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Fluctuating hydrodynamics of nanoscale interfacial flows
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Zhao, Chengxi (2020) Fluctuating hydrodynamics of nanoscale interfacial flows. PhD thesis, University of Warwick.
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Official URL: http://webcat.warwick.ac.uk/record=b3685530~S15
Abstract
Understanding the influence of thermal fluctuations on nanoscale interfacial flows is crucial to a range of modern and emerging technologies, such as in lab-on-a-chip technology and next generation 3D printing. In this thesis, effects of thermal fluctuations on two specific flows (nano-jets and bounded nano-films) are studied in detail with: (i) Molecular dynamics (MD) used as `numerical experiments'; and (ii) Landau-Lifshtz Navier-Stokes equations (LLNS, also known as fluctuating hydrodynamics equations) as an approximate, but numerically efficient, alternative. To pursue theoretical results and relatively cheap numerical solutions, further simplifications to LLNS equations, which use a long-wave approximation, are studied: (i) the stochastic lubrication equation (SLE) for nano-jets; and (ii) the stochastic thin-film equation (STFE) for bounded nano-films.
The famous Rayleigh-Plateau (RP) theory is re-evaluated and revised for the instability of nanoscale jets, where MD experiments demonstrate its inadequacy. A new framework based on the SLE is developed, which captures nanoscale flow features and highlights the critical role of thermal fluctuations at small scales. Remarkably, the model indicates that classically stable (i.e. `fat') liquid cylinders can be broken at the nanoscale, and this is confirmed by MD.
A simple and robust numerical scheme is then developed for the SLE, which is validated against MD for both the initial (linear) instability and the nonlinear rupture process. Particular attention is paid to the rupture process and its statistics, where the double-cone profile reported by Moseler & Landmann [1] is observed, as well as other distinct profile forms depending on the flow conditions. Comparison to the similarity solution in Eggers [2], a power law of the minimum thread radius against time to rupture, shows agreement only at low surface tension; indicating that surface tension cannot generally be neglected when considering rupture dynamics.
For bounded nano-films, STFEs are developed to accommodate substrate roughness and slip boundary conditions (BCs). An efficient solver with a new iteration method, verified by the theoretical models, is then developed to explore the nonlinear dynamics of nano-droplet spreading and coalescence. Numerical solutions of the spreading denote that the slip BC accelerates the process in both the deterministic and stochastic regimes, which is supported by the power laws of the similarity solutions derived. Additionally, thermal noise is shown to decelerate the coalescence, which is confirmed by MD.
Item Type: | Thesis (PhD) | ||||
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Subjects: | T Technology > TC Hydraulic engineering. Ocean engineering T Technology > TJ Mechanical engineering and machinery |
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Library of Congress Subject Headings (LCSH): | Hydrodynamics, Nanofluids, Thermal hydraulics, Heat flux | ||||
Official Date: | March 2020 | ||||
Dates: |
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Institution: | University of Warwick | ||||
Theses Department: | School of Engineering | ||||
Thesis Type: | PhD | ||||
Publication Status: | Unpublished | ||||
Supervisor(s)/Advisor: | Lockerby, Duncan A. ; Sprittles, James | ||||
Sponsors: | University of Warwick. Chancellor's International Scholarship ; Engineering and Physical Sciences Research Council | ||||
Extent: | xv, 129 leaves : illustrations | ||||
Language: | eng |
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