1. IBM Research Europe - Zurich, Säumerstrasse 4, CH-8803 Rüschlikon, Switzerland
2. Institute for Nano- and Microfluidics, TU Darmstadt, Peter-Grünberg-Strasse 10, D-64287 Darmstadt, Germany
3. Laboratory for Soft Materials and Interfaces, Department of Materials, ETH Zürich, Vladimir-Prelog-Weg 5, CH-8093 Zürich, Switzerland