In situ observation of the molecular ordering in the lubricating point contact area

Abstract

Stability analysis of ultrathin lubricant films via surface energy measurements and molecular dynamics simulationsThe organization of lubricant molecules confined between two solid surfaces when the lubricant film thickness is at the nanoscale is unknown. In this work, an ordering process of nematic liquid crystal molecules is observed by in situ polarized Raman spectroscopy of the lubricated point contact area. Our experimental results indicate that 4-n-pentyl-4 0 -cyanobiphenyl liquid crystal molecules orient along the rotation direction when the linear speed exceeds 12.6 mm/s, and the degree of order increases with linear speed. The relationship between the observed orientation and physical properties of the lubricant film is investigated. Isotropic orientation is observed at the outlet area of the contact region. The orientation behavior of liquid crystal molecules in a confined area is observed and the relationship between lubrication conditions and molecular orientation is discussed. V C 2014 AIP Publishing LLC. [http://dx.

Advances in thin film lubrication (TFL): From discovery to the aroused further researches

“…Raman spectrum is one of the most popular analysis techniques, as a result of the facility, high accuracy and tolerance to materials, which has been also utilized to achieve the aim of in-situ molecular monitoring during friction and shearing. Relative investigations on such systems have been reported by several groups separately, such as Granick and coworkers [52,53], Bongaerts et al [54], and Luo's group [55,56] (see Figure 6). By using such in-situ detection approach, the detection of ordered film in nano-confinement has been enabled.…”

“…By using such in-situ detection approach, the detection of ordered film in nano-confinement has been enabled. Zhang et al [55,56] investigated the behavior of confined liquid crystal molecules during sliding and rolling on the equipment based on polarization Raman spectroscopy, as shown in Figure 6. Both the alignment and the orientation were indicated to be closely related to the friction conditions.…”

Thin film lubrication in the past 20 years

“…Furthermore, by introducing the polarization Raman spectra to the tribometer system, as shown in Fig. 7(a), Luo's group [74,75] has successfully detected the molecular structure of both polar and nonpolar lubricant molecules in nano lubrication films, as shown in Fig. 7(b).…”

“…It should be noted that the results obtained during the lubricating process will be of great significance. The orientation of 5CB molecules varies with the speed and film thickness [74,90]. When the lubrication regime is transferred from TFL to EHL, the detected 5CB molecules change orientation, from perpendicular to the surface to parallel to the rolling direction, as a result of the shear effect.…”

“…Zhang et al [74,90] found that the degree of order of molecules in nano lubricating film is significantly affected by the polarity of the lubricant molecules. To measure both the film thickness and the molecular structures simultaneously, an in-situ system was developed by combing the interferometer and Raman spectra technique.…”