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Over the years we've received a number of
requests to provide reference materials, liquids and solids for use in
measuring contact angle and surface tension. We looked into reference
liquids several years ago but decided against offering them due to
myriad difficulties. First, many liquids have a limited shelf life and we
would end up disposing of large quantities of chemicals as they expire. Second, we became concerned about purity and the ability to
provide a product that could be verifiably pure. Lastly, it became
apparent that virtually all liquids useful for surface energy analysis
are readily available from other sources. Thus we decided then and
continue now to not offer liquids for reference or testing purposes. In
fact, we refer our customers to Fischer Scientific (http://www.fishersci.com/)
which is where we purchase test liquids (such as n-hexane, phenol, glycerol,
formamide, hexadecane, and deionized water) for our own lab.
Reference solids, by contrast, especially for contact angle, are not nearly as plentiful and with many solids, it's difficult to obtain pure traceable specimen samples which are certified. With metals in particular there are hundreds of different alloys, purities, surface finishes, plating and surface treatments - not to mention the problem of oxidation and patinas which can also greatly affect the contact angle. It is particularly challenging to identify a specific metal by alloy and surface condition that will reliably produce contact angle results within a narrow range over time and from one sample to the next. For this reason we decided not to offer any metal reference solids at this time.
One material, however, that is particularly robust, not subject to surface oxidation and plating issues, is available in high purity, is stable over time, is traceable to its manufacturing origins, is hydrophobic, and is certifiable is PTFE. PTFE, known by chemists as polytetrafluoroethylene, is known by most non-chemists as Teflon, a trademark name for a DuPont non-stick coating product line used mostly on frying pans and other cookware. As instrument designers, we find PTFE to be an excellent material for specialized parts: it's easy to machine, wears well, has excellent mechanical and electrical qualities and can handle high temperatures well. Our goniometers and accessories use a number of PTFE parts, from Teflon washers to adapters and fittings for fluid handling. Our microsyringe assembly is fabricated from solid PTFE.
Even PTFE, however, can present its share of challenges. There are many modified varietes: glass-filled PTFE may contain up to 20% glass which increases its wear resistance; static dissipative PTFE is modified with mica and sold as a semiconductor grade product - such as Semitron. There are many other varieties -- mechanical grade, electrical grade, weldable grade, etc. Additionally there are a number of surface modifications which affect roughness and wettability. Each variety and modification has its own unique strengths and properties -- which can affect the wettability and contact angle.
In the end we decided on Texolon® Brand High Performance Premium Virgin PTFE 8764 in molded sheets. We now offer certified reference samples to our customers. Each sample is identified by part number and serial number and includes the manufacturers' lot and billet numbers for traceability. Also included on the certificate are detailed mechanical and electrical properties - each one referenced to a specific ASTM (American Society for Testing and Material) method. A total of (10) ASTM results are included for such properties as elongation, compressive strength, and coefficient of linear thermal expansion. Next we took a series of contact angle measurements on the sample using deionized water and report on the certificate the range of results. The average contact angle for this material is 103° which is considered hydrophobic (but not superhydrophobic).
In recent months we've had a number of
requests for this type of product. If you were one of them, this item is
now available. The p/n is 100-27-01 and it can be purchased
directly from us or at our online store