Revealing of water surface pollution with liquid marbles

Floating of “liquid marbles” on the surface of water contaminated with organic compounds was studied. Marbles were obtained with spreading of hydrophobic polyvinylidene fluoride particles on the surface of water droplets. Marbles floated stably on the water surface, whereas they were destroyed on the water surfaces contaminated with silicon oil and kerosene. Limiting conditions of floating were studied experimentally with the water/alcohol mixtures.     

Capillary Interactions between a Probe Tip and a Nanoparticle

To understand capillary interactions between probe tips and nanoparticles under ambient conditions, a theoretical model of capillary forces between them is developed based on the geometric relations. It is found that the contribution of surface tension force to the total capillary force attains to similar order of magnitude as the capillary pressure force in many cases. It is also shown that the tip shape and the radial distance of the meniscus have great influence on the capillary force. The capillary force decreases with the increasing separation distances, and the variance of the contact angles may change the magnitudes of capillary forces several times at large radial distances. The applicability of the symmetric meniscus approximation is discussed.     

Capillary Forces between Submillimeter Spheres and Flat Surfaces at Constant Liquid Volumes

We investigate the capillary forces between submillimeter spheres and flat surfaces at constant liquid volumes theoretically and experimentally. An iterative method is used to estimate the capillary force with contact angles as the boundary conditions and the constant volume as a constraint. The theoretical analysis shows that the maximum capillary force between them decreases with the increase of the liquid bridge volume at small contact angles. The experimental results show that the force is smaller than the theoretical values at the initial separation distances. It is also observed that the force first increases and then decreases with an increasing separation distance in some cases. These phenomena of capillary forces hysteresis are explained according to the wetting hysteresis.     

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.     

Topological and chemical investigation on super-hydrophobicity of PTFE surface caused by ion irradiation

Super-hydrophobic PTFE surfaces were obtained by irradiation of 200 keV Xe⁺ ion with the fluence of 6.2×10¹³ ions/cm². The contact angle of water on such surface is as large as 161±3°. SEM and XPS were used to investigate how the topological and chemical changes affect the wettability of the irradiated surface. Needle like structures at nanometer scale caused by irradiation are considered to be the reason of the super-hydrophobicity. The formation of oxygen containing group and defluorination effect on the treated surface are inferred to have negative contribution to the hydrophobic optimization of PTFE surface.     

Titanium metallization of alumina ceramics by molten salt reaction

In this work, we reported a novel method that is flexible to metallize alumina ceramics with complex surface. Through Ti²⁺ disproportionation occurred in molten NaCl-KCl-K₂TiF₆ bath, the dense Ti layer was deposited on Al₂O₃ ceramics surface with strong adhesion. The effects of reaction temperature, time and initial K₂TiF₆ concentration on deposition rate were investigated. As-prepared coatings compose of bilayer structure of reactive Ti₂O phase, namely, the outer layer with coarse grains and the inner layer with fine grains. The wettability of eutectic Ag72Cu28 and Pb37Sn63 alloys with metallized Al₂O₃ ceramics was measured by using sessile drop method and compared with that of original ceramics. The results show that the metallized Al₂O₃ surface could be reactively wetted well with Ag72Cu28 and Pb37Sn63 alloys. The contact angles lowered to 35° and 8°, respectively, when temperature rose to 900 °C, showing significant enhancement of wettability after Ti metallization by molten salt reaction.     

Formation and mobility of droplets on composite layered substrates

A mesoscale fluid film placed on a solid support may break up and form droplets. In addition, droplets may exhibit spontaneous translation by modifying the wetting properties of the substrate, resulting in asymmetry in the contact angles. We examine mechanisms for droplet formation and motion on uniform and terraced landscapes, i.e., composite substrates. The fluid film stability, droplet formation and velocity are studied theoretically in the isothermal case using a lubrication approach in one spatial dimension. The droplet properties are found to involve contributions from both the terraced layer thickness and molecular interactions via the disjoining potential.     

Demonstration of Four Fundamental Operations of Liquid Droplets for Digital Microfluidic Systems Based on an Electrowetting-on-Dielectric Actuator

An electrowetting-on-dielectric actuator is developed, in which the liquid is sandwiched between top and bottom plates. For the bottom plate, silicon wafer is used as the substrate, the heavily phosphorus-doped polysilicon film is deposited by low pressure chemical vapour deposition as the microelectrode array, and thermally grown SiO2 film as the dielectric layer. The top p/ate is a glass plate covered with transparent and conductive indium tin oxide as the ground electrode. In addition, a Teflon AF1600 film is spun on the surface of both the plates as the hydrophobic layer. The experimental results show that when the gap height between two plates is 133μm, a prototype of the device is capable of creating, transporting, merging and dividing droplets of deionized water in an air environment with a 70V at lOHz voltage pulse. This is also established by simulations using the computational fluidic software of CFD-ACE＋.     

Monolayer passivation of silicon(0 0 1) surface by selenium

Monolayer passivation of the silicon(0 0 1) surface by selenium is investigated in an ultrahigh vacuum environment with a solid selenium source by reflection high-energy electron diffraction and residual gas analysis. It is found that precisely one monolayer of selenium is deposited on silicon(0 0 1) when the silicon substrate temperature is set slightly above the selenium source temperature and the passivation time ensures a little overdose of selenium above one monolayer. The temperature settings prevent selenium condensation on silicon(0 0 1), which makes selenium deposition on silicon(0 0 1) a thermodynamically self-limited process to exactly one monolayer.     

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.     

Superhydrophobic surfaces via electroless displacement of nanometric Cu layers by Ag

This paper explores the possibility of making hydrophobic and superhydrophobic surfaces from electroless displacement of Cu by Ag⁺, in the case where Cu oxidation is limited owing to Cu layers of nanometric thicknesses. The morphology of the Ag layers is studied by scanning electron microscopy for Cu thicknesses between 10 and 80 nm. The mapping of the elemental content of the layers by electron dispersive X-ray analysis also has been used to clarify the particle growing by diffusion limited aggregation. It is shown that the average size and the shape complexity of the Ag particles increase with the Cu thickness. The addition of dimethyl sulfoxide in the Ag⁺ aqueous solution improves the surface homogeneity, increases the particle density and decreases their sizes. The wetting behaviour of the surfaces, after grafting with octadecanethiol, has been studied from measurements of the contact angles of a drop of water. According to the thickness of the initial Cu layer and the morphology of the Ag layer, contact angles range between 110° and 154°. Superhydrophobic surfaces are obtained from 80 nm thick Cu layers.     

A Model for the Contact Angle of Liquid Droplets on Rough Surfaces

Up to now, measured results of the contact angle on rough surfaces have been explained usually based on the Wenzel equation （1936） and the Cassie-Baxter equation （1944）. However, these equations do not take into account considerations of liquid wetting behaviors on rough surfaces, and this leads to poor understanding of the mechanisms of contact between liquid droplets and rough surfaces （e.g. contact angle hysteresis）. We propose a new model for the contact angle of liquid droplets. By means of the present model, we can well understand the evperimental data which could not be well explained by the Wenzel equation and the Cassie-Baxter equation.     

Wetting and scanning force microscopy on rough polymer surfaces: Wenzel’s roughness factor and the thermodynamic contact angle

The influence of the surface roughness of polypropylene on the contact angle hysteresis is investigated by means of ethylene glycol drops in order to estimate the true Young’s equilibrium contact angle. A new relationship between the contact angle hysteresis and Wenzel’s contact angle is derived. In addition, the determination of Wenzel’s roughness factor by means of scanning force microscopy opens an alternative way to obtaining Young’s equilibrium contact angle without any surface manipulation. The experimental results presented verify this new approach. Received: 2 September 2002 / Accepted: 2 September 2002 / Published online: 5 March 2003 RID="*" ID="*"Corresponding author. Fax: +49-3328/352-452, E-mail: helmut.kamusewitz@gkss.de     

Large Slip Length over a Nanopatterned Surface

A thermodynamic method is employed to analyse the slip length of hydrophobic nanopatterned surface. The maximal slip lengths with respect to the hydrophobicity of the nanopatterned surface are computed. It is found that the slip length reaches more than 50 μm if the nanopatterned surfaces have a contact angle larger than 160°. Such results are expected to find extensive applications in micro-channels and helpful to understand recent experimental observations of the slippage of nanopatterned surfaces.     

Adsorption of Al on the Si(0 0 1) surface

The first-principles calculations have been presented to study the adsorption of aluminum (Al) on the Si(0 0 1)(2×1) surface. We have investigated the optimized geometries and electronic structures of the adatom-substrate system. The adsorption energy of the system has been calculated. The most stable adsorption sites were consequently determined to be HH site and T3+T4. It is shown that the Si-Si dimer is asymmetric on the reconstructed bare surface and become symmetric upon Al adsorption. In addition, the bond length of Si-Si was found to be considerably elongated in the adsorption system. It is found that the work function change obtained in our work is different from other previous results on the adsorption of alkali metals on the Si(0 0 1) surface. In order to investigate the relative stability of phases at different coverages, the surface formation energy of the adsorption system was calculated. To shed light on the nature of the Al-Si bond and the character of silicon surface, the density of states (DOS) and difference charge density of the system were evaluated.     

First-principles calculations of ethanethiol adsorption and decomposition on GaN (0 0 0 1) surface

The adsorption and decomposition of ethanethiol on GaN (0 0 0 1) surface have been investigated with first-principles calculations. The DFT calculations reveal that ethanethiol adsorbs dissociatively on the clean GaN (0 0 0 1) surface to form ethanethiolate and hydrogen species. An up limit coverage of 0.33 for ethanethiolate monolayer on GaN (0 0 0 1) surface is obtained and the position of the sulfur atom and the tilt angle of the thiolate chain are found to be very sensitive to the surface coverage. Furthermore, the reactivity of ethanethiol adsorption and further thermal decomposition reactions on GaN (0 0 0 1) surface is discussed by calculating the possible reaction pathways and ethene is found to be the major product.     

Scanning probe studies of water nucleation on aluminum oxide and gold surfaces

The nucleation of nanoscale water at surfaces in humid environments is sensitive to several factors, including the details of the surface morphology, ability of the surface to hydrate and the presence of contaminants. Tapping mode atomic force microscopy was used to investigate the nucleation process as a function of relative humidity (RH) on passive aluminum and gold thin films. Films exposed to the ambient environment prior to RH exposure showed discrete structures with lateral sizes ranging from 10 to 100 nm only at RH > 70%. These structures formed preferentially at grain boundaries, triple points and regions with significant topography such as protruding grains. The morphology of the passive aluminum surface is permanently altered at the sites where discrete structures were observed; nodules with heights ranging from 0.5 to 2 nm persist even after reducing the RH to <2%. The gold surface does not show such a permanent change in morphology after reducing the RH. Passive aluminum films exposed to high RH immediately after growth (e.g. no ambient exposure) do not show discrete structures even at the highest RH exposures of 90%, suggesting a hydrophilic surface and the importance of surface hydrocarbon contaminants in affecting the distribution of the water layer.     

Dewetting processes in a cylindrical geometry

Dewetting of liquid films of water-glycerol solutions of different viscosities has been studied experimentally in PVC cylindrical tubes. In contrast with plane surfaces, the dewetting capillary number Ca_vd increases with the film thickness h₀ over a large part of the experimental range and follows a same global trend independent of viscosity as a function of h₀. This increase is only partly explained by variations of the capillary driving force predicted in a recent theoretical work for a cylindrical geometry. An additional explanation is suggested, based on different spatial distributions of the viscous dissipation in the dewetting bump in the planar and cylindrical geometries. This mechanism is investigated for films of different thicknesses in a numerical model assuming a polynomial variation of the liquid thickness with distance in the bump region.     

Anti-icing performance of superhydrophobic surfaces

This article studies the anti-ice performance of several micro/nano-rough hydrophobic coatings with different surface chemistry and topography. The coatings were prepared by spin-coating or dip coating and used organosilane, fluoropolymer or silicone rubber as a top layer. Artificially created glaze ice, similar to the naturally accreted one, was deposited on the nanostructured surfaces by spraying supercooled water microdroplets (average size ∼80 μm) in a wind tunnel at subzero temperature (−10 °C). The ice adhesion strength was evaluated by spinning the samples in a centrifuge at constantly increasing speed until ice delamination occurred. The results show that the anti-icing properties of the tested materials deteriorate, as their surface asperities seem to be gradually broken during icing/de-icing cycles. Therefore, the durability of anti-icing properties appears to be an important point for further research. It is also shown that the anti-icing efficiency of the tested superhydrophobic surfaces is significantly lower in a humid atmosphere, as water condensation both on top and between surface asperities takes place, leading to high values of ice adhesion strength. This implies that superhydrophobic surfaces may not always be ice-phobic in the presence of humidity, which can limit their wide use as anti-icing materials.