Droplet Mode Research Articles

Arctic low-level boundary layer clouds: in situ measurements and simulations of mono- and bimodal supercooled droplet size distributions at the top layer of liquid phase clouds

Abstract. Aircraft borne optical in situ size distribution measurements were performed within Arctic boundary layer clouds with a special emphasis on the cloud top layer during the VERtical Distribution of Ice in Arctic clouds (VERDI) campaign in April and May 2012. An instrumented Basler BT-67 research aircraft operated out of Inuvik over the Mackenzie River delta and the Beaufort Sea in the Northwest Territories of Canada. Besides the cloud particle and hydrometeor size spectrometers the aircraft was equipped with instrumentation for aerosol, radiation and other parameters. Inside the cloud, droplet size distributions with monomodal shapes were observed for predominantly liquid-phase Arctic stratocumulus. With increasing altitude inside the cloud the droplet mean diameters grew from 10 to 20 μm. In the upper transition zone (i.e., adjacent to the cloud-free air aloft) changes from monomodal to bimodal droplet size distributions (Mode 1 with 20 μm and Mode 2 with 10 μm diameter) were observed. It is shown that droplets of both modes co-exist in the same (small) air volume and the bimodal shape of the measured size distributions cannot be explained as an observational artifact caused by accumulating data point populations from different air volumes. The formation of the second size mode can be explained by (a) entrainment and activation/condensation of fresh aerosol particles, or (b) by differential evaporation processes occurring with cloud droplets engulfed in different eddies. Activation of entrained particles seemed a viable possibility as a layer of dry Arctic enhanced background aerosol (which was detected directly above the stratus cloud) might form a second mode of small cloud droplets. However, theoretical considerations and model calculations (adopting direct numerical simulation, DNS) revealed that, instead, turbulent mixing and evaporation of larger droplets are the most likely reasons for the formation of the second droplet size mode in the uppermost region of the clouds.

Instability of the nth oscillation mode of a charged droplet in the presence of a uniform electrostatic field

The stability of the nth oscillation mode of a charged droplet in the presence of a uniform electro-static field is studied using nonlinear asymptotic calculations with the aid of the expansion in terms of two small parameters (dimensionless equilibrium deformation of a droplet and the ratio of the oscillation amplitude to radius). It is demonstrated that identical effects determine the droplet instability in the presence of electrostatic field for an uncharged droplet and a droplet with a charge that is less than the critical Rayleigh charge. The critical field parameter is saturated when the mode number increases but the corresponding asymptotic level that is parallel to the abscissa axis decreases with an increase in the droplet charge.

Evaluation of the Effect of Tube Pitch and Surface Alterations on Temperature Field at Sprinkled Tube Bundle

Water flowing on a sprinkled tube bundle forms three basic modes: It is the Droplet mode (liquid drips from one tube to another), the Jet mode (with an increasing flow rate droplets merge into a column) and the Membrane (Sheet) mode (with further increasing of falling film liquid flow rate columns merge and create sheets between the tubes. With sufficient flow rate sheets merge at this state and the tube bundle is completely covered by a thin liquid film). There are several factors influencing the individual mode types as well as heat transfer. Beside the above mentioned falling film liquid flow rate they are for instance tube diameters, tube pitches in a tube bundle or a physical condition of a falling film liquid. This paper presents a summary of data measured at atmospheric pressure at a tube bundle consisting of copper tubes of 12 milimeters diameter and of the studied tube length one meter. The tubes are positioned horizontally one above another with the tested pitches of 15, 20, 25 and 30 mm and there is a distribution tube placed above them with water flowing out. The thermal gradient of 15–40 has been tested with all pitches where the falling film liquid’s temperature at the inlet of the distribution tube was 15 °C. The liquid was heated during the flow through the exchanger and the temperature of the sprinkled (heater) liquid at the inlet of the exchanger with a constant flow rate about 7.2 litres per minute was 40 °C. The tested flow of the falling film liquid ranged from 1.0 to 13.0 litres per minute. Sequences of 180 exposures have been recorded in partial flow rate stages by thermographic camera with record frequency of 30 Hz which were consequently assessed using the Matlab programme. This paper presents results achieved at the above mentioned pitches and at three types of tube bundle surfaces.

Optical properties of size-resolved particles at a Hong Kong urban site during winter

Visibility degradation in Hong Kong is related to the city's serious air pollution problems. To investigate the aerosols' optical properties and their relationship with the chemical composition and size distribution of the particles, a monitoring campaign was conducted at an urban site in the early winter period (from October to December, 2010). The particle light scattering coefficient (Bsp) and absorption coefficient (Bap) were measured. Two collocated Micro-Orifice Uniform Deposit Impactor samplers (MOUDI110, MSP, USA) with nominal 50% cut-off aerodynamic diameters of 18, 10, 5.6, 3.2, 1.8, 1, 0.56, 0.32, 0.18, 0.1, and 0.056μm were used to collect size-resolved particle samples. The average Bsp and Bap were 201.96±105.82Mm−1 and 39.91±19.16Mm−1, with an average single scattering albedo (ωo) of 0.82±0.07. The theoretical method of light extinction calculation was used to determine the extinction of the size-resolved particulate matters (PM). The reconstructed light scattering coefficient correlated well with the measured scattering value in the Hong Kong urban area. Droplet mode (0.56–1.8μm) particles contributed most to the particle light extinction (~69%). Organic matter, ammonium sulphate and elemental carbon were the key components causing visibility degradation in the droplet (0.56–1.8μm) and condensation (0.1–0.56μm) size ranges. Five sources contributing to particle light extinction have been identified using positive matrix factorisation (PMF). Traffic/engine exhausts and secondary aerosols accounted for ~36% and ~32% of particle light extinction, respectively, followed by sea salt (15%). The remaining sources, soil/fugitive dust and tire dust, contributed by ~10% and 7%, respectively, to particle light extinction.

Lattice Boltzmann modeling of droplet condensation on superhydrophobic nanoarrays.

Droplet nucleation and growth on superhydrophobic nanoarrays is simulated by employing a multiphase, multicomponent lattice Boltzmann (LB) model. Three typical preferential nucleation modes of condensate droplets are observed through LB simulations with various geometrical parameters of nanoarrays, which are found to influence the wetting properties of nanostructured surfaces significantly. The droplets nucleated at the top of posts (top nucleation) or in the upside interpost space of nanoarrays (side nucleation) will generate a nonwetting Cassie state, while the ones nucleated at the bottom corners between the posts of nanoarrays (bottom nucleation) produce a wetting Wenzel state. The simulated time evolutions of droplet pressures at different locations are analyzed, which offers insight into the underlying physics governing the motion of droplets growing from different nucleation modes. It is demonstrated that the nanostructures with taller posts and a high ratio of post height to interpost space (H/S) are beneficial to produce the top- and side-nucleation modes. The simulated wetting states of condensate droplets on the nanostructures, having various geometrical configurations, compare reasonably well with experimental observations. The established relationship between the geometrical parameters of nanoarrays and the preferential nucleation modes of condensate droplets provides guidance for the design of nanoarrays with desirable anticondensation superhydrophobic properties.

One-year observations of size distribution characteristics of major aerosol constituents at a coastal receptor site in Hong Kong – Part 1: Inorganic ions and oxalate

Abstract. Size distribution data of major aerosol constituents are essential in source apportioning of visibility degradation, testing and verification of air quality models incorporating aerosols. We report here 1-year observations of mass size distributions of major inorganic ions (sulfate, nitrate, chloride, ammonium, sodium, potassium, magnesium and calcium) and oxalate at a coastal suburban receptor site in Hong Kong, China. A total of 43 sets of size-segregated samples in the size range of 0.056–18 μm were collected from March 2011 to February 2012. The size distributions of sulfate, ammonium, potassium and oxalate were characterized by a dominant droplet mode with a mass mean aerodynamic diameter (MMAD) in the range of ~ 0.7–0.9 μm. Oxalate had a slightly larger MMAD than sulfate on days with temperatures above 22 °C as a result of the process of volatilization and repartitioning. Nitrate was mostly dominated by the coarse mode but enhanced presence in fine mode was detected on winter days with lower temperature and lower concentrations of sea salt and soil particles. This data set reveals an inversely proportional relationship between the fraction of nitrate in the fine mode and product of the sum of sodium and calcium in equivalent concentrations and the dissociation constant of ammonium nitrate (i.e., (1/([Na+] + 2[Ca2+]) × (1/Ke')) when Pn_fine is significant (> 10%). The seasonal variation observed for sea salt aerosol abundance, with lower values in summer and winter, is possibly linked with the lower marine salinities in these two seasons. Positive matrix factorization was applied to estimate the relative contributions of local formation and transport to the observed ambient sulfate level through the use of the combined data sets of size-segregated sulfate and select gaseous air pollutants. On average, the regional/super-regional transport of air pollutants was the dominant source at this receptor site, especially on high-sulfate days while local formation processes contributed approximately 30% of the total sulfate. This work provides field-measurement-based evidence important for understanding both local photochemistry and regional/super-regional transport in order to properly simulate sulfate aerosols in air quality models.

Size distribution of carbonaceous aerosols at a high-altitude site on the central Tibetan Plateau (Nam Co Station, 4730 m a.s.l.)

The chemical composition and size distribution characteristics of atmospheric aerosols have important effects on the environment, human health and climate change. In this paper, we study the size distribution of carbonaceous aerosols at the remote and pristine site, Nam Co Monitoring and Research Station for Multisphere Interactions, in the inland Tibetan Plateau (TP) based on collected size-segregated aerosols during 2012. The samples were quantified using the thermal/optical (TOR) method. The overall average concentrations of OC and EC in TSP, PM9.0, PM2.1, and PM1.0 were 4.61μgm−3 and 0.19μgm−3, 4.52μgm−3 and 0.18μgm−3, 2.72μgm−3 and 0.11μgm−3, and 2.11μgm−3 and 0.09μgm−3, respectively. Generally, the highest concentration of OC and EC in different aerosol size occurred during winter. The low level of EC indicated that direct anthropogenic disturbances in the interior of the TP still remain insignificant.The size distributions of OC and EC concentrations presented bimodal variations. In winter, pre-monsoon, monsoon, and post-monsoon seasons, the peaks for OC were in droplet mode (0.43–0.65μm) and coarse mode (4.7–5.8μm); while in the monsoon period, the coarse mode shifted to a smaller size bin (3.3–4.7μm). The coarse mode may be due to dust particles while the droplet mode may be due to the growth process of particles. For EC, the peaks variations in coarse mode were as same as OC, while the other peaks were complicated: the peaks during winter, pre-monsoon, and monsoon seasons exhibited in droplet mode (1.1–2.1μm, 0.65–1.1μm, and 0.43–0.65μm, respectively), and in post-monsoon period, the peak located in condensation mode. The highest peak concentrations for OC and EC occurred in winter and the pre-monsoon period, while the lowest peak values in the monsoon and post-monsoon periods, respectively. The size distribution variations may be caused by deposition, gas/particles exchange, hygroscopic growth, external mixing, and secondary organic carbon formation. OC/EC ratios in aerosols over the TP mostly exhibit high values, which emphasizes the importance of OC over this region.

Oscillation dynamics of sessile droplets subjected to substrate vibration

Sessile droplets on a vibrating substrate are investigated focusing on axisymmetric oscillations with pinned contact line. Proper orthogonal decomposition is employed to identify the different modes of droplet shape oscillation and quantitatively assess the droplet oscillation and spectral response. We offer the first experimental evidence for the analogy of an oscillating sessile droplet with a non-linear spring mass damper system. The qualitative and quantitative agreement of amplitude-response and phase-response curves and limit cycles of the model dynamical system with that observed experimentally suggest that the bulk oscillations in the fundamental mode of a sessile droplet can be very well modeled by a Duffing oscillator with a hard spring, especially near the resonance. The red shift of the resonance peak with an increase in the glycerol concentration is clearly evidenced by both the experimental and predicted amplitude response curves. The influence of various operational parameters such as excitation frequency and amplitude and fluid properties on the droplet oscillation characteristics is adequately captured by the model.

Experimental study of falling-film mode transitions between horizontal tubes in CaCl2/water absorber

It is important to accurately predict the flow mode especially intertube falling-film mode of absorbent for its great impact on the heat and mass transfer in a horizontal tube absorber. In this paper, a test facility used to study falling-film outside horizontal tubes was built, and experiments were conducted to explore the intertube mode transitions with water-calcium chloride (CaCl2/water) solution in quiescent surroundings. The correlations which are more suitable for predicting CaCl2/water solution intertube mode transitions were developed. In general, the transitional Re will decrease with the solution concentration increasing, and its effects increase with the transitions from droplet mode to sheet mode. Hysteresis exists in all the mode transitions, and it is more obvious for low concentration solutions. The results may be an important support for modeling, designing and operating of CaCl2/water falling-film absorber.

Possible sources of two size-resolved water-soluble organic carbon fractions at a roadway site during fall season

To examine the formation pathways of two size-resolved water-soluble organic carbon (WSOC) fractions, a total of 16 sets of size-segregated aerosol samples were collected using a 10-stage Micro-Orifice Uniform Deposit Impactor (MOUDI) at a roadway site in Korea from September 29 to October 29, 2010. A XAD7HP (non-ionic aliphatic acrylic polymer) resin column was used to separate the filtered extracts into hydrophilic (WSOCHPI) and hydrophobic (WSOCHPO) WSOC fractions. Also the size distributions of water-soluble inorganic species and oxalate were examined to determine the formation pathways of size-resolved WSOCHPI and WSOCHPO fractions.The size distribution of WSOCHPI showed a dominant mode at 0.55 μm, while the WSOCHPO had dominant modes at both 0.17–0.32 μm and 0.55 μm. On the basis of the size distribution characteristics, it was found that the formation pathways of both WSOCHPI and WSOCHPO were dependent on the particle size; in the condensation mode (0.17–0.32 μm), both the WSOCHPI and WSOCHPO could be produced through atmospheric processes similar to those of SO42− and oxalate, which were derived from the gas-phase oxidations of organic compounds. In the droplet mode (0.55–1.8 μm), the cloud processing of both the organic compounds and biomass burning emissions could be a major pathway for the WSOCHPI formation. However, the droplet mode WSOCHPO was likely produced through cloud processing and heterogeneous reactions or aerosol-phase reactions. In the coarse mode (>3.1 μm), the WSOCHPI formation was more likely associated with soil-related particles (Ca(NO3)2 and CaSO4) than with sea-salt particles (NaNO3 and Na2SO4).

Variation of secondary coatings associated with elemental carbon by single particle analysis

The mixing state of elemental carbon (EC) with secondary species has been highlighted as a major uncertainty in assessing its climate impact. However, the extent to which secondary coatings are present on EC and the underlying processes remained poorly understood in China, where there is a high loading of EC produced from extensive usage of fossil fuels and biomass. A single particle aerosol mass spectrometer (SPAMS) was applied to detect the chemical compositions associated with EC at the Guangdong Atmospheric Supersite, China. Efforts were made to track the variation of secondary coatings on EC. It is the first report on the direct observation of secondary compositions associated with EC with high time resolution in China. The hourly average number of EC-containing particles accounted for ∼33% (21–50%) of total analyzed particles over the sampling period. EC was found to be extensively internally mixed with sulfate (97.4% in number), nitrate (89.5%), oxidized organics (69.6%), and/or ammonium (80%). The results also indicate that secondary processing on EC in condensation (0.2–0.5 μm) and droplet (0.7–1.1 μm) modes is different. Active photochemical formation of oxidized organics and ammonium sulfate during daytime, and formation of ammonium nitrate during nighttime led to a distinct diurnal circle of mixing state of EC in the condensation mode. However, the photochemical aging may have limited or negligible influence on the mixing state and growth of EC in the droplet mode. These findings improve the understanding of the evolution of physicochemical properties of EC, and may help to model its climate impact.

A thermodynamic basis for predicting falling-film mode transitions

Horizontal-tube, falling-film heat exchangers are used in many air-conditioning and refrigeration systems. Depending on the tube diameter and spacing, the flow rate, and fluid properties, when a liquid film falls over a series of horizontal tubes three distinct flow patterns can be manifested. These flow patterns are the droplet mode, the jet mode, and the sheet mode. A thermodynamic analysis is undertaken to predict the transitions between these modes. By seeking the condition corresponding to thermodynamic equilibrium between two neighboring modes, a scaling relation is developed for the transitional Reynolds number. This theoretical framework is used to provide the first explanation for the relationship between the transitional Reynolds number and modified Galileo number which has been previously based solely on experimental observations. This approach offers insight into the prevailing physics, and it suggests a tube-spacing effect on the mode transitions which has not previously been anticipated. Using limited data and prior results from the literature it is found that this effect is likely to exist. The implications of this thermodynamic approach to predicting two-phase flow patterns are discussed in terms of entropy generation minimization and transition hysteresis, as is its incompleteness.

Aerosol oxalate and its implication to haze pollution in Shanghai, China

A total of 238 samples of PM2.5 and TSP were analyzed to study the characteristics, sources, and formation pathways of aerosol oxalate in Shanghai in four seasons of 2007. The concentrations of oxalate were 0.07–0.41 μg/m3 in PM2.5 and 0.10–0.48 μg/m3 in TSP, respectively. Oxalate displayed a seasonal variation of autumn > summer > winter > spring in both PM2.5 and TSP and was dominantly present in PM2.5 in all samples. Correlation between oxalate and K+ and high ratio of oxalate/K+ suggested that biomass burning was a secondary source of aerosol oxalate in Shanghai, in addition to urban VOCs sources (vehicular and industrial emissions), especially in autumn. Secondary formation accounted for the majority of aerosol oxalate in Shanghai, which was supported by the high correlation of oxalate with nss-SO4 2−, K+ and NO3 −, proceeding from different mechanisms. Relatively high ambient RH together with high cloud cover was found benefiting the secondary formation of aerosol oxalate. The in-cloud process (aqueous-phase oxidation) was proposed to be likely the major formation pathway of aerosol oxalate in Shanghai, which was supported by the high correlation of oxalate with nss-SO4 2− and K+, dominant residence of oxalate in droplet mode and result of favorable meteorological condition analysis. High correlation of oxalate and NO3 − reflected the OH radical involved oxidation chemistry of the two species in the atmosphere and also suggested that gas-particle surface reactions and the evaporation–condensation process were both possible secondary formation pathways of aerosol oxalate in coarser particle mode (>1.0 μm). As a major water-soluble organic compound in aerosols, concentration of oxalate showed a distinct negative correlation to the atmospheric visibility, which implied that aerosol organic compounds could play an important role in haze pollution as well as in air quality in Shanghai.

Correlation between shape, evaporation mode and mobility of small water droplets on nanorough fibres

The dynamic wetting behaviour and the mobility of droplets on fibres is a very important factor in coating processes, textile fabrication, in self-cleaning processes and in the filtration of fluids. In principal, filter regeneration depends on the mobility of the droplets on the fibre surface. Mobile droplets tend to coalesce which greatly simplifies their removal from the filter. In this contribution mobility analyses of water droplets on monofilaments in air are performed. Studies of droplet evaporation on pure PET fibres and on nanorough fibres coated with SiO2 nanoparticles of diameters between 6nm and 50nm in a hydrophilic binder system were done. We show that the mobility of water droplets correlates with the droplet conformation which in turn is determined by the droplet–fibre interface. We demonstrate that fibre coatings can be used to tailor the conformation and mobility of water droplets. The smaller the nanoparticle diameters in the coating are, the smaller are the contact angles between water droplets and fibre and the better is the mobility of the droplets on the fibre. Our results allow a fast optimization of the fibre surface properties which are directly influencing the contact angle, the mobility and the coalescence of water droplets and thus filter regeneration.

SPH BASED OPTIMIZATION OF ELECTROWETTING-DRIVEN DIGITAL MICROFLUIDICS WITH ADVANCED ACTUATION PATTERNS

Fast and thorough mixing is a crucial operation of digital microfluidic devices, where discrete and small fluid portions are moved and processed. In this paper, we want to analyze and to optimize the mixing process by substituting conventional motion and superposing oscillatory and translational modes. An accurate multiphase smoothed particle hydrodynamics (SPH) discretization for incompressible flow is instantiated. Different harmonic excitation patterns for the solid–liquid surface energy are applied and their influence on droplet mode shapes, formation of eddies and the Shannon entropy of droplet fluid components are measured. We tailor enhanced actuation patterns which improve mixing grade and reduce mixing time.

Physical, Chemical and Optical Properties of Fine Aerosol as a Function of Relative Humidity at Gosan, Korea during ABC-EAREX 2005

The water uptake by fine aerosol in the atmosphere has been investigated at Gosan, Korea during ABC-EAREX 2005. The concentration of inorganic ion and carbon components, size distribution, and light scattering coefficients in normal and dry conditions were simultaneously measured for <TEX>$PM_{2.5}$</TEX> by using a parallel integrated monitoring system. The result of this study shows that ambient fine particles collected at Gosan were dominated by water-soluble ionic species (35%) and carbonaceous materials (18%). In addition, it shows the large growth of aerosol in the droplet mode when RH is higher than 70%. Size distribution of the particulate surface area in a wider size range (<TEX>$0.07-17{\mu}m$</TEX>) shows that the elevation of RH make ambient aerosol grow to be the droplet mode one around <TEX>$0.6{\mu}m$</TEX> or the coarse mode one, larger than <TEX>$2.5{\mu}m$</TEX>. Hygroscopic factor data calculated from the ratio of aerosol scattering coefficients at a given ambient RH and a reference RH (25%) show that water uptake began at the intermediate RH range, from 40% to 60%, with the average hygroscopic factor of 1.10 for 40% RH, 1.11 for 50% RH, and 1.17 for 60% RH, respectively. Finally, average chemical composition and the corresponding growth curves were analyzed in order to investigate the relationship between carbonaceous material fraction and hygroscopicity. As a result, the aerosol growth curve shows that inorganic salts such as sulphate and nitrate as well as carbonaceous materials including OC largely contribute to the aerosol water uptake.

Characterization of electrospray beams for micro-spacecraft electric propulsion applications

Abstract Electric propulsion for spacecraft offers many advantages compared to other traditional counterparts such as chemical propulsion. An electrostatic colloid thruster, well suited for small spacecrafts due to its inherent small size, is investigated. The underlying phenomena governing the plume, namely collisions and focusing are parameterized. The determining properties of the electrospray-based thruster, such as specific impulse, are measured for several propellants and demonstrated to be higher than cold-gas thrusters even for singly charged droplet mode. Moreover, the interesting concept of a dual colloid system with positively and negatively charged cone-jet beams was shown to be feasible by simulations and experiments.

Heat Transfer Performance for a Falling-Film on Horizontal Flat Tubes

Local and average heat transfer behavior for a falling film on horizontal flat tubes is explored through an experimental approach. Experiments are conducted using water, ethylene glycol, and their mixture (50% by volume) under different heat fluxes and tube spacing, with a range of flow rates that covers all flow modes. It is found that the local heat transfer coefficient decreases with distance from the top of the tube. The distribution of the heat transfer coefficient along the axial direction depends on the flow mode: it is constant for the sheet mode, shows small variations for the jet mode, and has variations as large as 20% for the droplet mode. Heat flux has almost no effect on the average Nusselt number within the experimental range. The average Nusselt number for the flat tube is close to that for round tubes in the droplet flow mode, however, in the jet and sheet modes the flat-tube Nusselt number is much larger than the round-tube Nusselt number. Boundary-layer theory is used to explain the local heat transfer coefficient distribution and the experimental data show good agreement with the boundary-layer theory for most cases. New curve fits for the average heat transfer coefficient for three flow modes at different tube spacing are provided and the maximum deviation of the data from the fit is less than 14%.

진동 평판 위 액적의 형상 진동 변화 및 모드 특성

Key Words: Droplet(액적), Lobe(둥근돌출부), Resonance Frequency(공진주파수), Secondary Droplet(미소액적), Shape Oscillation(형상진동)초록: 본연구는주기적인강제진동이가해지는액적의모드특성을실험적으로이해하는것을목적으로하고있다.액적의공진주파수예측을수행하여이론및실험적해석을통해두접근방법의타당성을 파악하였으며,초고속카메라를사용하여액적의다양한변형특성-모드형상,분리,미소액적의발생,그리고비틀림의특성을관찰하였다.이론해석및실험결과와의비교에있어공진주파수값의차이가약15%이하라는것이도출되었으며이러한차이의발생원인으로접촉선마찰,비선형벽고착,실험의불확실성등에큰영향을받는것으로판단된다.접촉선이고정되어있을경우와작은진폭조건하에서액적의모양은대칭형상을가졌으며,공진주파수에서의로브의크기는주변부주파수에서의로브크기보다더크게된다는점을확인하였다.Abstract: This study aims to understand the mode characteristics of a droplet under a periodic forcedvibration. To predict the resonance frequency of a droplet, theoretical and experimental approaches wereemployed. A high-speed camera was used to capture the various deformation characteristics of a droplet—mode shape, detachment, separated secondary droplet, and skewed deformation. The comparison between thetheoretical and the experimental approaches shows a ~10% discrepancy in the prediction of the resonancefrequency, which appears to be caused by the effect of contact line friction, nonlinear wall adhesion, andexperimental uncertainty. Owing to contact-line pinning and smaller amplitude, the droplet shape becomessymmetric and the size of each lobe at the resonance frequency exceeds that at the neighbor, which is out ofresonance.

Application of linear polarized light for the discrimination of frozen and liquid droplets in ice nucleation experiments

Abstract. We report on the development and test results of the new optical particle counter TOPS-Ice (Thermo-stabilized Optical Particle Spectrometer for the detection of Ice). The instrument uses measurements of the cross-polarized scattered light by single particles into the near-forward direction (42.5° ± 12.7°) to distinguish between spherical and non-spherical particles. This approach allows the differentiation between liquid water droplets (spherical) and ice particles (non-spherical) having similar volume-equivalent sizes and therefore can be used to determine the fraction of frozen droplets in a typical immersion freezing experiment. We show that the numerical simulation of the light scattered on non-spherical particles (spheroids in random orientation) considering the actual scattering geometry used in the instrument supports the validity of the approach, even though the cross-polarized component of the light scattered by spherical droplets does not vanish in this scattering angle. For the separation of the ice particle mode from the liquid droplet mode, we use the width of the pulse detected in the depolarization channel instead of the pulse height. Exploiting the intrinsic relationship between pulse height and pulse width for Gaussian pulses allows us to calculate the fraction of frozen droplets even if the liquid droplet mode dominates the particle ensemble. We present test results obtained with TOPS-Ice in the immersion freezing experiments at the laminar diffusion chamber LACIS (Leipzig Aerosol Cloud Interaction Simulator) and demonstrate the excellent agreement with the data obtained in similar experiments with a different optical instrument. Finally, the advantages of using the cross-polarized light measurements for the differentiation of liquid and frozen droplets in the realistic immersion freezing experiments are discussed.