Frictional Dissipation Pathways Mediated by Hydrated Alkali Metal Ions

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• 作者：Anastasia Gaisinskaya-Kipnis ; Liran Ma ; Nir Kampf ; Jacob Klein
• 刊名：Langmuir
• 出版年：2016
• 出版时间：May 17, 2016
• 年：2016
• 卷：32
• 期：19
• 页码：4755-4764
• 全文大小：578K
• 年卷期：0
• ISSN：1520-5827

文摘

Frictional energy dissipation between sliding solid surfaces in aqueous media may proceed by different pathways. Using a surface force balance (SFB), we have examined systematically how such dissipation is mediated by the series of hydrated cations M⁺ = Li⁺, Na⁺, and K⁺ that are trapped between two atomically smooth, negatively charged, mica surfaces sliding across the ionic solutions over many orders of magnitude loading. By working at local contact pressures up to ca. 30 MPa (∼300 atm), up to 2 orders of magnitude higher than earlier studies, we could show that the frictional dissipation at constant sliding velocity, represented by the coefficient of sliding friction μ_M+, decreased as μ_Li+ > μ_Na+ ≳ μ_K+. This result contrasts with the expectation (in conceptual analogy with the Hofmeister series) that the lubrication would improve with the extent of ionic hydration, since that would have led to the opposite μ_M+ sequence. It suggests, rather, that frictional forces, even in such simple systems, can be dominated by rate-activated pathways where the size of the hydration shell becomes a dissipative liability, rather than by the hydration-shell dissipation expected via the hydration lubrication mechanism.