A method for the direct measurement of surface tension of collected atmospherically relevant aerosol particles using atomic force microscopy

Abstract

Abstract. Accurate estimates of particle surface tension are required for models concerning atmospheric aerosol nucleation and activation. However, it is difficult to collect the volumes of atmospheric aerosol required by typical instruments that measure surface tension, such as goniometers or Wilhelmy plates. In this work, a method that measures, ex situ, the surface tension of collected liquid nanoparticles using atomic force microscopy is presented. A film of particles is collected via impaction and is probed using nanoneedle tips with the atomic force microscope. This micro-Wilhelmy method allows for direct measurements of the surface tension of small amounts of sample.This method was verified using liquids, whose surface tensions were known. Particles of ozone oxidized α-pinene, a well-characterized system, were then produced, collected, and analyzed using this method to demonstrate its applicability for liquid aerosol samples. It was determined that oxidized α-pinene particles formed in dry conditions have a surface tension similar to that of pure α-pinene, and oxidized α-pinene particles formed in more humid conditions have a surface tension that is significantly higher.

Aerosol Optical Tweezers Constrain the Morphology Evolution of Liquid-Liquid Phase-Separated Atmospheric Particles

“…In the literature, there is a previous direct measurement of the surface tension of a-pinene SOA performed using atomic force microscopy (AFM). 88 The authors measured a surface tension of 27.5 mN/m at 10% RH and 44.4 mN/m at 67% RH for a-pinene SOA. 88 Our observations are not consistent with such a strong RH dependence for the surface tension of SOA because the morphology determined using the AOT did not change with an RH of 12% versus 73%.…”

“…88 The authors measured a surface tension of 27.5 mN/m at 10% RH and 44.4 mN/m at 67% RH for a-pinene SOA. 88 Our observations are not consistent with such a strong RH dependence for the surface tension of SOA because the morphology determined using the AOT did not change with an RH of 12% versus 73%. This difference in surface tensions between the two experiments may be due to our different precursor concentrations and reaction conditions, and possible influences of the substrate the SOA was measured on using AFM.…”

Temperature‐ and Humidity‐Dependent Phase States of Secondary Organic Aerosols

“…Calculations are described elsewhere (Marsh et al, ; Rothfuss & Petters, ) and in the SI. Surface tension was assumed based on typical values for pure organic liquids (Korosi & Kovats, ) and α‐pinene‐derived SOA (Hritz et al, ). For 100 nm monomers the viscosity transition measurement is near 10 6 Pa·s.…”

Optical deformation of single aerosol particles

“…Due to the short times over which measurements are taken, droplet compositions remain below the solubility limit (5,38). Another method used to determine aerosol surface tension is atomic force microscopy (AFM) (39)(40)(41). This approach is able to access sizes below those that are feasible with optical trapping.…”