1,721,174 research outputs found
Long-wave Dynamics of Single- and Two-layer Flows
Full text linkThin-film flows are central to a number of industrial, biomedical and daily-life applications, which
include coating flow technology, enhanced oil recovery, microfluidics, and surfactant replacement
therapy. Though these systems have received a lot of attention in a variety of settings, the understanding
of the dominant physics governing the flows is not completely thorough; this is especially
true in cases where the free surface of the film or, in two-layer flows, the fluid-fluid interface is susceptible
to instabilities leading to the break-up of the film and the formation of fingering patterns.
The elucidation of the underlying mechanisms behind the onset of these instabilities is of utmost
importance to several industrial processes.
The work in this thesis focusses on modelling the dynamics of thin-film flows in the presence of
complexities; the latter arise from the presence of surface-active chemicals and spatial confinement.
The lubrication approximation, which is valid in the limit of small film aspect ratios, is used to
simplify the governing equations; this facilitates the derivation of an evolution equation for the
interfacial position. This methodology is employed extensively in the present thesis to examine co- and
counter-current two-layer flows in a closed, rectangular channel and the dynamics of a thin film
laden with surfactant, driven to climb up an inclined substrate.
In the two-fluid case, the dynamics of the flow are described by a single, two-dimensional, fourth-order
nonlinear partial differential equation. Analysis of the one-dimensional flow demonstrate the
existence of travelling-wave solutions which take the form of Lax shocks, undercompressive shocks,
and rarefaction waves. In unstably-stratified cases, a Rayleigh-Taylor mechanism spawns the formation
of large-amplitude capillary waves. A wide range of parameters is studied, which include
the density and viscosity ratios of the two fluids, the flow configuration (whether co- or counter-current),
the heights of the films at the channel ends and the channel inclination. The stability
of these structures to perturbations in the spanwise direction, is also examined through a linear
stability analysis and transient, two-dimensional numerical simulations. These analyses demonstrate
successfully that some of the structures observed in the one-dimensional flow are unstable to fingering
phenomena. In the case of the climbing film, two configurations are examined, namely,
constant flux and constant volume whereby the evolution equation for the interface is coupled to
convective-diffusive equations for the concentration of surfactant, present in the form of monomers
and micelles. The former are allowed to exist at the gas-liquid and liquid-solid interfaces, and in the
bulk; the latter can only be present in the bulk. For the constant flux case, the flow is simulated
by a continuously-fed uncontaminated fluid and surfactant at the flow origin allowed to spread on
a solid substrate which has been prewetted by a thin, surfactant-free precursor layer. The constant
volume configuration simulates the deposition of a finite drop, laden with surfactant, spreading on a
thin, uncontaminated film. In the absence of spanwise disturbances, the one-dimensional solutions
demonstrate how the climbing rate and the structural deformation of the film are influenced by
gravity, and physico-chemical parameters such as surfactant concentration (whether above or below
the critical micelle concentration), and rates of adsorption of monomers at the two interfaces. The
stability of the flow is examined through linear theory and transient solutions of the full, nonlinear,
two-dimensional system of equations revealing the growth of spanwise perturbations into full-length
fingers.
A brief introduction to the experimental design of an apparatus, aimed at validating channel flow
results, is also described. The objective of the experiment was to investigate the physical features
associated with the counter-current, pressure-driven flow of a gas-liquid system. Preliminary experimental
results revealed that upon perturbing the flow, an initially uniform liquid film becomes
unstable, resulting in the formation of fingers which elongated downstream as time progressed. Finally,
we conclude with recommendations for future work, representing natural extensions to the
theoretical work described in the present thesis
Predicting apparent slip at liquid-liquid interfaces without an interface slip condition
Full text linkWe show that if we include a density-dependent viscosity into the Navier-Stokes equations then we can describe, naturally, the velocity profile in the interfacial region, as we transition from one fluid to another. This requires knowledge of the density distribution (for instance, via Molecular Dynamics [MD] simulations, a diffuse-interface approach, or Density Functional Theory) everywhere in the fluids, even at liquid-liquid interfaces where regions of rapid density variations are possible due to molecular interactions. We therefore do not need an artificial interface condition that describes the apparent velocity slip. If the results are compared with the computations obtained from MD simulations, we find an almost perfect agreement. The main contribution of this work is to provide a simple way to account for the apparent slip at liquid-liquid interfaces without relying upon an additional boundary condition, which needs to be calculated separately using MD simulations. Examples are provided involving two immiscible fluids of varying average density ratios, undergoing simple Couette and Poisseuille flows
Drop count and size of 38 numerical simulation of a flat fan spray
No full textData refer to the drop size distribution used in the paper "Data-driven modelling for drop size distributions" by T. Traverso, T. Abadie, O. K. Matar, and L. Magri (arXiv link: https://arxiv.org/abs/2305.18049) Each of the 38 .csv file in this folder is associated with a different working condition of the nozzle. Specifically, the name 'alpha##_Re##_We##.csv' contains the working condition of the nozzle as - alpha## (## is the spray angle) - Re## (## is the Reynolds number) - We## (## is the Weber number) Each file contains as many raws as the number of drops. In the i-th raw, 1) the first element is the Volume of the i-th drop; 2) the second element is the estimated surface of the i-th drop with the method in equation (18) of [1]; 3) the third element is the equivalent diameter of the i-th drop (i.e., as if it was spherical - computed from the volume) The value of the Weber number found in the Arxiv paper is half of that reported here. The correct one is the one in this database. The paper will be corrected in due time. {"references": ["Traverso, T., Abadie, T., Matar, O. K., & Magri, L. (2023). Data-driven modelling for drop size distributions. arXiv preprint arXiv:2305.18049."]
Coupling frontal photopolymerization and surface instabilities for a novel 3D patterning technology
No full text
Role of heat generation and thermal diffusion during frontal photopolymerization
Full text linkFrontal photopolymerization (FPP) is a rapid and versatile solidification process that can be used to fabricate complex three-dimensional structures by selectively exposing a photosensitive monomer-rich bath to light. A characteristic feature of FPP is the appearance of a sharp polymerization front that propagates into the bath as a planar traveling wave. In this paper, we introduce a theoretical model to determine how heat generation during photopolymerization influences the kinetics of wave propagation as well as the monomer-to-polymer conversion profile, both of which are relevant for FPP applications and experimentally measurable. When thermal diffusion is sufficiently fast relative to the rate of polymerization, the system evolves as if it were isothermal. However, when thermal diffusion is slow, a thermal wavefront develops and propagates at the same rate as the polymerization front. This leads to an accumulation of heat behind the polymerization front which can result in a significant sharpening of the conversion profile and acceleration of the growth of the solid. Our results also suggest that a novel way to tailor the dynamics of FPP is by imposing a temperature gradient along the growth directio
Controlling frontal photopolymerization with optical attenuation and mass diffusion
Full text linkFrontal photopolymerization (FPP) is a versatile directional solidification process that can be used to rapidly fabricate polymer network materials by selectively exposing a photosensitive monomer bath to light. A characteristic feature of FPP is that the monomer-to-polymer conversion profiles take on the form of traveling waves that propagate into the unpolymerized bulk from the illuminated surface. Practical implementations of FPP require detailed knowledge about the conversion profile and speed of these traveling waves. The purpose of this theoretical study is to (i) determine the conditions under which FPP occurs and (ii) explore how optical attenuation and mass transport can be used to finely tune the conversion profile and propagation kinetics. Our findings quantify the strong optical attenuation and slow mass transport relative to the rate of polymerization required for FPP. The shape of the traveling wave is primarily controlled by the magnitude of the optical attenuation coefficients of the neat and polymerized material. Unexpectedly, we find that mass diffusion can increase the net extent of polymerization and accelerate the growth of the solid network. The theoretical predictions are found to be in excellent agreement with experimental data acquired for representative system
Slip at liquid-liquid interfaces
Full text linkWe address a problem of fundamental importance in the physics of interfaces, which is central to the description of multiphase fluid dynamics. This work is important to study interfaces in systems such as polymer melts and solutions, where velocity jumps have been observed and interpreted as a manifestation of slip. This is in violation of classical interfacial conditions that require continuity of velocity and has been remedied in the literature via use of ad hoc models, such as the so-called Navier slip condition. This paper suggests that it is possible to obviate completely the need for such an approach. Instead, we show that one simply requires knowledge of the density field and the molar fraction of the fluid components and the dependence of the viscosity on the density. This information can be obtained easily through molecular dynamics simulations
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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