Biomedical Engineering Reference
In-Depth Information
Figure 10.19
Energetic potential (a) and concentration profile of the particles (b). The height of
the energy barriers is much higher than thermal agitation. After a diffusion step of duration
t
off
, the
particles diffuse (c) and are trapped again when the potential is switched on (d).
them have diffused over a distance larger than the small side of the pattern of the
potential, on the other hand, because of the asymmetry of this pattern, the fraction
of the particles that have diffused over a distance larger than the large side of this
pattern is much smaller. As a result, a fraction of these particles will be trapped in
the minima next to the ones they previously occupied and, with the conventions
of Figure 10.19, more of them will shift to the left than to right (shadowed area
under the concentration peak in Figure 10.19(c)). By reiterating this process a large
number of times, one can then set these particles into motion over potentially large
distances whereas the only gradients present in the system are local. Hence, we have
solved here one of the main limitations of DEP by making it a transport technique
and not only an analysis technique. Furthermore, we have an adjustable “knob”:
t
off
, the time during which we let the particles diffuse is a control parameter for
these experiments.
The macroscopic motion of the particles is expected to vary exponentially with
their diffusion coefficient and thus with their size or their molecular weight which
is very promising for separating particles of different sizes [63].
This sawtooth potential can be of DEP nature [37, 65]. In particular, it can be
created by setting an ac. voltage between an electrode whose corrugations presents
the “good” properties of periodicity and asymmetry and a planar one [37]. Quali-
tatively, by a simple “tip effect,” the electric field is of much higher intensity on the
ridges than it is in the valleys. As a consequence, the asymmetry of the electrode
reflects itself on the intensity of the electric field and thus on the energetic potential.
This can be confirmed by a finite elements simulation of the electric field. When
experiencing the electric field at high enough frequencies, the beads are confined in
the valleys (negative DEP).
