Boundary lubrication with a liquid crystal monolayer

We study boundary lubrication characteristics of a liquid crystal (LC) monolayer sheared between two crystalline surfaces by nonequilibrium molecular dynamics simulations, using a simplified rigid bead-necklace model of the LC molecules. We consider LC monolayers confined by surfaces with three different atomic structures, subject to different shearing velocities, thus approximating a wide variety of materials and driving conditions. The time dependence of the friction force is studied and correlated with that of the orientational order exhibited by the LC molecules, arising from the competition between the effect of the structure of the confining surfaces and that of the imposed sliding direction. We show that the observed stick-slip events for low shear rates involve order-disorder transitions, and that the LC monolayer no longer has enough time to reorder at high shear rates, resulting in a smooth sliding regime. An irregular stick-slip phase between the regular stick-slip and smooth sliding is observed for intermediate shear rates regardless of the surface structure.

Figure 1

Top: Sketch of the rigid bead-necklace molecule, consisting of nine uniformly spaced interaction sites (beads) with the total length of the molecule equal to $4.8$ in the reduced units considered here (see text). Bottom: The geometry of the simulation system, with the $y$ axis normal to the page. Blue molecules are the LC mesogens, confined by two crystalline plates (black beads) to form a monolayer. The bottom plate is connected to a fixed point by a spring, while the top plate is subject to a normal force $F_n$, and is moving with a constant velocity $V$ along the $x$ direction.

Figure 2

Atomic structure of the three confining surfaces considered. (a) (100) surface. (b) (110) surface. (c) (111) surface.

Figure 3

The spring forces obtained at different driving velocities for LC molecules confined by (a) (100) surfaces, (b) (110) surfaces, and (c) (111) surfaces.

Figure 4

The velocities of the bottom plate at different driving velocities for LC molecules confined by (a) (100) surfaces, (b) (110) surfaces, and (c) (111) surfaces.

Figure 5

The Fourier transform of the spring forces at different driving velocities for LC molecules confined by (a) (100) surfaces, (b) (110) surfaces, and (c) (111) surfaces.

Figure 6

The snapshots of the LC monolayer confined by the (111) surfaces for the driving velocity of $V=0.01$.

Figure 7

The snapshots of the LC monolayer confined by the (110) surfaces for the driving velocity of $V=0.01$.

Figure 8

The snapshots of the LC monolayer confined by the (100) surfaces for the driving velocity $V=0.01$.

Figure 9

LC molecules confined by the (111) surfaces. The figures on the top row are the spring forces at different driving velocities: (a) $V=0.02$, (b) $V=0.03$. The figures in the middle row are the probabilities $P$ to find two parallel LC molecules. The figures on the bottom row show the time evolution of the average domain size.

Figure 10

LC molecules confined by (110) surfaces, with (a) $V=0.02$ and (b) $V=0.03$. The figures in the top row display the spring forces, while those in the bottom row show the corresponding values of the order parameter $S$.

Figure 11

The distribution of $|cos(\theta \left)\right|$ for LC molecules confined by (a) the (100) surfaces, (b) the (110) surfaces, and (c) the (111) surfaces.

Figure 12

The planar pair distribution function $\rho \left(r\right)$ of parallel LC molecules for a LC monolayer confined by (a) the (100) surfaces, (b) the (110) surfaces, and (c) the (111) surfaces.