Hou Jiapeng, Veeregowda Deepak H, de Vries Joop, Van der Mei Henny C, Busscher Henk J
Department of Biomedical Engineering, University of Groningen and University Medical Center Groningen, PO Box 196, 9700 AD Groningen, The Netherlands.
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.
J R Soc Interface. 2016 Oct;13(123). doi: 10.1098/rsif.2016.0554.
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.
水基润滑提供了廉价且环保的润滑方式，尽管在水基润滑中亲水性表面更受青睐，但在剪切过程中润滑表面往往无法保留水分子。我们在此表明，亲水性（水接触角为42°）的石英表面在水基润滑方面与疏水性更强的硅晶体表面（61°）具有相同的促进效果，而亲水性锗晶体表面（44°）的润滑效果最佳。因此，表面亲水性对于水基润滑来说并不充分。表面热力学分析表明，所有表面，无论其水基润滑情况如何，主要都是供电子的，这意味着水与其氢基团结合。X射线光电子能谱显示，提供最佳润滑效果的锗晶体表面由-O和=O官能团的混合物组成，而硅晶体和石英表面仅具有-O官能团。水中晶体的红外吸收带比较表明，亲水性锗表面的结合水层比疏水性硅晶体表面少，而锗晶体表面自由水的吸收带表明，锗晶体附近比硅晶体表面附近存在更明显的结构化自由水簇。因此，我们得出结论，结构化自由水簇的存在对于水基润滑至关重要。结构化水簇的普遍程度可以通过调节表面供电子基团与吸电子基团之间以及-O和=O官能团之间的比例来控制。
