Structure Development During Drying - a Review of the State of the Art

Author Last Name: 
Andrew Bayly
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United Kingdom

The review focusses on progress in understanding, measuring and modelling the structures formed when single droplets dry. In the initial period of drying concentration gradients are formed, these establish the basis for subsequent transformation to the solid particle structure. The P├ęclet number is becoming established as a useful metric for evaluating the magnitude of these concentration gradients and therefore interpreting trends seen in the final particle structure. The structures formed after drying depend on the nature of the phase transition of the material being dried. Three broad classes are discussed: colloidal suspensions, crystallising materials and skin forming materials. Recently significant progress has been made in the understanding of colloidal suspensions. Quantitative analysis and careful experimentation in the field has helped quantify and predict effects in model systems. These give insights that help the particle engineer design particles with the desired structure. That said there are still many open questions for this class of material, and micro-scale-modelling techniques are available and should be used to help address these questions. Crystallisation and film forming systems have seen less progress. Some work has been done looking at alternative crystallisation models, which give some interesting insights into amorphous-crystalline transitions; however there is a lot of opportunity to extend these further. Film forming systems have received little fundamental activity in recent years, however researchers are beginning to characterise material properties and link these to behaviour to explain observed phenomena. The whole area of materials characterisation in the non-equilibrium conditions seen during drying is a priority area for all material types, as the value of current modelling capability is limited by lack of reliable physico-chemical properties. The aerosol community has been active in developing new methods to address this need and their application should be extended to materials of industrial interest. Some of the recent developments in measurement techniques for free droplets has also been pushed by the aerosol community, beyond these methods there is still a need to extend and develop the state of the art in the area for better measurements of internal composition profiles. Drying of other particle systems is also discussed and key differences in the mechanisms driving the structure of dried thin films and sessile droplets are highlighted.