Structure and Undulations of Escin Adsorption Layer at Water Surface Studied by Molecular Dynamics

The saponin escin, extracted from horse chestnut seeds, forms adsorption layers with high viscoelasticity and low gas permeability. Upon deformation, escin adsorption layers often feature surface wrinkles with characteristic wavelength. In previous studies, we investigated the origin of this behavior and found that the substantial surface elasticity of escin layers may be related to a specific combination of short-, medium-, and long-range attractive forces, leading to tight molecular packing in the layers. In the current study, we performed atomistic molecular dynamics simulations of 441 escin molecules in a dense adsorption layer with an area per molecule of 0.49 nm2. We found that the surfactant molecules are less submerged in water and adopt a more upright position when compared to the characteristics determined in our previous simulations with much smaller molecular models. The number of neighbouring molecules and their local orientation, however, remain similar in the different-size models. To maintain their preferred mutual orientation, the escin molecules segregate into well-ordered domains and spontaneously form wrinkled layers. The same specific interactions (H-bonds, dipole–dipole attraction, and intermediate strong attraction) define the complex internal structure and the undulations of the layers. The analysis of the layer properties reveals a characteristic wrinkle wavelength related to the surface lateral dimensions, in qualitative agreement with the phenomenological description of thin elastic sheets.

Partial wetting of thin solid sheets under tension

We consider the equilibrium of liquid droplets sitting on thin elastic sheets that are subject to a boundary tension and/or are clamped at their edge. We study the geometrical–mechanical interplay through which the capillary force exerted by the droplet at the contact line modifies a pre-existing stress within the sheet, and characterize the parameter regimes in which this effect is large (non-perturbative) or small (perturbative).

Instability of thin films

We will first discuss the stability of liquid films deposited on solid surfaces with an emphasis on the nature of intermolecular forces and thermal fluctuations that conspire to generate complex morphologies. We will see how the global dewetting dynamics is driven by the solid–fluid interface and that dewetting can be a powerful tool to study the nanorheology of complex fluids, such as polymer melts in ultra thin films. In the second part, we will consider thin elastic sheets constrained by mechanical forces. The canonical example of such a system is given by a simple paper ball. We will see how the global geometry of these constraints drastically affects the final shape adopted by the sheet.

Kirigami actuators

Carefully tuning the location and arrangement of cuts within thin elastic sheets enables the design of mechanical actuators that scale down to atomically-thin 2D materials.