Water-drop spreading test

Several quantitative and qualitative tests are used for qualification of a solvent/cleaning process and for in-line quality assurance. Some tests, like the water-drop spreading test, are quick and simple. In this test, a 2 to 4 microliter droplet of water is applied to a substrate, and the contact angle it makes with the surface is measured using a goniometer. A contact angle... 

Wettability tests. Surface wettability may be readily assessed simply but subjectively by measurement of the contact angle. If the surface is clean, it is readily wetted and a drop of water will spread out rather than remain as a discrete droplet. This method cannot really be used to detect small variations in quality, but rather the gross effects. It does not lend itself to use on very rough or porous surfaces such as concrete. 

A standard test for cleanliness consists in watching what happens to a water drop deposited on the surface of the silicon. Water wets bare silicon, whereas it does not spread in the presence of impurities. One can also exhale onto the surface and watch the result. If the surface is clean, water vapor coats the silicon in the form of a homogeneous film that evaporates uniformly if not, a haze forms that disappears more slowly. This phenomenon has been studied in France it is called figure de souffle (breath pattern). 

Next let us consider those difficulties associated with the determination of the amount of material deposited on the surface. We have already noted that the method of depositing insoluble monolayers by spreading permits the accurate determination of n. Since the spreading technique requires solvent volatility, care must be exercised to prevent the stock solutions from changing concentration due to evaporation prior to their application to the surface. Also, precise microvolumetric methods must be used to dispense the solution on the aqueous surface since the quantity used is small. The solvent (as well as the solute) must be free from contaminants. There is also the possibility that the solvent will extract spreadable contaminants from the waxed surfaces of the float, barriers, and tray. Some workers advocate addition and evaporation of one drop at a time to minimize this. Oily contaminants may also reach the water surface from the fingers and from the atmosphere. These last sources are particularly hard to control Tests for reproducibility and blank compressions (i.e., moving the barrier toward the float on a clean surface) are the best evidence of their absence. 

Dissolve 46.5 g (45.5 ml, 0.5 ml) of aniline in a mixture of 126 ml of concentrated hydrochloric acid and 126 ml of water contained in a 1-litre beaker. Cool to 0-5 °C in a bath of ice and salt, and add a solution of 36.5 g (0.53 mol) of sodium nitrite in 75 ml of water in small portions stir vigorously with a thermometer and maintain the temperature below 10 °C, but preferably at about 5 °C by the addition of a little crushed ice if necessary. The diazotisation is complete when a drop of the solution diluted with 3-4 drops of water gives an immediate blue coloration with potassium iodide-starch paper the test should be performed 3-4 minutes after the last addition of the nitrite solution. Prepare a solution of 76 g (0.69 mol) of sodium fluoroborate (1) in 150 ml of water, cool and add the chilled solution slowly to the diazonium salt solution the latter must be kept well stirred and the temperature controlled so that it is below 10 °C. Allow to stand for 10 minutes with frequent stirring. Filter the precipitated benzenediazonium fluoroborate with suction on a Buchner funnel, drain well and wash the yellow solid with about 30 ml of ice-water, 15 ml of methanol and 30-40 ml of ether suck the solid as free as possible from liquid after each washing (2). Spread the salt upon absorbent filter paper and allow to dry overnight, if possible in a current of air. The yield of benzenediazonium fluoroborate is 60—65 g the pure salt melts with decomposition at 119-120 °C. 

In the authors experience, the above method will lead infallibly to the proper type of emulsion, and testing is therefore unnecessary. For those who wish to confirm success, however, the simplest way is to take a beaker half full of water and drop two small, separate drops of emulsion onto the water surface. The first drop always spreads somewhat, but the second will remain a discrete, white globule with no spreading at all if the emulsion is, indeed, water in oil. If the second drop disintegrates into bits and pieces that spread around over the surface of the water, the emulsion was oil in water, at least in part, and should be prepared afresh. Read the above instructions again first, though. 

Several techniques determine whether the continuous phase is oil or water. The simplest is the dilution method, in which a drop or two of the emulsion is added to water. If it is an oil-in-water emulsion it will spread and disperse. If it is water-in-oil it will remain as a drop 18). The dilution test can be effective, but care must be taken that sampling the emulsion does not itself determine the continuous phase. For instance, drawing a water-in-oil emulsion up through the capillary of a dropper can cause the emulsion to... 

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