title: Structured free-water clusters near lubricating surfaces are essential in water-based lubrication.
abstract: Water-based lubrication provides cheap and environmentally friendly lubrication and, although hydrophilic surfaces are preferred in water-based lubrication, often lubricating surfaces do not retain water molecules during shear. We show here that hydrophilic (42° water contact angle) quartz surfaces facilitate water-based lubrication to the same extent as more hydrophobic Si crystal surfaces (61°), while lubrication by hydrophilic Ge crystal surfaces (44°) is best. Thus surface hydrophilicity is not sufficient for water-based lubrication. Surface-thermodynamic analyses demonstrated that all surfaces, regardless of their water-based lubrication, were predominantly electron donating, implying water binding with their hydrogen groups. X-ray photoelectron spectroscopy showed that Ge crystal surfaces providing optimal lubrication consisted of a mixture of -O and =O functionalities, while Si crystal and quartz surfaces solely possessed -O functionalities. Comparison of infrared absorption bands of the crystals in water indicated fewer bound-water layers on hydrophilic Ge than on hydrophobic Si crystal surfaces, while absorption bands for free water on the Ge crystal surface indicated a much more pronounced presence of structured, free-water clusters near the Ge crystal than near Si crystal surfaces. Accordingly, we conclude that the presence of structured, free-water clusters is essential for water-based lubrication. The prevalence of structured water clusters can be regulated by adjusting the ratio between surface electron-donating and electron-accepting groups and between -O and =O functionalities.
authors.affiliation: Department of Biomedical Engineering, University of Groningen and University Medical Center Groningen, PO Box 196, 9700 AD Groningen, The Netherlands.
authors.forename: Jiapeng
authors.initials: J
authors.lastname: Hou
authors.affiliation: Department of Biomedical Engineering, University of Groningen and University Medical Center Groningen, PO Box 196, 9700 AD Groningen, The Netherlands Ducom Instruments Europe B.V., Center for Innovation, 9713 GX Groningen, The Netherlands.
authors.forename: Deepak H
authors.initials: DH
authors.lastname: Veeregowda
authors.affiliation: Department of Biomedical Engineering, University of Groningen and University Medical Center Groningen, PO Box 196, 9700 AD Groningen, The Netherlands.
authors.forename: Joop
authors.initials: J
authors.lastname: de Vries
authors.affiliation: Department of Biomedical Engineering, University of Groningen and University Medical Center Groningen, PO Box 196, 9700 AD Groningen, The Netherlands h.c.van.der.mei@umcg.nl.
authors.forename: Henny C
authors.identifier: 0000-0003-0760-8900
authors.initials: HC
authors.lastname: Van der Mei
authors.affiliation: Department of Biomedical Engineering, University of Groningen and University Medical Center Groningen, PO Box 196, 9700 AD Groningen, The Netherlands.
authors.forename: Henk J
authors.initials: HJ
authors.lastname: Busscher
mesh_terms: D057927:Hydrophobic and Hydrophilic Interactions; D054327:Lubricants / Q000737:chemistry*; D008956:Models, Chemical; D011791:Quartz / Q000737:chemistry*; D012822:Silicon Dioxide / Q000737:chemistry*; D014867:Water / Q000737:chemistry*
chemical_list: D054327:Lubricants; D014867:Water; D011791:Quartz; D012822:Silicon Dioxide
keywords: X-ray photoelectron spectroscopy; attenuated total reflection crystals; contact angles; surface chemical composition; surface thermodynamics; tribochemistry