Low sliding angles in hydrophobic and oleophobic coatings prepared with plasma discharge method Mirjami Kiuru * , Esa Alakoski Department of Physical Sciences, Accelerator Laboratory, University of Helsinki, P.O. Box 64, Gustaf Hallstromin katu 2 FIN-00014 Helsinki, Finland Received 5 November 2003; accepted 31 January 2004 Available online 9 March 2004 Abstract Extremely low sliding angle, 0.15 F 0.03j with 20 Al distilled water droplet, was measured on diamond-like carbon – polydimethylsiloxane hybrid (DLC – PDMS-h) coating (advancing and receding contact angles for water were h A /h R = 101j/99j). Diamond-like carbon –polymer hybrid (DLC– p-h) coatings were prepared with filtered pulsed arc discharge (FPAD) method using a graphite – polymer cathode. The polymers used in the hybrid coatings were polydimethylsiloxane (PDMS) and polytetrafluoroethylene (PTFE). Contact and sliding angles of these hydro- and oleophobic (i.e., water and oil repellent) new coatings were measured with water and oil (hexadecane and cooking oil). D 2004 Elsevier B.V. All rights reserved. Keywords: Hydrophobic phenomena; Contact angle; Sliding angle; DLC– p-h coating; Plasma discharge; Filtered pulsed arc discharge (FPAD) method 1. Introduction The Helsinki University Diamond Group has a long history in preparing thick, well-adherent and high-quality DLC 1 coatings [1–11]. Recently, we have studied surface modification, especially on non-stick and anti-soiling coat- ings. With the modified filtered pulsed arc discharge (FPAD) method developed in our laboratory, we have prepared coatings with low surface energy. These dia- mond-like carbon–polymer hybrid (DLC–p-h) coatings are hydro- and oleophobic [12]. Hardness of these coatings depends on the amount of diamond bonds in the final coating and this in turn depends on the process parameters during the deposition. Contact angle is typically the property measured for the non-stick coatings to estimate their surface energy. When surface energy is low, contact angle is high. A surface is said to be hydrophobic when the water contact angle is over 90j. The term oleophobicity is broader, and no strict definition can be found in the literature (oils tend to spread on surfaces, and they have much smaller contact angles than water has). Theories and studies over several decades concerning wettability can be found [13–25]. A water contact angle of only around 120j is achieved with material of the lowest surface energy (surface with regularly aligned closest hex- agonal packed –CF 3 groups) [18]; after this, any increase in contact angle is due to surface topography [19–23]. Surface roughness contributes to contact angle as the drop is pinned on the surface. The contact line of the drop can have a complex shape because of surface geometry or chemical heterogeneity, and this has its effect on contact angle. Chen et al. [19] state that if the surface really repels the droplet on it, the contact angle hysteresis is small, and the drop moves spontaneously or easily on horizontal or near- horizontal surfaces. Murase and Fujibayashi [24] also found that higher contact angles do not always correlate with smaller sliding angles, which would mean that hydropho- bicity has to be distinguished from the true repellency of water droplets. Our results have agreed with this, and the next step in our work with anti-soiling surfaces was to study sliding angles, i.e., the critical angle at which a droplet of 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.01.024 * Corresponding author. Tel.: +358-9-19150646; fax: +358-9- 19150736. E-mail address: mirjami.kiuru@helsinki.fi (M. Kiuru). 1 We use the more familiar and general term diamond-like carbon (DLC) instead of tetrahedral amorphous carbon (ta-C). www.elsevier.com/locate/matlet Materials Letters 58 (2004) 2213 – 2216