Biomedical Engineering Reference
In-Depth Information
stress at the surface of the comet. Numerically, the last equation together
with Equation (1.41) can be solved numerically to calculate the droplet shape.
By fitting experimentally determined profiles, one obtains for the parameters
σ
0
=
kTρ
+
32nN
/μ
m
2
and
v
p
1
.
4nm
/
sec (see Figure 1.10).
Figure 1.10.
Calculated drop profile (full line) compared with the experimentally
determined profile (dashed line). The inset shows the normal stress distribution along
the drop surface. Figure is reprinted from [42]. (Copyright (2004), with permission
from The American Physical Society).
As can be seen from the inset of Figure 1.10, the normal stress is positive
at the back of the drop thus pulling the liquid droplet. Recent experiments
confirm this [45]. This finding is in accordance with the theoretical prediction
made in [10] that the actin gel could pull at the rear of
Listeria
.
Experimentally, it was also found that saltatory motion of soft beads
(droplets) is different from that of hard beads [45]. Because on liquid droplets
the actin activator (VCA) detached from actin filaments is free to move over
the surface, the step size of the droplet motion depends on the diffusion time
τ
d
of VCA over the sphere's surface. Because
τ
d
increases with bead size, the
step size does as well. The opposite is true for hard beads: here, larger beads
have smaller step sizes (see Figure 1.11).
1.5.2 Properties of Comet Tails
Actin-based movement of beads is governed by the mechanical properties of
the actin network. Experimentally, these can be fine-tuned by changing the
concentrations of proteins involved in actin dynamics and assembly.
For example, the coating of polystyrene beads can be varied [46]. It was
found that the degree of filament alignment in the comet tails depends on
