64,903 research outputs found

    On phase change of a vapor bubble in liquid water

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    We consider a bubble of vapor and inert gas surrounded by the corresponding liquid phase. We study the behavior of the bubble due to phase change, i.e. condensation and evaporation, at the interface. Special attention is given to the effects of surface tension and heat production on the bubble dynamics as well as the propagation of acoustic elastic waves by including slight compressibility of the liquid phase. Separately we study the influence of the three phenomena heat conduction, elastic waves, and phase transition on the evolution of the bubble. The objective is to derive relations including the mass, momentum, and energy transfer between the phases. We find ordinary differential equations, in the cases of heat transfer and the emission of acoustic waves partial differential equations, that describe the bubble dynamics. From numerical evidence we deduce that the effect of phase transition and heat transfer on the behavior of the radius of the bubble is negligible. It turns out that the elastic waves in the liquid are of greatest importance to the dynamics of the bubble radius. The phase transition has a strong influence on the evolution of the temperature, in particular at the interface. Furthermore the phase transition leads to a drastic change of the water content in the bubble, so that a rebounding bubble is only possible, if it contains in addition an inert gas. In a forthcoming paper the equations derived are sought in order to close equations for multi-phase mixture balance laws for dispersed bubbles in liquids involving phase change. Also the model is used to make comparisons with experimental data on the oscillation of a laser induced bubble. For this case it was necessary to include the effect of an inert gas in the thermodynamic modeling of the phase transition

    On phase change of a vapor bubble in liquid water

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    We consider a bubble of vapor and inert gas surrounded by the corresponding liquid phase. We study the behavior of the bubble due to phase change, i.e. condensation and evaporation, at the interface. Special attention is given to the effects of surface tension and heat production on the bubble dynamics as well as the propagation of acoustic elastic waves by including slight compressibility of the liquid phase. Separately we study the influence of the three phenomena heat conduction, elastic waves, and phase transition on the evolution of the bubble. The objective is to derive relations including the mass, momentum, and energy transfer between the phases. We find ordinary differential equations, in the cases of heat transfer and the emission of acoustic waves partial differential equations, that describe the bubble dynamics. From numerical evidence we deduce that the effect of phase transition and heat transfer on the behavior of the radius of the bubble is negligible. It turns out that the elastic waves in the liquid are of greatest importance to the dynamics of the bubble radius. The phase transition has a strong influence on the evolution of the temperature, in particular at the interface. Furthermore the phase transition leads to a drastic change of the water content in the bubble, so that a rebounding bubble is only possible, if it contains in addition an inert gas. In a forthcoming paper the equations derived are sought in order to close equations for multi-phase mixture balance laws for dispersed bubbles in liquids involving phase change. Also the model is used to make comparisons with experimental data on the oscillation of a laser induced bubble. For this case it was necessary to include the effect of an inert gas in the thermodynamic modeling of the phase transitio

    Fluctuating hydrodynamics model for homogeneous and heterogeneous vapor bubble nucleation

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    At the molecular scale, even in conditions of thermodynamic equilibrium, the fluids do not exhibit a deterministic behavior. Going down below the micrometer scale, the effects of thermal fluctuations play a dominant role in the dynamics of the system, calling for a suitable description of thermal fluctuations. These models not only play an important role in physics of fluids, but a deep understanding of these phenomena is necessary for the progress of some of the latest nanotechnology. For instance the modeling of thermal fluctuations is crucial in the design of flow micro-devices, in the study of biological systems, such as lipid membranes, in the theory of Brownian engines and in the development of artificial molecular motor prototypes. Another problem with a huge technological impact is the phenomenon of nucleation – the precursor of the phase transition in metastable systems – in this context related to bubble formation in liquid-vapor phase transition. Vapor bubbles form in liquids by two main mechanisms: boiling, by increasing the tempe- rature over the boiling threshold, and cavitation, by reducing the pressure below the vapor pressure threshold. The liquid can be held in these metastable states (overheating and tensile conditions, respectively) for a long time without forming bubbles. Bubble nucleation is indeed an activated process, requiring a significant amount of energy to overcome the free energy barrier and bring the liquid from the metastable conditions to the thermodynami- cally stable state where vapor is observed. Depending on the thermodynamic conditions, the nucleation time may be exceedingly long, the so-called "rare- event" issue. Nowadays molecular dynamics is the unique tool to investigate such thermally activated processes. However, its computational cost limits its application to small systems (less than few tenth of nanometers) and to very short times, preventing the study of hydrodynamic interactions. The latter effects are crucial to understand the cavitation phenomenon in its entirety, starting from the vapor embryos nucleation up to the macroscopic motion. In this thesis a continuum diffuse interface model of the two-phase fluid has been embedded with thermal fluctuations in the context of the so-called Fluctuating Hydrodynamics (FH) and has been exploited to address cavitation. This model provides a set of partial stochastic differential equations, whose deterministic part is represented by the capillary Navier-Stokes equations and reproducing the Einstein-Boltzmann probability distribution for the macroscopic fields. This mesoscale approach enables the description of the liquid-vapor transition in extended systems and the evaluation of bub- ble nucleation rates in different metastable conditions by means of numerical simulations. Such model is expected to have a huge impact on the understanding of the nucleation dynamics since, by reducing the computational cost by orders of magnitude, it allows the unique possibility of investigating systems of realistic dimensions on macroscopic time scales. In addition, after the nucleating phase, the deterministic equations have been used to address the collapse of a cavitation nanobubble near a solid boundary, showing an unprecedented description of interfacial flows that naturally takes into account topology modification and phase changes (both vapor/liquid and vapor/supercritical fluid transformations)

    Gas-Vapor Interplay in Plasmonic Bubble Shrinkage

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    The understanding of the shrinkage dynamics of plasmonic bubbles formed around metallic nanoparticles immersed in liquid and irradiated by a resonant light source is crucial for the usage of these bubbles in numerous applications. In this paper we experimentally show and theoretically explain that a plasmonic bubble during its shrinkage undergoes two different phases: first, a rapid partial bubble shrinkage governed by vapor condensation and, second, a slow diffusion-controlled bubble dissolution. The history of the bubble formation plays an important role in the shrinkage dynamics during the first phase, as it determines the gas-vapor ratio in the bubble composition. Higher laser powers lead to more vaporous bubbles, while longer pulses and higher dissolved air concentrations lead to more gaseous bubbles. The dynamics of the second phase barely depends on the history of bubble formation, i.e. laser power and pulse duration, but strongly on the dissolved air concentration, which defines the concentration gradient at the bubble interface. Finally, for the bubble dissolution in the second phase, with decreasing dissolved air concentration, we observe a gradual transition from a R(t)(t0t)1/3R(t) \propto (t_0 - t) ^{1/3} scaling law to a R(t)(t0t)1/2R(t) \propto (t_0 - t) ^{1/2} scaling law, where t0t_0 is the lifetime of the bubble and theoretically explain this transition

    Simulations of bulk and confined bubble nucleation

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    The present thesis investigates, with atomistic simulations, vapor nucleation and liquid dynamics under nanoscale confinement. The main objective of this work is to go beyond the quasi-static classical picture of liquid-vapor phase transition, including kinetic and inertial effects. The performed simulations provide an accurate description of the phenomenon and a framework to interpret experimental observations. The dynamics of vapor nucleation is investigated in the pure bulk liquid and in confined conditions. In the last case, also wetting transition is studied. Particular attention is devoted to surfaces that combine textured geometries with an hydrophobic chemistry. These are able to stabilize vapor phase within surfaces asperities, producing a state in which liquid is suspended above the entrapped vapor pockets. In these conditions, remarkable properties arise that are collectively known as superhydropobicity. In this suspended state, known also as Cassie-Baxter state, the contact area between solid and liquid is reduced with respect to a flat surface and with respect to the textured surface in which the corrugations are flooded with the liquid. Moreover, the liquid presents a higher contact angle (CA), with a lower CA hysteresis and a reduced liquid-solid friction. Due to these properties, superhydrophobic surfaces are suitable for applications such as self-cleaning glass, window, andwallpaint. Theypreventmoistureaccumulation, helpanti-icing, andallowdropwisecondensationtoincreasetheheattransferefficiencyandwaterharvesting. These are all in-air applications. However, the presence of a large shear free liquid/gas interface suggested that super-hydrophobic surfaces can be used in many submerged applications, e.g. drag reduction, anti-friction, anti-adhesive, anti-corrosion, and boiling heat transfer. Cassie-Baxter state can be destabilized by changes in pressure and temperature, that produce the intrusion of the liquid within surface defects. The corresponding state in which the surface is completely wetted is known as Wenzel state. The loss of super-hydrophobic properties (Cassie-Baxter to Wenzel transition) has proved to be experimentally irreversible. It is therefore crucial to characterize both wetting and recovery mechanisms in order understand how to design surfaces supporting a robust Cassie-Baxter state, i.e. a suspended state that can resist to temperature and pressure fluctuations. Wetting transition and recovery of superhydrophobic state take place via vapor/liquid and liquid/vapor phase transitions occurring under confinement at the nanoscale within geometric defects. Over the last decades, a significant amount of experimental and theoretical work has been devoted to the study of confined liquidvapor transition. In spite of this, not much is known yet about the kinetics of the process. The contribution to the topic obtained during the three years of my PhD is presented in this thesis. The first part of the work has been devoted to develop and test Molecular Dynamics and Monte Carlo methods able to properly simulate multiphase systems. Indeed, it has been demonstrated that serious issues arise when the standard global barostats, developed to simulate bulk systems, are straightforwardly applied to systems with subdomains at different pressures, e.g. liquid and vapor domains during nucleation. A solution to overcome these artifacts has been proposed, consisting in the implementation of a local barostat that imposes a local force balance between a piston and the contacting liquid. With this approach, a more accurate prediction of the vapor nucleation barrier in a super-heated liquid has been obtained. Secondly, the simulation techniques developed at the first stage of my PhD work have been employed to study homogeneous bubble nucleation. At the liquid pressure andtemperaturehereinvestigated, thisphenomenonisarareevent: thewaitingtime to observe the inception of vapor formation is order of magnitude longer than the typical time that can be explored by atomistic simulations. This issue, that causes waste of computational resources, has been tackled by carefully selecting special techniques able to preserve kinetic and inertial effects during bubbles growth. With this approach, “dynamical” quantities have been estimated, e.g. the nucleation rate. Other two essential aspects have been addressed: the limits of theoretical expressions routinely used to evaluate the kinetic prefactor in Eyring equation for vapor nucleation; the relation between successful nucleation events and relevant observables, such as temperature and liquid velocity, at beginning and during bubble expansion. The last section of this thesis is focused on heterogeneous nucleation and wetting of super-hydrophobic surfaces. Recent theoretical and experimental studies have produced conflicting results in the characterization of the pathways by which liquid intrudes in pores. The disagreement resides, specifically, in the symmetry properties expected for the advancing meniscus shape. Experiments show a symmetric pathways, in which the liquid penetrates in the surface pores with an essentially flat meniscus, while quasi-static theories predict that the asymmetric pathway is more probable, in which the liquid entering in the surface cavities bend forming a bubble in a corner. My simulations have proved that inertial effects change the wetting and recovery path with respect the predictions of quasi-static approaches. This reconcile theory and experiments: when the transition is barrierless, as expected in experimental conditions in which only nearly spontaneous processes can be addressed, the more complete theory developed here predicts a symmetric wetting as observed in the experiments

    Deciphering the molecular mechanism of water boiling at heterogeneous interfaces

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    Water boiling control evolution of natural geothermal systems is widely exploited in industrial processes due to the unique non-linear thermophysical behavior. Even though the properties of water both in the liquid and gas state have been extensively studied experimentally and by numerical simulations, there is still a fundamental knowledge gap in understanding the mechanism of the heterogeneous nucleate boiling controlling evaporation and condensation. In this study, the molecular mechanism of bubble nucleation at the hydrophilic and hydrophobic solid–water interface was determined by performing unbiased molecular dynamics simulations using the transition path sampling scheme. Analyzing the liquid to vapor transition path, the initiation of small void cavities (vapor bubbles nuclei) and their subsequent merging mechanism, leading to successively growing vacuum domains (vapor phase), has been elucidated. The molecular mechanism and the boiling nucleation sites’ location are strongly dependent on the solid surface hydrophobicity and hydrophilicity. Then simulations reveal the impact of the surface functionality on the adsorbed thin water molecules film structuring and the location of high probability nucleation sites. Our findings provide molecular-scale insights into the computational aided design of new novel materials for more efficient heat removal and rationalizing the damage mechanisms

    Micro/nano structured phase change systems for thermal management applications

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    Phase change phenomena have been of interest mainly due to large latent heats associated with the phase transition and independency on external energy to drive the phase change process. When combined with micro/nano structures, phase change systems become a promising approach to address challenges in high heat flux thermal management. The objective of this thesis is to implement micro and nano structured surfaces for better understanding the underlying fundamentals of evaporation and boiling phase change heat transfer and enhancing the heat transfer performance. First, we study single bubble dynamics on superheated superhydrophobic (SHB) surfaces and the corresponding heat transfer mechanism of water pool boiling. Because of the large contact angle, such surfaces are ideal for correlating pool boiling with single bubble dynamics by accurately controlling the number of nucleation sites in a defined area. The fundamental parameters of single bubble dynamics are collected and put into the heat flux partitioning model. We find that latent heat transport and bulk liquid water convection contribute together to the heat removal on superhydrophobic surfaces. Next, we present a novel method to fabricate silicon nanowires by one-step metal assisted chemical etching (MACE) on micro-structured surfaces with desired morphologies. Patterned vertically aligned silicon nanowires are fabricated on dense cavity/pillar arrays due to trapped hydrogen bubbles serving as an etching mask. Uniformly grown silicon nanowires on structured surfaces can be fabricated if extra energy is introduced to remove the trapped bubbles. An enhanced pool boiling heat transfer performance on such structured surfaces is demonstrated. Finally, we study the ultimate limits of water evaporation in single 2D nanochannels and 1D nanopores. These ultimate transport limits are determined by the maximum evaporation fluxes that liquid/vapor interfaces can provide regardless of liquid supply or vapor removal rates. A hybrid nanochannel design is utilized to provide sufficient liquid supply to the evaporating meniscus and evaporated vapor is efficiently removed by air jet impingement or a vacuum pump. The effect of nanoscale confinement on evaporation flux has been investigated, with feature size ranging from 16 nm to 310 nm. An ultra-high heat flux of 8500 W/cm2 is demonstrated in a single 16-nm nanochannel at 40 °C.2017-09-09T00:00:00

    Phase Diagram of Water Confined by Graphene

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    The behavior of water confined at the nanoscale plays a fundamental role in biological processes and technological applications, including protein folding, translocation of water across membranes, and filtration and desalination. Remarkably, nanoscale confinement drastically alters the properties of water. Using molecular dynamics simulations, we determine the phase diagram of water confined by graphene sheets in slab geometry, at T = 300 K and for a wide range of pressures. We find that, depending on the confining dimension D and density σ, water can exist in liquid and vapor phases, or crystallize into monolayer and bilayer square ices, as observed in experiments. Interestingly, depending on D and σ, the crystal-liquid transformation can be a first-order phase transition, or smooth, reminiscent of a supercritical liquid-gas transformation. We also focus on the limit of stability of the liquid relative to the vapor and obtain the cavitation pressure perpendicular to the graphene sheets. Perpendicular cavitation pressure varies non-monotonically with increasing D and exhibits a maximum at D ≈ 0.90 nm (equivalent to three water layers). The effect of nanoconfinement on the cavitation pressure can have an impact on water transport in technological and biological systems. Our study emphasizes the rich and apparently unpredictable behavior of nanoconfined water, which is complex even for graphene
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