Introduction
According to the Fifth Assessment Report of the Intergovernmental Panel on
Climate Change, clouds and aerosols contribute the largest uncertainty to
understanding changes in climate (Boucher et al., 2013). Aerosols affect the
climate directly by reflecting or absorbing solar radiation, and indirectly
when they form cloud particles (Boucher et al., 2013). A major difficulty in
modeling particle nucleation and aerosol activation lies in determining
physical properties of particles on the nanoscale without precise knowledge
about chemical composition.
Experimental setup used to generate and collect oxidized α-pinene particles. The smog chamber is initially flushed with dry or wet
air. Once the relative humidity in the chamber is established, particles are
generated in the smog chamber by mixing α-pinene and ozone.
Resulting particles are either analyzed with the SMPS or sampled using the
cascade impactor.
Recent studies in particle nucleation and cloud droplet activation have used
various methods to estimate particle surface tension, which is a very
important parameter in modeling both processes (Duplissy et al., 2008; Kiss
et al., 2005; Laaksonen and McGraw, 1996; Moldanova and Ljungström, 2000;
Petters et al., 2009; Prisle et al., 2010; Sorjamaa et al., 2004; Wex et al.,
2009). Particle nucleation is described by the Kelvin equation (Laaksonen and
McGraw, 1996), which requires knowledge about surface tension of the
nucleating particle (Laaksonen and McGraw, 1996; Schmelzer et al., 1996). Not
surprisingly, direct measurement of the surface tension of particles near
activation state conditions has not been possible. Studies on nucleation
often rely on an assumption about the composition and use compiled values for
bulk surface tension including values extrapolated from other phases,
estimated or interpolated from similar compounds or simply assume
“physically reasonable values” (Daisey and Hopke, 1993; Moldanova and
Ljungström, 2000). Hansen et al. (2015), demonstrated the magnitude of
the error that can occur when commonly made assumptions about surface
tensions are used in models. A direct method of measuring the surface
tensions of particles immediately after nucleation is preferable to these
assumptions and would likely reduce the error in particle nucleation models.
Köhler theory is used to predict the properties of activating cloud
condensation nuclei (Köhler, 1936). The Köhler equation balances the
Kelvin effect with Raoult's Law in order to describe particle activation.
The Kohler equation applies equilibrium thermodynamics to describe the
process in which water vapor condenses to form liquid droplets.
lnpw(Dp)p0=4MwσwRTρwDp-6nsMwπρwDp3,
where T is absolute temperature, R ideal gas constant,
pw droplet water vapor pressure, p0 saturation
vapor pressure over a flat surface, σw droplet surface
tension, ρw density of pure water,
ns moles of solute, Mw molecular weight of
water, and Dp droplet diameter (Köhler, 1936). Thus, similar
problems arise in specifying physical properties used in the Kelvin term of
the Köhler equation. To date, there has been little consistency between
assumptions used for the activated particles' surface tensions. Many
researchers (Conant et al., 2002; Huff Hartz et al., 2005; Petters and
Kreidenweis, 2007; Prenni et al., 2007) have assumed that, at activation, the
particles consist mainly of water, so a surface tension of pure water was
used. Though this is a reasonable initial assumption, it neglects the
depressive effect of organic surfactants on the activating particles' surface
tensions (Facchini et al., 1999; Kiss et al., 2005). It is now generally
agreed upon that, for most activating particles with these surfactants, the
surface tension is reduced by about 10–15 % (Asa-Awuku et al., 2010;
Engelhart et al., 2008; Facchini et al., 2000; King et al., 2009). Several
methods have been used to predict this surface tension reduction. Some
researchers have collected particles and diluted them so as to allow for a
direct measurement using conventional instruments (Asa-Awuku et al., 2008;
Henning et al., 2005; Moore et al., 2008; Schwier et al., 2013). These values
were then extrapolated back to the initial concentration by fitting them to a
Szyskowski–Langmuir isotherm. Occasionally, surface tensions for the
particles have been back-calculated using Köhler theory analysis when all
other parameters are known or estimated (Asa-Awuku et al., 2010; Engelhart et
al., 2008). Others (Kiss et al., 2005; Raymond and Pandis, 2002) have
prepared solutions mimicking the bulk chemical composition of aerosol
particles and directly measured their surface tensions. However, none of
these methods directly measures the surface tension of the actual particles
in question.
Panel (a) shows an AFM surface scan of cleaned (no sample)
puck. The surface is rough at this microscopic level. Panel (b) shows an AFM
scan of the same puck after sampling. This image was collected in the sample
deposit region. It shows that the roughness gets filled in by the sample.
Panel (c) shows the centered vertical and horizontal traces from these
analyses. Significant roughness is observed on the steel that is not
observed on the collected sample. This indicates that the sample could flow;
i.e., it had liquid characteristics at the time of sampling.
Yazdanpanah (Yazdanpanah et al., 2008) has developed a method to measure the
surface tension of small (∼ 200 nm in diameter) droplets and films
using constant-diameter nanoneedle tips on the atomic force microscope. In
this work, we will show how his method has been adapted to accurately measure
the surface tensions of collected atmospheric aerosols.
Experimental methods
Particle generation
In this project, oxidized α-pinene particles were generated in a
1 m3 polytetrafluoroethylene (PTFE) smog chamber (Fig. 1). Particles
were formed in either “dry” (< 10 % RH) or “wet” (67 % RH)
conditions. To generate the “dry“ conditions, the chamber was flushed with
clean, dry air for several hours. Compressed air was cleaned using a TSI
3074B filtered air supply. To generate the “wet” conditions, clean air was
bubbled through water at 2 liters per minute (L min-1), filtered, and sent to the smog chamber. The
chamber was flushed with this humid airstream until a maximum relative
humidity was reached. Relative humidity was measured using a Vaisala HM337 Humidity and Temperature Transmitter. Neither the water content of the
particles nor the surface tensions of the particles generated under dry and
wet conditions are likely to be directly proportional to the relative
humidities (Jonsson et al., 2007).
During experiments, the dry, cleaned airstream was sent into the smog
chamber at 2 L min-1. This airstream could be diverted either through
a sample port or through an ozone generator (Poseidon Ozone Generator by
Ozotech) in series with a High-Efficiency Particulate Air (HEPA) filter before entering the chamber. An outlet
port from the chamber could be connected either to a scanning mobility
particle sizer (a 3080 TSI differential mobility analyzer in series with the
3775 TSI condensation particle counter) or a cascade impactor (I-1L Cascade
impactor by PIXE). Experiments were only conducted when the initial particle
concentration in the smog chamber was below
100 particles cm-3, as
measured by the scanning mobility particle sizer (SMPS).
At the start of each experiment, ozone was added to the smog chamber. If
particle counts in the smog chamber remained low after about 5 min,
indicating a chamber free of oxidizable volatile organic compounds,
5 µL of liquid α-pinene (97 % pure, Acros Organics)
would be then injected into a sample port, where it would be vaporized and carried into the
smog chamber. Ozone and α-pinene were added in a roughly 1 : 1
molar ratio; the high starting concentrations were necessary so that an
adequate particle volume would form for collection later. The resulting
oxidized α-pinene particles were allowed to age in the chamber for
90 min. The ozone–α-pinene system was selected because it is one of the
more, if not the most, characterized secondary organic aerosol (SOA) systems. Speciation and chemical
characterization results from similar systems have been reported by various
researchers (e.g., Jang and Kamens, 1999; Praplan et al., 2015; Tu et al.,
2016; Yu et al., 1999).
During the aging process, particle size distribution data were collected with
the SMPS. The SMPS sample flow rate was 0.3 L min-1 and the sheath
flow rate used on the differential mobility analyzer (DMA) was
3 L min-1. These settings allowed for collection of particle size
distribution data over the range of 15 to 660 nm. The low sampling flow rate
ensured that the smog chamber operated under positive pressure. The size of
the oxidized α-pinene particles followed a log-normal distribution,
whose center shifted to larger sizes over time. In the period where particles
aged, the modal diameter increased from around 120 to 200 nm. The most
significant changes in particle size distribution occurred in the first hour
after the α-pinene was introduced to the smog chamber. The 90 min
aging period ensured minimal changes in particle size distribution during
collection. A schematic of the experimental setup is shown in Fig. 1.
Surface tension of bulk liquids used for standardization, measured
by the Wilhelmy plate at 23.9 ∘C. Averages reported as
“average ± standard error (s/n)”. Pure oleic acid has a
surface tension of 32.79 dyn cm-2 at 20 ∘C (Chumpitaz et al.,
1999), and pure α-pinene has a surface tension of
26.0 dyn cm-1 at 25 ∘C (Daisey and Hopkey, 1993). Measured
values on Wilhelmy plate are close to reported values, considering
differences in purity and temperature.
Component
Surface tension (dyn cm-1)
Oleic acid
29.47
(90 % purity)
29.53
Average
29.50 ± 0.03
α-pinene
25.75
(97 % purity)
25.36
Average
25.6 ± 0.2
Particle collection
At 90 min after α-pinene was introduced to the smog chamber, the
outlet of the chamber was switched to feed to the cascade impactor. The
second smallest stage (L2) was used to collect the particles on a cleaned
steel disk. The 50 % aerodynamic cutoff diameter for this stage at
4 L min-1 was 40 nm. After 90 min a visible particle film had
collected on the disk. The particles deposit in a circular region
∼ 6 mm in diameter on the steel disc. The steel disc before and after
sample collection was imaged using a ScanAsyst and PeakForce Tapping-mode atomic force microscope (AFM)
microscope. These images are displayed in Fig. 2a and b. These images show
that the steel disk is rough. After sampling the surface is smoother
indicating that the sample flowed and filled in the roughness. Two traces,
centered vertically and horizontally, from each image are shown in Fig. 2c.
Sample analysis
A Veeco Multimode V AFM and NaugaNeedle NN-HAR-FM60
probes were used to analyze the particle film collected on the disk. The
probes consist of a flat, flexible cantilever, and a nanoneedle mounted
normally to the cantilever at its end. The Ga–Ag nanoneedles are shaped as
cylinders on the order of 100 nm in diameter and 10 µm in length.
A micro-Wilhelmy method developed by Yazdanpanah et al. (2008), described
below, was then used to measure the surface tension of the samples.
A typical force curve obtained using NaugaNeedle NN-HAR-FM60
probes and an atomic force microscope. The blue line indicates the probe
approaching the sample, and the red line indicates the probe retracting from
the sample. At point 1, the nanoneedle is approximately 1 µm from the
surface of the liquid sample. At point 2, the nanoneedle is just above the
surface of the liquid. At point 3, the nanoneedle has touched the liquid,
which wicks up and exerts a downward force on the probe. At point 4, the
nanoneedle begins to pull out of the liquid. At point 5, the liquid is just
about to break from the end of the nanoneedle, and the contact angle of the
liquid–needle interface approaches zero. At point 6, the nanoneedle has been
pulled out of the liquid sample. The probe retracts back to point 1.
The sample was analyzed with the AFM in force mode. In this mode, the AFM's
piezoelectric transducers push the sample film up to and away from the probe
with high precision. The downward force exerted on the probe was recorded by
the AFM as a function of its location relative to the film's surface. A force
curve obtained with the AFM is presented in Fig. 3.
In Fig. 3, the curve in blue illustrates the force exerted on the probe as it
approaches and touches the sample surface. The curve in red illustrates the
force exerted on the probe as it is pulled from the sample. If it is assumed
that only forces related to the surface tension of the liquid film are
exerted on the probe, then Eq. (2) will be
Fprobe=σ⋅L⋅cos(θ),
where σ is the surface tension of the sample, L is the wetted
perimeter of the tip, and θ is the contact angle between the fluid
and the tip. For a more complete derivation of this equation see Yum and Yu (2006).
Measured and calculated values obtained during three experiments.
In the first experiment, α-pinene was used as the standard, oleic
acid was used as a check standard, and the oxidized α-pinene
particles were generated in dry conditions. In the second experiment, oleic
acid was used as the standard, there was no check standard, and the oxidized
α-pinene particles were generated in dry conditions. In the third
experiment, α-pinene was used as the standard, there was no check
standard, and the oxidized α-pinene particles were generated in wet
conditions.
Standard
Check standard
Sample
oxidized α-pinene
particles
Experimental
Measured
Calculated
Measured
Calculated
Measured
Calculated
conditions
maximum
wetted tip
maximum
wetted tip
maximum
surface tension
force (nN)
perimeter (nm)
force (nN)
perimeter (nm)
force (nN)
(dyn cm-1)
– Particles generated
10.1
26.8
at < 10 % RH
9.7
377.0
11.0
373.8
10.3
27.4
– Standard: α-pinene
9.6
373.8
10.9
370.7
10.2
27.0
(97 % purity)
9.7
377.0
10.8
364.4
11.4
30.2
– Check standard: oleic
10.5
27.8
acid (90 % purity)
Average
375.9 ± 1.1
369.7 ± 2.8
27.8 ± 0.6
– Particles generated
at < 10 % RH
11.7
395.8
10.8
27.2
– Standard: oleic acid
11.8
399.0
10.6
26.7
(90 % purity)
11.7
395.8
10.6
26.8
– Check standard: none
Average
369.9 ± 1.0
26.9 ± 0.2
– Particles generated
21.4
43.3
at 67 % RH
12.7
496.4
21.2
42.9
– Standard: α-pinene
12.5
490.1
20.6
41.8
(97 % purity)
12.6
493.2
22.8
46.3
– Check standard: none
23.4
47.5
Average
493.2 ± 1.8
44.4 ± 1.1
Because the nanoneedle has a cylindrical geometry, the wetted perimeter, L,
is constant during all force measurements. This can be seen by the
near-constant negative force exerted on the probe when it is initially
retracting out of the sample. The increase in the downward force before the
nanoneedle is completely pulled from the sample is attributed to a decrease
in the contact angle. At the point the sample breaks away from the
nanoneedle, the contact angle is zero. When this angle is zero and the needle
is smaller than the capillary length (Uddin et al., 2011), Eq. (2) is as
follows:
σ=FprobeL.
For this project, Eq. (3) (Padday et al., 1975) was used, using the force
reading at the point the nanoneedle broke from the sample. This corresponds
to point 5 in Fig. 3. The magnitude of the force at the break-away step
suggests that the collected sample is liquid rather than a glassy or
amorphous solid observed for some oxidized volatile organic compound (VOC) systems.
Measured and approximated surface tensions of α-pinene
particles. Bulk α-pinene and dry, oxidized α-pinene
particles have a similar surface tension. Wet α-pinene particles
have a higher surface tension. Measurements from this study are shown in
italicized font, other values are given for context.
RH at particle creation (%)
Surface tension (dyn cm-1)
Description, source
n/a
25.6
Pure α-pinene, bulk
This experiment; Wilhelmy plate
< 10
27.5
Oxidized α-pinene particles
This experiment; AFM measurements
67
44.4
Oxidized α-pinene particles
This experiment; AFM measurements
> 100
61.7
Oxidized α-pinene particles, assume depressed
(activation)
surface tension of pure water
Engelhart et al. (2008)
> 100
72.5
Oxidized α-pinene particles, assume surface
(activation)
tension of pure water
Huff Hartz et al. (2005); Prenni et al. (2007)
Several aspects of the AFM system were calibrated daily before the collected
α-pinene particles were analyzed, typically during particle
collection. Because the AFM directly measures the deflection of the
cantilever, a force exerted on the nanoneedle could only be obtained after
calibrating the cantilever's deflection and determining its spring constant.
In the AFM, a laser is reflected off of the cantilever into a photodetector;
cantilever deflection is measured by the movement of the laser on the
photodetector. To calibrate this measurement, the probe was gently pushed
into a hard, steel surface. The slope of the force curve when the probe is in
contact with the surface indicates the observed cantilever deflection from
the photodetector (y axis of the force curve) versus the actual distance
the surface is moving the cantilever (x axis of the force curve). This
slope was entered into the AFM's operating program.
The spring constant of the tip was found using a thermal tune. The thermal
tune is a common method to calculate the spring constant using measurements of
the cantilever's response to thermal noise (Serry, 2005). The native Veeco
software was used to perform the thermal tune. After these calibrations, the
AFM will produce force curves that relate force and distance accurately.
In order to calculate surface tension from force data, the wetted perimeter
of the nanoneedle also had to be obtained. This was done by obtaining force
curves of liquid standards and using Eq. (3) to back-calculate the wetted
perimeter given force and surface tension information. Two liquid standards
were used: 90 % pure oleic acid (Sigma-Aldrich) and 97 % pure,
non-oxidized liquid α-pinene. The surface tensions of these two
standards were measured using a Wilhelmy plate (Sigma 703D, KSV Instruments
Ltd.); results are shown in Table 1. Measurements for the standards yielded
lower values compared to the literature for pure oleic acid and α-pinene. Because the standards were not completely pure, this was not
unexpected, and surface tension values obtained from the Wilhelmy plate were
used.
A summary of the steps used to calibrate and analyze samples on the AFM is
shown in Fig. 4.
Procedure used to determine the surface tension of oxidized
α-pinene particles using the AFM. The cantilever's spring constant
was determined (step 1), which allowed for the AFM to obtain force curves. Force
curves of a liquid standard were obtained (step 2), and the nanoneedle's
wetted perimeter was calculated with Eq. (3) given the standard's known
surface tension (step 3). Force curves of the oxidized α-pinene
sample were obtained (step 4), and its surface tension was calculated with
Eq. (3) given the nanoneedle's wetted perimeter (step 5). For initial tests,
a check standard was used to verify the validity of the wetted perimeter and
sample surface tension calculations (optional steps 6–7).
Results and discussion
Surface tension data were obtained for oxidized α-pinene particles.
The AFM's measurements and calculated values are presented in Table 2. Both
“dry” oxidized α-pinene particles and “wet” oxidized α-pinene particles were analyzed. The mean surface tension of “dry”
oxidized α-pinene particles was found to be 27.5 dyn cm-1 at
23 ∘C, with an average uncertainty of 1.1 dyn cm-1. This is
similar to the surface tension of pure α-pinene as reported in the
literature (Daisey and Hopke, 1993) and measured with our Wilhelmy plate. The
mean surface tension of “wet” oxidized α-pinene particles was found
to be 44.4 dyn cm-1 at 23 ∘C, with an uncertainty of
2.4 dyn cm-1.
The results presented in Table 2 include a set of standards, which were done
for every set of measurements. The purpose of this standard was to allow for the
determination of the perimeter of the nanoneedle. For the first set of
reported measurements, a check standard was also added to verify that the
perimeter measurement was correct. This check standard is not required for
each set of measurements. The surface tensions measured here were compared to
the surface tensions of the standards measured and presented in Table 1.
Table 3 compares the mean surface tensions of oxidized α-pinene
particles measured in this study with published estimates for the surface
tension of activating, oxidized α-pinene particles (Engelhart et al.,
2008; Huff Hartz et al., 2005; Prenni et al., 2007). Our results suggest that
the surface tension of dry, oxidized α-pinene particles is not very
different from the surface tension of its VOC precursor. It is also apparent
that the surface tension of oxidized α-pinene particles formed in
more humid conditions had a higher surface tension than oxidized α-pinene particles formed in dry conditions.
These results appear to be in agreement with current theory. It is generally
believed that the surface tension of an activating, oxidized α-pinene
particle is slightly lower than that of pure water, at 61.7 dyn cm-1
(Engelhart et al., 2008). This is due to the depressive effect of organic
surfactants in the droplets. The results from the particles generated at the
higher of two humidities suggest that the surface tension is between the
surface tension of pure water and the surface tension of the dry oxidized
α-pinene. This relationship is unlikely to be directly linear given
that additive surface tensions only apply to chemicals with similar
properties, which water and the organics produced from the oxidation of
α-pinene are not. Furthermore, the surface tension of the dry, oxidized
α-pinene particles was found to be similar to the surface tension of
pure α-pinene. This similarity in properties may be due to their
similar structures. Now that a method suitable for the direct measurement of
particle surface tension has been established, direct measurements of
particles with several other moisture contents should be taken to examine the
precise relationship between surface tension and moisture content in a
particle. With modifications to the particle generation technique, this
method can be used to experimentally measure the surface tension of
activating particles.