Squeeze-Out of Branched Alkanes on Graphite

Abstract

We study squalane and heptamethylnonane (HMN) confined between a conducting atomic force microscope tip and a graphite surface. Solvation layering occurs for both liquids but marked differences in the squeeze out mechanics are observed for ordered or disordered monolayers. The squalane monolayer at 25 C is an ordered solid, as verified by direct imaging, and the squeeze out can be modeled using elastic continuum mechanics. HMN is in a disordered state at 25 C and cannot be modeled as a single elastic asperity even in solid-solid contact because HMN liquid is trapped in the contact zone.

Lubricated friction around nanodefects

“…1). This view is also supported by computer simulations and other experimental studies that indicate a melting of the solid interfacial squalane layer in contact with graphite between 325 and 338 K (27,50).…”

“…Consequently, the data points at 298 K have been excluded. No clear dependence of  d on v (C) and T (D) is observed within experimental error, except for a local maximum around 323 K, which is ascribed to local melting instabilities in the confined squalane layer (27,50). Linear fitting of the data at each temperature yields no systematic trend (C).…”

Computational simulations of solvation force and squeezing out of dodecane chain molecules in an atomic force microscope

“…AFM tip is modeled by a gold tip to reflect the surface chemistry of the tip according to AFM experimental procedures. 22,23,51 We use the embedded atom method (EAM) potential to describe the dynamics of gold atoms in the AFM tip. The EAM model is an updated version developed by the force matching method.…”

Direct Observation of the Dynamics of Self-Assembly of Individual Solvation Layers in Molecularly Confined Liquids

“…Crucially, the distance between two consecutive jumps informs on the precise orientation of each molecular layer with respect to the surface. This experimental approach has revealed invaluable information about the molecular packing properties of a variety of chemically distinct liquids, encompassing nonpolar liquids with spherical structure such as octamethylcyclotetrasiloxane (OMCTS) [5], linear and branched alkanes [6,7], polar liquids such as short alcohols [8], a variety of ionic liquids [9,10], and even water [11,12], all exhibiting a priori similar force-induced rupture mechanisms. This progress notwithstanding, the reverse process encompassing the reversible reformation of each individual molecular layer, together with the quantification of the number of interactions involved in the rupture and reformation processes, remained completely elusive.…”