Droplet Mode Research Articles

Spray and mixing characteristics of liquid jet in a tubular gas–liquid atomization mixer

For the design and optimization of a tubular gas–liquid atomization mixer, the atomization and mixing characteristics of liquid jet breakup in the limited tube space is a key problem. In this study, the primary breakup process of liquid jet column was analyzed by high-speed camera, then the droplet size and velocity distribution of atomized droplets were measured by Phase-Doppler anemometry (PDA). The hydrodynamic characteristics of gas flow in tubular gas–liquid atomization mixer were analyzed by computational fluid dynamics (CFD) numerical simulation. The results indicate that the liquid flow rate has little effect on the atomization droplet size and atomization pressure drop, and the gas flow rate is the main influence parameter. Under all experimental gas flow conditions, the liquid jet column undergoes a primary breakup process, forming larger liquid blocks and droplets. When the gas flow rate ( Q g ) is less than 127 m 3 ·h −1 , the secondary breakup of large liquid blocks and droplets does not occur in venturi throat region. The Sauter mean diameter (SMD) of droplets measured at the outlet is more than 140 μm, and the distribution is uneven. When Q g > 127 m 3 ·h −1 , the large liquid blocks and droplets have secondary breakup process at the throat region. The SMD of droplets measured at the outlet is less than 140 μm, and the distribution is uniform. When 127 < Q g < 162 m 3 ·h −1 , the secondary breakup mode of droplets is bag breakup or pouch breakup. When 181 < Q g < 216 m 3 ·h −1 , the secondary breakup mode of droplets is shear breakup or catastrophic breakup. In order to ensure efficient atomization and mixing, the throat gas velocity of the tubular atomization mixer should be designed to be about 51 m·s −1 under the lowest operating flow rate. The pressure drop of the tubular atomization mixer increases linearly with the square of gas velocity, and the resistance coefficient is about 2.55 in single-phase flow condition and 2.73 in gas–liquid atomization condition. The breakup morphologies of liquid jet with different liquid and gas flow rates are photographed by a high-speed camera respectively. The liquid jet column swings and breaks up into large droplets and liquid blocks in the air flow. When flow through the Venturi throat, these large droplets and liquid blocks break up into smaller droplets. The particle size and velocity distribution of atomized droplets were measured by Phase-Doppler anemometry (PDA). Under the same gas flow rate condition, the droplet sizes at different measured points are basically the same, and the droplet size decreases gradually with the increase of gas velocity. With the increase of gas velocity, the droplet size distribution of each measured point is closer to a straight line, and the uniformity of atomized droplet distribution in the tube is better. With the value of the gas velocity 72 m 3 ·h −1 , the average droplet size is about 200 μm, and with the value of the gas velocity 216 m 3 ·h −1 , the average droplet size is about 50 μm. • Experiments were performed on droplet size and velocity in tubular atomizing mixer. • Breakup morphologies of liquid jet under different flow conditions were obtained. • The relationship between gas flow rate and droplet size was obtained. • Gas–liquid atomizing and mixing characteristics in tubular mixer were analyzed.

Numerical study of rheological behaviors of a compound droplet in a conical nozzle

The dynamics of compound droplets is more and more attractive because of their applications in a wide range of industrial and natural processes. This study aims to improve the understanding of dynamical rheological behaviors of a compound droplet moving in a nozzle with a conical shape in the downstream region via front-tracking-based simulations. The numerical results show that the compound droplet experiences three stages of deformation: the entrance stage (in front of the conical region), the transit stage (within the conical region), and the exit stage (in the exit of the nozzle). The droplet receives the maximum deformation in the axial direction during the transit stage, and the radially maximum deformation occurs during the exit stage. Because of the acceleration induced by the conical region, the inner droplet of the compound droplet can break up into smaller droplets during the exit stage. To reveal the transition between the finite deformation and the breakup, many parameters including the Capillary number Ca (varied in the range of 0.0125–1.6), the droplet size relative to the nozzle size R1/R0 (varied in the range of 0.2–0.9), the droplet radius ratio R21 (varied in the range of 0.3–0.8), the viscosity ratios μ21 and μ31 (varied in the range of 0.05–3.2), the interfacial tension ratio σ21 (varied in the range of 0.125–8.0), the conical angle α (varied in the range of 4°–34°) and the initial location of the inner droplet (i.e. the droplet eccentricity) are considered. From the finite deformation mode, the transition to the breakup mode of the inner droplet occurs when increasing any of Ca, R1/R0, R21 and α, or decreasing any of μ21 and σ21. The breakup mode is also enhanced when the inner droplet is initially located closer to the leading side of the outer droplet. However, varying μ31 induces no transition between these modes. The regime diagrams of these modes, based on these parameters, are also proposed.

COVID-19 preventive measures showing an unintended decline in infectious diseases in Taiwan

Most of the communicable diseases have contact, airborne and/or droplet mode of transsmission. Following the outbreak of COVID-19, the Taiwan government implemented the use of masks and sanitizer, as well as other preventive measures like social distancing for prevention. This public response likely contributed significantly to the decline in the outbreak of other infectious diseases.

Nanoliter-Scale Droplet-Droplet Microfluidic Microextraction Coupled with MALDI-TOF Mass Spectrometry for Metabolite Analysis of Cell Droplets.

The further miniaturization of liquid-phase microextraction (LPME) systems has important significance and major challenges for microscale sample analysis. Herein, we developed a rapid and flexible droplet-droplet microfluidic microextraction approach to perform nanoliter-scale miniaturized sample pretreatment, by combining droplet-based microfluidics, robotic liquid handling, and LPME techniques. Differing from the previous microextraction methods, both the extractant and sample volumes were decreased from the microliter scale or even milliliter scale to the nanoliter scale. We utilized the ability of a liquid-handling robot to manipulate nanoliter-scale droplets and micrometer-scale positioning to overcome the scaling effect difficulties in performing liquid-liquid extraction of nanoliter-volume samples in microsystems. Two microextraction modes, droplet-in-droplet microfluidic microextraction and droplet-on-droplet microfluidic microextraction, were developed according to the different solubility properties of the extractants. Various factors affecting the microextraction process were investigated, including the extraction time, recovery method of the extractant droplet, static and dynamic extraction mode, and cross-contamination. To demonstrate the validity and adaptability of the pretreatment and analysis of droplet samples with complex matrices, the present microextraction system coupled with MALDI-TOF mass spectrometry (MS) detection was applied to the quantitative determination of 7-ethyl-10-hydroxylcamptothecin (SN-38), an active metabolite of the anticancer drug irinotecan, in 800-nL droplets containing HepG2 cells. A linear relationship (y = 0.0305x + 0.376, R2 = 0.984) was obtained in the range of 4-100 ng/mL, with the limits of detection and quantitation being 2.2 and 4.5 ng/mL for SN-38, respectively.

Engineering Micropatterned Surfaces for Controlling the Evaporation Process of Sessile Droplets

Controlling the evaporation process of a droplet is of the utmost importance for a number of technologies. Also, along with the advances of microfabrication, micropatterned surfaces have emerged as an important technology platform to tune the wettability and other surface properties of various fundamental and applied applications. Among the geometrical parameters of these micropatterns, it is of great interest to investigate whether the arrangement of the patterns would affect the evaporation process of a sessile liquid droplet. To address this question, we fabricated four microhole arrays with different arrangements, quantified by the parameter of “eccentricity”. The results suggested that, compared to smooth substrates, the evaporation mode was not only affected by engineering the microhole arrays, but also by the eccentricity of these micropatterns. The values of contact angle hysteresis (CAH) were used to quantify and test this hypothesis. The CAH could partially explain the different evaporation modes observed on the microhole arrays with zero and non-zero values of eccentricity. That is, on microhole arrays with zero eccentricity, CAH of water droplets was comparatively low (less than 20 ° ). Consistently, during the evaporation, around 60% of the life span of the droplet was in the mixed evaporation mode. Increasing the eccentricity of the microhole arrays increases the values of CAH to above 20 ° . Unlike the increasing trend of CAH, the evaporation modes of sessile droplets on the microhole array with non-zero values of eccentricity were almost similar. Over 75% of the life span of droplets on these surfaces was in constant contact line (CCL) mode. Our findings play a significant role in any technology platform containing micropatterned surfaces, where controlling the evaporation mode is desirable.

Dynamic wetting behavior of droplets on the surface of porous materials with micro-airflow

<p indent=0mm>The dynamic wetting behavior characteristic of droplets is of significant importance in various industrial applications and high technologies, such as surface self-cleaning, surface drag reduction, anti-frosting, and droplet directional transportation. The modifications of coating surface energy and micro/nano structures on substrate surfaces are the conventional and indirect ways to manipulate the droplet dynamic wetting behavior. At present, many direct and external incentive ways, like substrate vibration, electrowetting and additional magnetic field, also have been proposed to control the droplet dynamic wetting behavior. In the present study, inspired from the phenomenon of droplet floating in the air without gravity, an innovative manipulation method had been proposed, which utilized the additional micro-airflow field as a momentum field to control the force distribution and wetting characteristic of droplets. The core idea of this innovative manipulation method was similar as the Leidenfrost phenomenon, which used the gas buoyancy to counter the droplet gravity effect. A porous substrate sintered by copper particles was used as the experimental sample, a micro-airflow field was located at the down side of porous substrate, and a droplet was wetted on the up side of porous substrate. The magnitude of upright momentum force could be changed by adjusting the pressure of micro-airflow field. Under different pressure of micro-airflow field, the dynamic wetting behaviors of droplets on porous substrate were visualized, and the change regulations of dynamic wetting parameters as a function of wetting time were analyzed. According to the results, three different dynamic wetting modes of droplets on the surface of porous materials with micro-airflow were identified, namely, intact infiltration mode, broken infiltration mode and non-infiltration mode. In the intact infiltration mode and broken infiltration mode, the droplets were finally in the state of complete infiltration, while in the non-infiltration mode, the droplets always maintained the suspension state similar as the super-hydrophobic wetting state. When the droplet was in the intact infiltration mode on the surface of the porous sample, the corresponding dynamic wetting process was simultaneously affected by gravity, surface tension, lifting force, adhesion force and capillary force. In this mode, the apparent contact angle and volume of droplet decreased exponentially at the initial stage during the dynamic wetting process. At the following stage, the decreasing amplitude of two dynamic wetting parameters became slower and presented a linear declining trend. The wetting area at the bottom of droplet rapidly increased to the maximum value at the initial stage, and then decreased slowly. However, when the ratio of droplet height to bottom diameter was less than 0.03, the decreasing amplitude of bottom wetting area increased rapidly, and the droplet would quickly approach to the state of complete infiltration. Besides, the maximum wetting area at the bottom of droplet increased with the increase of pressure difference driven by micro-airflow. The larger the pressure difference was, the smaller the changing range of apparent contact angle and volume of droplet in the same wetting time was, but the longer the whole dynamic wetting period was.

Droplet deformation and breakup in shear flow of air

The deformation and breakup of droplets in airflows is important in many applications of spray and atomization processes. However, the shear effect of airflow has never been reported. In this study, the deformation and breakup of droplets in the shear flow of air is investigated experimentally using high-speed imaging, digital image processing, and particle image velocimetry. We identify a new breakup mode of droplets, i.e., the butterfly breakup, in which the strong aerodynamic pressure on the lower part of the droplet leads to the deflection of the droplet and then the formation of a butterfly-shaped bag. A regime map of the droplet breakup is produced, and the transitions between different modes are obtained based on scaling analysis. The elongation and the fragmentation of the droplet rim are analyzed, and the results show that they are significantly affected by the shear via the formation and the growth of nodes on the rim.

Numerical and experimental study of local heat mass transfer characteristics of horizontal falling films of CaCl2 solution absorbing vapor from humid air

The local heat and mass transfer characteristics of the falling film over horizontal tubes in the presence of humid air are investigated thoroughly by numerical simulation method and experimental method. To improve the accuracy of the model, a laminar model or a k–ε turbulence model is employed depending on the film Reynolds number (Re). And the consideration of the heat transfer in the gas phase made the model can effectively reflect the complex flow behavior and adopt more practical boundaries. The simulation results were validated by the experimental data. The simulations demonstrate that the film Re effects on the outlet parameters and the inner solution temperature distribution are consistent with the experimental data. The temporal variations in all the average solution and gas parameters finally undergo periodic cycles in the droplet flow mode, whereas a constant trend is observed in the sheet flow mode. The average absorption rate in the falling-film regions in the droplet and sheet flow mode may be almost 6 and 10 times the average absorption rate in the inter-tube regions. The inter-tube absorption rates in the sheet flow mode show a distinct saw-tooth pattern. The solution parameters variations in the droplet mode are greater than those in the sheet mode, while the gas parameters variations are less apparent.

Rainfall Characteristics in the Mantaro Basin over Tropical Andes from a Vertically Pointed Profile Rain Radar and In-Situ Field Campaign

Information on the vertical structure of rain, especially near the surface is important for accurate quantitative precipitation estimation from weather and space-borne radars. In the present study, the rainfall characteristics, from a vertically pointed profile Radar in the Mantaro basin (Huancayo, Peru) are observed. In summary, diurnal variation of near-surface rainfall and bright band height, average vertical profiles of the drop size distribution (DSD), rain rate, radar reflectivity (Ze) and liquid water content (LWC) are investigated to derive the rainfall characteristics. Diurnal variation of rain rate and bright band height show the bimodal distribution, where frequent and higher rain rate occurred during the afternoon and nighttime, and more than 70% bright band height found between 4.3–4.7 km. The average vertical profiles of Ze show the opposite characteristics above and below the melting level (ML) and depend on the near-surface rain rate. For example, the average Ze profiles have a negative gradient above the ML, whereas below, the ML, the gradient depends on the near-surface rain rate. The rain rate and LWC show the opposite behavior, and both consist of a positive (negative) gradient below (above) the ML. The vertical growth of DSD parameters depend on the near-surface rain rate, and a higher concentration of large-sized of droplets are observed for higher near surface rain rate, however, the dominant modes of droplets are &lt;1 mm throughout the vertical column. However, the most significant variation in DSD growth is observed for near-surface rain rate ≥20 mm/h. These findings suggest using different retrieval techniques for near surface rain estimation than the rest of the vertical profile and high rain rate events. The improved understanding of the tropical Andes precipitation would be very important for assessing climate variability and to forecast the precipitation using the numerical models.

CFD Analysis of Falling Film Hydrodynamics for a Lithium Bromide (LiBr) Solution over a Horizontal Tube

Falling film evaporators are used in applications where high heat transfer coefficients are required for low liquid load and temperature difference. One such application is the lithium bromide (LiBr)-based absorber and generator. The concentration of the aqueous LiBr solution changes within the absorber and generator because of evaporation and vapor absorption. This causes the thermophysical properties to differ and affects the film distribution, heat, and mass transfer mechanisms. For thermal performance improvement of LiBr-based falling film evaporators, in-depth analysis at the micro level is required for film distribution and hydrodynamics. In this work, a 2D numerical model was constructed using the commercial CFD software Ansys Fluent v18.0. The influence of the liquid load corresponding to droplet and jet mode, and the concentration, on film hydrodynamics was examined. It was found that the jet mode was more stable at a higher concentration of 0.65 with ±0.5% variation compared to lower concentrations. The recirculation was stronger at a low concentration of 0.45 and existed until the angular position (θ) = 10°, whereas at 0.65 concentration it diminished after θ = 5°. The improved heat transfer is expected at lower concentrations due to lower film thickness and thermal resistance, more recirculation, and a higher velocity field.

Coalescence and sedimentation of liquid iron droplets during smelting reduction of converter slag with mechanical stirring

To improve the sedimentation efficiency of iron droplets during iron extraction process from converter slag by smelting reduction, a mechanical stirring is proposed and the coalescence and sedimentation behavior of iron droplets under experimental crucible scale are studied based on the established coalescence model. Coalescence mechanism of droplets is elucidated and the effects of fluid flow, rotating speed and structure of the impeller are examined. The model is validated by the high-temperature experiment results. The results show that with rotating flow in radial direction and “double roll” pattern in axial direction, the settling mode of droplets is no longer a monotonous vertical uniform sedimentation, but a spiral sedimentation with acceleration, deceleration and eventual uniform velocity stages. The mechanical stirring improves the sedimentation efficiency by 3.4% and reduces the total settling time by 6.8%. The coalescence mode with mechanical stirring is mainly due to the oblique collision, instead of the chase collision of the large droplet chasing the small one without stirring. Although the coalescence of the colliding droplet pairs becomes difficult due to the increased relative velocities of droplets, the mechanical stirring greatly improves the coalescence number of iron droplets by a factor of 5.0 because of the increased collision probability. With the impeller rotating speed increased from 50 to 80 rpm, the coalescence of colliding iron droplet pairs is more difficult but the cumulative coalescence number and sedimentation efficiency increase. The same trend is also obtained for the three different impeller structures of ‘Y’ shape, straight shape and cross shape. Therefore, higher rotating speed (80 rpm) and cross-shape impeller are more favored during smelting reduction process.

Interaction of Liquid Droplets in Gas and Vapor Flows

We investigated the conditions, characteristics, and outcomes of liquid droplet interaction in the gas medium using video frame processing. The frequency of different droplet collision outcomes and their characteristics were determined. Four interaction regimes were identified: bounce, separation, coalescence, and disruption. Collision regime maps were drawn up using the Weber, Reynolds, Ohnesorge, Laplace, and capillary numbers, as well as dimensionless linear and angular parameters of interaction. Significant differences were established between interaction maps under ideal conditions (two droplets colliding without a possible impact of the neighboring ones) and collision of droplets as aerosol elements. It was shown that the Weber number could not be the only criterion for changing the collision mode, and sizes and concentration of droplets in aerosols influence collision modes. It was established that collisions of droplets in a gaseous medium could lead to an increase in the liquid surface area by 1.5–5 times. Such a large-scale change in the surface area of the liquid significantly intensifies heat transfer and phase transformations in energy systems.

Evaluation of the IMPROVE formulas based on Mie model in the calculation of particle scattering coefficient in an urban atmosphere

To assess the uncertainties in particle scattering coefficient (bsp) estimated from using the original and revised IMPROVE formulas, particle mass size distribution, bulk PM2.5 and PM10 and their major chemical compositions, and bsp under dry condition were synchronously measured in urban Guangzhou during 2015–2016. The estimated bsp, mass concentrations (Cm), and mass scattering efficiencies (MSEs) of the dominant chemical species in the fine, condensation and droplet modes were compared between those calculated using the original and revised IMPROVE formulas and the Mie model. On annual average, the reconstructed bsp using the original IMPROVE formula, the revised IMPROVE formula, and the Mie model explained 81%, 91%, and 98% of the measured bsp at 550 nm, respectively. The better performance of the Mie model than the original and revised IMPROVE formula in the reconstructed bsp was mainly due to the inclusion of the unidentified chemical species which explained 14% of the measured bsp at 550 nm. The estimated percentage contributions of the dominant chemical species to the measured bsp differed by < 3% in most cases, except for (NH4)2SO4 between the Mie model and the revised IMPROVE formula (6%). By including the unidentified chemical species in the revised IMPROVE formula can best reconstructed bsp to within 5% of the measured value.

Falling liquid film periodical fluctuation over a superhydrophilic horizontal tube at low spray density

Falling film on horizontal tube banks is widely used in heat transfer exchangers and absorbers due to its high heat and mass transfer performance. The superhydrophilic surface can effectively ameliorate the wettability of the tube wall and maintain thin film even at a very low spray density. Both of them play the dominating role in heat and mass transfer process. In this paper, a thermal tracing method was employed to investigate the liquid spreading feature and film fluctuation characteristics over a horizontal superhydrophilic tube. The results showed that the liquid film spreading width for the superhydrophilic tube was two times higher than that of the plain surface. Interestingly, as the liquid film extended continuously along the tube surface, the spreading width exhibited little relevance to the spray density under the current experimental condition for the superhydrophilic tube. Furthermore, in the case of droplet mode, the increment of flow rate leaded to an increase in the frequency of the impacting droplet instead of the droplet volume. Finally, the periodic behavior of liquid film fluctuation in droplet mode was observed as well, which demonstrated higher fluctuation intensity than that of other flow modes. The superhydrophilic-aided drop mode is of value in exploring the efficient, precise, and feasible technique for falling liquid film on a horizontal tube especially for ultralow spray density.

Self-Sterilizing Laser-Induced Graphene Bacterial Air Filter.

Nosocomial infections transmitted through airborne, droplet, aerosol, and particulate-transported modes pose substantial infection risks to patients and healthcare employees. In this study, we demonstrate a self-cleaning filter comprised of laser-induced graphene (LIG), a porous conductive graphene foam formed through photothermal conversion of a polyimide film by a commercial CO2 laser cutter. LIG was shown to capture particulates and bacteria. The bacteria cannot proliferate even when submerged in culture medium. Through a periodic Joule-heating mechanism, the filter readily reaches >300 °C. This destroys any microorganisms including bacteria, along with molecules that can cause adverse biological reactions and diseases. These molecules include pyrogens, allergens, exotoxins, endotoxins, mycotoxins, nucleic acids, and prions. Capitalizing on the high surface area and thermal stability of LIG, the utility of graphene for reduction of nosocomial infection in hospital settings is suggested.

Parametric study of the collision modes of compound droplets in simple shear flow

Compound droplet collision has been found in various industrial and academic applications. Such colliding phenomena of two droplets in simple shear flow are numerically resolved by a front-tracking technique. Initially, each compound droplet, assumed cylindrical, contains one concentric inner droplet. They are separated at the lateral and vertical intervals denoted by Δx0 and Δy0. Because of the shear flow, the compound droplets interact with each other and exhibit three collision modes: passing-over, merging (i.e. coalescence) and reversing. These modes and their transition are affected by many parameters including the Reynolds Re and Capillary Ca numbers (based on the properties of the outer fluid), the viscosity ratios μ13 and μ23, the interfacial tension ratio σ12 of the inner to outer interfaces, the ratio of the radii of the inner to outer droplets R12 and the initial distance between them (in terms of Δx0 and Δy0). It is found that from a merging mode, decreasing Re from 2.51 to a value less than or equal to 1.0 induces a transition to a reversing mode, whereas, increasing Ca from 0.005 to a value greater than or equal to 0.04 leads to a transition to a passing-over mode. A transition from the merging to passing-over modes also appears when varying μ23 in the range of 0.1–10.0 or varying R12 in the range of 0.2–0.8. A transition from a passing-over mode to a reversing one is available when increasing Δx0 or decreasing Δy0. Three modes of collision all occur when μ13 is varied in the range of 0.1–10.0. However, the variation of σ12 does not induce any transition between different modes. Several phase diagrams in terms of Re versus Ca, or Δx0 versus Δy0 are also proposed to show the transitions between these modes.

2018년 11월 광주 지역에서 발생한 미세먼지 고농도 사례 시 수용성 성분의 크기 분포 조사

In this study, mass size distributions of ambient aerosol particles and their water-soluble organic and inorganic species were investigated under high pollution episode. To achieve the aim of this study, 24-hr integrated size-segregated aerosol samples were collected at an ubran site of Gwangju between November 02 and 16, 2018, and analyzed for mass, water-soluble organic carbon (WSOC), and ionic species. During study period, a high pollution episode of fine particulate matter (PM) occurred between November 02 and 07, possibly due to high pressure developed around Korean peninsula, extremely stable air conditions, and long-range transportation of aerosol particles from North Korea and northeastern China region. On November 02~03 when influence of local pollution dominated, ambient aerosol particles, WSOC, NO3-, SO₄2-, and NH4+ exhibited bi-modal size distributions with a condensation mode at a particle size of 0.32 μm and a droplet mode at a particle size of 1.0 μm in PM1.8. While, between November 05 and 07 when long-range transportation of PM was mixed with locally-produced pollution, they showed very pronounced droplet mode size distributions at 1.0 μm, without a condensation mode. NO3- concentration in PM1.8 during pollution episode increased significantly from 5.9 μg/m³ on November 02 to 21.5μg/m³ on November 06, while SO₄2- in PM1.8 increased from 1.6 to 5.8 μg/m³, indicating the dominance of local pollution during pollution episode. Furthermore, for WSOC, NO3-, SO₄2-, and NH4+, fraction of droplet mode size (PM0.55-1.8) to PM1.8 increased gradually as PM pollution got increased, while the condensation mode (PM0.17-0.32) contribution decreased. In addition, it was demonstrated that relative humidity could be an important factor promoting formation of droplet mode WSOC, NO3-, and SO₄2-.

Impact freezing modes of supercooled droplets determined by both nucleation and icing evolution

Supercooled large droplet (SLD), which can cause abnormal icing phenomenon like run-back ridged ice, poses a serious threat to aircraft safety. In impact freezing of a supercooled droplet, the coupling of impact dynamics and ice growth determines its freezing rate and heat transfer efficiency, which is responsible for the abnormal icing phenomenon. The knowledge of supercooled droplet impact freezing is vital for the development of aircraft anti-icing and safety technologies. However, the effect of icing evolution on impact freezing is not well understood yet, especially on surfaces with different properties. This work experimentally investigates the freezing process of supercooled droplets impinging on surfaces with different heat conduction properties. By observing the phenomenon of droplet impact-nucleation and ice growth with high-speed camera, the frozen morphology, spreading ratio and freezing time are recorded with different temperatures and surface heat conduction properties. The mechanism of impact freezing of supercooled droplet is analyzed in different nucleation and ice growth conditions. In experiment, two frozen morphologies of droplets are found: irregular shape on plexiglass and poached egg shape on metal surface, besides the previously discovered frozen morphologies (basin, pancake, semisphere). The former shape exists when the horizontal growth rate of ice is lower than retraction rate of droplet, while the latter one exists when the vertical growth rate of ice is low enough. These two morphologies (irregular shape and poached egg) would evolve into shapes of basin or pancake with the increase of horizontal and vertical growth rate of ice, when the supercooling and surface thermal conductivity rise. During this transformation, the freezing of droplet is mainly influenced by nucleation rather than ice growth. Theoretical analysis shows that the freezing of supercooled droplet is determined by both nucleation and icing evolution. The freezing mode is nucleation-dominated if the growth rate of ice is higher than retraction rate, and the freezing mode is ice growth-dominated in the opposite case.

Impact of particle number and mass size distributions of major chemical components on particle mass scattering efficiency in urban Guangzhou in southern China

Abstract. To grasp the key factors affecting particle mass scattering efficiency (MSE), particle mass and number size distribution, PM2.5 and PM10 and their major chemical compositions, and the particle scattering coefficient (bsp) under dry conditions were measured at an urban site in Guangzhou, southern China, during 2015–2016. On an annual average, 10±2 %, 48±7 % and 42±8 % of PM10 mass were in the condensation, droplet and coarse modes, respectively, with mass mean aerodynamic diameters (MMADs) of 0.78±0.07 in the droplet mode and 4.57±0.42 µm in the coarse mode. The identified chemical species mass concentrations can explain 79±3 %, 82±6 % and 57±6 % of the total particle mass in the condensation, droplet and coarse mode, respectively. Organic matter (OM) and elemental carbon (EC) in the condensation mode, OM, (NH4)2SO4, NH4NO3, and crustal element oxides in the droplet mode, and crustal element oxides, OM, and CaSO4 in the coarse mode, were the dominant chemical species in their respective modes. The measured bsp can be reconstructed to the level of 91±10 % using Mie theory with input of the estimated chemically resolved number concentrations of NaCl, NaNO3, Na2SO4, NH4NO3, (NH4)2SO4, K2SO4, CaSO4, Ca(NO3)2, OM, EC, crustal element oxides and unidentified fraction. MSEs of particle and individual chemical species were underestimated by less than 13 % in any season based on the estimated bsp and chemical species mass concentrations. Seasonal average MSEs varied in the range of 3.5±0.1 to 3.9±0.2 m2 g−1 for fine particles (aerodynamic diameter smaller than 2.1 µm), which was mainly caused by seasonal variations in the mass fractions and MSEs of the dominant chemical species (OM, NH4NO3, (NH4)2SO4) in the droplet mode. MSEs of the dominant chemical species were determined by their lognormal size-distribution parameters, including MMADs and standard deviation (σ) in the droplet mode.

Fabrication of Microchannels by Using the CO2 Galvo Laser Marking Machine and Thermo-mechanical Sealing Method

Abstract: Microchannel in microtechnology is a channel with a hydraulic diameter below 1 mm. Microchannels are primarily used in biomedical devices and microfluidic applications. Fabrication of microchannels has always been a complex task even at the world centres of excellence. This article addresses the fabrication techniques for creating microchannels using a 40W CO2 Galvo laser marking machine. It was able to control the channel dimensions by changing the power, scanning speed, and scanning time of the laser source. The results show that the created channel width increased as the laser power increased and the scanning speed decreased. Similarly, the channel depth increased as the laser power increased. Successfully tested in the laminar flow and droplet modes, the created microchannels were sealed using the thermo-mechanical method at 220oC. This is a new method for faster and cheaper production of microdevices that could be explored for sustainable development in the industry. The article concludes that with an appropriate solution, microchannels with minimal width and depth dimensions of 50 µm × 50 µm can be developed with channel roughness of 2-3µm.&#x0D; Keywords: Microfluidics, microchannels, CO2 marking machine, Galvo, mechanical sealing method.&#x0D;