Correlating 3D Morphology, Phase State, and Viscoelastic Properties of Individual Substrate-Deposited Particles

Abstract

Depending on the source and relative humidity, aerosols can have different compositional, morphological, and viscoelastic properties. Aerosol studies determining the relationship between these properties and their combined effect on the climate and environment are important. This work aims to correlate the 3D morphology, phase state, and viscoelastic properties of selected single-component chemical systems found in sea spray aerosol (SSA) that were substrate-deposited on a solid surface, studied with atomic force microscopy (AFM). Specifically, two inorganic salts (NaCl and MgSO4), four organic acids (malonic, glutaric, azelaic, and palmitic acids), three saccharides (glucose, sucrose, and raffinose), and lipopolysaccharide from Escherichia coli were studied. Furthermore, three inorganic–organic binary chemical mixtures (NaCl–malonic acid, NaCl–glucose, and MgSO4–glucose) at 1:3 and 3:1 mass ratio were studied. AFM imaging and force spectroscopy at 20% relative humidity were performed to record 3D height images of individual particles and measure force–distance plots, respectively. First, by utilizing combined relative indentation depth (RID) and viscoelastic response distance (VRD) data obtained from the force–distance plots, we establish quantitative framework toward differentiation of the solid, semisolid and liquid phase states of individual particles without prior knowledge of their chemical identity. Second, we show that the single particle aspect ratio (AR) of a wide range of compounds relevant to SSA is a measure of the extent of the particle spreading as a result of impaction with the solid substrate, which can be directly related to the RID and VRD results. Thus, we demonstrate that a quick height imaging and determination of a single particle AR can be used to assess the phase state. Therefore, we introduce the ability to semiquantitatively assess the phase states of individual substrate deposited particles of SSA-relevant compounds, irrespective of the microscopy technique used, which can subsequently be further validated by more quantitative AFM force spectroscopy.

The Sea Spray Chemistry and Particle Evolution study (SeaSCAPE): overview and experimental methods

“…The morphological categorization was performed qualitatively, analogous to previous studies. 23,76 Next, the relative distribution of morphologies for nSSA and hetSSA samples were compared (Fig. 11b).…”

Probing the Water Uptake and Phase State of Individual Sucrose Nanoparticles Using Atomic Force Microscopy

“… 57 For each force plot at a particular RH, the viscoelastic response distance (VRD) and indentation depth ( I ) at 10 nN were determined on the basis of the previously established method, as illustrated in Figure 3 . 57 , 65 The VRD values can be related to the particle viscoelastic nature, where higher values generally correspond to lower viscosity. The relative indentation depth (RID) at 10 nN was quantified by dividing the measured indentation depth at 10 nN by the particle height at the corresponding RH.…”

“…Previously, a quantitative framework was established to determine the phase state of individual sub-micrometer particles using the VRD and RID measurements. 65 Specifically, the RID measurement is used to differentiate between the semisolid and liquid phases, where a RID value equal or greater than 0.95 is indicative of a liquid and a value less than 0.95 is indicative of a semisolid or solid phase. The VRD measurement is used to differentiate between the solid and semisolid phases, where a VRD value less than 0.5 nm is indicative of a solid phase and a value greater than 0.5 nm is indicative of a semisolid phase.…”

Glassy aerosol may promote virus transmission

“…PeakForce QNM mode with a force threshold of 10 nN. As the tip was indented into the particles, the tip-particle separation distance and force recorded and used to infer viscoelastic properties of the particles 41,42 . The phase of the particles could then be determined as compared to phases of reference materials (NaCl, sucrose, malonic acid).…”