The demonstrated temporal Fano interference of light scattered by evaporating droplets of a nano-suspension can be a powerful tool to characterize interfaces of evaporating liquids.
Scattering of coherent light by a droplet of water fullerene suspension was investigated. Two light wavelengths were used simultaneously. The evolution of the radius and refractive index of a droplet was examined. Resonant scattering was detected and analysed by means of a simple model and some conclusions were drawn on the microscopic properties of the suspension. The study was supplemented with atomic force microscopy measurements of samples obtained by drying the suspension.
We have made advancements in a linear electrodynamic quadrupole trap (LEQT) to produce various nanoparticle microaggregates and drop patterns by evaporation-driven aggregation in levitating droplets of colloidal suspensions and to analyse their properties. The LEQT is mounted in a small chamber with well controlled atmospheric parameters, such as temperature and relative humidity, to trap droplets ranging from ∼70 μm down to ∼1 μm in diameter and provide a controlled environment to study formation of nanoparticle aggregates. Transient and final dry microstructures formed during the droplet drying life-time have been deposited on a silicon substrate placed in the bottom part of the trap and studied off-line with scanning electron microscopy (SEM). The experimental results are supported/supplemented by the numerical model of evaporation-driven aggregation under various conditions. The application of the CUDA technology allowed us to simulate aggregation of a few hundred thousand nanoparticles of several types. Using the LEQT we investigated highly-ordered aggregates of silica nanoparticles, composites of silica and gold nanoparticles as well as sodium dodecyl sulfate (SDS) crystalline microstructures and aggregates of silica nanoparticles with SDS. We observed the formation of microstructures at the successive stages of aggregation and under various experimental conditions. The observed final microstructures demonstrate the technical capabilities and versatility of the LEQT for its adaptation for droplet drying scale-up applications such as spray drying.
We have investigated fine details of evaporation of free microdroplets of liquid binary mixtures comprising ethylene glycol (EG), diethylene glycol (DEG), triethylene glycol (TEG), glycerol and water. The microdroplets were kept and studied in an electrodynamic trap. Several phenomena associated with their evaporation are identified, discussed and modelled analytically. In particular, we've observed distillation at the microscale manifesting as a sigmoid transition of the evaporation rate (surface change rate). Sigmoid transition is known to be a characteristic feature for the evolution of the population (amount) with limited resources. We have shown that the transition itself can be comprehended using a stationary evaporation model under instantaneous mixing conditions. The condition is discussed and justified. The more general findings are primarily exemplified by a practical case of DEG contaminated with water by considering a humid and a dry ambient atmosphere. The influence of the composition of the droplet and the ambient atmosphere on the initial (pre-transition) stage of evaporation is considered in a general manner. Three types of conditions are discussed concerning the presence of an admixture in liquid and vapour phases (exemplified by the DEG/water system): (i) "dry" liquid - dry atmosphere, (ii) "wet" liquid - dry atmosphere, and (iii) "wet" liquid - wet atmosphere. Case (i) has been successfully verified against the theoretical prediction. Case (ii) has the requirement of considering non-stationary liquid-in-liquid diffusion. Case (iii) has led to a study of evaporation of a liquid mixture microdroplet with the more volatile component in equilibrium with its vapour.
Evaporation of a droplet of pure water several micrometers in size was investigated. The droplet was levitated in an electrodynamic trap placed in a small climatic chamber. The evolution of the droplet and the evolution dynamics was studied by analyzing the coherent light scattering patterns with the aid of Mie theory. A numerical model of droplet evolution incorporating the kinetic effects near the droplet surface was constructed. By applying this model to the experimental data the mass and thermal accommodation coefficients were determined to be αC=0.12±0.02 and αT= 0.65±0.09. This model enabled to find the droplet temperature evolution and the relative humidity in the droplet vicinity with high precision as well.
