The drying and crystallisation of solution droplets is a problem of broad relevance, determining the micro-structures of particles formed in spray drying, the phase of particles delivered by, for example, aerosol formulations for inhalation therapies, and the impact of aerosols on radiative forcing and climate. The ephemeral nature of free-droplets, particularly when considering the drying kinetics of droplets with highly volatile constituents, has often precluded the accurate measurement of transient properties such as droplet size and composition, preventing the robust assessment of predictive models of droplet drying rates, nucleation and crystallisation. Here, we report novel measurements of the drying kinetics of individual aqueous sodium chloride solution droplets using an electrodynamic balance to isolate and trap single aerosol droplets (radius ~ 25 µm). The initial solution droplet size and composition is shown to be highly reproducible in terms of of drying rate and crystallisation time when examined over hundreds of identical evaporating droplets. We introduce a numerical model that determines the concentration gradient across the radial profile of the droplet as it dries, considering both the surface recession due to evaporation and the diffusion of components within the droplet. Drying induced crystallisation is shown to be fully determined for this system, with nucleation and instantaneous crystallisation occurring once a critical supersaturation level of 2.04 ± 0.02 is achieved at the surface of the evaporating droplet surface. This phenomenological model provides a consistent account of the timescale and surface concentration of free-droplet crystallisation on drying for the different drying conditions studied, a necessary step in progress towards achieving control over rates of crystallisation and the competitive formation of amorphous particles.

Key to resolving the scientific challenge of the glass transition is to understand the origin of the massive increase in viscosity of liquids cooled below their melting temperature (avoiding crystallisation). A number of competing and often mutually exclusive theoretical approaches have been advanced to describe this phenomenon. Some posit a bona fide thermodynamic phase to an 'ideal glass', an amorphous state with exceptionally low entropy. Other approaches are built around the concept of the glass transition as a primarily dynamic phenomenon. These fundamentally different interpretations give equally good descriptions of the data available, so it is hard to determine which-if any-is correct. Recently however this situation has begun to change. A consensus has emerged that one powerful means to resolve this longstanding question is to approach the putative thermodynamic transition sufficiently closely, and a number of techniques have emerged to meet this challenge. Here we review the results of some of these new techniques and discuss the implications for the existence-or otherwise-of the thermodynamic transition to an ideal glass.

Disks moving in a narrow channel have many features in common with the glassy behavior of hard spheres in three dimensions. In this paper we study the caging behavior of the disks which sets in at characteristic packing fraction φ d . Four-point overlap functions similar to those studied when investigating dynamical heterogeneities have been determined from event driven molecular dynamics simulations and the time dependent dynamical length scale has been extracted from them. The dynamical length scale increases with time and, on the equilibration time scale, it is proportional to the static length scale associated with the zigzag ordering in the system, which grows rapidly above φ d . The structural features responsible for the onset of caging and the glassy behavior are easy to identify as they show up in the structure factor, which we have determined exactly from the transfer matrix approach.

In the COVID-19 pandemic, among the more controversial issues is the use of masks and face coverings. Much of the concern boils down to the question—just how effective are face coverings? One means to address this question is to review our understanding of the physical mechanisms by which masks and coverings operate—steric interception, inertial impaction, diffusion, and electrostatic capture. We enquire as to what extent these can be used to predict the efficacy of coverings. We combine the predictions of the models of these mechanisms which exist in the filtration literature and compare the predictions with recent experiments and lattice Boltzmann simulations, and find reasonable agreement with the former and good agreement with the latter. Building on these results, we explore the parameter space for woven cotton fabrics to show that three-layered cloth masks can be constructed with comparable filtration performance to surgical masks under ideal conditions. Reusable cloth masks thus present an environmentally friendly alternative to surgical masks so long as the face seal is adequate enough to minimize leakage.

We model the thermodynamics of local structures within the hard sphere liquid at arbitrary volume fractions through the morphometric calculation of n-body correlations. We calculate absolute free energies of local geometric motifs in excellent quantitative agreement with molecular dynamics simulations across the liquid and supercooled liquid regimes. We find a bimodality in the density library of states where five-fold symmetric structures appear lower in free energy than four-fold symmetric structures, and from a single reaction path predict a relaxation barrier which scales linearly in the compressibility factor. The method provides a new route to assess changes in the free energy landscape at volume fractions dynamically inaccessible to conventional techniques.

The evaporation of liquid solution droplets and solute crystallization can be highly complex and is an important problem, particularly in spray drying where powdered products are produced from sprayed liquid droplets, such as in the food or pharmaceutical industries. In this work, we study the relationship between the evaporation rates of single levitated NaNO3 droplets under varying environmental conditions and the propensity for nucleation of NaNO3 crystals. We use a combination of an electrodynamic balance to study single-droplet evaporation kinetics, SEM imaging of dried particles, and modeling of the internal solute distribution inside a drying droplet. We show that the aqueous NaNO3 droplets exhibit broad distributions in the time that crystal nucleation is observed, droplet to droplet. The distribution of nucleation time is dependent upon environmental conditions such as the drying temperature, relative humidity (RH), and solute concentration. Even when evaporating in 0% RH, some droplets do not nucleate crystals in the time taken for all water to evaporate and dry to form an amorphous particle. We believe that this interplay between crystalline or amorphous particle formation is a result of the viscosity of aqueous NaNO3 solutions, which rises by several orders of magnitude as the concentration increases. We show that for droplets with an initial radius of ∼25 μm the propensity for aqueous NaNO3 droplets to nucleate crystals upon drying increases with a decreasing RH and increases with an increasing temperature in the range 278–306 K. This work demonstrates the importance of the drying kinetics on the propensity of evaporating droplets to nucleate crystals.

A quantitative understanding of the evaporative drying kinetics and nucleation rates of aqueous based aerosol droplets is important for a wide range of applications, from atmospheric aerosols to industrial processes such as spray drying. Here, we introduce a numerical model for interpreting measurements of the evaporation rate and phase change of drying free droplets made using a single particle approach. We explore the evaporation of aqueous sodium chloride and sodium nitrate solution droplets. Although the chloride salt is observed to reproducibly crystallise at all drying rates, the nitrate salt solution can lose virtually all of its water content without crystallising. The latter phenomenon has implications for our understanding of the competition between the drying rate and nucleation kinetics in these two systems. The nucleation model is used in combination with the measurements of crystallisation events to infer nucleation rates at varying equilibrium state points, showing that classical nucleation theory provides a good description of the crystallisation of the chloride salt but not the nitrate salt solution droplets. The reasons for this difference are considered.

In the COVID-19 pandemic, among the more controversial issues is the use of face coverings.To address this we show that the underlying physics ensures particles with diameters 1 µm are efficiently filtered out by a simple cotton or surgical mask. For particles in the submicron range the efficiency depends on the material properties of the masks, though generally the filtration efficiency in this regime varies between 30 to 60 % and multi-layered cotton masks are expected to be comparable to surgical masks.Respiratory droplets are conventionally divided into coarse droplets ( 5-10 µm) responsible for droplet transmission and aerosols ( 5-10 µm) responsible for airborne transmission. Masks are thus expected to be highly effective at preventing droplet transmission, with their effectiveness limited only by the mask fit, compliance and appropriate usage. By contrast, knowledge of the size distribution of bioaerosols and the likelihood that they contain virus is essential to understanding their effectiveness in preventing airborne transmission. We argue from literature data on SARS-CoV-2 viral loads that the finest aerosols ( 1 µm) are unlikely to contain even a single virion in the majority of cases; we thus expect masks to be effective at reducing the risk of airborne transmission in most settings.