Quantifying lubricant droplet spreading on a flat substrate using molecular dynamics

Abstract

Understanding the physical behavior of polymer-based lubricants on the nanoscale is of critical importance to a myriad of engineering applications and devices. We have used molecular dynamics simulations to quantitatively evaluate the physical mechanisms underlying perfluoropolyether lubricant spreading on a solid substrate. We quantify the effect of molecular mass, molecule length, and lubricant and substrate functional end groups on lubricant spreading. The results show that lubricant functional end groups play a critical role in lubricant spreading on the nanoscale. Lubricant spreading increases with increasing molecule length for lubricant with functional end groups, but decreases with the increase in molecule length for lubricant without functional end groups. In the former case, the fraction of the lubricant chain that is functional is the primary driving factor for lubricant spreading, while in the latter case, the molecular mass is most important. For both lubricants with and without functional end groups, spreading is inhibited by molecule entanglement beyond a critical molecule length, and spreading becomes independent of lubricant functional end groups and molecular mass.

Spreading Kinetics of Ultrathin Liquid Films Using Molecular Dynamics

“…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 substrates with nanoscale grooves using molecular dynamics

“…However, controlling spreading and coverage of an ultrathin liquid polymer film is challenging because surface forces, as opposed to body forces like gravity, dominate on the nanoscale. Several research groups, including ours, have studied polymer spreading on a flat substrate [5][6][7][8][9], whereas only a few research groups have studied polymer spreading on a surface with nanoscale topographical features, despite evidence that such features significantly affect polymer spreading.…”

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.…”