Increase In Droplet Concentration Research Articles

Visualization study on the dynamic behavior of shear thinning droplets impacting superhydrophobic spheres

The surface and droplet characteristics significantly affect the dynamic of droplet impact on solid surfaces. In present work, a platform is utilized to experimentally study the dynamic behavior of shear thinning fluid droplets impacting superhydrophobic spheres, where the effects of different concentrations, Weber numbers (We), particle size ratios on droplets impact are studied. The results showed that on the superhydrophobic sphere, as the shear thinning droplet concentration increases, the diffusion diameter increases, and the contact time decreases. For the same curvature, the contact time between the droplet and the sphere decreases with the increase of the We. When the We reaches 75 or above, the contact time remains constant with an increase in the We. For various curvature conditions, the contact time increases as the curvature increasing. When the diameter ratio of the sphere to the droplet is D∗≥6, the maximum diffusion coefficient remains almost constant. However, when D∗<6, the maximum diffusion dimensionless diameter significantly increases as the particle size ratio decreases. More importantly, a theoretical model considering gravity effect is presented to predict the maximum dimensionless diameter of shear thinning droplets on superhydrophobic spheres according to energy conservation. The predicted results are rationally agreement with experiments with the deviation within ±10 %.

Analysis of the heat transfer fluctuations in the Rayleigh-Bénard convection of concentrated emulsions with finite-size droplets.

Employing numerical simulations, we provide an accurate insight into the heat transfer mechanism in the Rayleigh-Bénard convection of concentrated emulsions with finite-size droplets. We focus on the unsteady dynamics characterizing the thermal convection of these complex fluids close to the transition from conductive to convective states, where the heat transfer phenomenon, expressed in terms of the Nusselt number Nu, is characterized by pronounced fluctuations triggered by collective droplet motion [F. Pelusi et al., Soft Matter, 2021, 17(13), 3709-3721]. By systematically increasing the droplet concentration, we show how these fluctuations emerge along with the segregation of "extreme events" in the boundary layers, causing intermittent bursts in the heat flux fluctuations. Furthermore, we quantify the extension S and the duration of the coherent droplet motion accompanying these extreme events via a suitable statistical analysis involving the droplet displacements. We show how the increase in droplet concentration results in a power-law behaviour of the probability distribution function of S and and how this outcome is robust at changing the analysis protocol. Our work offers a comprehensive picture, linking macroscopic heat transfer fluctuations with the statistics of droplets at the mesoscale.

Corrosion of steel in a CO2-containing solution droplet generated in wet gas pipelines studied by scanning Kelvin probe

A scanning Kelvin probe (SKP) was used to study corrosion of X52 pipeline steel under a CO2-containing solution droplet simulating the water condensate generated in interior of wet gas pipelines. The Volta potential measured by SKP was indicative of the feature of corrosion occurring under the droplet. Three potential zones were observed from the droplet center towards the transition and the side of the droplet, which were -0.1 V, -0.4 V and 0.1 V (Kelvin probe, kp), respectively, after 4 h of testing. The droplet side had the highest Volta potential, which was attributed to corrosion scale generation and precipitation due to a limited solution volume and achieved solubility limit of FeCO3 scale. The transition zone possessed the most negative Volta potential due to a higher corrosion activity than the droplet center with more supply and dissolution of CO2, while the droplet center had the thickest solution. The SKP measurements could estimate the volume evolution of the solution droplet with time at a relatively accurate scale. The solution droplet could maintain its topographic feature, i.e., a spherical cap shape, within 4 h of testing in this work. The increase in droplet concentration due to solution evaporation during testing affected the Volta potential, but slightly only.

Numerical Study of Bubble Cloud and Thermal Lesion Evolution During Acoustic Droplet Vaporization Enhanced HIFU Treatment.

Acoustic droplet vaporization (ADV) has been proven to enhance high intensity focused ultrasound (HIFU) thermal ablation of tumor. It has also been demonstrated that triggering droplets before HIFU exposure could be a potential way to control both the size and the shape of the thermal lesion. In this paper, a numerical model is proposed to predict the thermal lesion created in ADV enhanced HIFU treatment. Bubble oscillation was coupled into a viscoelastic medium in the model to more closely represent real applications in tissues. Several physical processes caused by continuous wave ultrasound and elevated temperature during the HIFU exposure were considered, including rectified diffusion, gas solubility variation with temperature in the medium, and boiling. Four droplet concentrations spanning two orders of magnitude were calculated. The bubble cloud formed from triggering of the droplets by the pulse wave ultrasound, along with the evolution of the shape and location of the bubble cloud and thermal lesion during the following continuous wave exposure was obtained. The increase of bubble void fraction caused by continuous wave exposure was found to be consistent with the experimental observation. With the increase of droplet concentration, the predicted bubble cloud shapes vary from tadpole to triangular and double triangular, while the thermal lesions move toward the transducer. The results show that the assumptions used in this model increased the accuracy of the results. This model may be used for parametrical study of ADV enhanced HIFU treatment and be further used for treatment planning and optimization in the future.

A model study of the effect of clustering on the last stage of drizzle formation

AbstractIn the final stage of growth, settling cloud drops become drizzle by collision and coalescence with the smaller droplets in their path. We study the impact of droplet clustering on the growth of settling collector drops by modelling it as an exponentially decaying correlation. Enhancement in collector drop growth is measured by the reduction in time needed for drizzle formation in the last stage. The effect of clustering is characterized by two dimensionless parameters C = δλL and K = (λ0L)−1, where δλ is the enhancement in the collision rate inside the clusters, L is the cluster size perceived by the settling collector drop and λ0 is the initial collision rate. Increasing C or K reduces the time needed for drizzle formation compared to the unclustered case. We find that for K ≥ 5, C ≤ 2 even a small increase in droplet concentration inside clusters can cause a significant enhancement in the collector drop growth. Further even for low turbulence intensities (low K) significant reduction in the time for drizzle formation is achieved for moderate values of C. These findings are relevant to the debate on the intensity of clustering needed to explain rapid drizzle formation.

The Importance of the Shape of Cloud Droplet Size Distributions in Shallow Cumulus Clouds. Part I: Bin Microphysics Simulations

Abstract In this two-part study, the relationships between the width of the cloud droplet size distribution and the microphysical processes and cloud characteristics of nonprecipitating shallow cumulus clouds are investigated using large-eddy simulations. In Part I, simulations are run with a bin microphysics scheme and the relative widths (standard deviation divided by mean diameter) of the simulated cloud droplet size distributions are calculated. They reveal that the value of the relative width is higher and less variable in the subsaturated regions of the cloud than in the supersaturated regions owing to both the evaporation process itself and enhanced mixing and entrainment of environmental air. Unlike in some previous studies, the relative width is not found to depend strongly on the initial aerosol concentration or mean droplet concentration. Nonetheless, local values of the relative width are found to positively correlate with local values of the droplet concentrations, particularly in the supersaturated regions of clouds. In general, the distributions become narrower as the local droplet concentration increases, which is consistent with the difference in relative width between the supersaturated and subsaturated cloud regions and with physically based expectations. Traditional parameterizations for the relative width (or shape parameter, a related quantity) of cloud droplet size distributions in bulk microphysics schemes are based on cloud mean values, but the bin simulation results shown here demonstrate that more appropriate parameterizations should be based on the relationship between the local values of the relative width and the cloud droplet concentration.

Linear and nonlinear optical properties of fluorescein sodium salt doped droplet

The Uv-Vis, fluorescence, Raman spectroscopy and Z-scan were used to study optical properties of fluorescein sodium salt (FSS) in AOT/Oil/Water microemaulsion. The FSS-microemulsion was prepared by two types of Oil (Heptane and Hexane) at two molar ratios of water to surfactant (X = 3 and 8), different droplet concentrations at constant ratios of dye in water (Y = FSS/water). In the AOT microemulsion media, both the emission and absorption spectra of the FSS present a red shift with respect to droplet concentration and water droplet size. A change is molecular structure was observed from Anion to Dianionic by increase of droplet concentration. The Raman spectra shows, the polarity of samples doesn’t change with change of droplet concentration. The ratios of the excited state to the ground state of dipole moments (μe/μg) of FSS-microemulsion were determined by perturbation theory. At the same dye concentrations in samples, the μe/μg depending on the oil type (Oil polarity) and size of water droplet (X) of the microemulsion. The dipole moment of the ground sate is decreasing by decrease of the droplet size and increase of the droplet concentration. The FSS in both microemulsion systems is localized in the interface of droplet because of the negative interface of AOT and the probabilities, the dye aggregation inside of the droplets change by the size of droplet and it can change the dipole moment of the ground state. The photophysics properties tend to bulk water with the increase of the droplet size. The nonlinear optical properties of FSS/AOT/Heptane/Water microemaulsion were investigated. The magnitude of the third-order nonlinear refractive index (n2) and nonlinear absorption coefficient (s) the second order hyperpolarizabilities (γR) and the real third order susceptibilities (χR) were measured using single-beam z-scan at different droplet concentration and two molar ratio. The results shows, the ratio of dipole moment (μe/μg) has behavior similar to the 1/γR.

Comments on “Photophysics of rhodamine B in the nanosized water droplets: A concentration dependence study”

This comment concerns some erroneous claims, which arise from the results and discussion in the recent article by A. Rahdar (Journal of Molecular Liquids 220 (2016) 395–403). The mixture of Rhodamine B with AOT/Water/n-Hexane was studied at two water to surfactant molar ratio (Y=10 and 8). Morevoer, The effect of droplet concentration was studied on fluorescence spectra of Rhodamine B-doped droplets. The size of droplet was changed by increase of water to surfactant molar ratio(Y). The results shows, by increase of droplet concentration, the fluorescence (νf) maxima wavenumbers was changed and the absorbance (νa) maxima wavenumbers was constant. We found, the value of νa−νf and νa+νf of Rhodamine B inside of droplets is quite different from the results that found before by A. Rahdar. By theory and experiments, the corrected value of the ratio of dipole moment of Rhodamine B in droplets was reported. We have proven, the ratio of dipole moment was changed between 1.82988 and 2.05972 by increase of droplet concentration at constant molar ratio Y=10. Moreover, the effect of droplet size is reported on the ratio of excited to ground states of dipole moment at Rhodamine B.

Modeling of the flow structure and heat transfer in a gas-droplet turbulent boundary layer

A numerical study of dynamics and heat/mass transfer in a gas-droplet turbulent boundary layer on a vertical flat plate is carried out. A large number of factors which affect the heat and mass transfer and the structure of thermal and concentration fields in a turbulent boundary layer is analyzed. It is shown that the increase in droplet concentration results in the intensification of heat transfer, as compared with the single-phase air flow. The comparison of this analysis with experimental data shows a qualitative and quantitative agreement between the calculated and experimental data.

Multi-variate probability density functions with dynamics for cloud droplet activation in large-scale models: single column tests

Abstract. Successful simulation of cloud-aerosol interactions (indirect aerosol effects) in climate models requires relating grid-scale aerosol, dynamic, and thermodynamic fields to small-scale processes like aerosol activation. A turbulence and cloud parameterization, based on multi-variate probability density functions of sub-grid vertical velocity, temperature, and moisture, has been extended to treat aerosol activation. Multi-variate probability density functions with dynamics (MVD PDFs) offer a solution to the problem of the gap between the resolution of climate models and the scales relevant for aerosol activation and a means to overcome the limitations of diagnostic estimates of cloud droplet number concentration based only on aerosol concentration. Incorporated into the single-column version of GFDL AM3, the MVD PDFs successfully simulate cloud properties including precipitation for cumulus, stratocumulus, and cumulus-under-stratocumulus. The extension to treat aerosol activation predicts droplet number concentrations in good agreement with large eddy simulations (LES). The droplet number concentrations from the MVD PDFs match LES results more closely than diagnostic relationships between aerosol concentration and droplet concentration. In the single-column model simulations, as aerosol concentration increases, droplet concentration increases, precipitation decreases, but liquid water path can increase or decrease.

The sensitivity of stratocumulus-capped mixed layers to cloud droplet concentration: do LES and mixed-layer models agree?

Abstract. The sensitivity of a stratocumulus-capped mixed layer to a change in cloud droplet concentration is evaluated with a large-eddy simulation (LES) and a mixed layer model (MLM). The strength of the second aerosol indirect effect simulated by the two model types agrees within 50% for cases in which the LES-simulated boundary layer remains well mixed, if the MLM entrainment closure includes the effects of cloud droplet sedimentation. To achieve this agreement, parameters in the MLM entrainment closure and the drizzle parameterization must be retuned to match the LES. This is because the LES advection scheme and microphysical parameterization significantly bias the entrainment rate and precipitation profile compared to observational best guesses. Before this modification, the MLM simulates more liquid water path and much more drizzle at a given droplet concentration than the LES and is more sensitive to droplet concentration, even undergoing a drizzle-induced boundary layer collapse at low droplet concentrations. After this modification, both models predict a comparable decrease of cloud liquid water path as droplet concentration increases, cancelling 30–50% of the Twomey effect for our case. The agreement breaks down at the lowest simulated droplet concentrations, for which the boundary layer in the LES is not well mixed. Our results highlight issues with both types of model. Potential LES biases due to inadequate resolution, subgrid mixing and parameterized microphysics must be carefully considered when trying to make a quantitative inference of the second indirect effect from an LES of a stratocumulus-topped boundary layer. On the other hand, even slight internal decoupling of the boundary layer invalidates the central assumption of an MLM, substantially limiting the range of conditions that MLM-predicted sensitivities to droplet concentration are meaningful.

Concentration dependence of shape and structure fluctuations of droplet microemulsions investigated by neutron spin echo spectroscopy

We describe dynamic modes that originate from shape and structure fluctuations in a droplet microemulsion system. The modes are decoupled by a contrast variation neutron scattering technique using the relative intermediate form factor method. The strategy of the method is analogous to the relative form factor method, which decouples the form and structure factors from the small-angle neutron scattering intensity [M. Nagao, Phys. Rev. E 75, 061401 (2007)]. First, we will briefly explain theoretical and experimental approaches to understanding neutron spin echo (NSE) data from droplet microemulsion systems. Then we will introduce the relative intermediate form factor method, which decouples shape and structure fluctuations. The concentration dependence of the droplet dynamics in a microemulsion system is used to elucidate the strengths of this method. The intermediate form and structure factors are successfully decoupled from an observed intermediate scattering function by NSE. The decay rate of the shape fluctuation modes linearly decreases, while the fluctuation amplitude increases as the droplet concentration increases. The first cumulant of the obtained intermediate structure factor shows a clear de Gennes narrowing behavior at a length scale corresponding to the interdroplet distance. However, in the high-momentum-transfer and longer-time regions, the first cumulant deviates from the intermediate structure factor. This result suggests the existence of other dynamic modes of structure fluctuations rather than the center-of-mass diffusion mode.

Atmospheric chemistry implications of the emission of biomass smoke

The health effects of combustion-generated particulates are well known and total particulates, PM10 ,P M2.5 and PAH are now routinely monitored in the atmosphere. More recently, it has been noted that particulates have an important impact on the climate and on rainfall. 1,2 While greenhouse gases absorb reflected infrared radiation from the earth, aerosols reflect solar radiation to space. Aerosols also have an important role in cloud formation, acting as nucleation sites. The increase in droplet concentration in clouds leads to an increase in the reflection of solar radiation, which results in cooling. Furthermore, if the moisture content of the cloud remains constant, but the number concentration of droplets increases, the droplet size decreases resulting in an increase in the lifetime and amount of clouds, as well as in decreased precipitation. 1 Theinfluence of carbonaceous aerosols is more difficult to predict since they are strong infrared absorbers of reflected radiation from the earth, but they also scatter solar radiation into space. Consequently, there has been recent interest in the influence of carbonaceous aerosols on climate change. In addition, the aerosols arising from combustion act as carriers for absorbed chemical species

Intermicellar material exchange in reverse micelles formed by ionic AOT and nonionic Igepal surfactants studied by means of pulse radiolysis. Influence of the temperature

Abstract The recombination of thiocyanate anion radicals, (SCN)2−, formed pulse radiolytically within the water pools of reverse micelles stabilized with anionic AOT and nonionic Igepal surfactants, was proved as an indicator reaction to study intermicellar exchange. It was found that the exchange process is slower inIgepal than in AOT reverse micelles with the same water to surfactant ratio. The apparent activation enthalpy and entropy of the exchange process were determined in different alkanes. For the AOT and Igepal reverse micelles the activation parameters increase with the droplet size, but for the AOT systems they do not significantly change with the increase of droplet concentration. For non-percolated systems the activation parameters for Igepal reverse micelles approach those for AOT reverse micelles. This result supports existing suggestions that the mechanism of intermicellar exchange does not differ in principle between reverse micelles stabilized with ionic and nonionic surfactants.

Effects of salinity and clay type on oil-mineral aggregation.

Effects of salinity and clay type on the aggregation process of crude oil and clay particles (oil-mineral aggregates--OMA) were studied in the laboratory. OMA were generated by shaking various oil/clay mixtures in water at 20 degrees C and at a pre-determined energy level. Shape, median and maximum sizes, size distribution and concentration of oil droplets forming OMA were measured using epi-fluorescence microscopy. Results showed that the median and maximum sizes and the concentration of mineral-stabilized droplets increase rapidly when salinity increases from zero to a critical aggregation salinity in the range of 1.2-3.5 ppt. The magnitude of the increase of droplet size is controlled by clay type, while the increase of droplet concentration is influenced by oil type. Size distributions of oil droplets are self-similar, but their magnitudes depend on salinity and oil type. The effect of salinity on droplet size distribution is strongly influenced by clay type. Empirical relationships are derived to calculate concentration and size distribution of mineral-stabilized droplets.

Analysis of smoke impact on clouds in Brazilian biomass burning regions: An extension of Twomey's approach

Satellite remote sensing of smoke aerosol‐cloud interaction during the recent Smoke, Clouds, and Radiation‐Brazil (SCAR‐B) experiment is analyzed to explore the factors that determine the magnitude of the cloud response to smoke aerosol. Analysis of 2 years worth of data revealed that the response is greatest in the north of Brazil where aerosol optical depth is smallest, and tends to decrease as one moves southward, and as aerosol optical depth increases. Saturation in this response occurs at an aerosol optical depth of 0.8 in 1987 and 0.4 in 1995. To explore the reasons for this, a framework is developed in which the satellite‐measured response can be compared to simple analytical models of this response and to numerical models of smoke aerosol‐cloud interaction. Three types of response are identified: (1) cloud droplet concentrations increase with increasing aerosol loading, followed by saturation in the response at high concentrations; (2) as in type 1, followed by increasing droplet concentrations with further increases in aerosol loading. This increase in droplet concentration is due to the suppression of supersaturation by abundant large particles, which prevents the activation of smaller particles. This enables renewed activation of larger particles when smoke loadings exceed some threshold; (3) as in type 1, followed by a decrease in droplet number concentrations with increasing aerosol loading as intense competition for vapor evaporates the smaller droplets. The latter implies an unexpected increase in drop size with increasing smoke loading. The conditions under which each of these responses are expected to occur are discussed. It is shown that although to first‐order smoke optical depth is a good proxy for aerosol indirect forcing, under some conditions the size distribution and hygroscopicity can be important factors. We find no evidence that indirect forcing depends on precipitable water vapor.

Cloud optical thickness and albedo retrievals from bidirectional reflectance measurements of POLDER instruments during ACE-2

The POLDER instrument is devoted to global observations of the solar radiation reflected by the Earth–atmosphere system. The airborne version of the instrument was operated during the ACE-2 experiment, more particularly as a component of the CLOUDYCOLUMN project of ACE-2 that was conducted in summer 1997 over the subtropical northeastern Atlantic ocean. CLOUDYCOLUMN is a coordinated project specifically dedicated to the study of the indirect effect of aerosols. In this context, the airborne POLDER was assigned to remote measurements of the cloud optical and radiative properties, namely the cloud optical thickness and the cloud albedo. This paper presents the retrievals of those 2 cloud parameters for 2 golden days of the campaign 26 June and 9 July 1997. Coincident spaceborne ADEOS-POLDER data from 2 orbits over the ACE-2 area on 26 June are also analyzed. 26 June corresponds to a pure air marine case and 9 July is a polluted air case. The multidirectional viewing capability of airborne POLDER is here demonstrated to be very useful to estimate the effective radius of cloud droplet that characterizes the observed stratocumulus clouds. A 12 μm cloud droplet size distribution appears to be a suitable cloud droplet model in the pure marine cloud case study. For the polluted case the mean retrieved effective droplet radius is of the order of 6–10 μm. This only preliminary result can be interpreted as a confirmation of the indirect effect of aerosols. It is consistent with the significant increase in droplet concentration measured in polluted marine clouds compared to clean marine ones. Further investigations and comparisons to in-situ microphysical measurements are now needed.

Measurements of Cloud Susceptibility

Abstract The sensitivity of warm stratocumulus cloud albedo to changes in droplet concentration, termed “cloud susceptibility,” is calculated using data from the UKMO Meteorological Research Flight. Stratocumulus clouds in the eastern Pacific, South Atlantic, subtropical regions of the North Atlantic, and around the British Isles are studied. The range of susceptibility measured is large and maritime clouds are shown to have the largest susceptibility. Numerical simulations of the changes in cloud radiative and microphysical properties with increasing droplet concentration are carried out. These highlight the high sensitivity of maritime clouds to changes in droplet concentration and the rapid reduction in sensitivity as the cloud droplet concentration increases.

The effect of anthropogenic sulfate aerosols on marine cloud droplet concentrations

Nonseasalt sulfate (nss SO 4 2- ) mass concentrations, cloud condensation nuclei (CCN) number concentrations, and cloud droplet concentrations in warm cumulus and stratocumulus clouds were simultaneously measured in situ in marine air masses on El Yunque peak in Puerto Rico. Our results show that CNN number concentrations (measured at 0.5% supersaturation) and nss SO 4 2- mass concentrations (in the range of ∼ 400–1700 ng m -3 ) are significantly correlated at this site. Droplet concentrations in the cumulus clouds studied do not show a discernible trend with nss SO 4 2- mass concentrations (in the range of ∼ 300–1400 ng m -3 ). In stratocumulus clouds, a small increase in droplet concentration with nss SO 4 2- mass concentrations in the range of ∼ 300–1100 ng m -3 was observed. We attribute the low sensitivities of the droplet number concentrations to nss SO 4 2- mass concentrations to the entrainment/mixing processes in these clouds. The magnitudes of the empirically derived sensitivities are considerably lower than those assumed in recent assessments of the effect of anthropogenic sulfate aerosols on cloud albedo. DOI: 10.1034/j.1600-0889.1994.t01-1-00005.x

The effect of anthropogenic sulfate aerosols on marine cloud droplet concentrations

Nonseasalt sulfate (nss SO 4 2- ) mass concentrations, cloud condensation nuclei (CCN) number concentrations, and cloud droplet concentrations in warm cumulus and stratocumulus clouds were simultaneously measured in situ in marine air masses on El Yunque peak in Puerto Rico. Our results show that CNN number concentrations (measured at 0.5% supersaturation) and nss SO 4 2- mass concentrations (in the range of ∼ 400–1700 ng m -3 ) are significantly correlated at this site. Droplet concentrations in the cumulus clouds studied do not show a discernible trend with nss SO 4 2- mass concentrations (in the range of ∼ 300–1400 ng m -3 ). In stratocumulus clouds, a small increase in droplet concentration with nss SO 4 2- mass concentrations in the range of ∼ 300–1100 ng m -3 was observed. We attribute the low sensitivities of the droplet number concentrations to nss SO 4 2- mass concentrations to the entrainment/mixing processes in these clouds. The magnitudes of the empirically derived sensitivities are considerably lower than those assumed in recent assessments of the effect of anthropogenic sulfate aerosols on cloud albedo. DOI: 10.1034/j.1600-0889.1994.t01-1-00005.x