Similarity solutions spreading film/drop

PLL-coated slides Always wear gloves during this procedure. Place the RNase-free slide on clean aluminum foil or a similar surface that has been wiped with absolute alcohol to avoid RNase contamination. With a pencil, mark the slide on the side to be coated, as PLL is transparent and impossible to detect once applied. Apply a small drop (approx 5 xL) of PLL solution to the slides. Spread a thin film of PLL over the whole surface. Either use the same method as for making blood films, or appose the surface of two slides to spread the drops of liquid and then slide them apart. The PLL film dries quickly (1-2 min) and the tissue sections can be picked up onto the coated surface immediately afterwards (see Note 1). 

Spreading and Translation. Examples of motions of drops on liquid surfaces are known in literature like the pentanol droplet of Nagai et al. (2005) or the aniline droplet of Chen et al. (2009), both on aqueous solutions. The aniline drop is similar to the present system in the sense that dissolution, evaporation and density effects are also involved. In this case evaporation and solubilization are relatively slow and the drop appears as a macroscopic drop coexisting with a surrounding precursor film. Two different kinds of motion are observed bee-like motion and circular motion that could last for hours. Motility was ascribed to a surface tension imbalance at the front and rear of the drop. This difference, revealed by a difference in contact angle, was attributed to the precursor film. The film is driven by a Marangoni flow and may lose its initial symmetry (by fluctuations or induced manually). The initiated translation is then sustained by the dissymmetry of the surface active film pointing to the rear of the drop. 

The mode of action of PDMS-hydrophobed silica antifoams in aqueous surfactant solutions has been extensively stndied by Denkov et al. [53] and reviewed in detail in Chapter 4. Essentially the hydrophobed silica particles rupture the so-called air-water-oil pseudoemulsion film, thereby enabling the oil to emerge into the air-water surface. It is known that once they emerge into the air-water surface, drops of PDMS oils usually initially spread over that snrface, exhibiting either complete wetting or pseudo-partial wetting behavior (see Section 3.6.2). This means that the oil spreads as either a thick duplex layer or spreads and breaks up into lenses in equilibrium with a thin oil layer. Since such behavior is ubiquitous with aqueous surfactant solntions, it is reasonable to expect similar behavior when PDMS oil drops are introduced into the gas-blood surface. It is not, however, known whether complete or pseudo-partial wetting behavior is to be expected. 

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