Abstract
A refined technique for observing the complete evaporation behaviour of free-falling droplets, from droplet generation to complete solvent evaporation, with ultra-high time resolution is introduced and benchmarked. High-resolution phase-delay stroboscopic imaging is employed to simultaneously resolve the evolving droplet morphology, geometric and aerodynamic diameters, throughout the evaporative lifetime with a user-controlled < μs timescale. This allows rapid, complex morphological changes, such as crystallisation events, to be clearly observed and the corresponding mechanisms to be inferred. The dried particles are sampled for offline SEM analysis and the observed morphologies compared to the inflight imaging. Density changes can be calculated directly from the deviation between the geometric and aerodynamic diameters. The full capabilities of the new technique are demonstrated by examination of the different evaporation behaviours and crystallisation mechanisms for aqueous sodium chloride droplets evaporating under different ambient relative humidity (RH) conditions. The crystallisation window, defined as the time taken from initial to complete crystallisation, is shown to be RH dependent, extending from 0.03 s at 20% RH and 0.13 s at 40% RH. The different crystallisation mechanisms observed during the experiments are also clearly reflected in the final structure of the dry particles, with multi-crystal structures produced at low RH compared to single-crystal structures at higher RH. It is anticipated that this technique will unlock measurements which explore the evaporation behaviour and crystallisation mechanisms for rapid, complex droplet drying events, and with increasingly non-ideal solutions, relevant to industrial applications.
Highlights
To quantify the transport of material released in a spray dryer or nuclear accident, and to understand the efficacy of filtration systems, the aerodynamic size of the particles must be understood.[17,18] The aerodynamic size depends on physical properties as well as the geometric size
The uncertainty measured for the geometric diameter does not exceed 0.5 mm and for the aerodynamic diameter generally remains below 1 mm throughout the 2 s window
We demonstrate the capability of the Falling Droplet Column (FDC) for exploring morphological changes during evaporation, in this case for aqueous NaCl droplets evaporating into a humidity below the efflorescence point (B45% relative humidity (RH)).[25]
Summary
To quantify the transport of material released in a spray dryer or nuclear accident, and to understand the efficacy of filtration systems, the aerodynamic size of the particles must be understood.[17,18] The aerodynamic size depends on physical properties as well as the geometric size. The dry particle size (at least the point at which a solid crust is formed) and morphology are influenced by the competition between solvent evaporation rate and solute diffusion, represented by the diffusion constant D This competition is characterised by the Peclet number Pe, defined in eqn (4).[23] k. We introduce the Falling Droplet Column (FDC), where the droplet chain technique has been extended to include high resolution imaging and significantly improved temporal resolution This technique allows detailed images of the evolving morphology to be collected throughout the full evaporative lifetime, from droplet generation to dry particle formation, spanning almost 6 orders of magnitude in time (o1 ms to B5 s). Paper mechanisms for inorganic solution droplets evaporating under different environmental conditions
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