Self-propelled microswimmers hold tremendous potential as autonomous agents to localize, pick-up and deliver nanoscopic objects, e.g., in bioremediation, drug-delivery and gene-therapy. Until now their behaviour has only been studied in homogenous environments. We demonstrate how they navigate through environments presenting complex spatial features, which more closely mimic the conditions inside cells, living organisms and future lab-on-a-chip devices. For example, when driven by an external force through patterned environment, some swimmers do not follow this force but steer along directions being clear of obstacles. This complex response to the environment can be exploited to characterize and sort, for example, chemotactic bacteria avoiding the need of stable chemical gradients.
Compared to Newtonian liquids, viscoleastic fluids are distinguished by rather long relaxation times which can be on the order of seconds. Accordingly, they can be easily brought out of equilibrium already by small shear flows, i.e. far below values where shear thinning or shear thickening occurs. As a consequence, a simple Markovian description is no longer valid.
Transport and flow phenomena in porous media occur in various fields of science and engineering, ranging from agricultural, biomedical, chemical and petroleum engineering to soil sciences. Despite the huge interest on this topic, relations between flow and transport properties of porous media and the corresponding microstructure are not fully understood.
Stereoisomeric molecules with opposite chirality, so-called enantiomers, often vary regarding their sensory, pharmacological and toxicological properties. Such enantiomer specific effects play a central role in the development, testing and evaluation of drugs, pesticides and food related products. Accordingly, efficient techniques for separation of chiral mixtures into enantiopure compounds are of enormous practical relevance.
Friction between solids is responsible for many phenomena like earthquakes, wear or crack propagation. Unlike macroscopic objects which only touch locally due to their surface roughness, spatially extended contacts form between atomically flat surfaces. They are described by the Frenkel-Kontorova model which considers a monolayer of interacting particles on a periodic substrate potential. In addition to the well-known stick-slip motion such models also predict the formation of kinks and antikinks which largely reduce the friction between the monolayer and the substrate.
In the dawn of the industrial revolution of the 18th century, thermodynamics emerged as an independent field of physics. To establish a comprehensive knowledge of steam engines and combustion motors, initially the main emphasis was to understand the transformation of heat to mechanical work. However, macroscopic thermodynamics is only able to describe large systems where the variety of internal degrees of freedom - one liter of gas contains approximately 1023 particles - allows one to neglect fluctuations. In accordance with the ongoing trends for miniaturization in general and the development of micromanipulation techniques like atomic force microscopy or optical tweezers in the late 1980's in particular, scientists have become able to investigate thermal systems on length and energy scales where fluctuations are not negligible.
Quasicrystals are somewhat paradoxical structures which exhibit many amazing properties distinguishing them from ordinary crystals. Although the atoms are not localized at periodic positions, quasicrystals posses perfect long-range order.
Critical Casimir Forces
When fluctuating fields are confined between two surfaces, long-ranged forces arise. Arguably, the most famous example is the quantum-electrodynamical Casimir force resulting from zero-point vacuum fluctuations between neutral, parallel, conducting plates. In 1978, Fisher and de Gennes realized that a thermodynamic analogue exists, the critical Casimir force, acting between surfaces immersed in a binary liquid mixture close to its critical point and generated by confinement of its concentration fluctuations.
Ultrasmall Force Measurements
The concept of force plays a central role in our understanding of nature. Due to the ongoing trend towards miniaturization, the investigation of forces relevant at microscopic and nanoscopic length scales therefore is an important field of research. Over the past decades several methods to measure ultra-small forces with resolution in the sub-pico Newton range have been developed.
The experimental realization of microscale devices able to manipulate small amounts of liquids (~nl-pl) is the basis for the development of so-called lab-on-a-chip systems, portable units that are capable of performing all types of chemical or biological analysis. Such systems consist of complex networks of multiple pumps, valves, mixers, reaction chambers, analyzers, and so on.
Click one of the links below to read more about Total Internal Reflection Microscopy, Acousto-Optic Deflectors, Interference Patterns and Optical Tweezers