ABSTRACT
The synthesis of ZnO nanoparticles proceeds through a complex sequence of precursor reactions, nucleation, and growth processes. For further advancement and control of nanoparticle synthesis, a detailed understanding of the mechanisms and kinetics is essential. With the recent advancement in X-ray scattering and spectroscopy methods, in situ experiments during nanoparticle synthesis can be performed, which provide important new insights into reaction and growth mechanisms. Here we use in situ small- and wide-angle X-ray scattering (SAXS, WAXS) coupled with UV-vis spectroscopy to investigate the nucleation and growth process of an oleate-based ZnO nanoparticle synthesis yielding narrowly disperse nanoparticles over the complete time scale from 30 s to 18 h. We find that the nucleation and early growth period during the first 1000 s can be quantitatively described by a classical homogeneous nucleation and growth mechanism. Furthermore, we identified a second growth phase where nanoparticle crystallization occurs, as indicated by the appearance of higher-order Bragg peaks and a pronounced shift of the absorption edge in the UV-vis spectra. The results are in very good agreement with recent studies on the use of the ZnO alkali hydroxide hydrolysis route. Thus, a very good understanding of the nucleation and growth mechanisms and kinetics of the most important ZnO synthesis routes has been established.
ABSTRACT
We present a microfluidic nozzle device for the controlled continuous solution blow spinning of ultrafine fibers. The device is fabricated by soft lithography techniques and is based on the principle of a gas dynamic virtual nozzle for precise three-dimensional gas focusing of the spinning solution. Uniform fibers with virtually endless length can be produced in a continuous process while having accurate control over the fiber diameter. The nozzle device is used to produce ultrafine fibers of perfluorinated copolymers and of polycaprolactone, which are collected and drawn on a rotating cylinder. Hydrodynamics and mass balance quantitatively predict the fiber diameter, which is only a function of flow rate and air pressure, with a small correction accounting for viscous dissipation during jet formation, which slightly reduces the jet velocity. Because of the simplicity of the setup, the precise control of the fiber diameter, the positional stability of the exiting ultrafine fiber and the potential to implement arrays of parallel channels for high throughput, this methodology offers significant benefits compared to existing solution-based fiber production methods.
ABSTRACT
Liquid microjets play a key role in fiber spinning, inkjet printing, and coating processes. In all of these applications, the liquid jets carry dispersed particles whose spatial and orientational distributions within the jet critically influence the properties of the fabricated structures. Despite its importance, there is currently no knowledge about the orientational distribution of particles within microjets and droplets. Here, we demonstrate a microfluidic device that allows to determine the local particle distribution and orientation by X-ray scattering. Using this methodology, we discovered unexpected changes in the particle orientation upon exiting the nozzle to form a free jet, and upon jet break-up into droplets, causing an unusual biaxial particle orientation. We show how flow and aspect ratio determine the flow orientation of anisotropic particles. Furthermore, we demonstrate that the observed phenomena are a general characteristic of anisotropic particles. Our findings greatly enhance our understanding of particle orientation in free jets and droplets and provide a rationale for controlling particle alignment in liquid jet-based fabrication methodologies.
ABSTRACT
Collagens are widely used as biomaterials in drug-delivery and tissue engineering applications due to their biodegradability, biocompatibility and hypoallergenicity. Besides gelatin-based materials, collagen microfibers are in the focus of biomedical research. Commonly, man-made fibers are produced by wet-spinning yielding fiber diameters higher than 8 µm. Here, assembly and continuous production of single collagen type I microfibers were established using a microfluidic chip. Microfluidics-produced microfibers exhibited tensile strength and Young's modulus exceeding that of fibers produced in classical wet-spinning devices and even that of natural tendon and they showed lower diameters. Their structural orientation was examined by polarized Fourier transform infrared spectroscopy (FTIR) showing fibril alignment within the microfiber. Cell culture tests using the neuronal cell line NG108-15 showed cell alignment and axon growth along the microfiber axes inaugurating potential applications in, for example, peripheral nerve repair.
