Currently, the impact of various phase states of aerosols on the climate is not well understood, especially for submicrometer sized aerosol particles that typically have extended lifetime in the atmosphere. This is largely due to the inherent size limitations present in current experimental techniques that aim to directly assess the phase states of fine aerosol particles. Herein we present a technique that uses atomic force microscopy to probe directly for the phase states of individual, submicrometer particles by using nanoindentation and nano-Wilhelmy methodologies as a function of relative humidity (RH) and ambient temperature conditions. When using these methodologies for substrate deposited individual sucrose particles, Young's modulus and surface tension can be quantified as a function of RH. We show that the force profiles collected to measure Young's modulus and surface tension can also provide both qualitative and quantitative assessments of phase states that accompany solid, semisolid, and liquid particle phases. Specifically, we introduce direct measurements of relative indentation depth and viscoelastic response distance on a single particle basis at a given applied force to quantitatively probe for the phase state as a function of RH and corresponding viscosity. Thus, we show that the three phase states and phase state transitions of sucrose can be identified and ultimately propose that this technique may also be used to study other atmospherically relevant systems.

Understanding the role of sea spray aerosol (SSA) on climate and the environment is of great interest due to their high number concentration throughout the Earth's atmosphere. Despite being of fundamental importance, direct surface tension measurements of SSA relevant sub-micrometer particles are rare, largely due to their extremely small volumes. Herein, atomic force microscopy (AFM) is used to directly measure the surface tension of individual sub-micrometer SSA particle mimics at ambient temperature and varying relative humidity (RH). Specifically, we probed both atmospherically relevant and fundamentally important model systems including electrolyte salts, dicarboxylic acids, and saccharides as single components and mixtures. Our results show that the single particle surface tension depends on RH or solute mole percentage and chemical composition. Moreover, for liquid droplets at and below 100 Pa s in viscosity, or at corresponding RH, we show good agreement between the AFM single particle and the bulk solution surface tension measurements at overlapping concentration ranges. Thus, direct surface tension measurements of individual particles using AFM is shown over a wide range of chemical systems as a function of RH, solute mole percentage, and viscosity than previously reported.

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.

Cocrystallization of a nonconductive, boron-based host with aromatic guests generates conductive cocrystals. Carrier mobilities of the cocrystals with either pyrene or tetrathiafulvalene were measured using conducting probe atomic force microscopy. The incorporation of the π-electron-rich aromatic guests results in electrically conductive cocrystals. The cocrystal with tetrathiafulvalene as a guest shows an approximately seven times higher charge carrier mobility than the cocrystal with pyrene.

Nano-dimensional crystals of aspirin generated through sonochemistry exhibit Young's modulus values an order of magnitude softer than macro-dimensional crystals.