6 research outputs found
Plasma-liquid interactions: a review and roadmap
Plasma-liquid interactions represent a growing interdisciplinary area of research involving plasma science, fluid dynamics, heat and mass transfer, photolysis, multiphase chemistry and aerosol science. This review provides an assessment of the state-of-the-art of this multidisciplinary area and identifies the key research challenges. The developments in diagnostics, modeling and further extensions of cross section and reaction rate databases that are necessary to address these challenges are discussed. The review focusses on non-equilibrium plasmas
Comparison of Approaches for Measuring the Mass Accommodation Coefficient for the Condensation of Water and Sensitivities to Uncertainties in Thermophysical Properties
We compare and contrast measurements of the mass accommodation
coefficient of water on a water surface made using ensemble and single
particle techniques under conditions of supersaturation and subsaturation,
respectively. In particular, we consider measurements made using an
expansion chamber, a continuous flow streamwise thermal gradient cloud
condensation nuclei chamber, the Leipzig Aerosol Cloud Interaction
Simulator, aerosol optical tweezers, and electrodynamic balances.
Although this assessment is not intended to be comprehensive, these
five techniques are complementary in their approach and give values
that span the range from near 0.1 to 1.0 for the mass accommodation
coefficient. We use the same semianalytical treatment to assess the
sensitivities of the measurements made by the various techniques to
thermophysical quantities (diffusion constants, thermal conductivities,
saturation pressure of water, latent heat, and solution density) and
experimental parameters (saturation value and temperature). This represents
the first effort to assess and compare measurements made by different
techniques to attempt to reduce the uncertainty in the value of the
mass accommodation coefficient. Broadly, we show that the measurements
are consistent within the uncertainties inherent to the thermophysical
and experimental parameters and that the value of the mass accommodation
coefficient should be considered to be larger than 0.5. Accurate control
and measurement of the saturation ratio is shown to be critical for
a successful investigation of the surface transport kinetics during
condensation/evaporation. This invariably requires accurate knowledge
of the partial pressure of water, the system temperature, the droplet
curvature and the saturation pressure of water. Further, the importance
of including and quantifying the transport of heat in interpreting
droplet measurements is highlighted; the particular issues associated
with interpreting measurements of condensation/evaporation rates with
varying pressure are discussed, measurements that are important for
resolving the relative importance of gas diffusional transport and
surface kinetics
Statistical Thermodynamic Model for Surface Tension of Organic and Inorganic Aqueous Mixtures
The surface composition
and tensions of aqueous aerosols govern
a set of processes that largely determine the fate of particles in
the atmosphere. Predictive modeling of surface tension can provide
significant contributions to studies of atmospheric aerosol effects
on climate and human health. A previously derived surface tension
model for single solute aqueous solutions used adsorption isotherms
and statistical mechanics to enable surface tension predictions across
the entire concentration range as a function of solute activity. Here,
we extend the model derivation to address multicomponent solutions
and demonstrate its accuracy with systems containing mixtures of electrolytes
and organic solutes. Binary model parameters are applied to the multicomponent
model, requiring no further parametrization for mixtures. Five ternary
systems are studied here and represent three types of solute combinations:
organic–organic (glycerol–ethanol), electrolyte–organic
(NaCl–succinic acid, NaCl–glutaric acid), and electrolyte–electrolyte
(NaCl–KCl and NH<sub>4</sub>NO<sub>3</sub>–(NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub>). For the NaCl-glutaric acid system,
experimental measurements of picoliter droplet surface tension using
aerosol optical tweezers show excellent agreement with the model predictions
Dynamic measurements and simulations of airborne picolitre-droplet coalescence in holographic optical tweezers
We report studies of the coalescence of pairs of picolitre aerosol droplets manipulated with holographic optical tweezers, probing the shape relaxation dynamics following coalescence by simultaneously monitoring the intensity of elastic backscattered light (EBL) from the trapping laser beam (time resolution on the order of 100 ns) while recording high frame rate camera images (time resolution <10 μs). The goals of this work are to: resolve the dynamics of droplet coalescence in holographic optical traps; assign the origin of key features in the time-dependent EBL intensity; and validate the use of the EBL alone to precisely determine droplet surface tension and viscosity. For low viscosity droplets, two sequential processes are evident: binary coalescence first results from the overlap of the optical traps on the time scale of microseconds followed by the recapture of the composite droplet in an optical trap on the time scale of milliseconds. As droplet viscosity increases, the relaxation in droplet shape eventually occurs on the same time scale as recapture, resulting in a convoluted evolution of the EBL intensity that inhibits quantitative determination of the relaxation time scale. Droplet coalescence was simulated using a computational framework to validate both experimental approaches. The results indicate that time-dependent monitoring of droplet shape from the EBL intensity allows for robust determination of properties such as surface tension and viscosity. Finally, the potential of high frame rate imaging to examine the coalescence of dissimilar viscosity droplets is discussed
Volatility and Oxidative Aging of Aqueous Maleic Acid Aerosol Droplets and the Dependence on Relative Humidity
The
microphysical structure and heterogeneous oxidation by ozone
of single aerosol particles containing maleic acid (MA) has been studied
using aerosol optical tweezers and cavity enhanced Raman spectroscopy.
The evaporation rate of MA from aqueous droplets has been measured
over a range of relative humidities and the pure component vapor pressure
determined to be (1.7 ± 0.2) × 10<sup>–3</sup> Pa.
Variation in the refractive index (RI) of an aqueous MA droplet with
relative humidity (RH) allowed the subcooled liquid RI of MA to be
estimated as 1.481 ± 0.001. Measurements of the hygroscopic growth
are shown to be consistent with equilibrium model predictions from
previous studies. Simultaneous measurements of the droplet composition,
size, and refractive index have been made during ozonolysis at RHs
in the range 50–80%, providing insight into the volatility
of organic products, changes in the droplet hygroscopicity, and optical
properties. Exposure of the aqueous droplets to ozone leads to the
formation of products with a wide range of volatilities spanning from
involatile to volatile. Reactive uptake coefficients show a weak dependence
on ozone concentration, but no dependence on RH or salt concentration.
The time evolving RI depends significantly on the RH at which the
oxidation proceeds and can even show opposing trends; while the RI
increases with ozone exposure at low relative humidity, the RI decreases
when the oxidation proceeds at high relative humidity. The variations
in RI are broadly consistent with a framework for predicting RIs for
organic components published by Cappa et al. (J. Geophys. Res. 2011, 116, D15204). Once oxidized,
particles are shown to form amorphous phases on drying rather than
crystallization, with slow evaporation kinetics of residual water
In Situ Observation on the Dynamic Process of Evaporation and Crystallization of Sodium Nitrate Droplets on a ZnSe Substrate by FTIR-ATR
Sodium
nitrate is a main component of aging sea salt aerosol, and its phase
behavior has been studied repeatedly with wide ranges observed in
the efflorescence relative humidity (RH) in particular. Studies of
the efflorescence dynamics of NaNO<sub>3</sub> droplets deposited
on a ZnSe substrate are reported, using an in situ Fourier transform
infrared attenuated total reflection (FTIR-ATR) technique. The time-dependence
of the infrared spectra of NaNO<sub>3</sub> aerosols accompanying
step changes in RH have been measured with high signal-to-noise ratio.
From the IR difference spectra recorded, changes of the time-dependent
absorption peak area of the O–H stretching band (<i>ν-</i>OH, ∼3400 cm<sup>–1</sup>) and the nitrate out-of-plane
bending band (ν<sub>2</sub>-NO<sub>3</sub><sup>–</sup>, ∼836 cm<sup>–1</sup>) are obtained. From these measurements,
changes in the IR signatures can be attributed to crystalline and
solution phase nitrate ions, allowing the volume fraction of the solution
droplets that have crystallized to be determined. Then, using these
clear signatures of the volume fraction of droplets that have yet
to crystallize, the homogeneous and heterogeneous nucleation kinetics
can be studied from conventional measurements using a steady decline
in RH. The nucleation rate measurements confirm that the rate of crystallization
in sodium nitrate droplets is considerably less than in ammonium sulfate
droplets at any particular degree of solute supersaturation, explaining
the wide range of efflorescence RHs observed for sodium nitrate in
previous studies. We demonstrate that studying nucleation kinetics
using the FTIR-ATR approach has many advantages over brightfield imaging
studies on smaller numbers of larger droplets or measurements made
on single levitated particles