Evaporation of a free microdroplet of a binary mixture of liquids with different volatilities.

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.

Surface diagnostics of evaporating droplets of nanosphere suspension: Fano interference and surface pressure.

The evaporation of a single, levitating microdroplet of glycols containing SiO2 nanospheres, both of similar refraction indices, was studied by observing changes in the interference pattern and intensities of polarized and depolarized scattered laser light. The evolution of the effective radius of the droplet has been found on the basis of Mie scattering theory supplemented by the "electrical weighting" measurement of droplet mass evolution. During formation of a layer of nanospheres on the droplet surface, the asymmetric Fano profile was observed which was found to be due to the destructive and constructive interference of overlapping processes: (i) the scattering on single nanospheres emerging on the droplet surface and (ii) the scattering on ensembles of closely spaced (comparing to the light wavelength) nanospheres of an evolving surface film. Therefore we report the first observation of the Fano interference in the time domain rather than in the spectral domain. The optical surface diagnostics was complemented with the thermodynamics-like analysis in terms of the effective droplet surface pressure isotherm and with numerical simulations illustrating evaporation driven changes in the distribution of nanospheres. The reported study can serve as the basis for a wide range of novel diagnostic methods for studying configuration changes in complex systems of nano- and microparticles evolving at the sub-wavelength scale.