Contact angle measurement in practice (1): Preparations and general conditions for contact angle measurements
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Technical Note Practical Contact Angle Measurement (1) Technical note: Industry section: Drop Shape Analyzer – DSA100 Method: Keywords: methods, sample preparation, contact angle, sessile drop, plate With care to accuracy: preparations and general conditions for contact angle measurements This is the first of a series of articles on practical contact angle measurement. In this and the following issues we will cover a wide range of topics concerning drop shape analysis: from preparations for the measurement through drop deposition conditions and up to the choice of the methods used for analyzing the drop shape and calculating the surface free energy. This first installment answers questions about the correct handling of the samples and test liquids as well as suitable ambient conditions. The wettability of a solid by a liquid can dramatically alter even if there is only a slight modification to the chemical or physical properties of the surface. This fact makes the contact angle a sensitive quantity in surface analysis. However, the contact angle also reacts very sensitively to unwanted alterations in the participating phases. Some easy-to-follow tips about sample preparation and information about »pitfalls« in contact angle measurement should ensure that this method produces reliable results. Sample contamination Each solid surface has its own particular history. This is why a »spoilt« surface usually can no longer be saved – after cleaning it is no longer the same surface as before the contamination. This is why unwanted alterations to the surface are to be avoided. Normally the greatest effects are seen when the surface comes into contact with fats and grease or surface-active substances. We focus on these two cases below. Contamination by fats The first important rule for contact angle measurement: never touch the surface with your fingers. Even the slightest traces of grease will have an effect on the contact angle results. KRÜSS GmbH | Borsteler Chaussee 85 | 22453 Hamburg | Germany | www.kruss.d

contact angle on fingerprints on the same sample Fig. 1: Water contact angles on a polycarbonate sample An often unexpected source of contaminants is compressed air produced by a compressor – if you want to dry a surface or remove dust particles by blowing them off with compressed air then it is essential that you use purified air. It is less well known that cigarette smoke is also unhealthy for contact angle measurements. The nicotinic acid contained in the smoke is very surface-active. Although smoking is not allowed in laboratories anyhow, even cigarette smoke from a neighboring office...

Special rules apply for the standard test liquids diiodomethane and ethylene glycol. The first decomposes in time due to the effects of light – this is why it is stored in amber glass bottles; it should no longer be used when a yellow-brown coloration can be clearly seen. In contrast, ethylene glycol – just like other diols or glycerol – is strongly hygroscopic; its surface tension alters as the amount of water it contains increases. Such substances should be stored under dry conditions in airtight containers. The containers used for all test liquids should never be left open for a long...