Spreading drop

The rate at which a drop spreads, say hr m/s, will be influenced by ... 

Rafai S, Sarker D, Bergeron V, Meunier J, Bonn D (2002) Superspreading aqueous sm-factant drops spreading on hydrophobic surfaces. Langmuir 18 10486-10488... 

The last two decades have seen enormous developments in catalyst discovery and optimization tools, notably in the area of high-throughput experimentation (HTE) and process optimization (5). However, the basic concept used for exploring the catalyst space in homogeneous catalysis has not changed Once an active catalyst complex is discovered, small modifications are made on the structure to try and screen the activity of neighboring complexes, covering the space much like an ink drop spreads on a sheet of paper. This is not a bad method, but can we do better with the new tools that are available today ... 

The second method is to calculate a stress that is appropriate to a particular situation of interest. An example of this would be the stress acting on a drop of material due to its own weight as it rests on a support medium. The force of gravity tends to make the drop spread out into a film, while its surface tension tends... 

When analyzing wetting it is important to know whether we are in a saturated vapor or if we are working under dry conditions. In the first case the solid surface is usually covered with a molecularly thin film of adsorbed liquid molecules and the drop spreads on this adsorbed film. In the dry case the solid surface is usually not covered by molecules of the liquid and is not in equilibrium. 

How does the drop spread out Does the color move in all directions Wait until all the water is the color of the food coloring. How long does it take for the water to become completely colored ... 

Put a drop of oil and a drop of water on a piece of wax paper. Tilt the wax paper so the drops run down the paper. Does the oil drop spread out in a thin layer Does the water stay together in a drop Does the oil drop move more slowly than a water drop The nonpolar oil is attracted to the nonpolar wax paper so it spreads out. The attraction between the oil drop and the wax paper makes the oil drop move more slowly than the water drop. The polar water is not attracted to the nonpolar wax paper so it stays in a drop. 

J. Yang, J. Koplik, and J. R. Banavar, Molecular dynamics of drop spreading on a solid surface, Phys. Rev. Lett. 67, 3539-3542 (1991). 

Figure 2.4. Successive images taken with a high speed camera of a Pb drop spreading on a Fe substrate at 370°C. The first image represents the falling drop before contact. The arrow shows the shadow of the drop on the solid substrate. Reprinted from (Ebrill 1999) with kind permission of the author.

A small contact angle implies that the drop spreads over the surface if the contact angle is zero, the surface will be wetted completely (Figure C2-8). This happens when the solid/vapour tension is much larger than the solid/liquid tension the system then avoids any solid/vapour interface. If the contact angle is equal to Jt, there is no wetting. This happens when the solid/liquid tension is much higher than the solid/vapour tension. The system then minimizes the liquid/vapour interface, as with a drop of mercury on paper. 

During the first 10 ptsec of drop impact, the initially spherical drop spreads out, bulging at the edges, then recoUing. By 20—80 psec, the drop reaches a static configm ation, roughly the same diameter as the final spot size, and then begins to shrink as fluid penetrates... 

CIJ inks need to work on numerous substrates and there is a relationship between drop spread on a substrate and print quality. The amount of drop spread depends upon a number of issues in a CIJ ink, including surface tension, viscosity, solvent evaporation rate, interaction with substrate, amount and type of polymer in the ink etc. You may wish to improve the print quahty of a certain ink-substrate combination by optimally adjusting the drop spread. A common method to do this is by adding a surfactant to the ink formulation. Depending upon the chemistry of the surfactant, it will either increase or decrease drop spread, and hence is a good mechanism for tuning print quality. There are many hundreds of surfactants available, and you can use all chemical types, including anionic, cationic, non-anionic forms. Specific examples include polyoxyethylene fatty ethers and diethylhexyl sodium sulphosuccinate. 

Surface treatment and substrate patterning can be employed to further reduce feature resolution. Eor example, drop spread can be controlled and significantly reduced by prepatterning the substrate with "dewetting patterns" that confine the ink to specific regions on the sirrface. Alternatively, Smith et al achieved printed lines 5-30 microns thick by creating channels in the substrate and then filling them with inkjetted ink. 

Wu X, Phan-Thien N, Fan X, Ng TY. (2003) A molecular dynamics study of drop spreading on a solid surface. Phys Fluids 15 1357-1362. 

Drop spreading The PolyJet process is based on the formation of liquid films, rather than single drops, during the brief time interval between the drop s contact with a surface and the moment of UV exposure. Therefore, the surface tension (and the viscosity) is of major importance. The surface tension affects the magnitude of... 

Thin films of the collagen and gelatin samples were formed on freshly flamed platinum sheets by the drop spreading method (I). A few grains... 

Drop spread from distilled water on platinum sheet and air dried in grease free container at 20°C. 

Drop-spread film swollen with distilled water. 

Drop spread from distilled water at 80°C i.e. gelatinized) and air dried at 80°C in greasefree container. 

Average contact angle values for various highly purified diagnostic liquids on films prepared from purified rat skin collagen are presented in Table II. Contact angles were measured with the films (a) drop spread from distilled water on a platinum sheet and air dried to a specularly smooth continuous film in a greasefree container at a temperature that never exceeded 20°C (b) prepared as in Method a but then reswollen with distilled water for at least 1 hr and measured while still completely water swollen in equilibrium with relative humidities of 50% and >99%... 

Drop spread from dichloracetic acid on platinum sheet and air dried at 20°C in covered, greasefree container. 

Assuming that coalescence time is equal to drop-formation time, and that the drop spreads at the interface of the two phases, exposing a fresh film of area A, it can be shown (S8) that the ratio of transfer efficiencies in the drop-formation and coalescence regions can be approximated by... 

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