David Y.H. Pui

Electrospraying of conducting liquids for monodisperse aerosol generation in the 4 nm to 1.8 μm diameter range

An electrospraying system operated in the cone-jet mode is shown to produce monodisperse droplets in a wide size range and with good monodispersity. These highly charged droplets are rapidly discharged in a radioactive neutralizer. The produced droplet size can be further reduced by the evaporation process without experiencing the Rayleigh breakup. The experiment is performed using sucrose solutions and the liquid electrical conductivity is controlled by adding small amounts of nitric acid. The size distribution is quite monodisperse having a geometrical standard deviation of 1.1. The mean droplet size produced can be varied from 40 nm to 1.8 μm by changing the liquid feed rate and electrical conductivity. Using 0.1% sucrose concentration solution, the residue particle size is reduced by a factor of ten, in the range from 4 nm to 0.18 μm. The parameters affecting the particle size distribution, the measured spraying current, and the operating envelope have also been obtained and quantified.

A submicron aerosol standard and the primary, absolute calibration of the condensation nuclei counter

Abstract A submicron aerosol generator has been described. The generator combines pneumatic atomization with electrostatic classification to produce uniform particles from 0.01 to 0.5 μm at concentrations up to 10 6 particles/cc. The particle diameter accuracy of the aerosols is about 2%, and the concentration accuracy about 5%. This submicron aerosol generator has been used to produce NaCl aerosols as absolute calibration standards for condensation nuclei counters. Calibration results are presented showing that the Environment/One (Model Rich 100) counter has a linear response at concentration levels up to 130,000 particles/cc, but the indicated concentration based on the accepted Nolan-Pollak standard is lower than the acutal concentration by a factor of 2.5. Above an aerosol concentration of 130,000 particles/cc, the instrument response becomes nonlinear, but the response is independent of particle size (0.011–0.15 μm in diameter) in both the linear and nonlinear ranges. Similar studies of the General Electric counter showed that the instrument response is linear to the highest concentration level (65,000) used in the studies. Results of these experimental findings are discussed, and their implications in coagulation-coefficient and particle-diffusion measurements are considered.

Performance evaluation of the electrical low-pressure impactor (ELPI)

The performance of the Electrical Low-Pressure Impactor (ELPI) has been evaluated using monodisperse aerosols. ELPI is a near real-time size analyzer consisting of an aerosol charger and a cascade impactor. Particle cut sizes of the cascade impactor and the charging efficiency of the charger were determined experimentally in the designed range of the instrument, i.e. 0.03–10 μm. The Stokes numbers of 50% particle cut sizes of all the impactor stages were found to vary from 0.421 to 0.483 with an average of 0.456 and standard deviation of 0.017. The collection efficiency curves were found to be steep with an average steepness (ratio of 70–30% and 30% collection efficiency) of 1.19. The charging efficiency of the charger was close to that specified by the manufacturer for particles smaller than 2 μm in diameter. For larger particles, a deviation from the manufacturer’s specifications was observed. It results from the high particle loss in the charger because of the small size of the charger, strong trapping electric field, and the perpendicular directions of the aerosol stream and the electric field in the charging zone. A comparison with SMPS for size distribution measurements was made in this study. The good agreement between the two measured size distributions shows the capability of the ELPI for near real-time particle size measurements.

Measurement of Atlanta Aerosol Size Distributions: Observations of Ultrafine Particle Events

As part of EPRIs Aerosol Research Inhalation Epidemiology Study (ARIES), measurements of aerosol size distributions in the 3 nm to 2

On the performance of the electrical aerosol analyzer

Abstract The electrical aerosol analyzer has been studied using monodisperse aerosols. The aerosols were generated by the electrostatic classification principle described by Liu and Pui (1974a) and by the photo-chemical conversion of gaseous SO2 into sulfuric acid droplets. The measuring range of the instrument was found to be from 0·006 to 1·0 μm in particle dia. and the sensitivity was found to vary from 0·01 to 400 pA/(106 particles/cm3) over the range of the instrument. Data are presented showing the performance of the device as a function of particle size and as a function of the operating conditions of the diffusion charger.

Experimental study of particle deposition in bends of circular cross section

The deposition efficiency of liquid particles in tube bends of circular cross section has been measured for flow Reynolds numbers of 100, 1000, 6000, and 10,000. The particle Reynolds number, Re p, was in the range 0.6–3.9 for the laminar flow cases (i.e., Re = 100 and 1000), whereas for the turbulent flow cases (i.e., Re = 6000 and 10,000) Re p was in the range 1.3–12.7. Bends constructed of stainless steel and glass tubes of different diameters were used. The experiments were performed using monodisperse aerosols generated by the vibrating orifice aerosol generator. The results were in good agreement with the theory of Cheng and Wang for Re = 1000, but differed from theory for Re = 100. For the turbulent cases, no dependence was found on the flow Reynolds number and an exponential curve of deposition efficiency versus Stokes number was fitted to the experimental results. A theoretical justification of the form of the curve is given in the paper.

Electrical neutralization of aerosols

The charge neutralization process for aerosol particles in a bipolar ionic atmosphere has been studied theoretically and experimentally. The degree of neutralization was found to depend on the parameter, Nt, where N is the ion concentration and t is the neutralization time. For particles in the 0·001–100, μm dia. range, the maximum Nt product needed for charge neutralization was found to be 6 × 106 and 1 × 105 (ions/ml) (sec) respectively for the two limiting cases studied, i.e. a ⪢ λanda < λ, where a is the particle radius and λ is the mean free path of the small ions. Experimental studies were made on four radioactive krypton-85 neutralizers with activities of 0·5,1, 2 and 10 mCi. The equilibrium ion concentration was found to range from 1·2 × 106to 8·8 × 106ions/ml. The performance of these devices as aerosol charge neutralizers was investigated using monodisperse DOP (di-octyl phthalate) aerosols of 2·53 μm dia. The results are in good agreement with the theoretical predictions.

The aerosol mobility chromatograph: A new detector for sulfuric acid aerosols

Abstract A new instrument has been developed for measuring sulfuric acid aerosols. The instrument is called an Aerosol Mobility Chromatograph since it is based on the electrical mobility of aerosol particles and operates in a way similar to that of the conventional liquid or gas Chromatograph. The particle diameter range of the instrument is from 0.005 to 0.2 μm and the sensitivity (for detecting monodisperse sulfuric acid aerosols), from 0.01 to 10 −5 μm −3 , depending upon the specific particle detector used. This paper describes the operating principle of the AMC and the performance characteristics of a prototype device developed at the Particle Technology Laboratory, University of Minnesota.

Experimental investigation of scaling laws for electrospraying : Dielectric constant effect

ABSTRACT Experiments were performed to investigate the effect of liquid dielectric constant on existing scaling laws for the electrospraying process. The variations of the droplet size and the emitted current were measured as a function of the dielectric constant for the electrospray operating in the cone-jet mode. Eight different solvents with dielectric constants, κ, ranging from 12.5 to 182 were tested. The residue particle size distributions were measured using a TSI scanning mobility particle sizer, (SMPS). The produced liquid droplet sizes were then calculated from the known solution concentrations. The results show that: (1) For the produced droplet size, Dd , experimental data are in agreement with the scaling law (G(κ) = 1.66κ −1/6) proposed by Ganan-Calvo et al. (1994) for solvents with high dielectric constants. The derivation in low dielectric constant cases may be that the assumption of Ganan-Calvo (1994) on the characteristic length may not hold for the present system; (2) for the emitted cu...

Slip Correction Measurements of Certified PSL Nanoparticles Using a Nanometer Differential Mobility Analyzer (Nano-DMA) for Knudsen Number From 0.5 to 83

The slip correction factor has been investigated at reduced pressures and high Knudsen number using polystyrene latex (PSL) particles. Nano-differential mobility analyzers (NDMA) were used in determining the slip correction factor by measuring the electrical mobility of 100.7 nm, 269 nm, and 19.90 nm particles as a function of pressure. The aerosol was generated via electrospray to avoid multiplets for the 19.90 nm particles and to reduce the contaminant residue on the particle surface. System pressure was varied down to 8.27 kPa, enabling slip correction measurements for Knudsen numbers as large as 83. A condensation particle counter was modified for low pressure application. The slip correction factor obtained for the three particle sizes is fitted well by the equation: C = 1 + Kn (α + β exp(−γ/Kn)), with α = 1.165, β = 0.483, and γ = 0.997. The first quantitative uncertainty analysis for slip correction measurements was carried out. The expanded relative uncertainty (95 % confidence interval) in measuring slip correction factor was about 2 % for the 100.7 nm SRM particles, about 3 % for the 19.90 nm PSL particles, and about 2.5 % for the 269 nm SRM particles. The major sources of uncertainty are the diameter of particles, the geometric constant associated with NDMA, and the voltage.