Numerical Investigation of Thermal-driven Thin Film Instabilities

• Author / Creator

  Mohammadtabar, Ali

• Instability, morphology, and dewetting of thin films have been the subject of intensive research due to their important industrial and technological applications, including decorative or protective coatings, an intervening media in liquid-liquid emulsions, and polymer films in soft lithography. Thin-film instability can be triggered either naturally through intermolecular forces or with external forces such as mechanical, thermal, and electric forces. Thermal-induced patterning of ultra-thin liquid films subjected to temperature gradient is investigated by characterizing the dynamics, instability, and pattern formation process. The majority of existing studies in the literature are restricted to lubrication approximation, which is only valid for the cases that the initial film thickness is smaller than the characteristic wavelength of induced instabilities. The long-wave approximation is not valid in the later stages of pattern evolution. Hence, in this thesis, the full governing equations of fluid flow and the thermally induced Marangoni effect are employed to track the polymer film's interface and the air bounding layer. First, a phase-field numerical model is developed to simulate the dynamic process of thermal-induced patterning. A systematic study on the impact of influential parameters has revealed an increase in the temperature gradient, thermal conductivity ratio, and initial thickness of the thin-film resulted in a shorter processing time and faster pattern formation. The newly developed numerical model more accurately predicts the characteristic wavelength than the linearized model. In the second part, the volume of fluid (VOF) and thin film (TF) methods are used to solve the governing equations. The results obtained from VOF are compared with the TF model in many cases to find the best model for predicting the characteristic wavelength for the growth of thermal-induced instabilities. This is followed by examining the effect of the protrusion width and the distance between the protrusions on the structures' final shape and interface evolution time in both VOF and TF models. Then, the linear theoretical relations for forming secondary pillars are presented based on the width of protrusions, their separation distance, and the inverse filling ratio. The number of pillars is found to increase with the protrusions’ width and distance between protrusions. In the last part, a mathematical model is developed to characterize the thermocapillary destabilization of the air–polymer interface for non-Newtonian polymer films. A power-law model is adopted to model shear-thickening/thinning fluids and overcome the assumption limitations, such as the independence of viscosity from the shear rate. The presented analyses in this Ph.D. thesis will advance existing knowledge in the thermal-induced patterning mechanism, providing practical implications for lowering the cost and time requirements in designing related experiments.

• Subjects / Keywords

  • Thermocapillary
  • Instability
  • Thin film
  • Phase field
  • Volume of Fluid
  • Micro/nanofabrication
  • non-Newtonian

• Graduation date

  Fall 2021

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-m0hk-q506

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.