Dynamics of thin films through molecular simulations
File(s)
Author(s)
Rahman, Muhammad Rizwanur
Type
Thesis
Abstract
Thin liquid films play pivotal roles in various natural phenomena and engineering processes. Yet, our understanding is limited by the scarcity of atomic insights. This work investigates the nanoscopic details of thin films and their rupture, and the transport of surfactants along the film surfaces. At this scale, experimental investigations are prohibitive; and conventional theoretical approaches are inadequate due to their reductive approximations. Therefore, this work employs molecular dynamics simulations which enables the examination at the most fundamental scale.
Present investigations of film interfaces through stress-cluster analysis reveals that higher temperature results in a more disconnected surface stress-field, contributing to an overall decrease in surface tension. Local variation of surface tension is responsible for Marangoni convection leading to rupture. The processes by which these rupture sites emerge - traditionally categorized as spinodal and heterogeneous rupture - are found to share similar molecular origin. Present study on the growth of rupture sites identified the limitations of continuum scale theories in explaining retraction rates for thinner films, and corrections are proposed that rectify the discrepancies. Notably, a spatio-temporal memory of rupture is identified, highlighting the concept of deterministic outcomes emerging from stochastic processes.
Finally, the transport mechanism of surfactant molecules, along the deforming surfaces of thin films and droplets, is investigated. The continuum scale transport equation is solved by employing a finite difference scheme, incorporating inputs from MD simulations. This model provides confirmation of the validity of the transport equation at the nanoscale. By uniquely confirming the applicability of the transport equation for a molecularly thin film, this study elucidates the long-debated relationship between the continuum and the nanoscale. Together, the findings from this research represent an important step in bridging the physics of fluids across scales, offering new insights to the dynamics of thin films.
Present investigations of film interfaces through stress-cluster analysis reveals that higher temperature results in a more disconnected surface stress-field, contributing to an overall decrease in surface tension. Local variation of surface tension is responsible for Marangoni convection leading to rupture. The processes by which these rupture sites emerge - traditionally categorized as spinodal and heterogeneous rupture - are found to share similar molecular origin. Present study on the growth of rupture sites identified the limitations of continuum scale theories in explaining retraction rates for thinner films, and corrections are proposed that rectify the discrepancies. Notably, a spatio-temporal memory of rupture is identified, highlighting the concept of deterministic outcomes emerging from stochastic processes.
Finally, the transport mechanism of surfactant molecules, along the deforming surfaces of thin films and droplets, is investigated. The continuum scale transport equation is solved by employing a finite difference scheme, incorporating inputs from MD simulations. This model provides confirmation of the validity of the transport equation at the nanoscale. By uniquely confirming the applicability of the transport equation for a molecularly thin film, this study elucidates the long-debated relationship between the continuum and the nanoscale. Together, the findings from this research represent an important step in bridging the physics of fluids across scales, offering new insights to the dynamics of thin films.
Version
Open Access
Date Issued
2024-08-07
Date Awarded
01/11/2024
License URL
Advisor
Dini, Daniele
Smith, Edward
Ewen, James
Sponsor
Beit Trust
Shell International Ltd
Publisher Department
Mechanical Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)