flairsLab - Film Rupture

Rupture Dynamics of Thin Films

The phases in a film's lifetime, dewetting and fluid memory

This study reveals that irrespective of the microscopic randomness, certain macroscopic outcomes are deterministic. We observed a short-term memory window - during the otherwise chaotic film break up process - when a future rupture event becomes already imprinted.

Abstract: Thin films, bubbles and membranes are central to numerous natural and engineering processes, i.e., in thin-film solar cells, coatings, biosensors, electro-wetting displays, foams, and emulsions. Yet, the characterization and an adequate understanding of their rupture is limited by the scarcity of atomic detail. We present here the complete life-cycle of freely suspended films using non-equilibrium molecular dynamics simulations of a simple atomic fluid free of surfactants and surface impurities, thus isolating the fundamental rupture mechanisms. Counter to the conventional notion that rupture occurs randomly, we discovered a short-term 'memory' by rewinding in time from a rupture event, extracting deterministic behaviors from apparent stochasticity. A comprehensive investigation of the key rupture-stages including both unrestrained and frustrated propagation is made - characterization of the latter leads to a first-order correction to the classical film-retraction theory. Furthermore, the highly resolved time window reveals that the different modes of the morphological development, typically characterized as heterogeneous nucleation and spinodal decomposition, continuously evolve seamlessly with time from one into the other.

Read this work on Communications Physics, Nature.

The Taylor-Culick Retraction

Once initiated, how a defect in a liquid film propagates over time and at what speed the film collapses have been investigated for decades. The mathematical frameworks developed over the years were successful in predicting the experimental observations, but only for thicker films, and measurably failed for the case of nanoscopic thin-films. This study filled the gap down to atomic scale.

Abstract: The rupture of thin films is described by the Taylor–Culick (TC) theory, but this is subject to widespread debate, particularly for films at the nanoscale. We use non-equilibrium molecular dynamics simulations to explore the validity of the assumptions used in continuum models by tracking the evolution of holes in a film. By deriving a new mathematical form for the surface shape and considering a locally varying surface tension at the front of the retracting film, we reconcile the original theory with our simulation to recover a corrected TC speed valid at the nanoscale.

Intrinsic Interface & A Mechanistic View of Surface Tension

The liquid-vapor interface is overwhelmingly abundant in our everyday life, but insanely complicated when looked at in detail. Specially, when zoomed in such that the molecular details become visible, the interface gets blurred by thermal motion. This investigation avoids the blurring by looking at the Intrinsic Interface. This also presents a mechanical perspective of the surface tension utilising stress networks, percolation theory and fractals.

Abstract: The evolution of the liquid–vapor interface of a Lennard-Jones fluid is examined with molecular dynamics simulations using the intrinsic sampling method. Results suggest clear damping of the intrinsic profiles with increasing temperature. Investigating the surface stress distribution, we have identified a linear variation of the space-filling nature (fractal dimension) of the stress clusters at the intrinsic surface with increasing surface tension or, equivalently, with decreasing temperature. A percolation analysis of these stress networks indicates that the stress field is more disjointed at higher temperatures. This leads to more fragile (or poorly connected) interfaces which result in a reduction in surface tension.