Structured free-water clusters near lubricating surfaces are essential in water-based lubrication

Jiapeng Hou, Deepak H Veeregowda, Joop de Vries, Henny C Van der Mei, Henk J Busscher

Research output: Contribution to journalArticleAcademicpeer-review

2 Citations (Scopus)

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.

Original languageEnglish
Article number20160554
Number of pages8
JournalJournal of the Royal Society Interface
Volume13
Issue number123
DOIs
Publication statusPublished - 1-Oct-2016

Keywords

  • tribochemistry
  • attenuated total reflection crystals
  • surface thermodynamics
  • contact angles
  • X-ray photoelectron spectroscopy
  • surface chemical composition
  • BOUNDARY
  • PASSIVATION
  • MEMBRANES
  • FRICTION

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