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Discrete Microfluidics

Motivated by applications in medicine, chemistry, and biotechnology, microfluidics tries to satisfy the increasing demand to handle smaller amounts of liquids in less time. We explore a discrete microfluidics which might allow using minute amounts of liquid e.g. for purposes in combinatorial chemistry or drug screening.

In microfluidics devices we dispense liquid in small emulsion droplets with picoliter volumes. These droplets serve as individual reaction container for microfluidics processing. The surrounding continuous phase typically has a very small volume fraction, thus the emulsion is called gel emulsion. The topology of this emulsion is equivalent to foam and offers several new possibilities: The lamellae between individual droplets can be targeted ruptured initiating the coalescence of the two droplets. Furthermore, different chemicals can be effectively mixed within droplets. The mixing occurs by a twisty flow pattern within droplets that is induced by the friction at the wall of the microfluidics channels. Provided the droplets are sufficiently small, the mixing occurs already very fast by diffusion and can be considered as instantaneous after rupturing the lamellae. The relation of the internal length scale of the liquid (droplet size) to the external length scale provided by the channel geometry determines the relative position of droplets within a droplet arrangement when flowing through a microfluidic channel. Varying the channel geometry locally or temporarily the relative positions of droplets can thus be manipulated. Even large droplets can be split in smaller ones that could repeatedly run through the same process. The large number of different reaction products might be stored and analyzed e.g. in a wide and shallow channel where the droplets form a honeycomb structure similar to standard micro arrays.

Left: droplet arrangement flowing through an appropriate microchannel geometry. The two row arrangement is transferred into a one row arrangement and back to a two row Arrangement.
Right: If a one row droplet arrangement arrives at a y-junction the droplets Are split in half. A two row arrangement arriving at a y-junction is reversibly split or combined like a zipper.

Using ferrofluids as continuous phase allows manipulating single droplets respectively the foam like topology by applying external magnetic fields. In this way, relative droplet positions can be selectively varied or microfluidics valves can be realized. Actually we also use the discrete microfluidics to e.g. synthesize silica particles for heterogeneous catalysis and to study fibrin gels.

Left: Fibriniogen gel in a „relaxed“ droplet. By elongating the droplet in a narrowed microchannel the fibrinogen gel is mechanically stretched.
Right: Scanning electron micrograph of microfluidically synthesized silica particles of abou 3 µm diameter and with an internal surface area above 800 m2/g.

Besides the applied, (bio-)technical and pharmaceutical aspects of the discrete microfluidics we are interested in a fundamental understanding of e.g. friction in thin emulsion lamellae up to “Newton Black Films”, the electrical field distribution and dynamics in case of electro coalescence, and topological re-arrangements of individual compartments. Furthermore, we explore the possibility to orient amphiphiles in well organized emulsion lamellae. The generation of such a gel-emulsion in a microfluidics device is equivalent to the generation of membrane stacks that are used for x-ray analysis. We try to use the self-assembly of molecules (and channel proteins) in emulsion lamellae to study electronic properties of single molecules.

Sketch of a refractive x-ray optics (CRL) made from foam lamellae. The foam lamellae will be continuously generated whereas the number of the foam lamellae and thus the focal length can be continuously varied.

In another project, we use the large number of regularly curved foam lamellae of monodisperse foam in a capillary as diffractive x-ray lens (compound refractive lens, CRL). Generating foam lamellae in-situ with variable separation distance we try to develop x-ray optics with continuously variable focal length that can be used for large x-ray energies and intensities

Funding

  • SFB 755 „Nanoscale Photonic Imaging“
  • DFG

Collaborations

  • Dr. M. Brinkmann, MPI-DS, Göttingen, Germany
  • Prof. Dr. S. Herminghaus, MPI-DS, Göttingen, Germany
  • Prof. Dr. A. Ott, Saarland University, Germany
  • Prof. Dr. T. Pfohl, U-Basel, Basel, Switzerland
  • Dr. C. Priest, The Wark, Adelaide, Australia
  • Prof. Dr. W. Maier, Saarland University, Saarbrücken, Germany
  • Prof. Dr. M. Mayor, U-Basel, Basel, Switzerland