Polymer spreading on substrates with nanoscale grooves using molecular dynamics

Abstract

Understanding how liquid polymer interacts with and spreads on surfaces with nanoscale texture features is crucial for designing complex nanoscale systems. We use molecular dynamics to simulate different types of polymer as they spread on substrates with a single nanoscale groove. We study how groove design affects the potential energy of a substrate and how this governs polymer spreading and orientation. Based on our simulations, we show that groove shape, polymer chemistry, and polymer molecule entanglement are the three parameters that determine polymer spreading on a nanoscale groove. We provide a molecular-level explanation of the underlying physical mechanisms, and we illustrate this fundamental understanding by designing a network of grooves to engineer user-specified polymer spreading and coverage. This work has implications for nanoscale systems and devices that involve the design of complex groove networks with an ultrathin polymer coating, including micro and nanoelectromechanical devices, nanoimprint lithography, flexible electronics, antibiofouling coatings, and hard disk drives.

Polymer Spreading on Unidirectionally Nanotextured Substrates Using Molecular Dynamics

“…The polymer spreads in a single groove and d ⊥ is unaffected by the texture, as we have previously reported in detail. 15 Figure S2 illustrates the number of grooves and peaks the polymer encounters during spreading on several example substrates. However, although perpendicular spreading depends on the number of grooves the polymer encounters, the groove shape is the dominant factor that determines anisotropic polymer spreading.…”

“…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.…”

“…The ability to create anisotropic spreading in particular, which enables liquid spreading to occur in a user-specified direction, finds application in micro/nanofluidics, flexible electronics, and drag reduction surfaces . Several research groups have shown both experimentally and numerically that unidirectionally textured substrates lead to anisotropic spreading, where liquids spread preferentially in the direction parallel to the texture. − Anisotropic spreading on unidirectionally textured substrates is most commonly explained by texture peaks, which impose a mechanical barrier that effectively “pins” the three-phase (solid/liquid/gas) contact line of the spreading liquid, inhibiting spreading in the direction perpendicular to the texture. − ,− The texture peaks are often described as energetic barriers that oppose wetting, which Yong and Zhang attribute to a high local atomic density at texture peaks, based on the results of molecular dynamics (MD) simulations (texture height of 1.85-23.77 Å). The liquid may be confined by texture peaks in the perpendicular direction and “squeezed” in the parallel direction.…”

“…The liquid may be confined by texture peaks in the perpendicular direction and “squeezed” in the parallel direction. Alternatively, the liquid may only be slowed by texture peaks, causing the contact line to advance in a stick-slip manner, pinning-depinning-repinning on subsequent texture peaks. ,,,, Conversely, others propose that anisotropic spreading is caused by texture grooves that accelerate spreading along the direction parallel to the texture, i.e., along the grooves. ,,− Fischer et al attribute this accelerated spreading in part to capillary action inside the grooves, based on experiments with sinusoidal textures (height of 8 or 16 μm). In addition, Zhang et al used Monte Carlo simulations and experiments (square-shaped textures with a height of 0.03-0.05 μm) to explain that an increased surface force exists inside a groove because it is surrounded by as many as three surfaces; i.e., a square groove is composed of a base and two side-walls.…”

Influence of Trapezoidal Cavity on the Wettability of Hydrophobic Surface: A Molecular Dynamics Study

“…Currently, there are many studies on surface wetting with different surface textures by molecular dynamics simulation because of its convenience and accuracy. − Noble et al studied the spreading behaviors of polymer on a substrate with a single groove and found that groove shape is one of the most important factors that determines the spreading behaviors of polymer. The transition between Wenzel state and Cassie state of droplets on a rough surface is studied by Koishi et al; the height of nanopillars and the spacing between nanopillars have a great impact on the transition between Wenzel state and Cassie state, and the free-energy barrier between the Wenzel state and Cassie state becomes higher with higher nanopillars.…”