Friction of the Liquid Crystal 8CB As Probed by the Surface Forces Apparatus

Abstract

The surface forces apparatus (SFA) was used to study the lubricating properties of an 8CB liquid crystal film confined between mica surfaces. After displacement of most of the liquid crystal by compression, a 10.2 Å thick layer of 8CB was found between mica surfaces in the range of applied pressures from 1 to 20 MPa. Under these conditions, we found that friction depends linearly on contact area in the 1−1.5 MPa range. We found that the critical shear stress was anisotropic, with values of 0.28 MPa for shear along the 8CB molecular axis and 0.42 MPa for shear in the perpendicular direction. This anisotropy was observed through all the ranges of load where slip occurred. At higher loads (1.5−2.7 MPa), the critical shear stresses increase to 0.6 and 1.2 MPa for parallel and perpendicular shear, respectively.

Boundary lubrication with a liquid crystal monolayer

“…A theoretical explanation of the dependence of the friction force on the molecular orientation has also been proposed [15], indicating that an increase in the shear velocity results in an alignment of the LC molecules with the flow, and consequently in a reduction of the friction * wei.chen@aalto.fi force. Such connections between the friction force and the molecular orientation have also been observed in surface force apparatus experiments on 8CB LCs confined by mica surfaces, where, e.g., an anisotropic critical shear stress has been observed [16]. Moreover, transitions between smooth and complex stick-slip sliding of branched hydrocarbons which can exhibit a liquid crystalline state under confinement have been observed [17].…”

Friction control with nematic lubricants via external fields

“…This is in qualitative agreement with experiments on systems such as 8CB LCs confined by mica surfaces, displaying, e.g., an anisotropic critical shear stress [23]. Thus, a possibility to control friction by tuning the LC orientation by external fields emerges.…”

“…In the context of LC lubricants, also applied pressure and sliding velocity of the confining surfaces are expected to play a role [16][17][18][19][20]. On the other hand, lubrication properties of LCs should depend on their ordering [21][22][23]. Thus, LCs provide a promising system to develop lubricants with controllable properties by applying external fields [24].…”

Dynamics of Layering Transitions in Confined Liquids

“…This leads to strong variations of the transmitted intensity, which are recorded using a CCD video camera with a microscope lens. The mica surfaces are mounted in crossed cylinder geometry inside a surface forces apparatus (SFA) that was described previously [10,11]. Normal forces are generated by magnetic fields produced by coils acting on a permanent magnet attached to a spring-support ͑spring constant ഠ 200 N͞m͒ holding the lower mica sample.…”