The Influence of the Mechanical Compliance of a Substrate on the Morphology of Nanoporous Gold Thin Films.

Nanoporous gold (np-Au) has found its use in applications ranging from catalysis to biosensing, where pore morphology plays a critical role in performance. While the morphology evolution of bulk np-Au has been widely studied, knowledge about its thin-film form is limited. This work hypothesizes that the mechanical compliance of the thin film substrate can play a critical role in the morphology evolution. Via experimental and finite-element-analysis approaches, we investigate the morphological variation in np-Au thin films deposited on compliant silicone (PDMS) substrates of a range of thicknesses anchored on rigid glass supports and compare those to the morphology of np-Au deposited on glass. More macroscopic (10 s to 100 s of microns) cracks and discrete islands form in the np-Au films on PDMS compared to on glass. Conversely, uniformly distributed microscopic (100 s of nanometers) cracks form in greater numbers in the np-Au films on glass than those on PDMS, with the cracks located within the discrete islands. The np-Au films on glass also show larger ligament and pore sizes, possibly due to higher residual stresses compared to the np-Au/PDMS films. The effective elastic modulus of the substrate layers decreases with increasing PDMS thickness, resulting in secondary np-Au morphology effects, including a reduction in macroscopic crack-to-crack distance, an increase in microscopic crack coverage, and a widening of the microscopic cracks. However, changes in the ligament/pore widths with PDMS thickness are negligible, allowing for independent optimization for cracking. We expect these results to inform the integration of functional np-Au films on compliant substrates into emerging applications, including flexible electronics.

Balance of Solvent and Chain Interactions Determines the Local Stress State of Simulated Membranes.

Characterization of the internal mechanical state of model lipid membranes is essential to understand the microscopic underpinnings of biological functions such as membrane fission and organelle shaping within the context of elastic theories such as the Helfrich framework. Here, we compute lateral stress or pressure profiles from molecular dynamics simulations of lipid bilayers and water-vacuum interfaces to understand the role that solvent treatment and force-field parametrization plays on the local mechanical features of membranes. We focus on two atomistic models, GROMOS 43A1-S3 and CHARMM36, and several variants of the MARTINI coarse-grained force-field, including the single-bead nonpolar water, three-point polarizable water, big multipole water, and solvent-free variants. Our results show that the various atomistic and coarse-grained force-fields produce contrasting lateral stress profiles as a result of the balance of solvent-solvent and solvent-solute forces at the hydrocarbon-water interface and fundamentally different treatment of pairwise (e.g., van der Waals, Coulomb, etc.) and multibody interactions (angles and torsions). Numerical integration of the second moment of the bilayer stress profiles indicates that different local distributions of repulsive and attractive stresses across the membrane, due to distinct force-field parametrizations, may result in substantial variations in macroscopic elastic properties.