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B: Fluid Interfaces, Colloids, Polymers, Soft Matter, Surfactants, and Glassy Materials
The Drying Kinetics of Salt Solution Droplets: Water Evaporation Rates and Crystallisation Florence K. A. Gregson, Joshua F. Robinson, Rachael E.H. Miles, Paddy Royall, and Jonathan Philip Reid J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b09584 • Publication Date (Web): 14 Dec 2018 Downloaded from http://pubs.acs.org on December 16, 2018
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The Journal of Physical Chemistry
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The Drying Kinetics of Salt Solution Droplets: Water
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Evaporation Rates and Crystallisation
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F. K. A. Gregson1, J. F. Robinson2, R. E. H. Miles1, C. P. Royall1,2 and J. P. Reid1,*
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1 School of Chemistry, University of Bristol, Bristol, BS8 1TS
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2 School of Physics, University of Bristol, Bristol, BS8 1TS
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[email protected] 7 8
Abstract
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The drying and crystallisation of solution droplets is a problem of broad relevance, determining the
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micro-structures of particles formed in spray drying, the phase of particles delivered by, for example,
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aerosol formulations for inhalation therapies, and the impact of aerosols on radiative forcing and
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climate. The ephemeral nature of free-droplets, particularly when considering the drying kinetics of
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droplets with highly volatile constituents, has often precluded the accurate measurement of transient
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properties such as droplet size and composition, preventing the robust assessment of predictive models
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of droplet drying rates, nucleation and crystallisation. Here, we report novel measurements of the drying
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kinetics of individual aqueous sodium chloride solution droplets using an electrodynamic balance to
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isolate and trap single aerosol droplets (radius ~ 25 µm). The initial solution droplet size and
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composition is shown to be highly reproducible in terms of of drying rate and crystallisation time when
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examined over hundreds of identical evaporating droplets. We introduce a numerical model that
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determines the concentration gradient across the radial profile of the droplet as it dries, considering both
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the surface recession due to evaporation and the diffusion of components within the droplet. Drying
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induced crystallisation is shown to be fully determined for this system, with nucleation and
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instantaneous crystallisation occurring once a critical supersaturation level of 2.04 ± 0.02 is achieved
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at the surface of the evaporating droplet surface. This phenomenological model provides a consistent
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account of the timescale and surface concentration of free-droplet crystallisation on drying for the
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different drying conditions studied, a necessary step in progress towards achieving control over rates of
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crystallisation and the competitive formation of amorphous particles.
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Introduction
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The drying of liquid droplets to form particles is an important process for a range of industries, from
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agriculture and inkjet printing to the production of pharmaceutics, cosmetics and food. 1–4 In addition
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the propensity of an aerosol droplet to crystallise into solid particles has been shown to dramatically 1 ACS Paragon Plus Environment
The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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influence the optical properties of atmospheric aerosols and, thus, their radiative forcing.5,6 The evolving
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heterogeneities in particle microstructure as a droplet evaporates can be a highly complex process. Heat
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and mass transfer can be strongly coupled.7 The competition between surface recession and internal
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flows such as convection, diffusion and Marangoni flows can result in an array of product morphologies,
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size and properties.8–10 For some solutes, nucleation and crystallisation is sufficiently rapid that dry
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crystalline particles result, dependent on drying rate; for other systems, drying may lead to amorphous
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glassy particles.11,12 The crystallisation of solutes present in a droplet is often the ultimate outcome of a
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droplet-drying processes. The time at which nucleation occurs within an evaporating droplet can be
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directly linked to the initial droplet size and solute concentration, allowing some control over the final
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crystalline particle size and morphology. Our objective here is to examine the drying kinetics and
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crystallisation of inorganic solution droplets. Through refined measurements of the drying rates of
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individual droplets, we will show that the crystallisation time and surface composition at crystallisation
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are highly reproducible over ranges of starting droplet composition and temperature, providing a
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phenomenological model for predicting the formation of crystalline particles.
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Although there has been considerable work on the study of droplets drying on surfaces, i.e. sessile
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droplets,13–15 far fewer studies have examined the drying kinetics and crystallisation of free droplets or
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aerosols. There are numerous unique experimental challenges associated with studying the drying
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dynamics of free droplets that are easily overcome when examining sessile droplets. Free droplets are
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ephemeral and difficult to follow in position unless a single particle is isolated and captured, such as
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with an optical trap,16 electrodynamic balance17 or acoustic trap.18 Even then, the timescales of
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evaporation processes can be 75% confirms that the transition at RH
