Friction Anisotropy and Asymmetry of a Compliant Monolayer Induced by a Small Molecular Tilt

Abstract

Lateral force microscopy in the wearless regime was used to study the friction behavior of a lipid monolayer on mica. In the monolayer, condensed domains with long-range orientational order of the lipid molecules were present. The domains revealed unexpectedly strong friction anisotropies and non-negligible friction asymmetries. The angular dependency of these effects correlated well with the tilt direction of the alkyl chains of the monolayer, as determined by electron diffraction and Brewster angle microscopy. The molecular tilt causing these frictional effects was less than 15 degrees, demonstrating that even small molecular tilts can make a major contribution to friction.

Material Anisotropy Revealed by Phase Contrast in Intermittent Contact Atomic Force Microscopy

“…Recently, anisotropic properties have been studied at the nanometer scale with atomic force microscopy (AFM) [1][2][3][4][5][6]. Measuring anisotropies with AFM requires breaking the rotational symmetry of the tip-sample interaction.…”

Friction between solids

“…Monolayers formed from self-assembly, such as alkanethiols and alkylsilanes, which are covalently bonded to the substrate, may be good candidates for boundary-layer lubricants. As a result, the mechanical and tribological properties of self-assembled monolayer (SAM) materials have been studied a great deal using scanning probe microscopies (Carpick & Salmeron 1997;Liley et al 1998;Harrison et al 2004 The properties of SAMs containing chains with diacetylene moieties have been studied using the AFM (Mowery et al 1999;Cheadle et al 2001). Because the crosslinked backbone (which is the result of exposure to UV radiation) demonstrates a chromatic response to elevated temperature and stress, these structures have the potential to be used as molecular sensors (Enkelmann 1984;Burns et al 2001).…”

“…For example, Liley et al (1998) used a lateral force microscope to measure friction of a lipid monolayer in the wearless regime. The monolayer contained domains with long-range orientational order and larger friction anisotropies and non-negligible friction asymmetries were measured.…”

Contribution of Slip and Cleavage in Friction and Wear at (1 0 -1 4) Surface of Magnesite (MgCO₃) Crystal

“…In nano-scale friction, however, clear asymmetry has been observed at single crystal surfaces having tilted S = O bonds of sulfate ions [12][13][14][15] , C = O bonds of carbonate ions 16) , or long-chain organic molecules adsorbed [17][18][19] . The asymmetry was explained using mechanical models of mass-spring systems 15,16) .…”