Dynamics in microfluidics measured by X-Ray scattering techniques

Small angle X‑ray scattering (SAXS) is a powerful technique to analyse characteristics of colloids, polymers and proteins. The large range of scattering vectors allows for investigations of dimensions in the range from a few ångstroms up to some hundred nanometres. Microfluidics incorporates the advantages of small sample volumes and the precise control of experimental parameters. It is thus an ideal tool to investigate a manifold of biological material and reactions. Besides an extensive variability in device fabrication, microfluidics offers easy and fast device production and high reproducibility. We combine X‑ray scattering techniques with microfluidics in order to quantitatively describe the dynamics of protein folding. Moreover we analysed the flow behaviour in specific microfluidic devices.
For this purpose, we developed a microfluidic device for fast mixing and X‑ray measurements. Soft lithography allowed us to produce microfluidic devices that were readily adaptable for SAXS experiments using synchrotron radiation or in‑house setups. By the use of parallel lamination and hydrodynamic focussing, we were able to reduce the diffusion path and thus drastically decrease the mixing time. A very low dead time of 1 ms or less, depending on the flow velocity, and high temporal resolutions are crucial for the study of fast reaction dynamics. The use of hydrodynamic focussing in y‑direction and specific flow‑defining geometries to focus in z‑direction results in minimal time dispersion (i.e. minimal velocity dispersion inside the sample), which is ideal for in‑house SAXS measurements. Accordingly, we were able to measure the dynamics of lysozyme folding with an in‑house setup and calculate the corresponding radii of gyration.
As microfluidic devices are used for various types of experiments, such as rheology, it is becoming more and more important to understand the flow dynamics in the channel. We took advantage of the newest generation of coherent synchrotron radiation to analyse the flow behaviour of complex fluids. Coherent X‑ray radiation grants the possibility of correlation spectroscopy, which allows measuring the flow dynamics of colloids. Common X‑ray photon correlation spectroscopy (XPCS) uses point detectors to collect the intensity and calculate the autocorrelation function. Here, we used a fast read‑out 2D X‑ray detector to collect full‑frame scattering intensity images. Unlike previous one‑dimensional analyses, we calculate the autocorrelation functions for the full, two‑dimensional q‑range (i.e. pixel‑by‑pixel for the full image). This leads to sequences of correlation images (one for each τ), or in other words, a correlation movie. The patterns revealed by these images depend strongly on the flow situation in the channel. Consequently, the correlation movies allowed us to determine diffusion constant, flow orientation and flow velocity under different flow scenarios. We could therefore derive information about the device anisotropy directly from the correlation images.
In essence, we developed a new microfluidic device to measure fast reaction dynamics and evolved a method to quickly analyse flow behaviour inside microfluidic channels.