Evaporative Drying of Droplets and the Formation of Micro-structured and Functional Particles and Films

Lead Research Organisation: Durham University
Department Name: Chemistry

Abstract

'Watching paint dry' is a metaphor for a boring and pointless activity. In reality, the drying of liquids is a complex process and the imperturbable appearance to the eye can hide a wealth of dynamics occurring inside the liquid. The effect of these internal processes is to change the distribution of materials in the deposit left after drying. We are all familiar with the coffee-ring effect, where split coffee dries to form a ring of solids at the edge of the spill - of little use if you are trying to coat a surface uniformly. This project is all about the drying of droplets, either in air or on a surface; one isolated droplet, two droplets merging or many droplets in a spray. We seek to understand how drops dry and how to control where the particles or molecules in the drop end up after the drop evaporates. When do you get a solid particle or a hollow particle? A round one or a spiky one? A uniform particle or one with shells? Or on a surface: a coffee-ring or a pancake? A uniform deposit, a layered one or a bull's eye? Are particles crystalline or amorphous, are different components mixed or separated? There are a myriad of possibilities for controlling the microstructure and properties of the final particle or film.

Drying is complicated for three main reasons. First, many transport processes (evaporation, heat flow, diffusion, convection) occur simultaneously and are strongly coupled. For example, in a small droplet of alcohol and water evaporating on a surface, the liquid inside the drop will flow around in a doughnut pattern tens of times each second. Second, the conditions in a drying droplet are often far from equilibrium. For example, a small water droplet in air or on a smooth clean surface can be cooled to -35 degrees C without freezing. So to understand drying one needs to understand the properties of fluids far from equilibrium. It is generally not possible to predict the final outcome of drying from the properties of simple solutions near equilibrium. Third, drops do not dry in isolation. They may merge or bounce, coalesce or chase each other across a surface. The evaporation of one droplet affects its neighbours. Moving droplets change the flow of air around other droplets, coupling the motion of droplets.

Why does anyone care, beyond the intellectual fascination with the bizarre outcomes of droplet drying? Drying of droplets turns out to be a rather important process in practical applications: spray painting, graphics printing, inkjet manufacturing, crop spraying, coating of seeds or tablets, spray cooling, spray drying (widely used in food, pharmaceutical and personal care products), drug inhalers and disinfection, to give a few examples. The physics and chemistry underlying all these applications is the same, but if manifests itself in different ways and the desired outcome varies between applications.

The first challenge addressed by this project is one of measurement: how do you work out what is going on in a droplet that is less than a tenth of a millimetre across and may dry in less than a second? We have already developed sophisticated measurement tools but will need to extend these further. Another challenge is one of modelling: to understand the drying process we need a theoretical framework and computer models to explain - and predict - experimental observations. We will begin looking at the fundamental processes occurring in single drops in air and on a surface and then explore what happens when drops interact or coalesce. This fundamental understanding will be fed into improved models of arrays, clouds or sprays of droplets that are encountered in most practical applications (such as spray coating, spray drying, inhalers or inkjet manufacturing).

We will use an Industry Club to engage with companies from a range of different sectors. This Club will provide a forum for sharing problems, ideas and solutions and for disseminating the knowledge generated in the project.

Planned Impact

Who benefits from this research?

The primary non-academic beneficiaries of this project will be companies who (i) employ evaporative drying in manufacturing processes or (ii) sell products in which evaporative drying occurs during the use of the product. These companies work across a wide-range of sectors including protective coatings, decorative coatings, graphic printing, display technologies, pharmaceutical production (including tablet coating, spray-drying, inhalers and nebulisers), crop protection, wound protection, disinfection, inkjet manufacturing, ceramic production, food production and personal care products.

There is a much wider set of beneficiaries which includes those whose quality of life is enhanced by improved products. There is also the potential for significant health and safety and environmental benefits.

How do they benefit?

Companies benefit from an improved ability to troubleshoot, optimise and control existing processes and from the ability to design new, innovative, products and processes faster and more effectively. We give several illustrative examples. Optimising the distribution of a metallic particles in a conductive ink would give finer lines and lower ohmic losses after sintering. Controlling coalescence of ink drops of different colours would allow high-resolution single-pass inkjet printing, making it competitive in speed with offset lithography. Uniform distribution of an antimicrobial agent in sprayed disinfectant would inhibit the growth of bacteria on hard surfaces in hospitals. Optimising the distribution, morphology and phase of active ingredient and adjuvant in a systemic pesticide would reduce the dosage of pesticide needed, while controlling the evaporation of droplets in flight would reduce drift and adverse environment impact. Control of the drop size distribution on inhalation from metered dose inhalers and nebulisers could lead to the optimal regional delivery of drugs to the respiratory track for enhanced efficacy. Improved scale-up and understanding of spray dried solids dispersion will get new, poorly soluble drugs to the patient faster. Models which predict the build-up of materials on the wall of the spray dryer can enable the formulators to assess manufacturability without costly, time consuming, large scale trials.

The project is designed to allow developmental activities to occur in parallel with the research programme (though separately funded) so that benefits to partners start to accrue before the end of the grant.

Other benefits to companies include sharing of best practice through the Industry Club, employment of trained personnel (> 10) from the project, access to new experimental tools and software. We note that membership of the Industry Club is limited to companies who have R&D in the UK, who manufacture in the UK or who play a significant role in the UK supply chain, thus enhancing the competitiveness of UK industry.

Potential long-term benefits to the consumer are equally diverse (beyond better quality products at lower cost). Examples include improved healthcare outcomes, e.g. reduced infection rates, faster recovery with improved wound dressings, controlled release of drugs, predictive models of manufacturing technologies which rapid development of the large numbers of formulations that personalised medicines will require, more effective personal care products with lower production and environmental/energy impact.

There are also significant health and safety benefits that arise from controlling evaporation, particle size and particle morphology. For example reduction in overspray and drift and suppression of dust generation from coatings. More efficient delivery of products in an outdoor environment (e.g. spray painting, crop spraying) also reduces negative impacts on the natural environment.

Publications

10 25 50
 
Description The grant is still in progress. Twelve papers have been published so far covering the following topics
(i) Evaporation of droplets in an electrodynamic trap, for pure liquids, binary solvent mixtures and salt solutions.
(ii) Measurement of dynamic surface tensions of surfactant solutions from the oscillation of falling droplets
(iii) Mapping of the parameter space of binary collisions between nanolitre droplets as a function of the relative size, viscosity, impact speed and impact velocity
(iv) Printing of emulsion droplets and the formation of microcapsules on surfaces.
(v) Control of coffee rings by sol gel transitions in silica nanoparticle suspensions.
Exploitation Route Findings are disseminated to our Industry Club and published in the literature. An international conference (Droplets 2019) was held in Durham in September 2019. Two research contracts have been entered into with industry partners to apply knowledge to specific industry problems.
Sectors Agriculture, Food and Drink,Electronics,Healthcare,Manufacturing, including Industrial Biotechology,Pharmaceuticals and Medical Biotechnology

 
Description Funding from Danone Nutrica: Measurement of evolving aerosol droplet mass for drying milk formulation droplets
Amount £36,000 (GBP)
Organisation Danone 
Sector Private
Country United Kingdom
Start 11/2019 
End 04/2020
 
Description Investigation into the drying and localisation of components in formulations
Amount £56,627 (GBP)
Funding ID RF030727 
Organisation Syngenta International AG 
Sector Private
Country Switzerland
Start 12/2019 
End 05/2020
 
Title Data from Surface Tensions of Picoliter Droplets with Sub-Millisecond Surface Age 
Description This data set provides the data underlying the figures in our publication "Surface Tensions of Picoliter Droplets with Sub-Millisecond Surface Age", Journal of Physical Chemistry A, 2019. 
Type Of Material Database/Collection of data 
Year Produced 2019 
Provided To Others? Yes  
 
Title Studies of Competing Evaporation Rates of Multiple Volatile Components from a Single Binary-Component Aerosol Droplet 
Description The simultaneous evaporation and condensation of multiple volatile components from multicomponent aerosol droplets leads to changes in droplet size, composition and temperature. Measurements and models that capture and predict these dynamic aerosol processes are key to understanding aerosol microphysics in a broad range of contexts. We report measurements of the evaporation kinetics of droplets (initially ~25 µm radius) formed from mixtures of ethanol and water levitated within a electrodynamic balance over timescales spanning 500 ms to 6 s. Measurements of evaporation into a gas phase of varied relative humidity and temperature are shown to compare well with predictions from a numerical model. We show that water condensation from the gas phase can occur concurrently with ethanol evaporation from aqueous-ethanol droplets. Indeed, water can condense so rapidly during the evaporation of a pure ethanol droplet in a humid environment, driven by the evaporative cooling the droplet experiences, that the droplet becomes pure water within 0.4 s. 
Type Of Material Database/Collection of data 
Year Produced 2019 
Provided To Others? Yes  
 
Title The Drying Kinetics of Salt Solution Droplets: Water Evaporation Rates and Crystallisation - J Phys Chem B (2018) 
Description 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. 
Type Of Material Database/Collection of data 
Year Produced 2018 
Provided To Others? Yes  
 
Description Evaporative Drying of Droplets 
Organisation Croda International
Country United Kingdom 
Sector Private 
PI Contribution Part of Industry Club
Collaborator Contribution Part of Industry Club
Impact See elsewhere on ResearchFish
Start Year 2016
 
Description Evaporative Drying of Droplets 
Organisation Procter & Gamble
Department Newcastle Innovation Centre
Country United Kingdom 
Sector Private 
PI Contribution P&G are part of the Industry Club
Collaborator Contribution P&G are part of the Industry Club
Impact See elsewhere on Researchfish entry
Start Year 2016
 
Description Syngenta Crop Protection Uk Ltd 
Organisation Syngenta International AG
Department Syngenta Crop Protection
Country United Kingdom 
Sector Private 
PI Contribution Part of Industry Club
Collaborator Contribution Part of Industry Club
Impact See elsewhere on ResearchFish
Start Year 2016
 
Description Casting Dispersions 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Other audiences
Results and Impact Talk at Casting Dispersions at the SCI in March 2018.
Year(s) Of Engagement Activity 2018
 
Description Droplet Coalescence 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Industry/Business
Results and Impact Topical workshop on droplet coalescence aimed at industrial partners on the Evaporating Droplets grant. This was organised by us in Durham - not merely participation.
Year(s) Of Engagement Activity 2017
 
Description Droplets 2017 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Other audiences
Results and Impact Talk at Droplets 2017 in Los Angeles on Printing of OLED Displays
Year(s) Of Engagement Activity 2017
 
Description Droplets 2019 International Conference 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Postgraduate students
Results and Impact One of the proposed engagement activities in our grant was to bid for and host the 2019 conference in the international conference series 'Droplets'. This conference was an opportunity to showcase the research carried out in the project and also more generally in the UK. There were 283 participants of whom 60% were non-UK. The feedback from delegates was very good and we believe that we gave an excellent impression of the strength of droplet research in Durham and in the UK as a whole.
Year(s) Of Engagement Activity 2019
URL https://droplets2019.co.uk/
 
Description Gordon Research Conference on Molecular Interactions and Dynamics 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Gordon Research Conference on Molecular Interactions and Dynamics at Stonehill College, Mass, US.
Year(s) Of Engagement Activity 2018
 
Description Invited talk at Respiratory Drug Delivery in Arizona, US, April 2018 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Invited plenary talk at Respiratory Drug Delivery in Arizona, US, April 2018
Year(s) Of Engagement Activity 2018
 
Description Invited talk at XXIst Symposium on Atomic, Cluster and Surface Physics, Obergurgl, Austria, 2018 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Invited talk at XXIst Symposium on Atomic, Cluster and Surface Physics, Obergurgl, Austria, 2018
Year(s) Of Engagement Activity 2018
 
Description Pittcon Conference, Orlando, Florida 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Invited talk at the annual Pittcon Conference, Orlando, Florida, February 2018.
Year(s) Of Engagement Activity 2018
 
Description Talk at AAAR 2019: Influence of Evaporation Rate and Suspended Solid Concentration on Dry Particle Formation from Evaporating Aerosol Microdroplets 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Talk at AAAR 2019 conference
Year(s) Of Engagement Activity 2019
 
Description Talk at AAAR 2019: Observing, Predicting and Controlling Crystalline and Amorphous Particle Formation in Evaporating Aqueous Aerosol Droplets 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Conference talk at the American Association for Aerosol Research
Year(s) Of Engagement Activity 2019
 
Description Talk at EAC 2019: From Droplet to Particle: Observing, Predicting and Controlling Crystalline and Amorphous Particle Formation in Rapidly Evaporating Aerosol Droplets 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Talk at European Aerosol Conference 2019
Year(s) Of Engagement Activity 2019
 
Description Talk at EAC 2019: Time-Resolved Studies of Aerosol Droplet Surface Tension and Coalescence 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Conference at talk at the European Aerosol Conference 2019 in Gothenberg Sweden.
Year(s) Of Engagement Activity 2019
 
Description Talk at EuroDrying 2019: PREDICTING AND CONTROLLING CRYSTALLISATION IN EVAPORATING AEROSOL SOLUTION DROPLETS 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Talk at Eurodrying 2019
Year(s) Of Engagement Activity 2019
 
Description Talk at Siminhale Workshop: The Microphysics of Aerosol Particle Transformations on Inhalation 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Talk at COST Action workshop for Siminhale project
Year(s) Of Engagement Activity 2019