The importance of organic aerosol particles in the environment has been long established, influencing cloud formation and lifetime, absorbing and scattering sunlight, affecting atmospheric composition and impacting on human health. Conventionally, ambient organic particles were considered to exist as liquids. Recent observations in field measurements and studies in the laboratory suggest that they may instead exist as highly viscous semi-solids or amorphous glassy solids under certain conditions, with important implications for atmospheric chemistry, climate and air quality. This review explores our understanding of aerosol particle phase, particularly as identified by measurements of the viscosity of organic particles, and the atmospheric implications of phase state.

The complex interplay of processes that govern the size, composition, phase and morphology of aerosol particles in the atmosphere is challenging to understand and model. Measurements on single aerosol particles (2 to 100 μm in diameter) held in electrodynamic, optical and acoustic traps or deposited on a surface can allow the individual processes to be studied in isolation under controlled laboratory conditions. In particular, measurements can now be made of particle size with unprecedented accuracy (sub-nanometre) and over a wide range of timescales (spanning from milliseconds to many days). The physical state of a particle can be unambiguously identified and its composition and phase can be resolved with a high degree of spatial resolution. In this review, we describe the advances made in our understanding of aerosol properties and processes from measurements made of phase behaviour, hygroscopic growth, morphology, vapour pressure and the kinetics of water transport for single particles. We also show that studies of the oxidative aging of single particles, although limited in number, can allow the interplay of these properties to be investigated. We conclude by considering the contributions that single particle measurements can continue to make to our understanding of the properties and processes occurring in atmospheric aerosol.

A tissue-based approach to in vitro drug screening allows for determination of the cumulative positive and negative effects of a drug at the tissue rather than the cellular or subcellular level. Skeletal muscle myoblasts were tissue-engineered into three-dimensional muscle with parallel myofibers generating directed forces. When grown attached to two flexible micro-posts (mu posts) acting as artificial tendons in a 96-well plate format, the miniature bioartificial muscles (mBAMs) generated tetanic (active) forces upon electrical stimulation measured with a novel image-based motion detection system. mBAM myofiber hypertrophy and active force increased in response to insulin-like growth factor 1. In contrast, mBAM deterioration and weakness was observed with a cholesterol-lowering statin. The results described in this study demonstrate the integration of tissue engineering and biomechanical testing into a single platform for the screening of compounds affecting muscle strength.

Introduction 4116 2. Theoretical Background and Framework for Atmospheric Aerosols 4117 2.1. Saturation Vapor Pressures 4117 2.2. Vapor−Liquid or Vapor−Solid Equilibria over Mixed Solutions 4118 2.3. Equilibria over Curved Surfaces 4118 2.4. Dynamic Evaporation and Condensation from and to an Aerosol Particle 4119 2.5. Ambient Partitioning 4120 3. Experimental Methods 4120 3.1. Knudsen-Cell-Based Methods 4121 3.1.1. Knudsen Mass Loss Methods 4121 3.

For the first time, a measurement of the viscosity of microparticles composed of Newtonian fluids has been made over a range of 12 orders of magnitude (10 À3 to 10 9 Pa s), extending from dilute aqueous solutions to the solid-like behaviour expected on approaching a glass transition. Using holographic optical tweezers to induce coalescence between two aerosol particles (volume <500 femtolitres), we observe the composite particle relax to a sphere over a timescale from 10 À7 to 10 5 s, dependent on viscosity. The damped oscillations in shape illustrate the interplay of surface capillary forces and bulk fluid flow as the relaxation progresses. Viscosity values estimated from the extrapolation of measurements from macroscopic binary aqueous solutions of sucrose are shown to diverge from the microparticle measurements by as much as five orders of magnitude in the limit of ultrahigh solute supersaturation and viscosity. This is shown to be a consequence of the sensitivity of the viscosity to the composition of the particles, specifically the water content, and the often incorrect compositional dependence on water activity that are assumed to characterise aerosols and amorphous phases under dry conditions. For ternary mixtures of sodium chloride, sucrose and water, the measured viscosities similarly diverge from model predictions by up to three orders of magnitude. The Stokes-Einstein treatment for relating the diffusivity of water in sucrose droplets to the particle viscosity is found to depart from the measured viscosities by more than one order of magnitude when the viscosity exceeds 10 Pa s and up to six orders of magnitude at the highest viscosities accessed. Coalescence is shown to proceed with unit efficiency even up to the highest accessible viscosity. These measurements provide the first comprehensive account of the change in a material property accompanying a transition from a dilute solution to an amorphous semi-solid state using aerosol particles to probe the change in rheological properties.

Atmospheric models generally assume that aerosol particles are in equilibrium with the surrounding gas phase. However, recent observations that secondary organic aerosols can exist in a glassy state have highlighted the need to more fully understand the kinetic limitations that may control water partitioning in ambient particles. Here, we explore the influence of slow water diffusion in the condensed aerosol phase on the rates of both condensation and evaporation, demonstrating that significant inhibition in mass transfer occurs for ultraviscous aerosol, not just for glassy aerosol. Using coarse mode (3-4 um radius) ternary sucrose/sodium chloride/aqueous droplets as a proxy for multicomponent ambient aerosol, we demonstrate that the timescale for particle equilibration correlates with bulk viscosity and can be ≫10 3 s. Extrapolation of these timescales to particle sizes in the accumulation mode (e.g., approximately 100 nm) by applying the Stokes-Einstein equation suggests that the kinetic limitations imposed on mass transfer of water by slow bulk phase diffusion must be more fully investigated for atmospheric aerosol. Measurements have been made on particles covering a range in dynamic viscosity from <0.1 to >10 13 Pa s. We also retrieve the radial inhomogeneities apparent in particle composition during condensation and evaporation and contrast the dynamics of slow dissolution of a viscous core into a labile shell during condensation with the slow percolation of water during evaporation through a more homogeneous viscous particle bulk.water uptake | whispering gallery modes | Raman spectroscopy | optical tweezers | viscous aerosol A tmospheric aerosol particles are typically complex mixtures of organic and inorganic species with correspondingly complex equilibria and temporal responses to changes in humidity. Secondary organic aerosols (SOA) continue to receive a great deal of attention due to their impact on radiative forcing, mainly through the indirect effect (1). Ambient aerosol typically contains a significant organic fraction, arising from the oxidation of volatile organic compounds (2). SOA has been largely thought of as existing as a liquid phase, or as a combination of a solid phase within a liquid droplet, but the reality is likely to be far more nuanced. Recently, Virtanen et al. demonstrated that ambient SOA particles can have similar mechanical properties to crystalline ðNH 4 Þ 2 SO 4 particles at 10-20% RH but exist as amorphous glasses to low relative humidity (RH) rather than forming crystalline phases (3). This picture is consistent with the conclusions of Mikhailov et al. (4) and Zobrist et al. (5), who suggested that the existence of glassy states may have profound consequences for the properties of atmospheric aerosol particles, particularly at low temperatures.The term glassy refers to an amorphous, highly viscous state with a dynamic viscosity (η) of greater than 1 × 10 12 Pa s and the mechanical properties of a solid (6). In thermodynamic terms, a glass is in a nonergodic metastable state...

The morphology of bi-phase aerosol particles containing phase separated hydrophobic and hydrophilic components is considered, comparing simulations based on surface and interfacial tensions with measurements made by aerosol optical tweezers. The competition between the liquid phases adopting core-shell and partially engulfed configurations is considered for a range of organic compounds including saturated and unsaturated hydrocarbons, aromatics, alcohols, ketones, carboxylic acids, esters and amines. When the solubility of the organic component and the salting-out of the organic component to the surface by the presence of concentrated inorganic solutes in the aqueous phase are considered, it is concluded that the adoption of a partially engulfed structure predominates, with the organic component forming a surface lens. The aqueous surface can be assumed to be stabilised by a surface enriched in the organic component. The existence of acid-base equilibria can lead to the dissociation of organic surfactants and to significant lowering of the surface tension of the aqueous phase, further supporting the predominance of partially engulfed structures. Trends in morphology from experimental measurements and simulations are compared for mixed phased droplets in which the organic component is decane, 1-octanol or oleic acid with varying relative humidity. The consequences of partially engulfed structures for aerosol properties are considered.

Summary The potential aerosolised transmission of severe acute respiratory syndrome coronavirus‐2 is of global concern. Airborne precaution personal protective equipment and preventative measures are universally mandated for medical procedures deemed to be aerosol generating. The implementation of these measures is having a huge impact on healthcare provision. There is currently a lack of quantitative evidence on the number and size of airborne particles produced during aerosol‐generating procedures to inform risk assessments. To address this evidence gap, we conducted real‐time, high‐resolution environmental monitoring in ultraclean ventilation operating theatres during tracheal intubation and extubation sequences. Continuous sampling with an optical particle sizer allowed characterisation of aerosol generation within the zone between the patient and anaesthetist. Aerosol monitoring showed a very low background particle count (0.4 particles.l −1 ) allowing resolution of transient increases in airborne particles associated with airway management. As a positive reference control, we quantitated the aerosol produced in the same setting by a volitional cough (average concentration, 732 (418) particles.l −1 , n = 38). Tracheal intubation including facemask ventilation produced very low quantities of aerosolised particles (average concentration, 1.4 (1.4) particles.l −1 , n = 14, p < 0.0001 vs. cough). Tracheal extubation, particularly when the patient coughed, produced a detectable aerosol (21 (18) l −1 , n = 10) which was 15‐fold greater than intubation (p = 0.0004) but 35‐fold less than a volitional cough (p < 0.0001). The study does not support the designation of elective tracheal intubation as an aerosol‐generating procedure. Extubation generates more detectable aerosol than intubation but falls below the current criterion for designation as a high‐risk aerosol‐generating procedure. These novel findings from real‐time aerosol detection in a routine healthcare setting provide a quantitative methodology for risk assessment that can be extended to other airway management techniques and clinical settings. They also indicate the need for reappraisal of what constitutes an aerosol‐generating procedure and the associated precautions for routine anaesthetic airway management.