CMDITR Review of Undergraduate Research - Pluto - University of ...

though the beads were moving because they
were caught up in the bulk flow of the medium
itself. Further experiments were done to
determine if this was indeed the case, and why.
Ethanol was used as a solvent instead of
deionized water, to see what effect changing the
medium would have. In this case, the
microbeads still moved towards the cathode, but
with a much lower velocity than in water.
Smaller, uncharged microbeads (2.25um)
were used initially to visualize the fluid flow
around immobile 12.07um beads that had
adhered to the surface. Their motion indicated
that there was turbulence around the immobile
beads. The very fact that these uncharged beads
were moving reconfirmed the idea that there was
bulk motion in the fluid near the surface, as they
could not be moving electrophoretically. And
interestingly, the smaller, uncharged beads
moved faster than the larger charged beads.
Monitoring beads in the solution away from
the bottom surface directly was difficult, as they
drifted in and out of focus and seemed to move
erratically, so we decided to test if the direction
of movement of beads along the bottom was
indicative of their movement in the rest of the
solution. To test this, the anode and cathode
were inspected under magnification before and
after exposing the beads to the electric field.
Surprisingly, the surface of the anode after the
run was covered with microbeads, while the
surface of the cathode was devoid of them. It
seemed as though the beads were moving one
way (to the anode) when away from the bottom
surface, and the other way (to the cathode) when
flush with the bottom surface. The fact that they
were sticking to the anode reaffirmed that they
were in fact carrying negative charge.
What could be causing this change in
direction? Perhaps the beads away from the
surface were moving to the anode
electrophoretically, dragging the water with them
and causing a counter-current along the bottom.
This was ruled out when a test using solely
uncharged beads showed them to move along the
bottom just as well as when they were in a
solution containing charged beads. In addition,
the bead solutions had always been very dilute,
so the beads being the cause of the bulk flow was
unlikely.
Then, in literature about capillary
electrophoresis, we came across a phenomenon
known as the double-layer effect. Along a glasswater
interface, hydroxyl ions in the water
interact with silanol groups on the glass surface,
giving the glass a negative charge. Mobile
protons in the solution are attracted to the
interface, so that there is a high concentration of
mobile positive charge carriers near the surface.
When a voltage is applied along the surface, the
protons move towards the cathode, dragging the
water with it and setting up a bulk flow (known
as electroosmotic flow, or EOF). Apparently,
this force was enough to overcome the
electrophoretic force acting on the negative
beads, and drag them along to the cathode. (Our
experiments indicate that something similar
happens with Pyrex and PDMS, albeit to a lesser
degree).
This phenomenon explained many of our
observations. The beads moved slower in
ethanol because ethanol does not dissociate to
the same degree into the hydroxyl ions and
protons that cause the double-layer effect. The
smaller, uncharged beads moved faster than the
larger, charged beads for two reasons: there was
no electrophoretic force acting on them to
oppose the EOF, and they were able to travel
fully inside the moving layer (apparently the
moving layer of protons in DI water is very
thin—the smaller beads could move within it,
while the larger beads were not fully submerged,
and felt more drag from the non-moving water
higher up from the bottom surface).
Anode
Cathode
CMDITR Review of Undergraduate Research Vol. 1 No. 1 Summer 2004 41