Nanostructured Polymer Films

Non-equilibrium flow in nanoscale geometries

Flow in nanoscale geometries is of importance in many areas as for example in chip production, printing techniques and bio-medical sensing devices.

The movement of sub-microliter droplets on solid supports is still far from being understood. Regarding applications, these solvents are rather solutions with polymers or particles being solved in the solvent. The movement of such binary droplets, however, is a very complex process involving several mechanisms. The solvent evaporates and increases the concentration of the colloidal particles. The increased evaporation near the contact line drives a convective flow within the drop that transports material towards the periphery. Additionally, an increased solute concentration and a decreased temperature near the three-phase contact line may trigger solutal and thermocapillary Marangoni flows. However, also the interaction with the substrate and transversal contact line instabilities have to be taken into account. Finally, capillary forces come into play as soon as the solution film has a comparable thickness as the colloidal particles diameter. Nano x-ray beams were then used to perform a quantitative analysis of the structure developing at the triple interface of a microdroplet during its evaporation. We were able to distinguish different ordering rates in the directions parallel and perpendicular to the substrate.

In order to investigate flow at an interface a new fluidic cell (central part fluidic channel) was designed and optimized for synchrotron GISAXS experiments and optical microscopy. Using our reciprocal space approach by means of microbeam GISAXS, we studied in-situ the influence of flow on deposition of thin films of colloidal particles in a fluidic channel (see figure In this way the concentration fluctuations were kept under control and the system was restricted to a single fluid-substrate interface. To the best of our knowledge this experiment was the first GISAXS characterization ever performed on a flowing system. It not only showed that the technique is able to detect particles at an interface buried in a fluid in motion but we also established that in these conditions the growth of highly ordered thin films displayed a change towards a denser packing regime after the deposition of an initial layer.

The experimental investigation focuses on non-equilibrium flow in nanoscale geometries to be generated by topographic or chemical surface structuring. The fluidics will be influenced by confinement given by nano-channels, which are produced via nano-scale phase separation of copolymers, by different surface functionalities, which may be similarly spatially structured, or by surface gradients, which are obtained with mixed polymer brushes. Flow will be investigated with solutions containing polymers and different liquids thus providing a tunable model system for the investigation of non-equilibrium flow. By variation of polymer concentration the viscosity is adjusted. and by the presence of a source or drain for the solution the non-equilibrium flow is controlled. Channeled substrates provide a confinement of the flowing solution into the restricted space of the channels and flow is investigated by microscopic and scattering techniques as a function of control parameters such as channel width, channel roughness or channel morphology. The surface can be modified by chemical reaction to provide specific functionality. For instance hydrophobic or hydrophilic behavior can be achieved, thus changing interaction between substrate and components of the solution. Chemically structured or gradient substrates provide a spatial variation of this interaction which with respect to nanofluidics could show a similar effect as channeled substrates. Thus both topographic and chemically structured functionalized model surfaces will be compared with respect to their influence on nanofluidics of polymer solutions and mixed liquids to provide a better understanding and possibly control of flow at nanoscopic level.

Selected Publications:

  • 1. N.Hermsdorf, K.Sahre, P.Volodin, M.Stamm, K.J.Eichhorn, S.Cunis, R.Gehrke, P.Panagiotou, T.Titz, P.Müller-Buschbaum
    Supported particle track etched polyimide membranes: A grazing incidence small-angle x-ray scattering study;
    Langmuir 20, 10303 (2004)
  • 2. N. Hermsdorf, M. Stamm, S. Förster, S. Cunis, S.S. Funari, R. Gehrke, P.Müller-Buschbaum
    Self Supported Particle Track Etched Polycarbonate Membranes as Templates for Cylindrical Polypyrrole Nanotubes and Nanowires: An X-Ray Scattering and Scanning Force Microscopy Investigation;
    Langmuir 21, 11987 (2005)
  • 3. P.Müller-Buschbaum, E.Bauer, S.Pfister, S.V.Roth, M.Burghammer, C.Riekel, C.David, U.Thiele
    Creation of multi-scale stripe-like patterns in thin polymer blend films;
    Europhys. Lett. 73, 35 (2006)
  • 4. P.Müller-Buschbaum, E. Bauer, E.Maurer, K.Schlögl, S.V.Roth, R.Gehrke
    A new route to large-area ordered polymeric nano-channel arrays;
    Appl.Phys.Lett. 88, 083114 (2006)
  • 5. J-.F.Moulin, S.V.Roth, P.Müller-Buschbaum
    Flow at interfaces: a new device for x-ray surface scattering investigations;
    Rev.Sci.Instr. 79, 015109 (2008) link