Date

2017

Document Type

Dissertation

Degree

Doctor of Philosophy

Department

Mechanical Engineering

First Adviser

Webb, Edmund B.

Other advisers/committee members

Blythe, Philip A.; Kazakia, Jacob Y.; DuPont, John N.

Abstract

Droplet wetting and spreading across a solid substrate is a fascinating fluid mechanics phenomenon. Recently, the behavior of nano-fluids, or fluid suspensions containing nanoparticles, has garnered tremendous attention for applications in advanced manufacturing. Despite previous contributions, much remains to be understood about the wetting and spreading of nano-suspension drops especially on the fundamental mechanisms involved in the spreading process. However, due to the rapid contact line advancement and nano-scale droplet size, both experiments and continuum scale simulations could not provide ways to resolve the problem. Here, we use the fundamental time and length scale classical molecular dynamics to reveal the atomistic scale details about interfacial thermodynamics and associated forces during droplet wetting and spreading, i.e. to explore the related thermo-physical phenomena associated with capillary flow. The first interest in my dissertation is to study the mechanisms of rapid contact line advance during the early stage of droplet spreading, i.e. the inertial regime spreading. Inertial regime spreading occurs at the earliest moment immediately following contact between a droplet and solid surface. It is at this point when a drop is most out of equilibrium and therefore experiences the highest capillary forces. For low viscosity liquids with high wettability, high contact line velocities are observed during this stage. Meanwhile, much remains unknown about mechanisms governing such rapid capillary flow during this stage; additionally, because the very leading edge of an advancing contact line is of vanishing physical size, it is expected that phenomena controlling wetting kinetics are atomic scale in nature. In my work, molecular dynamics simulations on metallic Pb-Cu systems were performed to study the early stage spreading. A counterintuitive result from our MD simulations is that even nanometer scale metallic drops exhibit a regime of wetting that is dictated by inertial effects. Therefore, mechanisms observed in atomic simulations may provide insight to corresponding mechanisms for inertial regime spreading of macro-scale droplets. We introduce a Tolman length corrected surface tension to account for liquid/vapor interface curvature effects that manifest in small drops. In addition, for inertial spreading on low advancing contact angle surface, a second nanoscale effect is observed which is unique to this surface and related to curvature gradients along a significant portion of the liquid/vapor interface. After accounting for all the nanoscale size effects, data from inertial spreading of nanodrops could be well described by continuum inertial wetting theory. Additionally, we explore the fundamental mechanisms in controlling the rapid contact line advancement during the inertial spreading. It is demonstrated that high contact line velocity is abetted by structural ordering of a liquid layer adjacent to the solid. Meanwhile, a tensile strain in this layer, which is most pronounced nearest to the contact line, may also play a role.For the second interest of my dissertation, wetting and spreading of nano-suspension drops are investigated using the same metallic system. The concept of assembling ordered arrays of nanoparticles on a substrate surface via suspension droplet wetting and subsequent evaporation has fueled a large body of research in this area. Self-pinning is a phenomenon intrinsic to the advancement or retraction of liquid/solid/vapor three-phase contact lines for nano-fluid droplets; in such cases, particles entrained to the contact line halt its motion, preventing the system from reaching equilibrium. Depending on the desired application, this can be either detrimental (e.g. preventing complete coating of a substrate by the suspension) or beneficial (e.g. stabilizing non-equilibrium droplet morphologies). Another relevant phenomenon is de-pinning, where an initially halted contact line is able to separate from the pinning particle and continue its advance (or retraction) across the surface. While deterministic engineering of nanoparticle distribution requires thorough understanding of the thermodynamics and associated wetting kinetics of nanosuspension droplets, quantitative understanding of forces acting on suspended nanoparticles is needed; however, such measurements remain experimentally inaccessible. Herein, we present results from molecular dynamics simulations of nanosuspension droplets spreading on solid surfaces, with emphasis on revealing forces on suspended particles. For a wetting system of identical liquid, solid and particle chemistry yet significant difference in advancing contact angles, self-pinning is observed for low θ_adv Cu(111) whereas de-pinning occurs at high θ_adv Cu(001). The role of contact angle in determining likelihood for self-pinning is investigated on fundamental time and length scales. Meanwhile, relations between contact line velocity and advancing contact angle are discussed from atomic scale computation results. For the pinning case, the precursor film is observed to be continually advance across the surface even when the droplet is pinned. However a single layer of liquid on the outer facet of the particle surfaces is observed which manifests a rate limiting step for the precursor film advance. For the de-pinning case, we examine the contact line region carefully during de-pinning, especially the fluid flow profile, advancing contact angle and droplet morphology. Furthermore, forces associated with such behaviors are presented. We found that significant forces in the direction normal to the substrate surface due to liquid atoms confined between the particles and substrate dominate the forces on nanoparticles such that forces are interface dominated. To advance this work, the roles of particle size, particle loading and interaction strength between particle and substrate are examined. Results presented illustrate how particle size and loading affect spreading kinetics and how this connects to dynamic droplet morphology and relevant forces that exist nearby the contact line region. Increased particle size in simulations permits a more detailed investigation of particle/substrate interfacial contributions to behavior observed at the advancing contact line. A transition from de-pinning to pinning is observed and interpreted in terms of the increasing capillary force between suspended nanoparticles and the solid/liquid/vapor interfaces. At higher nanoparticle concentrations, particle pile up occurs and particle/particle interactions become relevant, leading to pinning and non-equilibrium droplet morphology. By tuning down the interaction strength between particle and substrate, particle is not able to form strong bonds with the underlying substrate, thus contact line pinning does not occur in this case. In concluding, MD simulations reveal fundamental mechanisms of inertial spreading related to particle-free drops as well as suspension droplet behavior. Thermodynamic parameters controlling observed kinetics are elucidated. The implications of results presented are discussed. We also address those features in LAMMPS that permit relatively straightforward extraction of forces on suspended particles during suspension droplet wetting simulations. We finally highlight how results such as those herein can help improve the predictive capability of continuum-scale computational methods typically used to model particle suspension droplet behavior and thus ultimately benefit related engineering practices.

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