SPECIAL PHYSICAL MATHEMATICS SEMINAR


TOPIC:	AC ELECTRO-OSMOTIC FLOW ON POLARIZED ELECTRODES

SPEAKER:	YUXING BEN
		University of Notre Dame

ABSTRACT:

There is growing interest in the micro-fluidic community in electrokinetic
phenomena of colloidal or biological suspensions subjected to
time-dependent applied electric field.  For example, AC electric fields
have been applied at micro-electrodes to pump liquid electrolytes and to
concentrate or separate collidal particles and cells.  I will present my
analysis of AC electrokinetic flow on polarized electrodes, with the hope
that it will lead to better device design.  Two mechanisms are shown to
drive flow on the electrode pair, either of which can dominate depending
on the conditions.  At voltages below the ionization voltage, capacitive
charging of ions from the bulk electrolyte by the external field
dominates.  Above this ionization voltage, Faradaic charging prevails.
These two mechanisms induce opposite fluid flows and vortex circulations,
have different field screening features at low frequencies and scale
differently with respect to the applied voltage.  I will address some
important physical phenomena and devices we have produced by exploiting
these different features, with emphasis on the following two topics.

Electrokinetic flows and vortices due to capacitive charging can collect
particles from the bulk and the electrode surface and focus them into two
distinct linear aggregates along their stagnation lines, where the normal
flow is away from the electrode.  With a voltage increase and a subsequent
transition to Faradaic charging, the flow reverses direction and the lines
are erased.  By analyzing the single-layer Cauchy integral, I predicted
the position of the stagnation lines to be at 1/$\sqrt{2}$ of the width of
a wide electrode, which is favorably compared to experimental imaging.
We are currently developing a long-range bacteria trap based on this
mechanism.

As Faradaic charging scales exponentially with respect to the voltage
instead of the quadratic scaling of capacitive charging, it offers a
stronger pumping mechanism than the capacitive pumps proposed by earlier
researchers.  I will present a new Faradaic micro-pump design from our
group that also exploits field amplification by geometric singularities
and the distinct flow direction of the Faradaic mechanism.


DATE:		FRIDAY, MARCH 5, 2004

TIME:		2:30 PM

LOCATION:	Building 2, Room 338


Refreshments at 3:30 PM in Building 2, Room 349.


Massachusetts Institute of Technology, Department of Mathematics
Cambridge, MA  02139