3.9 Experimental methods
Figure 3.13: Set-up for micro particle image velocimetry (µPIV) as it is used at DTU
Aqua. (A) A small volume of water (blue) is imaged in an inverted microscope,
where the focal plane denes the slice in which the ow is observed.
(B) Two images are taken shortly after each other at time t and t0. The
image plane that here shows a haptophyte surrounded by tracer particles
is divided into small interrogation windows. In each interrogation
window the average particle displacement d is calculated by image crosscorrelation
and the velocity in this window is obtained as v = d=(t0 t).
Sketch (B) adapted from Rael et al. 2007. Haptophyte images by courtesy
of Lasse Tor Nielsen, DTU.
automatised particle tracking software has been developed for the post-processing of
video sequences Crocker and Weeks; Crocker and Grier, 1996; Drescher et al., 2009.
Interesting alternatives are 3D life-tracking tools, which were mainly developed by
Berg 1971 for the tracking of bacteria Berg and Brown, 1972; Stephens et al., 2008.
In these methods the position of the freely swimming organism is determined, while
or shortly after it is recorded such that it can be kept in the frame of view for a long
time by moving either the microscope stage or the camera accordingly to mirror its
3.9.2 Flow measurements
The observation of ows around organisms relies on tracers that are assumed to follow
the ow faithfully and which can be visualised and tracked to reconstruct the ow
As a qualitative method to visualise ow patterns, one can use coloured or uorescent
dye which, for example, has been used to make the vortices visible that are created by
jet propulsion in swimming salps (gure 3.12 B) Sutherland and Madin, 2010b.
The quantitative ow measurements around small organisms are usually based on
particle imaging. The ow is related to the motion of particulate tracers that are
distributed in the ow. Depending on the situation it can be advantageous to follow
individual particles with a method called particle tracking velocimetry (PTV), for