Microfluidics at high-intensity X-ray sources: from microflow chips to microfluidic liquid jet systems

Summary Microfluidics enables the precise control of liquids on the nanoliter scale. These very well defined flow conditions make this technology predestined for fundamental investigations at microfocused X-ray sources. With recent developments in synchrotron technology and with the advent of free electron lasers (FEL), very exciting possibilities arise, such as serial femtosecond X-ray nanocrystallography. This on-going technological progress also gives rise to the question of how to design and make sample environments that are compatible with the increasingly brilliant and highly intense X-ray beams. By developing X-ray compatible microflow chips and microfluidic liquid jet devices, which are optimized for the combination of microbeam X-ray scattering and microfluidics, this thesis contributes crucially to the advent of a powerful experimental methodology that is suitable for the investigation of nanostructures, particle alignment, protein nanocrystals and the in situ study of kinetics. After covering the fundamental details of microfocused X-rays and microfluidics individually, this thesis reviews the combination of these technologies extensively. This review includes the past and current device fabrication approaches, their up- and down-sides with respect to X-ray applications & processability as well as the related successful application examples. Further, different types X-ray compatible microfluidic devices have been developed and produced by using soft lithography which gives precise and highly reproducible design control over features of the microchannel geometry. These developed types of X-ray compatible microfluidic devices include closed-microchannel systems and open liquid jet systems. Devices of the first, closed type have already been operated successfully at the 3rd generation synchrotron PETRA III (DESY) at the Micro- and Nanofocus X-ray scattering beamline MiNaXS/P03. One example of the microfluidic small-angle X-ray scattering (MF-SAXS) experiments revealed the striking effect, that after passing a narrow section, anisotropic wormlike particles are rotated perpendicular to the flow direction, keeping this orientation over the remaining length of the channel. This phenomenon has then been studied excessively using various techniques including MF-SAXS, microparticle image velocimetry and different kinds of microscopy such as scanning electron-, light-, polarization-, high speed video-, fluorescence-, confocal laser scanning-microscopy. Additionally, the microfluidic systems have been studied using computational fluid dynamics (CFD) simulations that help to understand the fluid flow, non-linear problems or enabled the optimization of the microchannel geometries. As a result, the non-linear scientific problem of non-Newtonian fluids in confined geometries is now well understood and the related experimental control parameters have been identified and quantified. This flow-alignment of cylindrical, wormlike or fibrous structures is central to many processing steps such as in the production of fibers, during injection molding or the flow of cells and proteins through thin capillaries. Another example of a closed system demonstrates the high sample efficiency of microfluidic grids. These devices are merely millimeters of size, shear-inducing and require only 2-5 µl of sample for the shearing and X-ray study of a polymer nanocomposite material. The second microfluidic device type is based on an open nozzle geometry and produces small liquid jets with µm-diameters (0.9 to 5 μm) at very low flow rates (150 to 1000 µl h-1) under atmospheric or vacuum conditions. The presented microfluidic liquid jet devices are based on the gas dynamic virtual nozzle (GDVN) design which enables reliable and essentially clogging-free jetting over long periods of time. Further, these devices are easy to produce using established soft-lithographical techniques which enable precise and reproducible microchannel design control that is critical for the liquid jet optimization at small flow rates. This design control is demonstrated by the easy integration of additional microfluidic features, such as jet-in-jet flow focusing or dense arrays of multiple adjacent liquid jet nozzles on a single device, without the need of additional production steps. The microfluidic liquid jet system has also been studied in great detail using various microscopic techniques (see above) as well as CFD-simulations. Along with the variation of experimental parameters and nozzle geometries, these analyses and have lead to a better understanding of the fluid dynamic behavior of the liquid jet in microfluidic devices and to the control of jet diameters and droplet breakup types. The mentioned features (reliability, small sample consumption, etc.) and the open geometry design make this microfluidic liquid jet system highly relevant for the establishment as a sample environment at X-ray FELs. These facilities deliver X-ray pulses that are ultrashort (fs-range) and so enormously intense that a full diffraction pattern is recorded from a single pulse while the sample explodes in the process and turns into a glowing plasma (ca. 60,000 K). Hence, static samples or closed flow geometries are incompatible with these next generation X-ray sources which underlines the importance of the liquid jet approach. The broad scientific scope of microfluidic concepts and lithographic microfabrication techniques have also been demonstrated by creating solutions for other non-X-ray applications. The examples for this include the CFD-simulation of a non-linear scientific problem of a spray drying device, i.e. its internal fluid structure interaction, or the design & fabrication of microfeatured stamps for the microcontact printing of spherical polyelectrolyte brushes. Another example demonstrates the combination of nanotechnology and microstructuring techniques for developments towards sensoric applications.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Universität Bayreuth

Year

2013

Author dc:creator

• Trebbin, Martin

Contributors dc:contributor

• Förster, Stephan

Repository record source_url

https://epub.uni-bayreuth.de/id/eprint/29/

OAI identifier oai:identifier

oai:epub.uni-bayreuth.de:29