In recent years the trend of miniaturization, which had been associated exclusively with the microelectronics industry for a long time, reached the areas of mechanical and fluid mechanical engineering. Like in other areas of technology, the miniaturization of fluid handling systems reduces costs, increases the efficiency of processes, allows for automation, and for the integration of many process steps in a single complex device. In the course of miniaturization, new functionalities and new physical properties of liquids arise because the different physical effects govern than on the macro scale. With decreasing system sizes, surface-to-volume ratio increases. As a result, surface energies, or more generally interfacial energies, become increasingly important. At even smaller scales, typically on the nanometer scale, the conventional continuum description breaks down and the granular character of matter, i.e. its composition of individual molecules, becomes apparent.
What are the ultimate physical limits of miniaturization in the field of fluid mechanics? How can fluid structures be created, characterized, and manipulated on a submircometer scale? How do the physical properties of liquids evolve if they are confined to geometries that approach characteristic molecular length scales? How do these microscopic phenomena affect the macroscopic mechanical properties of complex systems, such as colloidal suspensions, emulsions, or even living cells? These are the questions that govern the research activities of the PCF group.
SPECIFIC RESEARCH ACTIVITIES
Current research projects can be classified in three general topics: (i) nanofluidics, (ii) (electro)wetting and microfluidics, and (iii) mechanical properties of complex fluid systems.
In the experiments on nanofluidics and nanotribology, we confine liquids in idealized slit pores made of two parallel atomically smooth walls at a variable separation of a few molecular diameters. Due to the interaction with the solid walls, the molecules in thin liquid films arrange in layers parallel to the walls instead of the random positional order in the bulk. The ultimate goal in these experiments is to gain fundamental insights into both equilibrium and transport properties at the boundary between molecular motion and continuum flow. The experiments are expected to contribute to the fundamental understanding of processes as diverse as lubrication in molecularly thin liquid films (e.g. for applications in micro- or nano-electromechanical motors (MEMS or NEMS)), fluid flow in porous media, transport in biological systems such as ion channels, lubrication in joints, to name just a few. On a practical level, the experiments involve various surface preparation and characterization methods as well as a variety of optical measurement techniques, including multiple beam interferometry, video microscopy, and laser-induced fluorescence.
The second set of projects deals with new concepts for microfluidics, mainly based on an active (electrical) control of the wettability of surfaces, i.e. of surface energies. Making use of the so-called electrowetting effect, liquid droplets can be transported on solid surfaces using an entirely electrical actuation. Interesting physical problems include the controlled generation, characterization, and manipulation of liquid micro- and nanostructures as well as their interaction with electric fields and/or externally imposed pressure-driven flow fields. Applications include lab-on-a-chip devices for biochemical and biomedical diagnostics. The experiments involve various surface preparation and patterning techniques (photolithography, soft lithography) as well as optical measurement techniques, such as fluorescence microscopy and high speed video imaging. Numerical modeling of the experiments plays a crucial role.
Many products of our daily lives such as cosmetic creams, milk products, or paints classify as complex fluid systems. The mechanical properties of such emulsions or dispersions are largely determined by the properties and/or the state of aggregation of the microscopic droplets or particles within the system. We use a variety of techniques such as confocal scanning laser microscopy (CSLM), atomic force microscopy, and classical rheological measurements in order to relate the macroscopic flow behavior of these systems to the structure and the mechanical properties on the microscopic level of the individual particles.