Viscotaxis: microswimmer navigation in viscosity gradients

The survival of many microorganisms, like \textit{Leptospira} or \textit{Spiroplasma} bacteria, can depend on their ability to navigate towards regions of favorable viscosity. While this ability, called viscotaxis, has been observed in several bacterial experiments, the underlying mechanism remains unclear. Here, we provide a framework to study viscotaxis of self-propelled swimmers in slowly varying viscosity fields and show that suitable body shapes create viscotaxis based on a systematic asymmetry of viscous forces acting on a microswimmer. Our results shed new light on viscotaxis in \textit{Spiroplasma} and \textit{Leptospira} and suggest that dynamic body shape changes exhibited by both types of microorganisms may have an unrecognized functionality: to prevent them from drifting to low viscosity regions where they swim poorly. The present theory classifies microswimmers regarding their ability to show viscotaxis and can be used to design synthetic viscotactic swimmers, e.g.\ for delivering drugs to a target region distinguished by viscosity

Phototaxis of synthetic microswimmers in optical landscapes

Many microorganisms, with phytoplankton and zooplankton as prominent examples, display phototactic behaviour, that is, the ability to perform directed motion within a light gradient. Here we experimentally demonstrate that sensing of light gradients can also be achieved in a system of synthetic photo-activated microparticles being exposed to an inhomogeneous laser field. We observe a strong orientational response of the particles because of diffusiophoretic torques, which in combination with an intensity-dependent particle motility eventually leads to phototaxis. Since the aligning torques saturate at high gradients, a strongly rectified particle motion is found even in periodic asymmetric intensity landscapes. Our results are in excellent agreement with numerical simulations of a minimal model and should similarly apply to other particle propulsion mechanisms. Because light fields can be easily adjusted in space and time, this also allows to extend our approach to dynamical environments.Comment: 10 pages, 7 figure

How "Facts" Shaped Modern Disciplines: The Fluid Concept of Fact and the Common Origins of German Physics and Historiography

This history of the concept of fact reveals that the fact-oriented practices of German physicists and historians derived from common origins. The concept of fact became part of the German language remarkably late. It gained momentum only toward the end of the eighteenth century. I show that the concept of fact emerged as part of a historical knowledge tradition, which comprised both human and natural empirical study. Around 1800, parts of this tradition, including the concept of fact, were integrated into the epistemological basis of several emerging disciplines, including physics and historiography. During this process of discipline formation, the concept of fact remained fluid. I reveal this fluidity by unearthing different interpretations and roles of facts in different German contexts around 1800. I demonstrate how a fact-based epistemology emerged at the University of G\"ottingen in the late eighteenth century, by focusing on universal historian August Ludwig Schl\"ozer and the experimentalist Georg Christoph Lichtenberg. In a time of scientific and political revolutions, they regarded facts as eternal knowledge, contrasting them with short-lived theories and speculations. Remarkably, Schl\"ozer and Lichtenberg construed facts as the basis of Wissenschaft, but not as Wissenschaft itself. Only after 1800, empirically minded German physicists and historians granted facts self-contained value. As physics and historiography became institutionalized at German universities, the concept of fact acquired different interpretations in different disciplinary settings. These related to fact-oriented research practices, such as precision measurement in physics and source criticism in historiography