Microorganisms play a vital part in the life of our planet. They not only lead in amounts, but also being responsible for many physiologically important processes they serve the whole ecosystem of Earth. With sizes ranging from nanometers to hundreds of microns, they come in all sorts of shapes and different behaviour. Since they have first been discovered, many works have been devoted to the understanding of their motion. Despite their morphological differences, the unifying feature of their motion is the fact that they live and move in a fluid-like environment (such as water) which makes exploration of the hydrodynamics of microscopic object a lively and developing area of research.

Water at such small scales behaves in a manner different to our everyday understanding. In particular, for motion characteristics typical for bacteria, the viscosity of the surrounding fluid dominates completely over the fluid inertia. Since macroscale swimming of fish (as well as people) generally relies on inertial mechanisms, bacteria have had to come up with different strategies for their motion using filament-like appendages, called cilia and flagella, to propel and stir the suspending fluid. By a perfect mathematical analogy, motion of microscale organisms in water can be imagined to be identical to the motion of large objects (macroscale, like humans) moving in a fluid of much greater viscosity, such as honey. This offers an insight into the dynamics of microworld by looking at slow motion of viscous fluids within the mathematical framework of the so-called low Reynolds number flows. The Reynolds number – a dimensionless parameter quantifying the relative importance of inertia to viscosity in a flow of given characteristics – is negligibly small both in the case of very viscous liquids (honey!) and very small scales (bacteria).

There is a rich zoology of phenomena related to bacterial motion, ranging from the understanding of propulsion and fluid flow generated by particular organisms given their shape and swimming gait, to their collective interactions. Hydrodynamic interactions are related to the fact that disturbances of the fluid caused by motion of a bacterium influence the motion of many surrounding organisms, and lead to complex many-body interactions. As a notable example, with the advent of optical microscopy techniques, it has been observed experimentally quite a long time ago that bacteria and eukaryotic flagellates swimming in close proximity tend to synchronise the beating pattern of their appendages, which is now understood to be possible in a large part due to the viscous interaction of these filaments via the fluid.

The author’s research in the area of small-scale hydrodynamics encompasses many problems related to the mathematical understanding of fluid flows in situations of biological relevance, in particular in systems in geometric confinement. Biologically relevant processes often occur in restricted geometries, i.e. close to walls, cell membranes, microchannels, and in crowded and complex environments. By building simplified models of such systems we gain insight into the basic underlying physical mechanisms. In such situations, the frictional forces acting on the suspended organisms are anisotropically enhanced by the proximity of an interface. This has an impact on the observed dynamics and is known e.g. to curve the trajectories of swimming bacteria and influence the process of achieving synchrony of their beating flagella. The ultimate objective of author’s research is to better understand the role of hydrodynamic interactions with interfaces and their inherently anisotropic character in processes of biological relevance.

**About the Author**

Maciej Lisicki is a theoretical physicist. He has graduated in 2015 with a PhD from the University of Warsaw in collaboration with Juelich Research Centre (Germany) working with Bogdan Cichocki and Jan Dhont on statistical physics of colloidal suspensions. Since then, he was a Crighton Fellow and is now conducting his postdoctoral research as a Mobility Plus Fellow at the University of Cambridge, working on biophysial fluid dynamics in the group of Eric Lauga. Apart from science, he greatly enjoys the outdoors, art and culture, and chasing life’s little pleasures.