Microscopic swimmers at surfaces

Both in soft matter and in biology, there are numerous examples of swimmers and self-propelled particles, which have a typical size in the range of a few nano- to several micro-meters, so that both low-Reynolds-number hydrodynamics and thermal fluctuations are essential to determine their dynamics. Well-known biological examples are sperm cells which are propelled by a snake-like motion of their tail, bacteria like E. coli which move forward by a rotational motion of their spiral-shaped flagella, and listeria which are propelled by an actin tail which is formed by actin polymerization at their surface. In soft matter systems, synthetic self-propelled particles have been designed to perform directed motion. Examples are here bimetallic nanorods which are driven by different chemical reactions at the two types of surfaces, or connected chains of magnetic colloidal particles on which a snake-like motion is imposed by an external magnetic field. Both in soft matter and biological systems, surfaces and walls are ubiquitous. For example, microfluidic devices have by large surface to volume ratios. Similarly sperm in the female reproductive tract find themselves in strongly confined geometries. Already in 1963 Rothschild found that sperm accumulate at surfaces. Thus surfaces strongly influence the dynamics of swimmers and self-propelled particles. We present a combined simulation study of sperm and of self propelled nanorods near surfaces. We show how surface adhesion emerges in both systems and how they are related. Scaling arguments are presented to understand the nanorods behaviour. Importance of hydrodynamic interactions is discussed for both systems.