64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011;
Baltimore, Maryland
Session G26: Minisymposium: Electrokinetic Flows about Ion-Selective Surfaces
8:00 AM–10:10 AM,
Monday, November 21, 2011
Room: 329
Chair: Udi Yariv, Technion - Israel Institute of Technology
Abstract ID: BAPS.2011.DFD.G26.5
Abstract: G26.00005 : Electrokinetics over liquid/liquid interfaces*
9:44 AM–10:10 AM
Preview Abstract
Abstract
Author:
Todd M. Squires
(Chemical Engineering Department, University of California, Santa Barbara)
Since liquid-liquid interfaces flow in response to an applied stress, one might expect
electrokinetic flows at liquid-liquid interfaces to be significantly higher than over
liquid-solid interfaces. The earliest predictions for the electrophoretic mobility of
charged mercury drops -- distinct approaches by Frumkin and Levich (1946), and
Booth (1951) -- differed by ${\cal O}(a/\lambda_D)$, where $a$ is the radius of the
drop and $\lambda_D$ is the Debye screening length. Seeking to reconcile this
rather striking discrepancy, Levine (1973) showed double-layer polarization to be
the key ingredient. Without a physical mechanism by which electrokinetic effects are
enhanced, however, it is difficult to know how general the enhancement is --
whether it holds only for liquid metal surfaces, or more generally, for all
liquid/liquid surfaces.
By considering a series of systems in which a planar metal strip is coated with either
a liquid metal or liquid dielectric, we show that the central physical mechanism
behind the enhancement predicted by Frumkin and Levich (1946) is the presence of
an unmatched electrical stress upon the electrolyte-liquid interface, which
establishes a Marangoni stress on the droplet surface and drives it into motion. The
source of the unbalanced electrokinetic stress on a liquid metal surface is clear --
metals represent equipotential surfaces, so no field exists to drive an equal and
opposite force on the surface charge. This might suggest that liquid metals
represent a unique system, since dielectric liquids can support finite electric fields,
which might be expected to exert an electrical stress on the surface charge that
balances the electric stress. We demonstrate, however, that electrical and osmotic
stresses on relaxed double-layers internal to dielectric liquids precisely cancel, so
that internal electrokinetic stresses generally vanish in closed, ideally polarizable
liquids. The enhancement for liquid mercury drops can thus be expected quite
generally over clean, ideally polarizable liquid drops. More broadly, the ability to
reliably engineer liquid interfaces in microfluidic systems, then, may provide a path
to significantly enhanced electrokinetic flows.
*Research done in collaboration with A. J. Pascall
To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2011.DFD.G26.5