Direct observation of the velocity contribution to friction in monolayer-coated micromachines

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• 作者：Sameer S. Shroff ; Maarten P. de Boer ; ^{mpdebo@andrew.cmu.edu" class="auth_mail" title="E-mail the corresponding author}
• 关键词：Friction ; MEMS ; Monolayers ; Micromachines
• 刊名：Extreme Mechanics Letters
• 出版年：2016
• 出版时间：September 2016
• 年：2016
• 卷：8
• 期：Complete
• 页码：184-190
• 全文大小：1260 K

文摘

Using a newly developed in-situ platform combined with high-speed imaging up to 200,000 frames per second, we measure friction versus velocity in a micromachined interface from 0.1 to 3000 class="mathmlsrc">title="View the MathML source" class="mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S2352431616300256&_mathId=si15.gif&_user=111111111&_pii=S2352431616300256&_rdoc=1&_issn=23524316&md5=13114441693647b0f06b559b7b2e7bf7">class="imgLazyJSB inlineImage" height="10" width="10" alt="View the MathML source" style="margin-top: -5px; vertical-align: middle" title="View the MathML source" src="/sd/grey_pxl.gif" data-inlimgeid="1-s2.0-S2352431616300256-si15.gif">class="mathContainer hidden">class="mathCode">$\mu$m/s. The steady-state friction coefficient (friction at constant velocity) of a boundary lubricant-coated interface decreases up to 300 class="mathmlsrc">title="View the MathML source" class="mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S2352431616300256&_mathId=si15.gif&_user=111111111&_pii=S2352431616300256&_rdoc=1&_issn=23524316&md5=13114441693647b0f06b559b7b2e7bf7">class="imgLazyJSB inlineImage" height="10" width="10" alt="View the MathML source" style="margin-top: -5px; vertical-align: middle" title="View the MathML source" src="/sd/grey_pxl.gif" data-inlimgeid="1-s2.0-S2352431616300256-si15.gif">class="mathContainer hidden">class="mathCode">$\mu$m/s, then increases. The friction increase is confirmed in an independent test in which velocity is increased instantaneously. Previous single asperity molecular dynamics simulations of monolayer boundary lubricants indicate that friction is independent of velocity, while experiments and simulations of periodic potentials indicate a monotonically increasing trend. We directly bridge spatial and temporal scales using rate–state constitutive laws, and attribute the decrease in friction to the decrease in asperity lifetime (a state effect), and the increase to the emerging dominance of interfacial viscosity (a rate effect).