Droplet Motion Research Articles

A Numerical Investigation into the Spread Characteristics of a Human Virus-Carrying Droplet in a Classroom Environment.

In public health, the transmission characteristics and laws of highly infectious virus-carrying particles in the air environment have become a hot topic. The study on the spread characteristics of human virus-carrying droplets in a typical densely populated space is necessary. As such, a classroom space lattice Boltzmann method (LBM) model with a dense population is established to simulate and analyze the spreading and diffusing behavior of pathogenic droplets. The results show that the dispersion density is mainly affected by the mainstream wind direction in the area of concern, and particle aggregation is more likely to form in the area close to the wind disturbance. Due to the dense thermal plumes, the droplet movement is a clear convergence towards the upper space of the classroom. This could explain the fact that people living above confirmed cases are now more likely to be infected.

Lattice Boltzmann Study on the Motion of Dual Droplets in Microchannels With Contact Angle Hysteresis

The contact angle hysteresis is defined as the difference between the advancing and receding contact angles, which is an important phenomenon in the two-phase flow on the wetting surface. In this paper, an improved pseudo-potential lattice Boltzmann (LB) Multiphase flow model, combined with geometric wetting boundary conditions, is employed to study the motion behavior of two droplets in micro-channel with different contact angle hysteresis. We have mainly studied the effects of capillary number, wettability, the width of contact angle hysteresis window, the initial distance between two droplets and the relative size of droplets on the dynamic behavior of two droplets in the micro-channel. The research results show that the increase of capillary number is conducive to the movement of droplets, but not conducive to the discharge of droplets from the pipeline, and the influence of the capillary number on the upstream droplet is greater than that on the downstream droplet. On the other hand, the larger the contact angle hysteresis window, the slower the droplet motion and deformation, but the more obvious about the deformation, and the two droplets merge earlier but discharge the pipeline later; In addition, with the increase of the initial distance between the two droplets, the merging-fracture mode of the two droplets changes from first merging and then fracture to in-process fracture and first fracture and then merging. Correspondingly, when the initial distance between the two droplets is small, the time for the two droplets to completely discharge the channel decreases slowly with the increase of the initial distance, while when the initial distance between the two droplets is large, the discharge time of the two droplets first decreases rapidly with the increase of the distance, and then remains basically unchanged; Finally, the results also show that the larger the relative size difference between upstream and downstream droplets, the more unfavorable it is for droplets to discharge from the pipeline.

Photoinduced collective motion of oil droplets and concurrent pattern formation in surfactant solution

Photoinduced collective motion of oil droplets and concurrent pattern formation in surfactant solution

Evaluation of the distribution of COVID-19 particles in an Isolation room to reduce the possibility of transmission via CFD simulation

The outbreak and prolonged COVID-19 pandemic has caused a population decline as well as a profound impact on the global economy, the COVID virus spreads highly in the air through the process of sneezing, contact leads to many dangers to the health and safety of people around the world. Many simulation studies have been carried out to predict the risk of spread, as well as to find solutions to limit the infection when spreading sneeze droplets in the air. In this study, the motion and distribution of droplets containing coronaviruses emitted by coughing or sneezing in the isolation room at Ho Chi Minh City, Vietnam National University were investigated using ANSYS Fluent software. The airflow in the isolation room was simulated by a 3D turbulence model and energy equation using the finite volume method (FVM) with a domain of isolation room solved for appropriate boundary conditions. The effect of ventilation airflow speed and the size of droplets on the distribution of particles in the air were investigated by the Lagrangian particle trajectory analysis method. The CFD analysis result showed that the velocity distribution, turbulent kinetic energy, and flow dynamics had strongly affected the reducing rate of average droplet concentration in the isolation room. Specifically, the study focused on the dispersion of liquid droplets containing the virus under fixed operating conditions of ventilation systems and exhaust fans, with a flow rate of 840 m3/h. Under these conditions, the retention time of liquid droplets was determined to be 37.5 seconds, corresponding to droplet diameters ranging from 5 to 100 micrometres. The results indicate that the presence of particles in the room gradually becomes diluted over time due to the continuous circulation of air. The ability of air to diffuse within the vicinity of the occupant's position and the limited spread of small particles within the room demonstrate that the operating conditions are suitable.

The Whole Theory of This Universe—A Step Forward to Einstein, Part-3<sup>rd</sup>: “The Universal Theory of Visible and Invisible Universe”

We believe that the universe is of two types: visible and invisible. Nothing is at rest between the invisible and visible universes. All microscopic bodies as well as all macroscopic bodies are in motion along curved paths (i.e. in circles). The universe inside an atom is as vast as the visible universe. An atom consists of millions of particles or particle galaxies which contain central energy pools or central energy cores. Energy pools present in the centre of the invisible universe inside atomic or subatomic particles from which particles and energy are continuously interconverting. In a dense central energy pool, two opposite charges are created due to the swirling motion of microscopic energy droplets. Small microscopic energy droplets may swirl either clockwise or anti-clockwise to produce microscopic tornadoes which are non-superimposable mirror images of each other and gain the property of positive and negative charges. Hence, the electrostatic force is originated between these two opposite charges, which are then changed into a pair of particles, i.e. catitron, which carries a positive charge, and anitron which carries a negative charge. All the other millions of subatomic particles or particle galaxies are produced in the same way. So, the electrostatic force is the basic force, and all other forces originate from this basic electrostatic force of attraction. When charged particles move, they produce an oscillating electric field, and the spinning of these particles produces oscillating magnetic fields. These oscillating electric and magnetic fields are perpendicular to each other and, by their interaction, an oscillating gravitational field is produced which is also perpendicular to both the oscillating electric field and the oscillating magnetic field. The Earth’s axial tilt, which causes the Earth’s precessional motion, is caused by the parallel alignment of the Earth’s magnetic field with the magnetic field of the Sun. Gravity is not a cause of space-time curvature, but gravity causes space-time curvature. Space-time curvature is nothing but a curved path around a heavy object. The Universal Theory of Visible and Invisible Universe—The Whole Theory of This Universe—A Step Forward to Einstein, opens new windows in the challenging fields of science and research, i.e. visible and invisible universe, universe inside an atom, what is the stuff of the entire universe? What will happen at the end of this whole universe?

Investigation of flow characteristics on porous gas diffusion layer microstructure that generated with binder and polytetrafluoroethylene distribution

The drainage properties of a gas diffusion layer (GDL) are essential factors in the performance of proton exchange membrane fuel cells. The GDL consisting of a three-dimensional (3D) carbon paper microstructure was developed and meshed with pore-scale reconstruction models in this paper. Localized binder and polytetrafluoroethylene (PTFE) structures were added to the carbon paper microstructure through 3D morphological imaging processing. The monitoring data of 1000 planes were multi-peaky fitted as a function of gas permeability and height to amend the macroscopic porous medium model. We analyzed drainage properties under different contact angles (θ) for the carbon paper with binder and PTFE. We described the mutual intrusion of moisture and air in GDL under different pressure differences. The results show that the pore-scale reconstruction model has the advantages of describing the flow in GDL accurately and with details, detecting low-flow resistance channels that spontaneously formed in GDL, and describing the variation of permeability as a function of location. In a hydrophobic environment, the liquid film connected to a GDL is challenging to split spontaneously. At the same time, the splitting motion of discrete droplets is more prominent than that of the liquid film. The pressure that enables complete water intrusion into the GDL is between 1 and 10 MPa.

Lattice Boltzmann Modeling of Cholesteric Liquid Crystal Droplets Under an Oscillatory Electric Field

We numerically study the dynamics of quasi-two dimensional cholesteric liquid crystal droplets in the presence of a time-dependent electric field, rotating at constant angular velocity. A surfactant sitting at droplet interface is also introduced to prevent droplet coalescence. The dynamics is modeled following a hybrid numerical approach, where a standard lattice Boltzmann technique solves the Navier-Stokes equation and a finite difference scheme integrates the evolution equations of liquid crystal and surfactant. Our results show that, once the field is turned on, the liquid crystal rotates coherently triggering a concurrent orbital motion of both droplets around each other, an effect due to the momentum transfer to the surrounding fluid. In addition the topological defects, resulting from the conflict orientation of the liquid crystal within the drops, exhibit a chaotic-like motion in cholesterics with a high pitch, in contrast with a regular one occurring along circular trajectories observed in nematics drops. Such behavior is found to depend on magnitude and frequency of the applied field as well as on the anchoring of the liquid crystal at the droplet interface. These findings are quantitatively evaluated by measuring the angular velocity of fluid and drops for various frequencies of the applied field.

DOES INTERNAL CONVECTION PROMOTE MICRO-EXPLOSION OF W/O EMULSION DROPLETS?

W/O emulsions are considered as a beneficial alternative to the use of fossil hydrocarbon fuels and not treated vegetable oils. The presence of a dispersed phase composed of water allows to reduce pollutant emissions and also increases combustion efficiency because of the so-called micro-explosion phenomenon. This micro-explosion has a strong stochastic behavior, and its realization depends on several parameters such as water droplet size distribution and motion during heating. This paper aims to provide a better understanding of the impact of the water droplet behavior on the atomization rate. PLIF technique is used to visualize and track the motion of the water droplets dispersed inside heated emulsion drops. Natural convection of the water droplets is observed, and its impact on coalescence and atomization rate is investigated. Emulsion with small size distributions and high droplet velocity tends to show a low or nonexistent micro-explosion rate, while coarse emulsions show an important coalescence rate, resulting in a high micro-explosion occurrence. Finally, a dimensionless indicator measuring dispersed-phase motion intensity is proposed.

An electrothermal platform for active droplet manipulation.

The smart control of droplet transport through surface structures and external fields provides exciting opportunities in engineering fields of phase change heat transfer, biomedical chips, and energy harvesting. Here we report the wedge-shaped slippery lubricant-infused porous surface (WS-SLIPS) as an electrothermal platform for active droplet manipulation. WS-SLIPS is fabricated by infusing a wedge-shaped superhydrophobic aluminum plate with phase-changeable paraffin. While the surface wettability of WS-SLIPS can be readily and reversibly switched by the freezing-melting cycle of paraffin, the curvature gradient of the wedge-shaped substrate automatically induces an uneven Laplace pressure inside the droplet, endowing WS-SLIPS the ability to directionally transport droplets without any extra energy input. We demonstrate that WS-SLIPS features spontaneous and controllable droplet transport capability to initiate, brake, lock, and resume the directional motion of various liquid droplets including water, saturated NaCl solution, ethanol solution, and glycerol, under the control of preset DC voltage (∼12 V). In addition, the WS-SLIPS can automatically repair surface scratches or indents when heated and retain the full liquid-manipulating capability afterward. The versatile and robust droplet manipulation platform of WS-SLIPS can be further used in practical scenarios such as laboratory-on-a-chip settings, chemical analysis and microfluidic reactors, paving a new path to develop advanced interface for multifunctional droplet transport.

Model studies on motion of respiratory droplets driven through a face mask

Face masks are used to intercept respiratory droplets to prevent spreading of air-borne diseases. Designing face masks with better efficiency needs microscopic understanding on how respiratory droplets move through a mask. Here we study a simple model on the interception of droplets by a face mask. The mask is treated as a polymeric network in an asymmetric confinement, while the droplet is taken as a micrometer-sized tracer colloidal particle, subject to driving force that mimics the breathing. We study numerically, using the Langevin dynamics, the tracer particle permeation through the polymeric network. We show that the permeation is an activated process following an Arrhenius dependence on temperature. The potential energy profile responsible for the activation process increases with tracer size, tracer bead interaction, network rigidity and decreases with the driving force and confinement length. A deeper energy barrier led to better efficiency to intercept the tracer particles of a given size in the presence of driving force at room temperature. Our studies may help to design masks with better efficiency.

Theoretical study of the influence of the changing environment on the process of rainfall irrigation

The technology of sprinkler irrigation has a higher coefficient of land use compared to that of regular irrigation, due to the absence of irrigation erosion, the prevention of unnecessary absorption of water into the soil, the high degree of mechanization and automation of the irrigation process, the saving of equipment and fuel costs, the availability of irrigation in areas with uneven relief, the uniform moistening of the root layer. It has the advantage of achieving the same growth and development of the plant in all parts of the field. At the same time, there are opportunities to achieve savings as a result of giving mineral fertilizers in the form of suspension during irrigation. This article examines the distribution of water droplets on the field surface during sprinkler irrigation and the influence of wind speed on the intensity of irrigation. Water drop motion and wind motion were analyzed in one coordinate system. As a result, a mathematical model of the resistance coefficient of the environment and the movement of a water droplet in a variable environment was developed.

Proposal of a new slit-lamp shield for ophthalmic examination and assessment of its effectiveness using computational simulations.

This study aimed to use computational models for simulating the movement of respiratory droplets when assessing the efficacy of standard slit-lamp shield versus a new shield designed for increased clinician comfort as well as adequate protection. Simulations were performed using the commercial software Star-CCM+. Respiratory droplets were assumed to be 100% water in volume fraction with particle diameter distribution represented by a geometric mean of 74.4 (±1.5 standard deviation) μm over a 4-min duration. The total mass of respiratory droplets expelled from patients' mouths and droplet accumulation on the manikin were measured under the following three conditions: with no slit-lamp shield, using the standard slit-lamp shield, and using our new proposed shield. The total accumulated water droplet mass (kilogram) and percentage of expelled mass accumulated on the shield under the three aforementioned conditions were as follows: 5.84e-10 kg (28% of the total weight of particle emitted that settled on the manikin), 9.14e-13 kg (0.045%), and 3.19e-13 (0.015%), respectively. The standard shield could shield off 99.83% of the particles that would otherwise be deposited on the manikin, which is comparable to 99.95% for the proposed design. Conclusion: Slit-lamp shields are effective infection control tools against respiratory droplets. The proposed shield showed comparable effectiveness compared with conventional slit-lamp shields, but with potentially enhanced ergonomics for ophthalmologists during slit-lamp examinations.

Acoustofluidics for simultaneous droplet transport and centrifugation facilitating ultrasensitive biomarker detection.

Trace biological sample detection is critical for the analysis of pathologies in biomedicine. Integration of microfluidic manipulation techniques typically strengthens biosensing performance. For instance, using isothermal amplification reactions to sense trace miRNA in peripheral circulation lacks a sufficiently complex pretreatment process that limits the sensitivity of on-chip detection. Herein we propose an orthogonal tunable acoustic tweezer (OTAT) to simultaneously actuate the transportation and centrifugation of μ-droplets on a single device. The OTAT enables diversified modes of droplet transportation such as unidirectional transport, multi-direction transport, round-trip transport, tilt angle movement, multi-droplet fusion, and continuous centrifugation of the dynamic droplets simultaneously. The multiplicity of modalities enables the focusing of a loaded analyte at the center of the droplet or constant rotation about the center axis of the droplet. We herein demonstrate the OTAT's ability to actuate transportation, fusion, and centrifugation-based pretreatment of two biological sample droplets loaded with miRNA biomarkers and multiple mixtures, as well as facilitating the increase of fluorescence detection sensitivity by an order of magnitude compared to traditional tube reaction methods. The results herein demonstrate the OTAT-based droplet acoustofluidic platform's ability to combine a wide range of biosensing mechanisms and provide a higher accuracy of detection for one-stop point-of-care disease diagnosis.

Spatiotemporal distribution prediction of coughing airflow at mouth based on machine learning - Part I: Study on boundary conditions at mouth in numerical simulation of cough airflow

In the post epidemic era, the movement and distribution of pathogenic airflow and droplets produced by cough in the building space have been widely studied. Due to the limitations of research methods, there are few detailed research data on the temporal and spatial distribution of boundary conditions during cough, which is the basis of research and the key boundary conditions of computer simulation. Previous experiments have obtained cough airflow velocity distribution away from the mouth. This study aims to infer detailed data at mouth for CFD boundary conditions based on these experimental data. This is the first part of the research. Based on experiments, the types of parameters contained in the boundary conditions near the mouth during coughing are discussed. The main parameters are determined, including the maximum velocity of the mouth air flow, and the distribution function of the ejected air flow, among others, and the approximate value range. Different parameter combinations are used as boundary conditions for simulation, and with various combinations, database of conditions are obtained. Preliminary machine learning is performed on these databases, and boundary condition data consistent with experimental results are inferred. The study demonstrates that when the velocity distribution of the air flow at mouth satisfies the normal distribution function on the central vertical two-dimensional profile, the maximum velocity of the mouth air flow is 15m/s. Part 2 will use the complex neural network model to fit and infer more accurate boundary condition. The findings of this study can provide more accurate boundary conditions for simulating pathogenic airflow, as well as a supplementary database for epidemiological research.

Contactless acoustic tweezer for droplet manipulation on superhydrophobic surfaces.

Droplet manipulation on superhydrophobic surfaces (DMSS) without conventional pipetting is an emerging liquid handling technology, which can be potentially used for diagnostic, analysis, and synthetic processes. Despite notable progress, controlling droplet motion on superhydrophobic surfaces by contactless acoustic waves is rarely reported. Herein, we report a contactless acoustic tweezer (CAT) for DMSS based on establishing ultrasonic standing wave between an ultrasound transducer (UST) and a superhydrophobic substrate to manipulate droplets without physical contact. The CAT utilizes acoustic radiation forces to trap and move droplets on superhydrophobic surfaces, which allows for precise and controllable movement of droplets by controlling the movement of the UST. Small droplets with volume less than 20 μL can be levitated in mid-air for out-plane manipulation, and large droplets with volume up to 500 μL can be trapped for in-plane manipulation. Experimental results demonstrate the versatility of the CAT for manipulating droplets with various compositions and volumes on various superhydrophobic substrates, offering a versatile and cross-contamination-free liquid handling approach for applications, including but not limited to high-throughput surface-enhanced Raman scattering.

A low-temperature digital microfluidic system used for protein-protein interaction detection.

The occurrence, development and prediction of various biological processes and diseases are inseparable from the protein-protein interaction (PPI), so it is extremely meaningful to perfect PPI networks. However, shortcomings of traditional detection methods, such as protein degradation, long detection time, complex operation, poor automation and high cost, restrict the rapid development of PPI networks. Here, a low-temperature digital microfluidic (LTDMF) system-based PPI detection box (LTDMF-PPI-Box) was developed to achieve rapid, lossless and efficient PPI detection. It consists of a PMMA shell, LTDMF-PPI and an integrated temperature control system. LTDMF reduces the PPI detection time from tens of hours to 1.5 hours by programmatically controlling the movement of droplets. Moreover, an integrated thermoelectric cooler (TEC) ensures an operating temperature of 4 °C, resulting in a protein protection up to 90%. The interaction between RILP protein and Rab26 protein which has a close connection to insulin secretion was demonstrated as a prototype to illustrate the feasibility of the LTDMF-PPI-Box. LTDMF with automation characteristics is capable of meeting the requirement of high-throughput screening of interacting proteins; therefore, the LTDMF-PPI-Box is expected to accelerate the establishment of the PPI network in the future.

Spectroscopic studies on CVD-grown monolayer, bilayer, and ribbon structures of WSe2 flakes

The spectroscopic properties of APCVD-grown monolayer, bilayer, and ribbon structures of WSe2 flakes are investigated in detail. The synthesis pathway of the ribbon structures is interpreted on the basis of droplet motion.

WALL EFFECTS ON THE THERMOCAPILLARY MIGRATION OF ISOLATED DROPLET IN LIQUIDS

In this paper, computational fluid dynamics (CFD) is used to analyze and represent droplet motion in a stationary fluid. Continuity conservation equations involving two-phase flow are solved in the commercial software package Ansys-Fluent that uses the volume-of-liquid (VOF) method to observe the liquid/liquid interface in two- and three-dimensional domains which has proven to be a very effective and valuable tool for studying the phenomenon of a multiphase interaction in zero-gravity condition. The flow is driven by the Marangoni effect caused by temperature differences. This causes the droplet to move from the cooler region to the warmer region. The results show that the scaled velocity of the droplet decreases with the increase of the thermal Marangoni number (Ma<sub>T</sub>), which is consistent with the results of previous experimental observations from the literature. Calculations are performed on cylinders with an inner diameter of Dr = 15, 17.5, 20, 25, 30, 35, 40, 50, 60, and 70 mm keeping the same 10 mm diameter for all calculations. Due to the column-wall effect, the droplet migration rate decreases only when the ratio of column diameter to droplet diameter Dr/db (AR) is less than 2. When Dr/d<sub>droplet</sub> is greater than 2, the effect of column diameter on droplet speed is negligible. No significant distortion in the shape of the droplet was observed and the droplets were spherical for all column widths.

AI-assisted modeling of capillary-driven droplet dynamics

Abstract In this study, we present and assess data-driven approaches for modeling contact line dynamics, using droplet transport on chemically heterogeneous surfaces as a model system. Ground-truth data for training and validation are generated based on long-wave models that are applicable for slow droplet motion with small contact angles, which are known to accurately reproduce the dynamics with minimal computing resources compared to high-fidelity direct numerical simulations. The data-driven models are based on the Fourier neural operator (FNO) and are developed following two different approaches. The first deploys the data-driven method as an iterative neural network architecture, which predicts the future state of the contact line based on a number of previous states. The second approach corrects the time derivative of the contact line by augmenting its low-order asymptotic approximation with a data-driven counterpart, evolving the resulting system using standard time integration methods. The performance of each approach is evaluated in terms of accuracy and generalizability, concluding that the latter approach, although not originally explored within the original contribution on the FNO, outperforms the former.

Theoretical study of a flow-through humidifier with a rotary water spray

Purpose. To formulate a mathematical description of the process of air humidification in a flow-through apparatus with a rotary water sprayer (as an automation object). Methods. The analytical method of research is applied, which is based on determining the dynamic characteristics of heat and mass transfer processes during the interaction of water droplets with an airflow. Results. A mathematical description of the dynamics of a flowing air humidifier with a rotary water sprayer is formulated as a mathematical model of the unsteady mode of heat and mass transfer of sprayed water with an air stream and a mathematical model of the movement of a droplet as a material particle in a vertical uneven air flow. The trajectories of the droplets in the air flow are determined and the air humidification graphs are obtained. The obtained solution to the system of equations allows us to determine the influence of design and operating parameters on the dynamic properties of the air humidifier. Conclusions 1. A dynamic mathematical model of heat and mass transfer in a flow-through apparatus of ideal mixing has been developed. 2. The analytical dependences of the change in air parameters in time on the operating and design parameters of the apparatus were obtained. 3. The kinetic parameters of the droplet motion are determined. Keywords: air flow, droplet, humidifier, rotary disk, heat transfer, moisture content.