Chemistry Reference
In-Depth Information
even though slight collective effects already lead to an enhancement of the effective
diffusivity. Further, the walls in the simulations are present much farther away (10
times the swimmers size) than in the experiments, where they might slow down
the droplets due to friction. However, it would be very interesting to look for any
threshold areal density in the experiments similar to what was seen in the simulations.
7.3.2 Swimmer-Tracer Scattering
The hydrodynamic flowfields not only have consequences for the interaction between
swimmers, but also for the distributions of passive particles (tracers) suspended in the
medium. The passive particles are in a way 'scattered' by the swimmers involving an
interplay of diffusion and advection due to the flow fields of the swimmers. An under-
standing of the swimmer-tracer interactions is important to understand processes
such as nutrient distribution and uptake in biological settings. Previous studies have
demonstrated the enhanced diffusion of Brownian tracer particles in the presence of
a dense suspension of swimming bacteria [
21
,
22
]. In fact, in the simulations shown
in Fig.
7.4
, it can be seen that the diffusivity of the passive tracers is enhanced with
the increasing swimming activity. Further, it has been shown experimentally that
the statistics of these passive tracers in suspensions of flagellated eukaryotic cells
exhibits a non-Gaussian form. In this section, we present the dynamics of the tracer
particles around squirming droplets. Rather than the statistical aspects, we focus here
on only on the geometry of the tracer trajectories.
The tracer particles that we use for the experiments are 200nm polystyrene beads
that are coated with a fluorescent dye. Therefore, their dynamics in fluid in the
absence of any flow is expected to be purely Brownian. However, in the presence of
a flow, as caused by the droplet squirmer, in addition to the diffusion, their motion
also depends on the flow field set up by the swimmer and the path of the swimmer
itself. For the droplet squirmer, the paths of the tracers (black lines) in the rest frame
of the droplet are shown in the left panel of Fig.
7.5
. Clearly, the tracer tracks follow
the flow around the swimmer and reveal, in effect, the flow field of the swimmer
which is similar to what we discussed in the previous chapter. However, when the
tracers are observed in the reference frame of the laboratory, their paths reveal loopy
structures as shown in the right panel of Fig.
7.5
. Particularly interesting are the paths
of the tracers closest to the squirmer path, which form closed loops. From a flow
field similar to that shown in the left panel, it is not obvious at first glance that the
tracers should form these closed loop structures.
In order to understand the tracer paths, we look closely into dynamics of a single
tracer in the path of the squirmer, which forms a closed loop as shown in the top
panel of Fig.
7.6
. Snapshots of the swimmer position and the trajectory of a single
tracer, in red, show that a loop gets closed as the swimmer passes the tracer. Beyond a
distance of
m, the tracer dynamics are Brownian. As the swimmer approaches
closer, the tracer follows the flowfield and correspondingly its velocity increases. The
∼
150
µ
