Drop Impacts with Liquid Pools and Layers

Impact of liquid layer thickness on the dynamics of nano- to sub-microsecond phenomena of nanosecond pulsed laser ablation in liquid

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.

A numerical study on splash of oblique drop impact on wet walls

Bubbles dispersed in thin liquid layers are ubiquitous and play important roles in the heat and mass transfer in nature and industrial processes including the energy, chemical, and biology engineering; thus, understanding the dynamics of bubbles confined in a liquid layer remains an important topic in multiphase flows. Here, we report the dynamics of bubble formation from a submerged orifice in a thin liquid layer with a thickness comparable to the bubble size. First, four flow regimes, detachment, jetting bursting, non-jetting bursting, and hole-opened bursting, are observed experimentally and their typical features are analyzed. Then, the evolutions of bubble size at different regimes are studied, and a quasi-static force model is proposed to predict the bubble size, which agrees well with the experimental results. We demonstrate the role of the capillary force exerted by the liquid film in the bubble cap and bubble geometry confined in the liquid layer in modulating the bubble size. Finally, a regime map regarding the liquid layer thickness and surface tension is provided and the criteria between different regimes are discussed based on the bubble geometry analysis and energy balance. Our experimental investigation and theoretical analysis provide insights into the formation and bursting dynamics of bubbles confined in a liquid layer.

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

Investigations into the phenomenology of convection flows next to the interface between two liquids have been carried out in laboratory experiments using various liquids. Then convection flows have been observed in industrial tests during continuous casting. The results show that the motional velocity of volume elements next to the interface due to disturbances of interfacial tension (produced by mass and charge transfer) depends on liquid layer thickness and on liquid properties. A new dimensionless number is introduced to describe this manner of convective flow; it is also suitable for evaluation of experimental results. Furthermore, a dimensionless function is theoretically developed to describe the relation between convective flows near the interface in the slag layer and mass transport.Casting powders melt on the surface of the liquid metal forming a liquid slag layer. Samples taken during casting have revealed convective flows in the flux layer and mass exchange with the liquid metal. Coefficients of the developed dimensionless function have been determined empirically for Ti‐transport from the interface into the flux layer.

A free bubble reaching the liquid surface usually bursts and then forms a liquid jet with drops ejected. While bubble-mediated jetting is a topic widely studied, few investigations deal with the jet produced by a growing bubble. Here, we report and characterize a novel phenomenon, named periodic bubbling-bursting, that can develop when a continuous stream of gas penetrates through a thin liquid layer. This behavior is complex with a characteristic frequency and can be divided into three stages from bubbling to cavity collapse and jetting. We show that increasing the liquid layer thickness and gas velocity leads to a larger bubble. However, the effect is strongly coupled with the orifice diameter and a scaling law of the bubble rupture radius is derived. Subsequently, we demonstrate that the collapsing cavities exhibit shape similarity and deduce the dependence of pinch-off height and opening angle of the conical cavity on the bubble rupture radius and liquid layer thickness. This enables us to disentangle three different neck-pinching mechanisms at play in pinch-off. Accordingly, gravity shapes the cavity and participates in the capillary wave selection that strongly modulates the jet formation. With increasing layer thickness, the jet first becomes fat and small and then ends up thinner and higher, detaching more and smaller droplets. We present a simple scaling law for the jet velocity which involves the liquid layer thickness to the power 1/2. Finally, a phase diagram for jet breakup and no breakup is built with respect to the initial Weber and Bond numbers.

The problem of thermocapillary deformation and film breakdown in a thin horizontal layer of viscous incompressible liquid with a free surface is considered. The deformable liquid layer is locally heated. The problem of thermocapillary deformation of the locally heated horizontal liquid layer has been solved numerically for two-dimensional unsteady case. The lubrication approximation theory is used. Capillary pressure, viscosity and gravity are taken into account. Evaporating rate is supposed to be proportional to the temperature difference between the liquid and ambient. Heat transfer in the substrate is also simulated. The numerical algorithm for the joint solution of the energy equation and the evolution equation for the thickness of liquid layer has been developed. The model predicts the thermocapillary deformation of the liquid surface and the formation of dry spots. The dynamics of liquid surface, the dry spots formation and the velocity of the contact line have been calculated. The deformation of the free surface has been calculated for different values of the heating power and thickness of the liquid layer. The effect of surface tension coefficient and wetting contact angle on the velocity of the contact line motion has been analyzed. It has been obtained that the velocity of the contact line increases with the increase of the wetting contact angle value and of the surface tension coefficient.

2 creates a need for a heat transfer design model which incorporates enough physical detail to yield accurate predictions while being sufficiently simplified to allow its use in routine design computations. The spray cooling group at West Virginia University is pursuing a coordinated program of laboratory experiments and computational simulations to develop a Monte Carlo-based spray cooling model that will satisfy this requirement. This paper reports progress in both the laboratory and computational fluid dynamics (CFD) phases of this study, including comparisons between single droplet impact results at We values of 161 and 633 to validate the CFD simulations and experimental measurements. A key goal of the present work is to determine the thickness of the thin liquid layer remaining in the crater formed when a liquid droplet impacts a surface covered by a preexisting liquid film. The volume of liquid in this thin liquid film in the droplet impact crater is one factor that will influence the onset of critical heat flux (CHF). Based on the present data the preexisting static film thickness is observed to have more of an effect than the initial drop parameters on the minimum liquid volume under the crater, but the crater lifetime depends on the initial drop conditions. The comparisons between experiments and simulations for these two cases show promise, but more refinement in experimental and computational technique are needed to achieve more consistent determinations of the volume of liquid under a crater.

We present the rationale, methods, and results of modeling of thin film organic liquids on various substrates. These liquids may coat surfaces (substrates) either as a result of their production, dispersal via aerosols or spills. Identification of unknown coated surfaces using either reflectance or emittance spectroscopy cannot be accomplished simply through reference to reflectance signature libraries since neither the thickness of the liquid layer nor the substrate type is known beforehand and both contribute to the signature. Liquid spectral libraries offer the complex index of refraction (n,k) as a function of wavelength which by itself is useful only for thick (bulk) liquid layers via computation of reflectance and transmittance coefficients using the Fresnel equations. Thin liquid layers both reflect and refract incident light in combination with reflectance from the substrate. We show modeling of various organic liquids on substrates using commercial thin film design and modeling software, as well as Monte Carlo ray tracing software to demonstrate the variety of potential signatures encountered that depend on the thickness of the liquid layer as well as the characteristics of the substrate (metal or dielectric). These substrates give rise to transflectance behavior, while many dielectric substrates have rich absorption features that provide complex signatures that combine attributes of both the liquid and the substrate. Knowledge of the complex index of refraction of both target liquids and substrates is essential in order to synthesize spectra necessary in the application of target identification algorithms.

Phenomenological study and comparison of droplet impact dynamics on a dry surface, thin liquid film, liquid film and shallow pool

A Lattice Boltzmann Approach to Multi-Phase Surface Reactions with Heat Effects

This thesis focuses on the high-speed impact of liquid drops on dry smooth surfaces and thin liquid films in the presence of a strong gas flow using experimental and numerical methods. A flywheel experiment was designed to study such challenging conditions by analyzing the impact with different drop diameters, velocities, liquids and gas mixtures. A sophisticated algorithm was also developed for image analysis and to handle a large number of experimental data. Selected experimental cases were complemented with numerical simulations, which provided insights on the flow around and inside the impacting drop. The results of the drop impact on dry surfaces indicate that the splashing scenario depends strongly on the physical properties of the liquids and not on the surrounding gas or the kinematic impact conditions. It was also demonstrated that the mechanism of gas entrapment at the early stage of the impact is not responsible for the splashing of drops; however, the physical properties of the surrounding gas influenced the spreading lamella and the ejection of secondary droplets. The results demonstrated that the prompt splash is indeed well described by the Rayleigh-Taylor instability in the spreading lamella. The drop impact on wetted surfaces always generates a chaotic and thin corona, which bends and deforms itself during the entire splashing process. Contrary to the splashing on dry surfaces, the break-up of the corona can be attributed to at least three different instabilities: rim-bending, Rayleigh-Taylor, and Rayleigh-Plateau instabilities. Those instabilities generate a host of small secondary droplets in a wide range of sizes and velocities. It was found that the thin ejected corona leads to hole formation and the eventual detachment from its base at the last stage of impact. The thickness of the corona was also estimated using two different theoretical methods. The outcome of high-speed impact on dry and wetted surfaces were quantified in this thesis. The results of the theoretical, experimental, and numerical analyses were combined to describe completely the formation of the secondary droplets. The presented methods allow the prediction of the splashing phenomenon for a drop impacting dry or wetted surfaces at low and high speed.

ABSTRACTPromoting both high functional group preservation and high deposition rates is a major challenge in plasma deposition. This study explores a dynamic method using nanosecond pulsed plasma discharges to polymerize thin liquid layers of 2‐hydroxyethyl methacrylate (HEMA). The influence of plasma pulse frequency (200 ns) and liquid layer thickness on chemical structure retention, polymeric growth, and thin film growth rate is examined. High‐resolution mass spectrometry (HRMS) highlights that plasma curing of liquid monomer layers achieves significantly better chemical structure preservation and polymeric growth, along with deposition rates an order of magnitude higher than traditional vapour‐phase methods. This work provides valuable insights and guidelines for plasma‐induced free‐radical polymerization of liquid layers into functional thin solid films.

This study systematically investigates the dynamics of particle–plate collisions across varying liquid layer depths, aiming to elucidate the influence of liquid layer thickness on impact processes, rebound behavior, and impact forces. Using a custom-built experimental setup combining high-speed imaging and multi-point dynamic pressure sensors, the effects of particle size, initial velocity, and liquid layer thickness on collision dynamics were examined. Results reveal that the liquid layer depth significantly alters liquid bridge morphology and rebound characteristics. For thin liquid layers (h/d &amp;lt; 0.4), collisions are dominated by lubrication and capillary effects, resulting in high restitution coefficients (e). As the liquid layer thickness increases to the shallow regime (0.4 ≤ h/d ≤ 1.2), e decreases sharply due to enhanced viscous dissipation and inertial damping. An exponential decay model fitted to the experimental data shows that e decreases exponentially with increasing h/d, with the decay rate closely related to the initial impact velocity. Additionally, measurements of impact forces in the liquid layer show that thicker liquid layers more effectively cushion the impact, resulting in significantly reduced forces. This work enhances understanding of the role of liquid layer thickness in particle–plate collisions, with implications for multiphase flow transport and engineering design.