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A measurement technique has been developed to measure the contact angle with of a confocal device. This technique has the unique advantage of allowing to perform both topography and contact angle measurements in the same location, therefore avoiding any shift in the sample positioning between the two measurements and ensuring the proper location of both measurements in the same area of the sample, thus enhancing the evaluation of the surface energy of the surface.
Specifically, this technique uses the confocal device to measure some parameters of the drop, such as the height (ℎ) and the apparent diameter (), in a top-view configuration. The drop volume is already known and small enough to discard gravity effects, so the shape of the drop can be approximated by a truncated sphere. Several purely geometric calculations are available to calculate the radius de of the drop and subsequently, the contact angle.
This work reports the first results of the ongoing validation study of this technique and the several mathematical calculations employed to extract the contact angle value. These initial measurements were performed for a hydrophobic surface with water as a measurement liquid. The contact angles for different set of drops for this sample were also measured by a commercial contact angle meter in side-view configuration, with the same liquid and drop dimensions, in order to verify the validity and the accuracy of the presented technique. This validation of the calculation of the contact angle is the first step for the further validation of the developed measurement method for the surface energy determination.
We have tested an in-line polariscopic arrangement to obtain a 2D map of the tension distribution in the bulk of the lens. This test is performed in the first 30 seconds after the injection molding process for two main reasons: first the stress values are still high because the lenses do not have enough time to relax the internal tensions and obtain the final shape, and second, we can remove the wrong lenses in the first moments and introduce only the good lenses in the annealing stage.
The proposed instrument is based in a transmission polariscopic arrangement. A collimated light beam is used to illuminate the sample, once the light crosses the sample, it is collected with an afocal system and the image is recorded in a CMOS sensor. Selecting an afocal system to capture the image is a useful decision because the lateral magnification can be maintained when small changes in the sample position are introduced. However the produced lenses can vary their focal lengths from on series to another. To avoid problems with the change of the focal length, the lens is introduced in a matching index and the polariscopic measurement is done. The proposed polariscopic arrangement uses two lineal polarizers, one acting as polarizer and the other acting as analyzer. This system instead of using one lineal polarizer and a lineal polarizer with an extra lambda/4 plate provides us an extra degree of freedom, enabling the possibility to put a certain degree of polarization in a well determined position of the lens, in our case the center of this lens.
The aim of this study is to select the minimum number of sets polarizer-analyzer and the right wavelengths to obtain a sure selection of the right lens. The preliminary results show that use two different wavelengths 470 & 627 nm is a good option to obtain a robust image. The second free variables that must be adjusted to obtain good values is the minimum number of set polarizer-analyzer necessary to obtain confident results. In our first tests it seems that recording only at 0, 15, 30 and 45 degrees is enough to get good results.
Mathematica description and first results for a PMMA lens are presented, however the number of measurements must be diminished to obtain an easy in-line implementation
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