Droplet meniscus

A sharp transition from the liquid droplet/ meniscus to the liquid film is impossible (arrow in Fig. 1) otherwise, the capillary pressure will be infinite. Hence, there is a smooth transition from the flat equilibrium liquid film on the solid surfaces to the spherical droplet, as shown in Fig. 1. 

Such boundary layers exist in proximity to any interface solid-liquid, liquid-liquid, and liquid-air. In the vicinity of the apparent three-phase contact line (Fig. 3), those boundary layers overlap. The overlapping of boundary layers is the physical phenomenon which results in the existence of surface forces. Let the thickness of the boundary layers be 8. In the vicinity of the three-phase contact line, the thickness of a droplet/ meniscus, h, is small enough, that is, /i 8, and, hence, boundary layers overlap (Fig. 3), which results in the creation of disjoining pressure. A similar situation occurs at a contact of two particles in a liquid (Fig. 4). The abovementioned characteristic scale of boimdary layer thickness, 8 10 cm, determines the characteristic thickness where the disjoining pressure acts. 

In the absence of (electro)chemical reactions at the substrate, the solution of a set of equations (Equation 19.4) gives the access to electrical properties of the pipette and the liquid droplet (meniscus) between the probe and the substrate. This is essential for modeling the overall... 

Once again, the smallness of the Ca means that the surface tension is much more powerful over the most part of the droplet/meniscus, and hence, the droplet/ meniscus has a spherical shape everywhere except for a vicinity of the apparent three-phase contact line. The size of this region, Z is estimated in this chapter in Section 3.2. It will be shown that the following inequality is satisfied ... 

While Eq. III-18 has been verified for small droplets, attempts to do so for liquids in capillaries (where Rm is negative and there should be a pressure reduction) have led to startling discrepancies. Potential problems include the presence of impurities leached from the capillary walls and allowance for the film of adsorbed vapor that should be present (see Chapter X). There is room for another real effect arising from structural peiturbations in the liquid induced by the vicinity of the solid capillary wall (see Chapter VI). Fisher and Israelachvili [19] review much of the literature on the verification of the Kelvin equation and report confirmatory measurements for liquid bridges between crossed mica cylinders. The situation is similar to that of the meniscus in a capillary since Rm is negative some of their results are shown in Fig. III-3. Studies in capillaries have been reviewed by Melrose [20] who concludes that the Kelvin equation is obeyed for radii at least down to 1 fim. 

A special form of crevice attack can occur at a waterline or at the edges of water droplets. At the water surface, a meniscus region is present where surface tension causes water to climb up the metal surface it contacts. In effect, a crevice is formed between the air-liquid and liquid-metal interface at the meniscus. Oxygen concentration is high at the meniscus due to the greater accessibility of this region to the air. The meniscus region becomes cathodic to the adjacent metal surface. Corrosion occurs just below the meniscus, and chloride, if present, is... 

Hydrogen first liquefied in sufficient quantity to show a meniscus (J. Dewar) following earlier observations of mists and droplets by others, 1877-85. 

Although most of the water in the meniscus evaporates once the tip has been retracted, residual structures can be observed in a radius of several tens of micrometers (depending on humidity and contact time) around the original contact point. For the tip radius and loads used in these experiments, the contact radius is approximately 10 A. The residual structures are in the form of flat islands and sometimes droplets. In our first experiments the perturbation created by a brief tip contact was not fuUy appreciated. Accidental tip contacts during approach of the tip to the surface do often occur. In such cases the tip is subsequently moved to an adjacent area, several micrometers away, to study the unperturbed surface. However, as stated already, the perturbed areas can extend over tens of micrometers away from the contact point. Droplets can be observed when the relative humidity is... 

TVansfer the sin e crystal quickly to the measurement cell while holding a droplet of quenching water on the electrode surface. The single crystal electrodes were lifted to make meniscus, which prevented the side surface of the electrode from touching the electrolyte. 

Figure J. Preparation of a welJ-defined single-crystal electrode by four steps, (a) Mirror-rinished crystal is annealed under gas flame, and (b) quenched in air and then pure water, (c) The crystal is covered with a droplet of pure water to protect it from pollution by impurities in air and (d) is introduced into the cell, where the crystal contacts the meniscus of the electrolyte solution. (From Ref. 25.)...

Besides such electrical applications as water-repellent surface films, there are many applications where water repellency is an end in itself. Treated glass windowpanes or windshields are not wet by muddy water, and so they stay clean longer and are easier to clean whenever washing is required. Rain docs not flood the surface of a treated windshield but stands in small droplets when the car is in motion the air stream blows the droplets off, leaving the glass clear. Treated tumblers are easier to wash and dry treated chemical glassware holds water with a flat meniscus treated vitreous enamel does not stain easily and has improved luster. Cloth and paper may be made water-repellent for protection against rain, and it is possible that all clothes could be so treated. 

The dead-space above the sample in the dart-like tip is much reduced by the use of the type of micropipette which uses fine capillary tips. The Oxford Ultramicro-sampler is an example. Use of this device completely overcomes the solvent expulsion problem. The principal disadvantage from the point of view of electrothermal atomisation with a graphite furnace is that its maximum capacity is 5 pi. This may be too little for the sensitivity of some elements. The tip material provided with this syringe is still prone to droplet formation, but this can be replaced by PTFE capillary tubing of the correct dimensions, e.g. Polypenco size TW 24, which appears to overcome the problem completely. Good precision should be attainable with tips made from this material, provided the tips are cut across at 90° to the tube axis. Chamfered tips give rise to a variable position of the meniscus with consequent loss of reproducibility. 

For vitreous solids, such as Si02, viscosity decreases strongly before reaching the melting point. In this case, the solid meniscus can be formed by viscous flow and the height h can reach easily measurable sizes in quite short times. For example, wetting of a Ni alloy droplet on a Si02 substrate at 1743 K is associated with the... 

Ejection of a droplet from a meniscus by a focused ultrasound pulse. 

A microfiuidic well plate that makes use of method 4 is described in Section 2.3 and method 6 is explained in Section 3.1 of this article. Method 5, i.e. the acoustic ejection of a droplet from a meniscus, is described in Ref. 10. The interested reader finds a comprehensive tutorial on the physico-chemistry of fluids in microstruc-tured systems and the basic principles of microfiuidic operations and technologies in Ref. 11. 

Electrospray, also called electrohydrodynamic or electrostatic spray, is an atomization technique in which liquids are dispersed solely by the application of high voltages. A simple electrospray setup is shown in Fig. 4. A liquid flows into a metal capillary tube charged to the kilovolt range and emerges from the tip as a conical meniscus, known as a Taylor cone, due to the intense electric field (Fig. 5). An unstable jet extends continuously from the apex of the cone and disperses into charged droplets further downstream. Electrosprays have been used in industrial... 

ESI nebulization involves a variety of electrochemical processes at the needle and at the counter electrode [27, 30]. The ESI interface can be considered as a electrochemical cell, in which part of the ion transport takes place through the gas phase (Figure 6.1). In positive-ion mode, an emichment of positive electrolyte ions occurs at the solution meniscus as the result of an electrophoretic charge separation. The liquid meniscus is pulled into a cone which emits a fine mist of droplets with an excess positive charge. Charge balance is attained by electrochemical oxidation at the capillary tip and reduction at the counter electrode. The topic arose significant discussion in 2000 and the discussion partners continued to disagree on the role of electrochemistry inESI-MS [39]. 

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