Birth of a bubble: drop impact onto a thin liquid film for an immiscible three-fluid system

This thesis presents the numerical simulation of fluid dynamics, as well as heat and mass transfer for drop impingement on a hot solid surface for low and high ambient pressures. The technical application ranges from effective thermal management strategies using spray cooling, safety aspects in high pressure nuclear reactors to process technology in chemical or food industry. It is reported in literature that wetting characteristics depend on the ambient pressure. Drop splash is suppressed at low ambient pressure. High ambient pressure encourages compressibility effects. The compressibility of both the liquid and vapour phase increases with increasing pressure. Thereby, the effects of compressibility on drop impingement is of interest. Up to now, no attempt has been made to investigate a full pressure range for the evaporative drop impingement process. In order to provide insights into evaporative drop impingement processes under various ambient pressures, numerical simulations are performed. CFD simulations are conducted using a finite volume discretisation method solving the Navier-Stokes equations. The volume of fluid method is utilised to resolve two-phase flow. The solver accounts for compressible fluid flow, heat and mass transfer due to evaporation across the free liquid-vapour interface, evaporation in the vicinity of the three-phase contact line, as well as for heat conduction within the solid substrate. The dynamic contact angle is implemented using a subscale model. Effects of low and high ambient pressure on the three-phase contact line are investigated in the well established so-called micro region model. The focus is the non-splashing drop-wall collision in a non-boiling, single-component evaporation regime. Ambient pressure ratios ranging between p/pcr = [8*10^{-3} ... 0.5], Reynolds and Weber numbers ranging between Re = [600 ... 1300] and We = [10 ... 50] are investigated. The wall temperature is above saturation but below Leidenfrost temperature. The wall superheat is in the order of 10 K. Different parameter studies are dedicated to investigate the influence of low and high ambient pressures on the evaporative drop impact processes. Within one parameter study, dimensional drop impact parameters are kept constant, such as drop diameter, impact velocity and wall superheat. Caused by the variation in ambient pressure, material properties of the fluid change. Consequently, non-dimensional groups are changing, indicating a shift in dominant forces. Another parameter study keeps non-dimensional groups constant. Further parameter studies focus on the influence of the vapour phase on the drop impact outcome, especially for high ambient pressure. Within this work, results are presented for different length scales. The modelling of the vicinity of an evaporating three-phase contact line indicates a strong influence of the ambient pressure on the apparent contact angle and the heat being transferred in the micro region. For increasing pressure, the contact angle increases whereas the transferred heat has a local maximum within the investigated pressure range. For the macro-scale drop impingement process, strong influence on the fluid dynamics and heat transfer is identified. In summary, numerical simulations of the evaporative drop impact and the modelling of micro-scale thermodynamic effects for low and high ambient pressure are investigated in the present thesis. The results increase the understanding of the influence of pressure on the fluid dynamics, as well as the heat and mass transfer. Correlations for the maximum spreading ratio, spreading duration, as well as transferred energy and mass are reported. The findings are expected to improve design concepts for technical applications within the investigated parameter range.

This paper presents a numerical and experimental study on hydrodynamic behavior of thin liquid films in rectangular domains. Three-dimensional computer simulations were performed using the lattice Boltzmann equation method (LBM). The liquid films laying on solid and liquid substrates are considered. The rupture of liquid films in computations is initiated via the thermocapillary (Marangoni) effect by applying an initial spatially localized temperature perturbation. The rupture scenario is found to depend on the shape of the temperature distribution and on the wettability of the solid substrate. For a wettable solid substrate, complete rupture does not occur: a residual thin liquid film remains at the substrate in the region of pseudo-rupture. For a non-wettable solid substrate, a sharp-peaked axisymmetric temperature distribution induces the rupture at the center of symmetry where the temperature is maximal. Axisymmetric temperature distribution with a flat-peaked temperature profile initiates rupture of the liquid film along a circle at some distance from the center of symmetry. The outer boundary of the rupture expands, while the inner liquid disk transforms into a toroidal figure and ultimately into an oscillating droplet.We also apply the LBM to simulations of an evolution of one or two holes in liquid films for two-layer systems of immiscible fluids in a rectangular cell. The computed patterns are successfully compared against the results of experimental visualizations. Both the experiments and the simulations demonstrate that the initially circular holes evolved in the rectangular cell undergoing drastic changes of their shape under the effects of the surface tension and gravity. In the case of two interacting holes, the disruption of the liquid bridge separating two holes is experimentally observed and numerically simulated.

In this chapter drop impacts onto a liquid layer of the same liquid as in the drop are considered. The chapter begins with consideration of such weak drop impacts on a liquid layer that they result only in capillary waves propagating over the surface. An interesting feature of these waves is that they are self-similar (Section 6.1). In the following Section 6.2 crown formation in strong (high-velocity) impacts onto thin liquid films is considered. Normal and oblique impacts of a single drop onto a wet wall are studied, as well as crown–crown interaction in sprays impacting the wall. Also, the evolution of the free rim on top of the crown is described. Then, in Section 6.3 drop impacts onto a thick liquid layer are considered and the dynamics of the crater formation is explained. Drop impacts onto a wet wall leave a residual liquid film on the wall which is addressed in Section 6.4. Drop impacts onto deep liquid pools produce a plethora of interesting morphological structures considered in Section 6.5. In the following Section 6.6 bending instability of a free rim is considered and the splashing mechanism is discussed. Splashing resulting from impacts of drop trains one-by-one is discussed in Section 6.7, where its physical mechanism and the link to splashing of a single drop impacting onto a liquid layer are elucidated. Several other regimes of drop impact are also mentioned. Drop Impact onto Thin Liquid Layer on a Wall: Weak Impacts and Self-similar Capillary Waves Consider patterns of capillary waves propagating over the free surface of a thin liquid film from the point where it was impacted normally by a tiny droplet or a stick (Fig. 6.1), as an example of a relatively weak (low-velocity) impact. For scales of the order of several millimeters the gravity effect on these waves is negligibly small, and for time scales of the order of several milliseconds viscosity effects can also be neglected.

We study the thinning and drainage of the intermediate liquid film between a bubble and a solid surface at close proximity in the presence of a surface acoustic wave (SAW) in the solid. Specifically, we employ the diffraction of light to observe a long air bubble confined in a solid rectangular channel filled with silicone oil. This setup, constituting a two-dimensional physical model of thin film drainage, allows us to analyze the influence of a SAW on the rate of thinning of the micron-thick liquid film separating the bubble and the solid substrate. The viscous penetration of the SAW into the liquid imposes a convective drift of mass, redistributing the fluid in the film against capillary resistance and producing a net drift of liquid out of the film. The rate of drainage of liquid from the film increases by one to several orders of magnitude in comparison to the rate of drainage due to the Laplace pressure of the bubble alone. The experimental findings agree well with a newly developed theory describing the SAW-enhanced drainage as a competition between the capillary flow and SAW-induced streaming.

Numerical investigation of oblique impact of multiple drops on thin liquid film

Numerical study of flow and heat transfer during a high-speed micro-drop impact on thin liquid films

The formation of the waves on a thin liquid water film was analytically investigated by studying its shear mode stability. The inclined angle of the substrate is limited to 8°. The purpose of analytical solution is to determine the maximum growth rate of the generated wave as well as its corresponding wave number, which is realized by solving the Orr-Sommerfeld equations for both gas and liquid phases with the corresponding boundary conditions. The results of wave formations on a surface with a thin liquid film of de-icing are validated by previous experimental data as well as compared with Yih’s theoretical analysis [7]. Studies have also conducted on the effect of surface tension or liquid film depth on the stability of a thin liquid film flowing along a solid substrate.

Due to the non-axisymmetric potential of the central bar, in addition to their characteristic arms and bar, barred spiral galaxies form a variety of structures within the thin gas disk, such as nuclear rings, inner spirals, and dust lanes. These structures in the inner kiloparsec are extremely important in order to explain and understand the rate of black hole feeding. The aim of this work is to investigate the influence of stellar bars in spiral galaxies on the thin self-gravitating gas disk. We focus on the accretion of gas onto the central supermassive black hole and its time-dependent evolution. We conducted multi-scale simulations simultaneously resolving the galactic disk and the accretion disk around the central black hole. In all the simulations we varied the initial gas disk mass. As an additional parameter we chose either the gas temperature for isothermal simulations or the cooling timescale for non-isothermal simulations. Accretion was either driven by a gravitationally unstable or clumpy accretion disk or by energy dissipation in strong shocks. Most of the simulations show a strong dependence of the accretion rate at the outer boundary of the central accretion disk (r< 300 pc) on the gas flow at kiloparsec scales. The final black hole masses reach up to ~109 M⊙after 1.6 Gyr. Our models show the expected influence of the Eddington limit and a decline in growth rate at the corresponding sub-Eddington limit.

A fuel injector with multi-swirl nozzles for finely atomizing a fuel spray in a gasoline engine was developed. The multi-swirl nozzles are composed of swirl chambers placed in the upstream of each orifice which is an outlet of injector. The swirl flow produced by the swirl chambers forms thin liquid films on the walls of the orifices by centrifugal force, and these films break up into small droplets outside the orifices. In this study, fuel flow within the swirl nozzle was simulated by using the volume of fluid (VOF) method, and the simulated velocities and width of the thin liquid film were used to predict the droplet size and spray angle of the fuel spray. Miyamoto’s atomization model was applied to connect a circumferential velocity and a width of the liquid film at the orifice outlet with the droplet size. It was found that the droplet size and spray angle of multi-swirl nozzles can be predicted by using the VOF method and Miyamoto’s atomization model.

In recent years, several gas-phase odor biosensors have been developed. Sensitive cells and multiple sensing elements were investigated for enhancing the performance of those biosensors. The thickness of the liquid layer that covers the cells, however, has rarely been so far explored. Recently we utilized the feedback control method to maintain a constant liquid film. Now we intended to step further about the alteration and estimation of this liquid film. Here, we developed a gas phase odor biosensor with variable liquid film thickness. The liquid layer thickness change was reflected as the impedance change between two electrodes. Thinner liquid film always resulted in greater response and faster peak time. For realizing such a thin liquid film state, withdrawing the buffer solution step by step was more preferred. Furthermore, the sample concentration dependency curves under normal and thin liquid layer conditions proved thin liquid film improved response evidently. Moreover, the weakened stability caused by thin liquid film was explained. Finally, we calculated the liquid film thickness according to the feedback control signal and validated the estimation result by manual operation. It revealed a steady liquid film thinner than 100 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> was implemented in this system. This study demonstrated the advantages of thin liquid layer and provided an access to estimation and fine adjustment of liquid film thickness in gas phase odor biosensors.

We have studied the dynamic structure of thin (approximately a few nanometers) liquid films of a nearly spherical, nonpolar molecule tetrakis(2-ethylhexoxy)silane (TEHOS) by using a combination of atomic force microscopy (AFM) and fluorescence correlation spectroscopy (FCS). Ultra-sensitive interferometer-based AFM was used to determine the stiffness (force gradient) and the damping coefficient of the liquid film. The experiments show oscillations in the damping coefficient with a period of approximately 1 nm, which is consistent with the molecular dimension of TEHOS as well as previous X-ray reflectivity measurements. Additionally, we performed FCS experiments for direct determination of the molecular dynamics within the liquid film. From the fluctuation autocorrelation curve, we measured the translational diffusion of the probe molecule embedded within the fluid film formed on a solid substrate. The autocorrelation function was best fitted with two components, which indicate that the dynamics are heterogeneous in nature. However, the heterogeneity is not as pronounced as had been previously observed for molecularly thin liquid films sandwiched between two solid substrates.

Drainage kinetics, thickness, and stability of water-in-oil thin liquid emulsion films obtained from asphaltenes, heavy oil (bitumen), and deasphalted heavy oil (maltenes) diluted in toluene are studied. The results show that asphaltenes stabilize thin organic liquid films at much lower concentrations than maltenes and bitumen. The drainage of thin organic liquid films containing asphaltenes is significantly slower than the drainage of the films containing maltenes and bitumen. The films stabilized by asphaltenes are much thicker (40-90 nm) than those stabilized by maltenes (∼10 nm). Such significant variation in the film properties points to different stabilization mechanisms of thin organic liquid films. Apparent aging effects, including gradual increase of film thickness, rigidity of oil/water interface, and formation of submicrometer size aggregates, were observed for thin organic liquid films containing asphaltenes. No aging effects were observed for films containing maltenes and bitumen in toluene. The increasing stability and lower drainage dynamics of asphaltene-containing thin liquid films are attributed to specific ability of asphaltenes to self-assemble and form 3D network in the film. The characteristic length of stable films is well beyond the size of single asphaltene molecules, nanoaggregates, or even clusters of nanoaggregates reported in the literature. Buildup of such 3D structure modifies the rheological properties of the liquid film to be non-Newtonian with yield stress (gel like). Formation of such network structure appears to be responsible for the slower drainage of thin asphaltenes in toluene liquid films. The yield stress of liquid film as small as ∼10(-2) Pa is sufficient to stop the drainage before the film reaches the critical thickness at which film rupture occurs.

The Strength of Long-Range Forces across Thin Liquid Films

Exploring and controlling the physical factors that determine the topography of thin liquid dielectric films are of interest in manifold fields of research in physics, applied mathematics, and engineering and have been a key aspect of many technological advancements. Visualization of thin liquid dielectric film topography and local thickness measurements are essential tools for characterizing and interpreting the underlying processes. However, achieving high sensitivity with respect to subnanometric changes in thickness via standard optical methods is challenging. We propose a combined imaging and optical patterning projection platform that is capable of optically inducing dynamical flows in thin liquid dielectric films and plasmonically resolving the resulting changes in topography and thickness. In particular, we employ the thermocapillary effect in fluids as a novel heat-based method to tune plasmonic resonances and visualize dynamical processes in thin liquid dielectric films. The presented results indicate that light-induced thermocapillary flows can form and translate droplets and create indentation patterns on demand in thin liquid dielectric films of subwavelength thickness and that plasmonic microscopy can image these fluid dynamical processes with a subnanometer sensitivity along the vertical direction.

Numerical simulation of Taylor bubble formation in a microchannel with a converging shape mixing junction