Impact of pinning of the triple contact line on electrowetting performance

Pinning of the triple contact line adversely affects electrowetting on dielectric. Electrowetting response of substrates with contact angle hysteresis ranging from 1° to 30° has been characterized, and the results are interpreted within the framework of electromechanics corrected for pinning. The relationship between contact angle hysteresis, threshold potential for liquid actuation, and electrowetting hysteresis is quantified. Our results demonstrate that a modified electrowetting equation, based on balance of forces (including the pinning forces) acting on the triple contact line and on the drop, describes the electrowetting response of substrates with significant contact angle hysteresis. Finally, the surface properties of PDMS Sylgard 184 were found to be influenced by the electric field.     

Anomalous contact angle hysteresis of a captive bubble: advancing contact line pinning

Contact angle hysteresis of a sessile drop on a substrate consists of continuous invasion of liquid phase with the advancing angle (θ(a)) and contact line pinning of liquid phase retreat until the receding angle (θ(r)) is reached. Receding pinning is generally attributed to localized defects that are more wettable than the rest of the surface. However, the defect model cannot explain advancing pinning of liquid phase invasion driven by a deflating bubble and continuous retreat of liquid phase driven by the inflating bubble. A simple thermodynamic model based on adhesion hysteresis is proposed to explain anomalous contact angle hysteresis of a captive bubble quantitatively. The adhesion model involves two solid–liquid interfacial tensions (γ(sl) > γ(sl)′). Young’s equation with γ(sl) gives the advancing angle θ(a) while that with γ(sl)′ due to surface rearrangement yields the receding angle θ(r). Our analytical analysis indicates that contact line pinning represents frustration in surface free energy, and the equilibrium shape corresponds to a nondifferential minimum instead of a local minimum. On the basis of our thermodynamic model, Surface Evolver simulations are performed to reproduce both advancing and receding behavior associated with a captive bubble on the acrylic glass.     

Using the fact that wetting is contact line dependent

Two series of experiments, both involving contact line pinning, are reported that were designed using the contact line perspective of wetting and require this perspective to explain the observed results. Perspectives based on contact areas, for example, Wenzel's and Cassie's, are not useful in either of these experimental situations. In the first type of experiment described, sessile water drops were pinned on low contact angle hysteresis surfaces using 40 different shape/size lithographed hydrophilic features. Hydrophilic arcs (sections of circles), short wedges (pointed to the center of the circle), long wedges (pointed to the opposite side of the circle), and the upper outlines of the short and long wedges were prepared and studied. These features were based on circles with diameters of 4 and 6 mm and arcs of 30°, 60°, 90°, and 120°. The volume of water that could be pinned depends on the linear shape of the portion of the feature that interacts with the receding contact line and not on the feature area. In the second type of experiment, thin hydrophilic contact lines were used to support films of water (puddles and kinetically trapped thin films) on water-repellent surfaces and used to control the shape (both 2D and 3D) of these thin films and puddles. Elongated water puddles, 60 mm long and 4 mm wide, were prepared using contact line patterns with line widths of 500, 250, and 100 μm. Curved puddles, geometric shapes, letters of the English alphabet, and puddles with variable liquid thicknesses (heights) were also prepared.     

Modeling of the moving deformed triple contact line: influence of the fluid inertia

For partial wetting, motion of the triple liquid-gas-solid contact line is influenced by heterogeneities of the solid surface. This influence can be strong in the case of inertial (e.g., oscillation) flows where the line can be pinned or move intermittently. A model that takes into account both surface defects and fluid inertia is proposed. The viscous dissipation in the bulk of the fluid is assumed to be negligible as compared to the dissipation in the vicinity of the contact line. The equations of motion and the boundary condition at the contact line are derived from Hamilton's principle. The rapid capillary rise along a vertical inhomogeneous wall is treated as an example.     

A new device for checking surface contamination based upon electrical measurement

Surface treatments, such as plating, painting, marking or assembling with adhesives, require reliable surface cleanliness. Industrial workshops cannot use sophisticated techniques, and there is a need for fast and reliable tests. Very simple drop or marker tests exist, but they provide only go — no go information. Measurements of electrical surface potential decay have been found to be convenient, and a commercial device for metal surfaces is now available, based upon this principle. Positive electrical charges are deposited upon the metal through corona discharge, and the resulting surface potential is measured by an electrostatic probe; its decay is followed over 5–10s. Usually, the relationship between the logarithm of the potential and the decay time is linear; a microcomputer calculates the two parameters of this law, which are related to the nature and thickness of the contaminating layer. Flatness of the surface is not required, making it possible to control parts with various geometries. It is easy to compare the contamination of a surface with a standard, and to decide whether the surface can be considered sufficiently clean. The device is rather small, inexpensive and very simple to operate; it is designed for quality control in industry. Developments for plastic parts are under progress, and valuable results have already been obtained, e.g. for ageing of epoxy parts.     

A micropump controlled by EWOD: wetting line energy and velocity effects

A Laplace pressure gradient between a droplet and a liquid meniscus was utilized to create an on-demand constant flow rate capillary pump. Electrowetting on dielectric was implemented to induce the pressure gradient in the microchannel. For an initial droplet volume of 0.3 μL and a power of 12 nW a constant flow rate of 0.02 μL s(-1) was demonstrated. The effects of the wetting line energy on the static contact angle and the wetting line velocity on the dynamic contact angle in the pump operation were studied. Sample loading on-demand could be achieved by regulating an electric potential.     

Warping of triple line in the wetting of B4C by a Cu‐1 at.% Cr alloy

The wetting of polycrystalline B₄C by a molten Cu‐1 at.% Cr alloy was investigated at 1373 K using a modified sessile drop method. The triple line warped after the contact angle reached a certain degree. The mechanism for this unusual phenomenon was presumably ascribed to the large disparity in the wettability for the liquid on the CrB₂ reaction layer and the precipitated graphite phase, as well as to their formation kinetics, rather than to the capillary flow driven by the surface tension gradient, as observed in the Cu/Si system. Copyright © 2010 John Wiley & Sons, Ltd.     

Environmental scanning electron microscopy study of the fine structure of the triple line and cassie-wenzel wetting transition for sessile drops deposited on rough polymer substrates

The wetting of rough honeycomb micrometrically scaled polymer substrates was studied. A very strong dependence of the apparent contact angle on the drop volume has been established experimentally. The environmental scanning electron microscopy study of the fine structure of the triple line is reported first. The triple line is not smooth and prefers grasping the polymer matrix over air holes. The precursor rim surrounding the drop has been observed. The revealed dependence of the apparent contact angle on the drop volume is explained by the transition between the pure Cassie and combined Wenzel-Cassie wetting regimes, which is induced by capillarity penetration of water into the holes of relief.     

VOF simulations of the contact angle dynamics during the drop spreading: Standard models and a new wetting force model

Introduction

In this study,a novel numerical implementation for the adhesion of liquid droplets impacting normally on solid dry surfaces is presented. The advantage of this new approach, compared to the majority of existing models, is that the dynamic contact angle forming during the surface wetting process is not inserted as a boundary condition, but is derived implicitly by the induced fluid flow characteristics (interface shape) and the adhesion physics of the gas–liquid-surface interface (triple line), starting only from the advancing and receding equilibrium contact angles. These angles are required in order to define the wetting properties of liquid phases when interacting with a solid surface.

Methodology

The physical model is implemented as a source term in the momentum equation of a Navier-Stokes CFD flow solver as an “adhesion-like” force which acts at the triple-phase contact line as a result of capillary interactions between the liquid drop and the solid substrate. The numerical simulations capture the liquid–air interface movement by considering the volume of fluid (VOF) method and utilizing an automatic local grid refinement technique in order to increase the accuracy of the predictions at the area of interest, and simultaneously minimize numerical diffusion of the interface.

Results

The proposed model is validated against previously reported experimental data of normal impingement of water droplets on dry surfaces at room temperature. A wide range of impact velocities, i.e. Weber numbers from as low as 0.2 up to 117, both for hydrophilic (θ_adv = 10° – 70°) and hydrophobic (θ_adv = 105° – 120°) surfaces, has been examined. Predictions include in addition to droplet spreading dynamics, the estimation of the dynamic contact angle; the latter is found in reasonable agreement against available experimental measurements.

Conclusion

It is thus concluded that theimplementation of this model is an effective approach for overcoming the need of a pre-defined dynamic contact angle law, frequently adopted as an approximate boundary condition for such simulations. Clearly, this model is mostly influential during the spreading phase for the cases of low We number impacts (We < ˜80) since for high impact velocities, inertia dominates significantly over capillary forces in the initial phase of spreading.     

A new type of hybrid magnetic semiconductor based upon polymeric iodoplumbate and metal-organic complexes as templates

A new type of one-dimensional organic-inorganic hybrid [M(en)3]Pb2I6 [M = Mn (1), Fe (2), Zn (3), Ni (4); en = ethylenediamine] has been obtained and structurally characterized by X-ray crystallography, which opens a new approach to construct hybrid magnetic semiconductors. The results of optical absorption spectra and theoretical calculations for compounds 1-3 reveal a quantum confinement effect, and the variable-temperature magnetic susceptibility measurements for 1, 2, and 4 indicate ferromagnetic, antiferromagnetic, and paramagnetic behavior, respectively.     

A new approach in small-angle neutron scattering: A method of triple isotopic substitutions

A new method is proposed for studying the structure and mutual arrangement of selected components in multicomponent particles. Here the difference between the scattering curve of a solution containing two types of structurally identical particles (differing only in the degree of deuteration) and the scattering curve of a solution containing particles of a third type (deuterated to an intermediate degree) is considered. This difference scattering curve differs only by a numerical multiplier from the “vacuum” scattering curve of the particle in which the scattering density is equal to the difference between the scattering densities of the particles of the first and second types. This means that any particle component which is deuterated (protonated) to the same degree in the particles of three different types does not contribute to the difference scattering curve and, consequently, is “invisible” for neutrons. The difference scattering curve depends neither on the isotopic content of the solvent nor on the inter-particle interference and particle association. Possible applications of this method are discussed.     

The dynamik contact angle II. Investigation of the adsorption mechanism based on the measurement of the preceding contact angles

The structure of the adsorption layer at the solid/gas interface is characterized, as a function of conditioning concentration, by the measurement of preceding contact angles. The contact angles were determined tensiometrically (plate method) and cinema tographically (capillary rise method) in the system glass or mercury/n-dodecyl ammonium chloride solution/air, respectively. In the dependence of contact angle on concentration, four regions are provable. These regions correlate with the surfactants, which are bound to adsorption in a heteropolar mode or by van der Waals forces of interaction, with the formation of layer-like coverage and with bilayers. Special attention was given to the fact that loosely bound surfactants are transferred from the solid/gas interface to the liquid/gas interface and cause a reduction of the preceding contact angle.Publication No. 1077 from the Research Institut of Mineral Processing, Academy of Sciences of the GDR, Freiberg, G.D.R.     

Experimental investigation on wetting mechanism for coal dust with different metamorphic degree based on infrared spectrum and 13C-NMR

Respirable coal dust accounts for heart and respiratory diseases of coal miners such as asthma, pneumoconiosis, and black lung disease. What is more, coal dust explosion seriously affects coal mine safety production and coal miners' life safety. Generally, dust suppressants are commonly applied in coal mines. However, current dust suppressants are not working effectively. To develop a better dust suppressant, we attempt to explore the factors affecting the wettability of coal dust under different metamorphic levels from the essence of coal dust wetting mechanism in this paper. Specifically, we use Fourier transform infrared (FTIR) spectroscopy and nuclear magnetic resonance (NMR) to reveal the microstructure of coal from two aspects of functional group and carbon skeleton structure and obtain the micro information of the surface functional group types, quantity, and carbon structure of coal with different degrees of metamorphism, as well as the change rule of functional group of coal sample with coal rank and the law of carbon increase and deoxidization of coal metamorphism. After that, we acquired the structural parameters of coal by the NMR experiments and fitted the quantitative mathematical relationship between the microstructure parameters and wettability of coal through SPSS and ORIGIN software. Finally, this paper constructs an evaluation model for the influencing factors of coal dust wettability, explains the influence degree of different coal dust structure on coal dust wettability, improves the coal dust wettability mechanism, and provides more quantitative research ideas and methods for the control of coal dust.     

A new interpretation of the Baylis-Hillman mechanism

[reaction: see text] On the basis of reaction rate data, we have proposed a new mechanism for the Baylis-Hillman reaction involving the formation of a hemiacetal intermediate. We have determined that the rate-determining step is second order in aldehyde and first order in DABCO and acrylate. We have shown that this mechanism is general to aryl aldehydes under polar, nonpolar, and protic conditions using both rate data and two isotope effect experiments.     

A new mechanism for the Favorskii rearrangement

The title reaction was investigated by the use of ONIOM-RB3LYP calculations. A reaction system composed of alpha-chlorocyclohexanone, a methoxide ion and 8 MeOH solvent molecules was adopted. Two reaction channels, the semibenzilic acid mechanism (A) and cyclopropanone mechanism (B), were compared. B is found to be more favorable than A. The rate-determining step of B is the (MeOH)(3) addition transition state (TS3B) to the cyclopropanone intermediate. While TS3B involves a concerted function of MeO(-) addition and proton relays, it has a large activation energy. A new route was found, where the chloride ion evolved at the cyclopropane formation step (TS2B) works as a nucleophile to the cyclopropanone intermediate. Thus, a cyclopentane-carbonyl chloride intermediate is formed with a small activation energy. A new cyclopropanone mechanism is proposed.     

A new mechanism for metal-catalyzed stannane dehydrocoupling based on alpha-H-elimination in a hafnium hydrostannyl complex

The hafnium hydrostannyl complex CpCpHf(SnHMes(2))Cl (Cp = eta(5)-C(5)Me(5)) has been synthesized from the reaction of CpCpHf(H)Cl with Mes(2)SnH(2). This compound has been identified as an intermediate in the metal-catalyzed dehydrocoupling of Mes(2)SnH(2) to the distannane Mes(2)HSnSnHMes(2) (Mes = 2,4,6-trimethylphenyl). The dehydrocoupling in this system appears to occur by elimination of :SnMes(2) from CpCpHf(SnHMes(2))Cl, with Sn-Sn bond formation proceeding via insertion of the stannylene into a Sn-H bond.     

A combinatorial scaffold approach based upon a multicomponent reaction

A combinatorial scaffolding procedure for the synthesis and spatial arrangement of tripartite structures was developed. [reaction: see text]     

A new spin-specific mechanism of attractive vibronic interaction

An interesting new type of spin-specific vibronic coupling is found. The non-adiabatic coupling induces an effective attractive interaction which softens the orginal repulsive interaction between a pair of electrons. The relation with the Cooper pair of superconducting electrons is discussed.