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Novel AutoCAD method for performing surface energetics analyses.

Article Excerpt
This study reports the development of an economical apparatus and method for measuring contact angles of liquids on solid surfaces using a digital microscope and AutoCAD [C] 2005 drafting software. Digital images (JPG) of 1.0[micro]1 droplets for a probe liquid triad (ultrapure water, diiodomethane, ethylene glycol) resting on the substrates of interest are produced. Using the AutoCAD[C] design window, images of droplets were overlaid with lines and shapes matching all profiles of the images. Once these reference lines were applied, contact angles ([theta]) of the probe liquids at the three phase boundary were determined using AutoCAD[C] software and geometric principals. Multiple surfaces were analyzed using this method, including N-type silicon wafers, glass microscope slides, 130 [Angestrom] thickness gold layers electrosprayed onto both N-type silicon wafers and glass slides, and a carboxyl-terminated self-assembled monolayer (SAM) deposited onto the gold layers. Ten replicates of each surface and treatments were tested independently to determine repeatability of the procedure. Contact angles determined for probe liquids were consistent and comparable to literature values. Surface free energy calculations for solids were calculated with the van Oss-Chaudhury-Good equation and were likewise consistent with literature values. This method of determining the contact angles of probe liquids on solid surfaces has proven reliable in confirming surface energy predictions based on the chemical and physical properties of those surfaces tested. Results support the premise that expensive optical apparatus and associated software are not required to perform accurate comparisons of surface wetting characteristics.

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Numerous fields of science and engineering require detailed understanding of material surface characteristics in order to select or properly prepare surfaces. A common and familiar example is that paints are many times formulated for very specific applications. Spreading and adhesion of paint onto a target surface strongly influences the appropriateness of that paint for particular surfaces. For these reasons paint intended for coverage of wood surfaces are often unsuitable for application to plastic or metal surfaces. Additionally, considerations must also be made concerning the surface characteristics of dried paints: Dried paints formulated for applications such as metal surfaces exposed to outdoor conditions should have low surface energies in order to repel precipitation and thus protect the underlying materials from oxidation. In another entirely different and strongly contrasting utilization, surface wetting characteristics of plant and tree leaves have also been studied by environmental toxicologist to demonstrate exposures to automobile exhaust (Pal et al. 2002) and exposure to sulfur and deposition of heavy metals (Turunen et al. 1997).

Under certain circumstances, observing changes in wetting properties of solid surfaces may be the only rapid and reliable method for confirming the presence or absence of modifications made to those surfaces. The application of self assembled monolayers (SAMs) to gold-coated silica or glass is one such situation. A readily accessible and inexpensive technology capable of imaging these chemical monolayers is currently unavailable. Yet, by observing the contact angles of probe liquids of known polarities on these surfaces pre-and post-application of a SAM, researchers can compare changes in surface energies and determine the degree of success of the procedure. Hydrophobic/hydrophilic interactions between the solid surfaces and the applied probe liquids rapidly allow inferences to be made regarding any observed changes.

Since surface energetics analyses and tensiometry are essential to many fields and have been in use for decades, many methods of acquiring and analyzing contact angles have been developed, and much debate has gone into determining which methods produce the most accurate and precise results. In fact, So many methods exist that to list and describe each here would not give fair treatment to any, and would be beyond the scope of any single manuscript. A review of...

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