1.2 Hydrodynamics of small organisms
analytical studies followed, which investigated the hydrodynamics of small organisms.
The main focus from a physical point of view has been optimal modes of swimming
and other physical aspects such as the properties of microswimmer suspensions, where
pairwise swimmer interactions as well as macroscopic patterns of such active uids are
studied Elgeti et al., 2011; Lauga and Powers, 2009.
Multipole expansions of the creeping ow have been an important mathematical tool
to study the ows around small organisms. Such mathematical tools allow the approximation
of far eld ow and characteristic hydrodynamic patterns around swimmers
and allow us to estimate forces on slender swimming appendages using fundamental
ow singularities Gray and Hancock, 1955; Brennen and Winet, 1977; Lauga and Powers,
2009. The solid boundary of swimmer bodies is usually modelled with zero ow
velocity at the surface (no-slip), while the squirmer model Lighthill, 1952; Blake, 1971
as an exception approximates the movement of many short appendages (cilia) as an
eective slip velocity on the swimmer surface. The hydrodynamics and prey capture
by lter feeders have mainly been studied using the well established tools of hydrosol
and aerosol theory Rubenstein and Koehl, 1977, which mainly focus on eciency
measures relevant to industrial lters Pich, 1966; Dorman, 1966.
There are only few model organisms such as Escherichia coli, Chlamydomonas sp.
and Volvox sp., the physics of which are being studied extensively Berg, 2004; Goldstein,
2015, while the morphological and functional variety of ecologically important
species is often neglected in hydrodynamics studies Jones, 1994; Sleigh, 1981. Further
the trade-os between dierent functions are usually not explicitly explored, but dierent
mechanisms and their optimisation are rather discussed separately. Many questions
regarding emerging patterns in planktonic ecosystems and trade-os for small marine
organisms, which have been addressed mainly by biologists and ecologists, are related
to physical mechanisms Rubenstein and Koehl, 1977; Fenchel, 1986, 1988; Shimeta,
1993; Acu~na et al., 2011; Kirboe, 2011; Kirboe et al., 2014; Andersen et al., 2016.
The relevant issues lead to many unanswered questions that range from the function
and structure of planktonic food webs and the specic trade-os for swimming and
resource acquisition modes to the emergence of gelatinous plankton. Those problems
are often more broad and complex than the straight-forward optimisation tasks that
might be posed from a narrow physical viewpoint. However, after the essential parameters
are disentangled, the mechanisms can often be traced back to simple physical
interactions of individuals through and with the ocean water. It is thus an exciting
and promising opportunity for a physicist to delve into those unknown realms and to
tackle unanswered ecological questions with the tools of physics Guasto et al., 2012.
Our main goal is thus not necessarily to develop completely new tools, but to use
and combine existing methods and results from physics in order to study the survival
strategies of individual organisms. The rst Ph.D. study at the Centre for Ocean
Life using this approach was conducted by Navish Wadhwa 2015, who studied the
hydrodynamical traits and trade-os of zooplankton with a focus on swimming and