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17 pages, 20440 KiB

Open AccessArticle

Dynamics of a Water Droplet Impacting an Ultrathin Layer of Oil Suspended on a Pool of Water

by Amir Dehghanghadikolaei, Bilal Abdul Halim, Ehsan Khoshbakhtnejad and Hossein Sojoudi

Fluids 2024, 9(4), 82; https://doi.org/10.3390/fluids9040082 - 25 Mar 2024

Viewed by 752

This study investigates water droplets impacting a two-layered pool, consisting of a deep pool of water above which an ultrathin a suspended layer of silicone oil is present. Initially, the difference between the impact dynamics of water droplets on ultrathin and thick layers [...] Read more.

This study investigates water droplets impacting a two-layered pool, consisting of a deep pool of water above which an ultrathin a suspended layer of silicone oil is present. Initially, the difference between the impact dynamics of water droplets on ultrathin and thick layers of oil were studied. It was found that the existence of an ultrathin layer of oil changes the impact characteristics such how aggressively the jet rises, how the dimensions of the impact impression change, and how the jets are broken down on their tops. Then, in a series of experiments on ultrathin layers of oil, the droplet size, the velocity of the droplets upon impact, and the viscosity of the oil layers were changed to observe and measure the characteristic dimensions of the formed craters and the jets. It was observed that when the viscosity of oil layers decreased to a minimum of 1 (cSt), the jet height and crater sizes increased to their maximum value. In addition to the effect of the oil viscosity, it was found that the droplet size and the release heights of the droplets were in the next orders of significance in determining the impact dynamics. The impacts were also characterized qualitatively by specifically looking into the crown and crater formations, pinch-off modes in jets, and number of formed secondary droplets. As well as the quantitative conclusion, it was found that the major affecting parameter in changing each of these qualities was the viscosity of the suspended oil layer. Full article

(This article belongs to the Special Issue Contact Line Dynamics and Droplet Spreading)

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16 pages, 5776 KiB

Open AccessArticle

Influence of Weber Number on Crown Morphology during an Oblique Droplet Impact on a Thin Wall Film

by Jonathan Lukas Stober, Maurizio Santini and Kathrin Schulte

Fluids 2023, 8(11), 301; https://doi.org/10.3390/fluids8110301 - 16 Nov 2023

Viewed by 1423

Abstract

Spray impacts can be found in several technical applications and consist of many single droplets, which impact under different trajectories on wetted walls. This study investigates the asymmetric crown morphology resulting from an oblique impact (

α =

60°) of a single droplet [...] Read more.

Spray impacts can be found in several technical applications and consist of many single droplets, which impact under different trajectories on wetted walls. This study investigates the asymmetric crown morphology resulting from an oblique impact (

α =

60°) of a single droplet on a horizontal and quiescent wall film of the same liquid. A droplet generator with an accelerated needle releases the droplets (

D =

1.5 mm) in a controlled trajectory on a thin film (

h_{f} / D =

0.2). The impact process is recorded from two perspectives with two synchronized high-speed cameras. Varying the Weber number within the splashing regime reveals distinct crown morphologies, which are described in detail. For

W e <

500, a single central finger develops at the front of the crown, with subsequent detachments of secondary droplets. At higher

W e

(>500), a collision of the crown with the wall film shortly after impact introduces disturbances into the rim, leading to two fingers in the middle of the front crown. A further increase in

W e

(>600) intensifies the crown–film interaction, resulting in an early ejection of tiny droplets and a complete breakup of the front rim. The influence of

W e

on the crown morphology during an oblique impact is also compared to the normal impact (90°). This study paves the way for a classification of impact regimes and a comprehensive picture of the oblique impact process, which deserve more investigation. Full article

(This article belongs to the Special Issue Contact Line Dynamics and Droplet Spreading)

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15 pages, 4247 KiB

Open AccessArticle

Numerical Simulation of Dropwise Condensation of Steam over Hybrid Surfaces via New Non-Dimensional Heat Transfer Model

by Giulio Croce and Nicola Suzzi

Fluids 2023, 8(11), 300; https://doi.org/10.3390/fluids8110300 - 15 Nov 2023

Cited by 1 | Viewed by 1272

Abstract

Dropwise condensation (DWC) of steam over hybrid hydrophobic–hydrophilic surfaces is numerically investigated via a phenomenological, Lagrangian model. The full non-dimensionalization of the heat transfer model, needed to determine the droplet growth, allows for generalization of computational results. Hybrid surfaces characterized by recursive geometries [...] Read more.

Dropwise condensation (DWC) of steam over hybrid hydrophobic–hydrophilic surfaces is numerically investigated via a phenomenological, Lagrangian model. The full non-dimensionalization of the heat transfer model, needed to determine the droplet growth, allows for generalization of computational results. Hybrid surfaces characterized by recursive geometries are implemented via the introduction of proper boundary conditions. The numerical size distribution of both the large and the small droplet populations, crucial for development of simplified, statistically sound models, is compared with empirical and theoretical correlations. Then, the validation with experimental data involving DWC over an hybrid surface is successfully conducted and the heat flux is enhanced under different operating conditions via hybrid geometry optimization. Full article

(This article belongs to the Special Issue Contact Line Dynamics and Droplet Spreading)

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35 pages, 8400 KiB

Open AccessArticle

Fine Flow Structure at the Miscible Fluids Contact Domain Boundary in the Impact Mode of Free-Falling Drop Coalescence

by Yuli D. Chashechkin and Andrey Yu. Ilinykh

Fluids 2023, 8(10), 269; https://doi.org/10.3390/fluids8100269 - 28 Sep 2023

Viewed by 1082

Abstract

Registration of the flow pattern and the matter distribution of a free falling liquid drop in a target fluid at rest in the impact mode of coalescence when the kinetic energy (KEn) of the drop exceeds its available surface potential energy (ASPe) was [...] Read more.

Registration of the flow pattern and the matter distribution of a free falling liquid drop in a target fluid at rest in the impact mode of coalescence when the kinetic energy (KEn) of the drop exceeds its available surface potential energy (ASPe) was carried out by photo and video recording. We studied the evolution of the fine flow structure at the initial stage of the cavity formation. To carry out color registration, the observation field was illuminated by several matrix LED and fiber-optic sources of constant light. The planning of experiments and interpretation of the results were based on the properties of the complete solutions of the fundamental equations of a fluid mechanics system, including the transfer and conversion of energy processes. Complete solutions of the system of equations describe large-scale flow components that are waves or vortices as well as thin jets (ligaments, filaments, fibers, trickles). In experiments, the jets are accelerated by the converted available surface potential energy (ASPe) when the free surfaces of merging fluids were eliminated. The experiments were performed with the coalescence of water, solutions of alizarin ink, potassium permanganate, and copper sulfate or iron sulfate drops in deep water. In all cases, at the initial contact, the drop begins to lose its continuity and breaks up into a thin veil and jets, the velocity of which exceeds the drop contact velocity. Small droplets, the size of which grows with time, are thrown into the air from spikes at the jet tops. On the surface of the liquid, the fine jets leave colored traces that form linear and reticular structures. Part of the jets penetrating through the bottom and wall of the cavity forms an intermediate covering layer. The jets forming the inside layer are separated by interfaces of the target fluid. The processes of molecular diffusion equalize the density differences and form an intermediate layer with sharp boundaries in the target fluid. All noted structural features of the flow are also visualized when a fresh water drop isothermally spreads in the same tap water. Molecular diffusion processes gradually smooth out the fast-changing boundary of merging fluids, which at the initial stage has a complex and irregular shape. Similar flow patterns were observed in all performed experiments; however, the geometric features of the flow depend on the individual thermodynamic and kinetic parameters of the contacting fluids. Full article

(This article belongs to the Special Issue Contact Line Dynamics and Droplet Spreading)

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10 pages, 3173 KiB

Open AccessArticle

Fingering Instability of Binary Droplets on Oil Pool

by Koji Hasegawa and Yuya Kishimoto

Fluids 2023, 8(5), 138; https://doi.org/10.3390/fluids8050138 - 25 Apr 2023

Cited by 1 | Viewed by 1443

Abstract

The interfacial instability of a complex fluid in a multiphase flow system is ubiquitous in both nature and industry. We experimentally investigated the spreading and interfacial instability dynamics of a binary droplet (a water and 2-propanol (IPA) mixture) on an immiscible (sunflower oil) [...] Read more.

The interfacial instability of a complex fluid in a multiphase flow system is ubiquitous in both nature and industry. We experimentally investigated the spreading and interfacial instability dynamics of a binary droplet (a water and 2-propanol (IPA) mixture) on an immiscible (sunflower oil) pool. For droplets of 40 wt% IPA solution on sunflower oil, fingering instability occurred at the spreading liquid front. To reveal the interfacial characteristics of the spreading and fingering processes, we analyzed the interplay among the speed, diameter, and number of fingers on the spreading front. Based on our observations, the finger length, wavelength between the fingers, head length, and neck length were quantified. Our experimental results clearly demonstrate that fingering instability can be driven by the capillary effect for a liquid–liquid system as well as the Plateau–Rayleigh instability. We hope that our results will inspire further experimental and numerical investigations to provide deeper insights into the interfacial dynamics of multicomponent droplets in a liquid pool. Full article

(This article belongs to the Special Issue Contact Line Dynamics and Droplet Spreading)

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23 pages, 8321 KiB

Open AccessArticle

Thin Film Evaporation Modeling of the Liquid Microlayer Region in a Dewetting Water Bubble

by Ermiyas Lakew, Amirhosein Sarchami, Giovanni Giustini, Hyungdae Kim and Kishan Bellur

Fluids 2023, 8(4), 126; https://doi.org/10.3390/fluids8040126 - 04 Apr 2023

Cited by 4 | Viewed by 2023

Abstract

Understanding the mechanism of bubble growth is crucial to modeling boiling heat transfer and enabling the development of technological applications, such as energy systems and thermal management processes, which rely on boiling to achieve the high heat fluxes required for their operation. This [...] Read more.

Understanding the mechanism of bubble growth is crucial to modeling boiling heat transfer and enabling the development of technological applications, such as energy systems and thermal management processes, which rely on boiling to achieve the high heat fluxes required for their operation. This paper presents analyses of the evaporation of “microlayers”, i.e., ultra-thin layers of liquid present beneath steam bubbles growing at the heated surface in the atmospheric pressure nucleate of boiling water. Evaporation of the microlayer is believed to be a major contributor to the phase change heat transfer, but its evolution, spatio-temporal stability, and impact on macroscale bubble dynamics are still poorly understood. Mass, momentum, and energy transfer in the microlayer are modeled with a lubrication theory approach that accounts for capillary and intermolecular forces and interfacial mass transfer. The model is embodied in a third-order nonlinear film evolution equation, which is solved numerically. Variable wall-temperature boundary conditions are applied at the solid–liquid interface to account for conjugate heat transfer due to evaporative heat loss at the liquid–vapor interface. Predictions obtained with the current approach compare favorably with experimental measurements of microlayer evaporation. By comparing film profiles at a sequence of times into the ebullition cycle of a single bubble, likely values of evaporative heat transfer coefficients were inferred and found to fall within the range of previously reported estimates. The result suggests that the coefficients may not be a constant, as previously assumed, but instead something that varies with time during the ebullition cycle. Full article

(This article belongs to the Special Issue Contact Line Dynamics and Droplet Spreading)

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17 pages, 3933 KiB

Open AccessArticle

Influence of the Deposition Rate and Substrate Temperature on the Morphology of Thermally Evaporated Ionic Liquids

by Rita M. Carvalho, Cândida Neto, Luís M. N. B. F. Santos, Margarida Bastos and José C. S. Costa

Fluids 2023, 8(3), 105; https://doi.org/10.3390/fluids8030105 - 22 Mar 2023

Cited by 1 | Viewed by 1657

Abstract

The wetting behavior of ionic liquids (ILs) on the mesoscopic scale considerably impacts a wide range of scientific fields and technologies. Particularly under vacuum conditions, these materials exhibit unique characteristics. This work explores the effect of the deposition rate and substrate temperature on [...] Read more.

The wetting behavior of ionic liquids (ILs) on the mesoscopic scale considerably impacts a wide range of scientific fields and technologies. Particularly under vacuum conditions, these materials exhibit unique characteristics. This work explores the effect of the deposition rate and substrate temperature on the nucleation, droplet formation, and droplet spreading of ILs films obtained by thermal evaporation. Four ILs were studied, encompassing an alkylimidazolium cation (C_nC₁im) and either bis(trifluoromethylsulfonyl)imide (NTf₂) or the triflate (OTf) as the anion. Each IL sample was simultaneously deposited on surfaces of indium tin oxide (ITO) and silver (Ag). The mass flow rate was reproducibly controlled using a Knudsen cell as an evaporation source, and the film morphology (micro- and nanodroplets) was evaluated by scanning electron microscopy (SEM). The wettability of the substrates by the ILs was notably affected by changes in mass flow rate and substrate temperature. Specifically, the results indicated that an increase in the deposition rate and/or substrate temperature intensified the droplet coalescence of [C₂C₁im][NTf₂] and [C₂C₁im][OTf] on ITO surfaces. Conversely, a smaller impact was observed on the Ag surface due to the strong adhesion between the ILs and the metallic film. Furthermore, modifying the deposition parameters resulted in a noticeable differentiation in the droplet morphology obtained for [C₈C₁im][NTf₂] and [C₈C₁im][OTf]. Nevertheless, droplets from long-chain ILs deposited on ITO surfaces showed intensified coalescence, regardless of the deposition rate or substrate temperature. Full article

(This article belongs to the Special Issue Contact Line Dynamics and Droplet Spreading)

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10 pages, 1325 KiB

Open AccessArticle

Molecular Dynamics of Nanodroplet Coalescence in Quasi-Saturated Vapor

by Dmitry Beloborodov and Aleksey Vishnyakov

Fluids 2023, 8(2), 77; https://doi.org/10.3390/fluids8020077 - 20 Feb 2023

Cited by 2 | Viewed by 1116

Abstract

The dynamics of coalescence of small Lennard–Jones droplets as a function of droplet size and temperature is explored with molecular simulations. Droplet sizes vary from several hundred to several thousand molecules, and three different temperatures are explored. As the droplets establish contact, a [...] Read more.

The dynamics of coalescence of small Lennard–Jones droplets as a function of droplet size and temperature is explored with molecular simulations. Droplet sizes vary from several hundred to several thousand molecules, and three different temperatures are explored. As the droplets establish contact, a liquid-like bridge between them forms and grows, ultimately leading to a complete coalescence. The dynamics of the bridge growth are consistent with the “collective molecular jumps” mechanism reported in the literature rather than with the continuous interpretation of the coalescence process in terms of capillary and viscous forces. The effective coalescence time shows a linear growth with the droplet sizes. The influence of the larger droplet size is weaker but non-negligible. Surprisingly, practically no dependence of the coalescence time on the temperature is observed. Comparison of the coalescence times with the droplet lifespan in a suspension shows that for reasonably dense suspensions and small droplet sizes, the coalescence time becomes significant and should be accounted for in the theoretical models of aggregation. Full article

(This article belongs to the Special Issue Contact Line Dynamics and Droplet Spreading)

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Review

Jump to: Research

19 pages, 3971 KiB

Open AccessReview

Physics of Dynamic Contact Line: Hydrodynamics Theory versus Molecular Kinetic Theory

by Alireza Mohammad Karim and Wieslaw J. Suszynski

Fluids 2022, 7(10), 318; https://doi.org/10.3390/fluids7100318 - 30 Sep 2022

Cited by 6 | Viewed by 2694

Abstract

The dynamic contact line plays a key role in various fields of interfacial physics, including bioprinting, nano-scale printing, three-dimensional printing, biomaterials, tissue engineering, smart materials, flexible printed electronics, biomedicine, and healthcare. However, there is still a lack of thorough physical understanding of its [...] Read more.

The dynamic contact line plays a key role in various fields of interfacial physics, including bioprinting, nano-scale printing, three-dimensional printing, biomaterials, tissue engineering, smart materials, flexible printed electronics, biomedicine, and healthcare. However, there is still a lack of thorough physical understanding of its real behavior in numerous complex problems in nature and technology. The dynamic contact line exhibits a complex conformation in real-life fluid dynamics problems. Therefore, this review presents two main long-standing models that describe the physics of the dynamic contact line: hydrodynamics theory and molecular kinetics theory. Next, the role of the dynamic contact line in current advanced technologies is discussed. Finally, this review discusses future research directions to enhance the power of current physical models of the dynamic contact line. Full article

(This article belongs to the Special Issue Contact Line Dynamics and Droplet Spreading)

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