J. Archer

Formation of Highly Ordered Spherical Aggregates from Drying Microdroplets of Colloidal Suspension

The formation of highly ordered spherical aggregates of silica nanoparticles by the evaporation of single droplets of an aqueous colloidal suspension levitated (confined) in the electrodynamic quadrupole trap is reported. The transient and final structures formed during droplet evaporation have been deposited on a silicon substrate and then studied with SEM. Various successive stages of the evaporation-driven aggregation of nanoparticles have been identified: formation of the surface layer of nanoparticles, formation of the highly ordered spherical structure, collapse of the spherical surface layer leading to the formation of densely packed spherical aggregates, and rearrangement of the aggregate into the final structure of a stable 3D quasi-crystal. The evaporation-driven aggregation of submicrometer particles in spherical symmetry leads to sizes and morphologies of the transient and final structures significantly different than in the case of aggregation on a substrate. The numerical model presented in the article allows us to predict and visualize the observed aggregation stages and their dynamics and the final aggregates observed with SEM.

High-precision temperature determination of evaporating light-absorbing and non-light-absorbing droplets.

Models describing evaporation or condensation of a droplet have existed for over a century, and the temporal evolutions of droplet radius and temperature could be predicted. However, the accuracy of results was questionable, since the models contain free parameters and the means of accurate calibration were not available. In previous work (Hołyst et al. Soft Matter 2013, 9, 7766), a model with an efficacious parametrization in terms of the mean free path was proposed and calibrated with molecular dynamics numerical experiment. It was shown that it is essentially possible to determine reliably the temperature of a steadily evaporating/condensing homogeneous droplet relative to ambient temperature when the evolution of the droplet radius is known. The accuracy of such measurement can reach fractions of mK. In the case of an evaporating droplet of pure liquid, the (droplet) temperature is constant during the stationary stage of evaporation. In this paper, we show that, in many cases, it is also possible to determine the temporal evolution of droplet temperature from the evolution of the droplet radius if the droplet (initial) composition is known. We found the droplet radius evolution with high accuracy and obtained the evolution of droplet temperature (and composition) for droplets of (i) a two-component mixture of pure liquids; (ii) solutions of solid in liquid, one that is non-surface-active and another that is; and (iii) suspensions of non-light-absorbing and light-absorbing particles.

Optical diagnostics of surfaces of single evaporating liquid microdroplet of solutions and suspensions

Experimental elastic-scattering characteristics of single evaporating liquid microdroplet of solution and suspension are reported. The microdroplets studied were composed of: (i) silica (SiO2) nanoparticles (225 nm radius) dispersed in diethylene glycol (DEG) liquid suspension (ii) DEG and Sodium dodecyl sulfate (SDS) solution, and (iii) SiO2 nanoparticles dispersed in DEG and SDS solution. We observed regular Mie type fringes from (i), (ii) and (iii) at the initial stages of the evaporation process followed by intensity fluctuations (speckles) for (i) and surface reflections (blinking of scattered light intensities) for (ii) during the inclusions surface layer formation. For (iii), Mie-type fringes, and a mixture of surface reflections (blinking of scattered light intensities) and intensity fluctuations (speckles) was observed. The changes in the intensities and polarization of the scattered light showed characteristic stages of evaporation driven processes occurring at the droplet surface. The observed phenomenon carry information about the inclusions’ mean distances, size, and stages of aggregation of SiO2 nanoparticles and crystallization of SDS nanocrystallites on the droplet surface. Additionally, we deposited samples of the final dried composite microobject on a silicon substrate and analyzed with SEM. The study provide different surface diagnostic methods of configuration changes in complex systems of nano-and microparticles evolving at the sub-wavelength scale and serves as an alternative method for studying stages of droplet with submicron inclusions evaporation processes.

Optical diagnostics of a single evaporating droplet using fast parallel computing on graphics processing units

Abstract We report on the first application of the graphics processing units (GPUs) accelerated computing technology to improve performance of numerical methods used for the optical characterization of evaporating microdroplets. Single microdroplets of various liquids with different volatility and molecular weight (glycerine, glycols, water, etc.), as well as mixtures of liquids and diverse suspensions evaporate inside the electrodynamic trap under the chosen temperature and composition of atmosphere. The series of scattering patterns recorded from the evaporating microdroplets are processed by fitting complete Mie theory predictions with gradientless lookup table method. We showed that computations on GPUs can be effectively applied to inverse scattering problems. In particular, our technique accelerated calculations of the Mie scattering theory on a single-core processor in a Matlab environment over 800 times and almost 100 times comparing to the corresponding code in C language. Additionally, we overcame problems of the time-consuming data post-processing when some of the parameters (particularly the refractive index) of an investigated liquid are uncertain. Our program allows us to track the parameters characterizing the evaporating droplet nearly simultaneously with the progress of evaporation.

Analysis of Surface Layer Properties of Evaporating Microdroplet of Aqueous SiO 2 Nanospheres Suspension with Sodium Dodecyl Sulfate

We studied the evolution of interference patterns and intensities of polarized and depolarized scattered laser light on a single evaporating microdroplet of suspension composed of silica (SiO2) spherical nanospheres (225 nm radius) and sodium dodecyl sulfate (SDS) dispersed in diethylene glycol (DEG). Our experimental set-up description and procedures can be found elsewhere in (Jakubczyk D, Derkachov G, Kolwas M, Kolwas K, J Quant Spectrosc Radiat Trans 126:99–104, 2012; Kolwas M, Kolwas K, Derkachov G, Jakubczyk D, Phys Chem Chem Phys 17:6881–6888, 2015; Jakubczyk D, Kolwas M, Derkachov G, Kolwas K, Zientara M, Acta Phys Pol-Ser A Gen Phys 122:709, 2012).

Evaporation-Driven Aggregation of Nanoparticles in a Free Droplet: Spherical Symmetry in Nanostructured Material

We have investigated the evaporation-driven aggregation of nanoparticles during the slow-drying process of single colloidal droplets levitating in the electrodynamic trap. The final, spherical quasi-crystal composed of nanoparticles is an interesting nanostructured material of wide range of possible applications (plasmonics, metamaterials, colloidal crystals).