Liquid Droplet Deposition Research Articles

Evaluation of droplet deposition parameters based on the Genetic-Otsu algorithm.

Pesticide spraying is a cost-effective way to control crop pests and diseases. The effectiveness of this method relies on the deposition and distribution of the spray droplets within the targeted application area. There is a critical need for an accurate and stable detection algorithm to evaluate the liquid droplet deposition parameters on the water-sensitive paper (WSP) and reduce the impact of image noise. This study acquired 90 WSP samples with diverse coverage through field spraying experiments. The droplets on the WSP were subsequently isolated, and the coverage and density were computed, employing the fixed threshold method, the Otsu threshold method, and our Genetic-Otsu threshold method. Based on the benchmark of manually measured data, an error analysis was conducted on the accuracy of three methods, and a comprehensive evaluation was carried out. The relative error results indicate that the Genetic-Otsu method proposed in this research demonstrates superior performance in detecting droplet coverage and density. The relative errors of droplet density in the sparse, medium, and dense droplet groups are 2.7%, 1.5%, and 2.0%, respectively. The relative errors of droplet coverage are 1.5%, 0.88%, and 1.2%, respectively. These results demonstrate that the Genetic-Otsu algorithm outperforms the other two algorithms. The proposed algorithm effectively identifies small-sized droplets and accurately distinguishes the multiple independent contours of adjacent droplets even in dense droplet groups, demonstrating excellent performance. Overall, the Genetic-Otsu algorithm offered a reliable solution for detecting droplet deposition parameters on WSP, providing an efficient tool for evaluating droplet deposition parameters in UAV pesticide spraying applications.

Molecular-level studies of extracellular matrix proteins conducted using atomic force microscopy.

Extracellular matrix (ECM) proteins provide anchorage and structural strength to cells and tissues in the body and, thus, are fundamental molecular components for processes of cell proliferation, growth, and function. Atomic force microscopy (AFM) has increasingly become a valuable approach for studying biological molecules such as ECM proteins at the level of individual molecules. Operational modes of AFM can be used to acquire the measurements of the physical, electronic, and mechanical properties of samples, as well as for viewing the intricate details of the surface chemistry of samples. Investigations of the morphology and properties of biomolecules at the nanoscale can be useful for understanding the interactions between ECM proteins and biological molecules such as cells, DNA, and other proteins. Methods for preparing protein samples for AFM studies require only basic steps, such as the immersion of a substrate in a dilute solution or protein, or the deposition of liquid droplets of protein suspensions on a flat, clean surface. Protocols of nanolithography have been used to define the arrangement of proteins for AFM studies. Using AFM, mechanical and force measurements with tips that are coated with ECM proteins can be captured in ambient or aqueous environments. In this review, representative examples of AFM studies are described for molecular-level investigations of the structure, surface assembly, protein-cell interactions, and mechanical properties of ECM proteins (collagen, elastin, fibronectin, and laminin). Methods used for sample preparation as well as characterization with modes of AFM will be discussed.

Analysis of the influence of swirling flow on the boiling heat transfer characteristics of two-phase flow

The Eulerian-Eulerian (EE) simulation approach + GENeralized TwO Phase (GENTOP) concept and two-fluid three-flow field with droplet were used to simulate the heat and mass transfer. Analysis encompassed the examination of steam and liquid phase distributions, as well as flow pattern transitions. Heat transfer enhancements were quantified using a comprehensive heat transfer factor. The disturbance intensity of the secondary flow was characterized by the absolute vortex flux. Results revealed a correlation between the intensity of the secondary flow and axial length, indicative of progressive fluid disturbance enhancement. Additionally, the maximal motion of the steam phase vortex was observed both near the wall and channel center. The EE+GENTOP concept exhibited superior capability in reproducing multi-scale flow including small-scale dispersed bubbles and large-scale continuous steam (coalescence and break-up). For investigations into the entrainment and deposition of liquid droplets in annular flow, the selection of a two-fluid three-flow field with the droplet model is recommended.

CFD modeling of two-phase annular flow toward the onset of liquid film reversal in a vertical pipe

This paper presents a CFD simulation to film reversal phenomena in an annular flow in a vertical pipe of diameter 76.2 mm by performing a 2D axisymmetric numerical simulations using ANSYS Fluent® version 16.1 commercial software. In a vertical annular flow, the gas phase is driven by the pressure gradient along the pipe while the liquid is, mainly, driven by the shear force acting at the liquid-gas interface. When the gas velocity decreases, the liquid film will start falling down causing a problem known as "liquid loading" or film reversal phenomenon.The aim is to predict the onset of film reversal so as to allow necessary measures to be taken to mitigate the problem of liquid loading. Simulations were made using air and water as the main working fluids in a 3 m long pipe. The model results agreed well when compared with experimental results, from literature, regarding pressure gradient measurements and critical velocity predictions. Due to the presence of the viscous sublayer, the shear stress fluctuates with high amplitude in the vicinity of the pipe wall where the liquid phase dominates and gradually reduces towards the pipe center. The mass transfer at the interface (entrainment and deposition of liquid droplets) may also affect the shear stress and its space and time variations.

Extraction of Aerosol-Deposited Yersinia pestis from Indoor Surfaces To Determine Bacterial Environmental Decay

Public health and decontamination decisions following an event that causes indoor contamination with a biological agent require knowledge of the environmental persistence of the agent. The goals of this study were to develop methods for experimentally depositing bacteria onto indoor surfaces via aerosol, evaluate methods for sampling and enumerating the agent on surfaces, and use these methods to determine bacterial surface decay. A specialized aerosol deposition chamber was constructed, and methods were established for reproducible and uniform aerosol deposition of bacteria onto four coupon types. The deposition chamber facilitated the control of relative humidity (RH; 10 to 70%) following particle deposition to mimic the conditions of indoor environments, as RH is not controlled by standard heating, ventilation, and air conditioning (HVAC) systems. Extraction and culture-based enumeration methods to quantify the viable bacteria on coupons were shown to be highly sensitive and reproducible. To demonstrate the usefulness of the system for decay studies,Yersinia pestis persistence as a function of surface type at 21 °C and 40% RH was determined to be >40%/min for all surfaces. Based upon these results, at typical indoor temperature and RH, a 6-log reduction in titer would expected to be achieved within 1 h as the result of environmental decay on surfaces without active decontamination. The developed approach will facilitate future persistence and decontamination studies with a broad range of biological agents and surfaces, providing agent decay data to inform both assessments of risk to personnel entering a contaminated site and decontamination decisions following biological contamination of an indoor environment. Public health and decontamination decisions following contamination of an indoor environment with a biological agent require knowledge of the environmental persistence of the agent. Previous studies on Y. pestis persistence have utilized large liquid droplet deposition to provide persistence data. As a result, methods were developed to deposit aerosols containing bacteria onto indoor surfaces, reproducibly enumerate bacteria harvested from coupons, and determine surface decay utilizing Y. pestis The results of this study provide foundational methods required to evaluate surface decay of bacteria and potentially other biological agents, such as viruses, in aerosol particles as a function of surface type and environment. Integrating the data from both aerosol and liquid deposition surface decay studies will provide medical and public health personnel with a more complete understanding of agent persistence on surfaces in contaminated areas for assessment of health risks and to inform decontamination decisions.

Behaviour of spreading molten metal drops deposited by fusion

Liquid droplet deposition on solid surfaces has an important role in the industrial and research activities. The behaviour of such deposit is influenced by volume and interfacial phenomena and involves a large number of mechanisms such as gravity effect, mass transfer, capillary forces, and wetting. For the case of metal deposition the analysis of the problem is more complex because of the importance of thermal effects, involving steep gradients and phase changes. A unique experimental approach is presented in order to study the evolution of the spreading of a large drop of liquid metal called “macro-drop”. The objective of this work is to supply qualitative and quantitative information during the deposit of liquid metal in relation with process parameters. The overall shape of the macro-drop, especially its spreading and contact angles are studied in detail. The gradual spreading of the macro-drop is mainly governed by mass and heat transfers. The initial rapid spreading is due to kinetic energy of depositing droplets and direct arc heating on the solid target. All experimental results are analysed in the light of process parameters to identify the physical mechanisms involved and appreciate their effects on the behaviour of such a macro-drop.

Inkjet printing as a possible route to study confined crystal structures

Inkjet printing is a technique for the precise deposition of liquid droplets in the pL-volume range in well-defined patterns. Previous studies have shown that inkjet printing is attractive in polymer technology since it permits the controlled deposition of functional polymer surfaces. We suggest that the technique might also be useful for studying crystallization, in particular confined crystallization. Inkjet printing is a non-contact deposition method with minimal risk of contamination, which allows the exact deposition of both polymer solutions and polymer melts. This paper demonstrates the possibility of utilizing the technique to create surfaces where polymer chains form isolated small structures. These structures were confined by both the low polymer content in each droplet and the time constraint on crystal formation that arose as the result of the rapid solvent evaporation from the pL-sized droplets. In theory, inkjet printing enables the exact deposition of systems with as few as a single polymer chain in the average droplet. With appropriate instrumentation, the versatile inkjet technology can be utilized to create whole surfaces covered with polymer structures formed by the crystallization of small, dilute and rapidly evaporating droplets. 110pL droplets of a 10−6gL−1 poly(ε-caprolactone) solution in 1-butanol have been deposited and studied by atomic force microscopy. Small structures of ca. 10nm thickness and ca. 50nm diameter also seemed to exhibit crystalline features. Some of the small structures had unusual rectangular forms whilst others were interpreted to be early precursors to six-sided single crystals previously observed for poly(ε-caprolactone). The unusual forms observed may have resulted from the entrapment of crystal structures into metastable phases, due to the limited amount of polymer material present and the rapid evaporation of the droplets.

Evidence for Family-Meakin Dynamical Scaling in Island Growth and Coalescence during Vapor Phase Deposition

Using real-time grazing-incidence small-angle x-ray scattering, we find that the processes of island formation and coalescence during the room-temperature vapor phase deposition of aluminum lead to dynamical scaling of the evolving surface morphology. The scaling is quantitatively consistent with the self-similarity predicted by the Family-Meakin model, which was developed to describe liquid droplet deposition, growth, and coalescence. The Family-Meakin model assumes only that atomic diffusion over the substrate between islands or droplets is negligible and that diffusion between impinging islands or droplets is sufficient to give complete coalescence. Therefore the dynamical scaling morphology evolution identified here may be common in the initial stages of those solid film growth processes which proceed by island formation and growth.

Four degree of freedom liquid dispenser for direct write capillary self-assembly with sub-nanoliter precision

Capillary forces provide a ubiquitous means of organizing micro- and nanoscale structures on substrates. In order to investigate the mechanism of capillary self-assembly and to fabricate complex ordered structures, precise control of the meniscus shape is needed. We present a precision instrument that enables deposition of liquid droplets spanning from 2 nl to 300 μl, in concert with mechanical manipulation of the liquid-substrate interface with four degrees of freedom. The substrate has sub-100 nm positioning resolution in three axes of translation, and its temperature is controlled using thermoelectric modules. The capillary tip can rotate about the vertical axis while simultaneously dispensing liquid onto the substrate. Liquid is displaced using a custom bidirectional diaphragm pump, in which an elastic membrane is hydraulically actuated by a stainless steel syringe. The syringe is driven by a piezoelectric actuator, enabling nanoliter volume and rate control. A quantitative model of the liquid dispenser is verified experimentally, and suggests that compressibility in the hydraulic line deamplifies the syringe stroke, enabling sub-nanoliter resolution control of liquid displacement at the capillary tip. We use this system to contact-print water and oil droplets by mechanical manipulation of a liquid bridge between the capillary and the substrate. Finally, we study the effect of droplet volume and substrate temperature on the evaporative self-assembly of monodisperse polymer microspheres from sessile droplets, and demonstrate the formation of 3D chiral assemblies of micro-rods by rotation of the capillary tip during evaporative assembly.

Core Heat Transfer Coefficients Immediately Downstream of the Rewetting Front during Anticipated Operational Occurrences for BWRs

A heat transfer coefficient (HTC) model was developed for the prediction of post-boiling transition (post-BT) behavior that might occur during anticipated operational occurrences (AOOs) for boiling water reactors (BWRs). The model development was based on measurements of heat transfer coefficient, liquid droplet deposition rate, and droplet concentration in our experiments conducted at high pressure. The model focused on the heat transfer near the rewetting front where the cooling by droplet deposition significantly affects the propagation behavior of a liquid film. The correlation by Sugawara was validated for the prediction of the deposition by using the experimental data. The model was also expressed as a function of the distance from the rewetting front to use in analytical models for the rewetting propagation. Both expressions of the present model successfully predicted our experimental data simulating the BWR thermal-hydraulic conditions.

ICONE19-43536 Development of CFD Analysis Method Based on Droplet Tracking Model for BWR Fuel Assemblies

It is well known that the minimum critical power ratio (MCPR) of the boiling water reactor (BWR) fuel assembly depends on the spacer grid type. Recently, improvement of the critical power is being studied by using a spacer grid with mixing devices attaching various types of flow deflectors. In order to predict the critical power of the improved BWR fuel assembly, we have developed an analysis method based on the consideration of detailed thermal-hydraulic mechanism of annular mist flow regime in the subchannels for an arbitrary spacer type. The proposed method is based on a computational fluid dynamics (CFD) model with a droplet tracking model for analyzing the vapor-phase turbulent flow in which droplets are transported in the subchannels of the BWR fuel assembly. We adopted the general-purpose CFD software Advance/FrontFlow/red (AFFr) as the base code, which is a commercial software package created as a part of Japanese national project. AFFr employs a three-dimensional (3D) unstructured grid system for application to complex geometries. First, AFFr was applied to single-phase flows of gas in the present paper. The calculated results were compared with experiments using a round cellular spacer in one subchannel to investigate the influence of the choice of turbulence model. The analyses using the large eddy simulation (LES) and re-normalisation group (RNG) k-ε models were carried out. The results of both the LES and RNG k-ε models show that calculations of velocity distribution and velocity fluctuation distribution in the spacer downstream reproduce the experimental results qualitatively. However, the velocity distribution analyzed by the LES model is better than that by the RNG k-ε model. The velocity fluctuation near the fuel rod, which is important for droplet deposition to the rod, is also simulated well by the LES model. Then, to examine the effect of the spacer shape on the analytical result, the gas flow analyses with the RNG k-ε model were performed for three kinds of spacers of actual fuel assemblies: a lattice type spacer, a round cell type spacer, and a grid spacer with mixing vanes, respectively. The results indicate that the swirling flow is generated by the mixing vane, and the turbulence energy increases near the surface of fuel rod. The Lagrangian method was applied to droplet behavior in turbulent gas flow around the round cell type spacer and that around a mixing vane spacer as trial analyses. It was confirmed that liquid droplet deposition is promoted in downstream from the mixing vane spacer because of swirling flow.

Core Heat Transfer Coefficients Immediately Downstream of the Rewetting Front during Anticipated Operational Occurrences for BWRs

A heat transfer coefficient (HTC) model was developed for the prediction of post-boiling transition (post-BT) behavior that might occur during anticipated operational occurrences (AOOs) for boiling water reactors (BWRs). The model development was based on measurements of heat transfer coefficient, liquid droplet deposition rate, and droplet concentration in our experiments conducted at high pressure. The model focused on the heat transfer near the rewetting front where the cooling by droplet deposition significantly affects the propagation behavior of a liquid film. The correlation by Sugawara was validated for the prediction of the deposition by using the experimental data. The model was also expressed as a function of the distance from the rewetting front to use in analytical models for the rewetting propagation. Both expressions of the present model successfully predicted our experimental data simulating the BWR thermal-hydraulic conditions.

Capillary-based liquid microdroplet deposition

Liquid droplet deposition through a capillary onto a substrate is studied. The application of pressure into the capillary causes a liquid meniscus to form at the outlet. Touching the substrate with the liquid meniscus and subsequent capillary retraction gives liquid deposition on the substrate. Theoretical and experimental studies of the steady liquid bridge structure between the capillary and substrate identified the range of parameters when deposition of small droplets (no blot) can be performed. Experiments revealed that in this range of parameters the size of deposited liquid droplets is less than 10%–15% of inner diameter of the capillary.

Visualization of flow boiling of liquid nitrogen in a vertical mini-tube

The flow boiling patterns of liquid nitrogen in a vertical mini-tube with an inner diameter of 1.931 mm are visualized with a high-speed digital camera. The superficial gas and liquid velocities are in the ranges of 0.01–26.5 m/s and 0.01–1.2 m/s, respectively. Four typical flow patterns, namely, bubbly, slug, churn and annular flow are observed. Some interesting scenes about the entrainment and liquid droplet deposition in the churn and annular flow, and the flow reversal with the indication of negative pressure drop, are also presented. Based on the visualization, the two-phase flow regime maps are obtained. Compared with the flow regime maps for gas–water flow in tubes with similar hydraulic diameters, the region of slug flow in the present study reduces significantly. Correspondingly, the transition boundary from the bubbly flow to slug flow shifts to higher superficial gas velocity, and that of churn to annular flow moves to lower superficial gas velocity. Moreover, time-averaged void fraction is calculated by quantitative image-digitizing technique and compared with various prediction models. Finally, three kinds of oscillations with long-period and large-amplitude are found, possible explanation for the oscillations is given by comparing the instantaneous flow images with the data of pressure, mass flux and temperature recorded synchronously.

Near‐wall gas‐droplet flows with liquid‐film formation

AbstractA self‐consistent asymptotic theory of a near‐wall aerodisperse flow with the formation of a liquid film due to inertial deposition of liquid droplets on the body surface is constructed. With the reference to the examples of low‐speed mist flows near plane and curvilinear solid surfaces, asymptotically different situations are analyzed. The two‐fluid equations of the mist flow are solved simultaneously with the equations of the liquid film, on the outer edge of which the distributed mass, momentum, and energy fluxes (found from the solution of the outer problem) are specified. It is shown that depending on the values of governing parameters the flow in the film should be described by either boundary‐layer or creeping‐flow equations in the layer with a priori unknown thickness. For the film on the frontal surface of a blunt body, the film thickness and the friction and heat transfer coefficients are found numerically. The conditions in which the presence of the film significantly reduces the heat fluxes are determined and the possibility of the realization of steady‐state flow regimes in the film is demonstrated. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Computer Simulations of Homogeneous Deposition of Liquid Droplets

Understanding the deposition of liquid droplets on surfaces is essential in many environmental and industrial processes. In this paper, we describe computer simulations of homogeneous deposition of liquid droplets on an ideal, smooth, and horizontal solid surface. The statistical evolution of droplet deposition and growth processes are investigated. It is found that three basic events, namely, deposition, incorporation, and coalescence, produce droplets of different sizes and that the droplet size polydispersity is continuously increasing with time leading to a bimodal distribution. By considering the total number of droplets N t o t on the surface versus time, we demonstrate that four growth regimes must be considered. These regimes reflect the relative influence of the three events during the deposition process. Variation with time of the surface coverage Γ as a function of the contact angle θ between the droplets and the surface is also investigated. The effect of a variable contact angle on the value of surface saturation and dynamical growth is calculated. Finally, by considering the total mass of the deposited material and surface coverage, some guidelines to achieve an efficient droplet deposition process and surface coverage versus total droplet mass are proposed.

Experimental Investigation of the Deposition of Liquid Droplets on the Walls of a Vertical Cylindrical Channel from Wall Gas-Droplet Jets

The results are given of an experimental investigation of the formation of liquid films from wall gas-droplet jets under quasi-isothermal conditions. The experiments are performed in the initial segment of a round cylindrical tube under conditions of varying cocurrent-flow parameter m = ρsws/ρ0w0 and concentration of liquid droplets. The instantaneous thickness of the liquid film is measured by a capacitive sensor. The distributions of the liquid-film thickness and of the mass rate of particle deposition along the length of the channel are obtained. The pattern of deposition is compared with available data for a stabilized two-phase flow in a tube. Reasons for the difference of the laws of liquid-droplet deposition from wall gas-droplet jets are discussed.

Investigation of Capillary Forces Using Atomic Force Microscopy

An atomic force microscope tip, coated with a small amount of liquid silicone, was used to investigate the wetting and capillary bridging forces on various low- and high-energy surfaces. The low-energy surfaces were prepared by reacting alkyl and perfluoroalkyl functional silanes with a silicon wafer (Si/SiO2). Force−distance scans in air revealed that the silicone fluid forms ductile capillary bridges on the low-energy methyl and perfluoromethyl surfaces, whereas a tight bridge is formed on silica. Further studies on a silicon wafer possessing a gradient of surface energy shed more light on the relationship between surface wettability and capillary forces. These observations can be modeled in a general way using the Young−Laplace equation. The understanding of these capillary interactions at nanoscopic levels may have important applications, especially in the controlled deposition of liquid droplets on surfaces.

Developments in the modelling of horizontal annular two-phase flow

Several authors have considered the extension from vertically upward flow to horizontal flow of a model for annular two-phase flow which allows separate descriptions of liquid droplet deposition and entrainment. However, solutions to the equations which result have either not been obtained or have been found to be mathematically and physically unacceptable. In this paper it is shown how to obtain acceptable solutions. In addition, various physical mechanisms which could improve the model are assessed. Of these it is shown that a variable deposition flux could lead to significant improvement between theory and experiment. A model for the variable deposition flux is then developed and is shown to give results at least in qualitative agreement with known experimental results. The inclusion of the variable deposition rate in the governing equations leads to encouraging results for film thickness and film flowrate distributions.

Slag and other liquid behavior on vertical surface at near-freezing temperature

Deposition of liquid droplets from turbulent stream to vertical surface and the subsequent transient behavior of the liquid layer are analyzed for the surface temperatures near the freezing point of the liquid. General wave behavior of the equations governing the liquid layer is elucidated. The analysis is applied to the problem of slag layer accumulation on the passage walls of a magnetohydrodynamic regenerative heat exchanger using the coal combustion product as the heat source. The wave behavior predicts the emergence of an accumulation shock that leads to clogging of the passages for certain cyclic operations.