquid phase hydrodynamics. The momentum exhange between the bubbles and the liquid phase is modeled with a source term equals to the volumetric buoyancy force acting on the gas in the bubble stream. The volumetric buoyancy force accounts for in line interactions between bubbles through the average gas volume fraction in the gas liquid column which depends on the size and the rising velocity of bubbles. The fluid flow within an open-top rectangular enclosure is further investigated by particle image velocimetry for a bubble stream rising in a water-glycerol solution. The measured fluid velocities in a vertical plane are compared with the predictions of the numerical model over a wide range of fluid viscosity (43 mPa s-800 mPa s) and gas flow rates. Finally, the recirculation flows resulting from the interaction of two neighbouring vertical bubble streams are studied. Received: 23 July 1997 / Revised: 19 December 1997 / Accepted: 11 May 1998     

The effect of substrate roughness on the static friction of CuO nanowires

The dependence of static friction on surface roughness was measured for copper oxide nanowires on silicon wafers coated with amorphous silicon. The surface roughness of the substrate was varied to different extent by the chemical etching of the substrates. For friction measurements, the nanowires (NWs) were pushed by an atomic-force microscope (AFM) tip at one end of the NW until complete displacement of the NW was achieved. The elastic bending profile of a NW during this manipulation process was used to calculate the ultimate static friction force. A strong dependence of static friction on surface roughness was demonstrated. The real contact area and interfacial shear strength were estimated using a multiple elastic asperity model, which is based on the Derjaguin–Muller–Toporov (DMT) contact mechanics. The model included vertical elastic flexure of NW rested on high asperities due to van der Waals force.     

Formation of gold nanowires through self-assembly during scanning force microscopy

Gold films with a nominal thickness of 5–40 monolayers were grown on dielectric substrates and imaged by scanning force microscopy (SFM). The films originally consisted of well-separated or densely packed clusters. During imaging in contact mode, the morphology of the films changed drastically. At low coverage, i.e. Θ<10 monolayers, the well-known stripes originating from mobile clusters, eventually accumulated into larger aggregates, were observed. In contrast, at larger coverage, highly ordered structures consisting of one-dimensional wires evolved during scanning. They often were parallel with equal separation, i.e. well-defined periodicity, over distances of several μm. Typically, the wires were 5–10 nm high and 50–100 nm wide. Investigations of Au films prepared at varying temperature on different dielectric substrates allow us to suggest a self-assembling mechanism for wire formation in which gold is periodically collected by the SFM tip and redeposited as soon as a critical amount is reached. Received: 22 February 1999 / Accepted: 2 March 1999 / Published online: 7 April 1999     

Magnetic force microscopy signal of flux line above a semi-infinite type II-superconductor

We have examined a single flux line in the semi-infinite type-II superconductor. The stray magnetic field of the flux line has been calculated. We have found that the vertical force exerted on a magnetic force microscopy (MFM) tip from the flux line is measurable by currently existing MFM. Two types of magnetic tips were taken into consideration, solid and thin film tips. For example, with a Cobalt film of the thickness of 100 nm and 30 nm on a tip, we found a vertical force of 4*10^–10 N and 1.5*10^–10 N, respectively. The lateral force exerted on a tip by the flux line was also calculated. The lateral force must be small enough to prevent the flux line from becoming depinned.     

Dynamic modelling of the deformed contact line under partial wetting conditions: Quasi-static approach

We study numerically the motion of contact lines in the context of the “Wilhelmy plate" experiment: a vertical solid plate is withdrawn at constant velocity from a bath of liquid. We apply the contact line dissipation quasi-static model to the relaxation of an initially periodically deformed contact line. The obtained numerical data are compared to the experimental results [1] showing a good agreement.     

Limiting phenomena for the spreading of water on polymer films by electrowetting

This paper is about fundamental limitations in electrowetting, used as a tool for spreading water solutions on hydrophobic surfaces, like the surface of a polymer film. Up to which point can an electric voltage decrease the contact angle? The first limitation comes when using pure water, above a threshold voltage, little droplets are emitted at the perimeter of the mother drop. We present an analysis of the drop contour line stability, involving competition between electrostatic and capillary forces, which is compatible with observations. The use of salted water solutions suppresses this instability, then one faces a second limitation: the evolution of the contact angle saturates before complete wetting. We show that this saturation is caused by ionisation of the air in the vicinity of the drop edge. We analyse the luminescence induced by gas ionization and measure the related electrical discharges. We explain how air ionization suppresses the driving force for electrowetting and how it induces the formation of an hydrophillic ring around the drop.     

Morphologies of diblock copolymer thin films before and after crystallization

Spin-coated thin films of about 100nm of low-molecular-weight hydrogenated poly(butadiene-b- ethyleneoxide) (PB_h-PEO) diblock copolymers have been crystallized at various constant temperatures. Crystallization has been observed in real time by light microscopy. Detailed structural information was obtained by atomic force microscopy, mainly enabled by the large viscoelastic contrast between amorphous and crystalline regions. The behavior in thin films is compared to the bulk properties of the polymer. Crystallization started from an annealed microphase separated melt where optical microscopy indicated a lamellar orientation parallel to the substrate. A small difference in the length of the crystallizable block produced significantly different crystallization behavior, both in the bulk and in thin films. For thin films of the shortest diblock copolymer (45% PEO content) and for an undercooling larger than about 10 degrees, crystallization created vertically oriented lamellae. These vertical lamellae could be preferentially aligned over several micrometers when crystallization occurred close to a three-phase contact line. Annealing at temperatures closer to the melting point or keeping the sample at room temperature for several months allowed the formation of a lamellar structure parallel to the substrate. A tentative interpretation based on kinetically caused chain folding and relaxation within the crystalline state, with implications on general aspects of polymer crystallization, is presented. Received 19 March 1999 and Received in final form 14 December 1999     

Spatial variations of the mean and statistical quantities in the thermal boundary layers of turbulent convection

We have studied the dynamics of the contact line of a viscous liquid on a solid substrate with macroscopic random defects. We have first characterized the friction force f₀ at microscopic scale for a substrate without defects; f₀ is found to be a strongly nonlinear function of the velocity U of the contact line. In presence of macroscopic defects, we find that the applied force F(U) is simply shifted with respect to f₀(U) by a constant: we do not observe any critical behavior at the depinning transition. The only observable effect of the substrate disorder is to increase the hysteresis. We have also performed realistic numerical simulation of the motion of the contact line. Using the same values of the parameters as in the experiment, we find that the experimental data is qualitatively well reproduced. In light of experimental and numerical results, we discuss the possibility of measuring a true critical behavior.Received: 6 October 2003, Published online: 19 February 2004PACS: 46.65. + g Random phenomena and media - 64.60.Ht Dynamic critical phenomena - 68.08.Bc Wetting     

Impact of electric fields on the speed of contact line in vertical deposition of diluted colloids

We report experimental results on the influence of electric fields on the contact line dynamics of the vertical deposition of water-based diluted colloidal suspensions. We measure the speed of macroscopically receding contact line for different initial concentrations and applied voltages. We explain the observed behavior via the electrophoretic effect in the region near the contact line. The electrophoretic effect induces a concentration gradient along the direction of the applied field which influences the morphology of the dried deposit of colloidal particles. Thus the applied field has an effect on the receding contact line through morphological formation and its transition.     

Droplet suction on porous media

We study the forced aspiration of small ( mm) and large ( cm) liquid drops, deposited on prewetted porous membranes, and pumped mechanically with a constant current J. Two kinds of membranes are used where the pores are i) disconnected, cylindrical and calibrated or ii) interconnected “sponge-like”. Whatever the size of the drops and the intensity J of the current, two suction regimes are observed versus time: 1) a “locked” regime, when the drop is pinned, with a dynamic contact angle decreasing from advancing () to finite receding () contact angle; 2) an “unlocked” regime, where the contour line recedes with a constant contact angle closed to . In both regimes, the shape of the drop remains quasistatic, during the suction process, i.e. a spherical cap for small drops and a flat “gravity pancake” for large ones. Received 19 January 2000     

Self-assembled and fluorescence enhancement of semiconductor nanoparticles induced by surfactant adsorption

When semiconductor colloidal CdS nanoparticles and nonylphenol are mixed together in dimethyl sulfoxide at room temperature, a self-assembling process is induced. In the course, the size tunable properties of CdS nanoparticles are amplified. A blue shift in the emission spectrum and a strong photoluminescence enhancement are observed without significant change in the absorption features of the colloidal nanoparticles. These results are attributed to the adsorption of nonylphenol onto the nanoparticles surface and to the association process of the surfactant molecules. The surfactant adsorption process provides a nanoparticle surface passivation and induces an associative phase that enlarges the photoluminescence stability. This strategy opens the possibility to improve simultaneously physicochemical and photoluminescence properties of nanocrystals in solution as well as to control their deposition on two-dimensional surfaces.     

Pattern formation during the drying of a colloidal suspension

Receding contact lines of colloidal suspensions are studied in the presence of drying, inside Hele-Shaw cells. At high velocity the contact line movement is continuous and the particle deposition is uniform. At small velocity, a periodic pinning-unpinning of the contact line is observed leading to a patterning of the substrate. We focused on the correlation between the deposition pattern and the pinning force that grows during the pinning. Our results strongly indicate that this pinning force is proportional to the macroscopic slope of the deposit and accounted by a simple capillary balance.     

Nonlinear dependence of the contact angle of nanodroplets on contact line curvature

We have measured the contact angle of microsized and nanosized alkane droplets partially wetting a model substrate using true noncontact atomic force microscopy. The large range of droplet sizes accessible using this technique allowed us to determine the contact line curvature dependence of the contact angle with unprecedented accuracy. Whereas previous studies aimed at explaining such a dependence by a line tension effect, our results and calculations on a model system exclude such an effect and point to an extreme sensitivity to weak substrate heterogeneities confirmed by numerical simulations.