Terraced spreading of nanometer-thin lubricant using molecular dynamics

Abstract

6Ultra-thin lubricant films are essential in the design of nanoscale systems and devices as surface 7 effects become increasingly important on the nanoscale. We have used molecular dynamics simulations to 8 quantify terraced spreading of perfluoropolyether lubricant on a flat substrate as a function of polymer 9 chain length, lubricant thickness, and functional end groups of the lubricant and the substrate. In addition, 10we have investigated the physical mechanisms that drive terraced lubricant spreading on a flat substrate. 11The results show that terraced lubricant spreading follows a process of diffusion and instability, where 12 functional lubricant end groups are attracted to other functional end groups to form clusters that organize 13 into layers. These distinct layers of functional end groups cause the lubricant thickness profile to take on a 14 terraced shape, where layers correspond to the locations at which terraced formations occur. The presence 15 of functional end groups determines the locations of both layer and terrace formations, and greatly affects 16 lubricant spreading. 17

Spreading Kinetics of Ultrathin Liquid Films Using Molecular Dynamics

“…This agrees with previously reported velocity profiles that indicate slip of the precursor film at the substrate interface. 32 We also note that our observation of ν ≈ 1/3 is in agreement with the theoretical predictions of Liao et al 18 for a precursor film spreading from a droplet in the case of continuum flow driven by disjoining pressure and with intensive slip at the liquid−solid interface, the same conditions verified in our simulations.…”

“…We simultaneously evaluate the droplet thickness profile to determine whether a central droplet is present (see insets), which we define as a thickness greater than the precursor film thickness of 3σ, where σ is the diameter of one bead. 32 In this paper, we adopt the convention in Figures 2−4 that solid and hollow markers indicate that a central droplet is present or has depleted, respectively. We also depict in one of the insets in Figure 2a a molecular view of the precursor film that has spread to a thickness of approximately one bead.…”

“…The polymer molecules do not remain entangled because a molecule length of N = 10 is below the critical entanglement length, initially established for a primitive chain by Kremer et al 36 and previously identified for our model as N ≈ 20. 32,37 The insets in Figure 2b show the corresponding pressure (side view, averaged over 500 000 time steps) and entanglement (top view) maps (color scale provided). The pressure map insets depict an area of high pressure at the center base of the droplet at the inception of spreading, which creates a pressure difference that drives the molecules outward.…”

Polymer Spreading on Unidirectionally Nanotextured Substrates Using Molecular Dynamics

“…The bead mass is 0.2 kg/mol; thus, the molecular weight is 2 kg/mol. The potential function interactions that define the system are similar to validated potentials used in previous research and are discussed in detail in the Supporting Information (SI). ,− ,− Gravity is negligible at this scale and, therefore, not included in our model.…”

“…However, no publications seem to exist that provide a complete molecular-level explanation of how nanoscale textures affect polymer spreading and how texture peaks and texture grooves alter the wettability of a substrate. Thus, the objective of this work is to quantify polymer spreading on unidirectional nanotextured substrates and to provide a molecular-level explanation of polymer spreading on nanotextured substrates, as compared to a flat substrate, which we have studied previously. − We perform MD simulations of liquid polymer spreading on rigid substrates with unidirectional nanotexture features of various shapes and sizes.…”