Among the unique properties of microfluidic devices is the ability to move liquids via electroosmosis. When the solid-liquid interface acquires a surface charge, an electrical double layer is formed as ions in the electrolyte solution align preferentially based on their charge. When this occurs, an electric field applied parallel to the wall will induce fluid flow.
Modeling and predicting the electrokinetic properties of microfluidic substrates that lead to electroosmosis is inherently difficult. The surface charges are a function of chemical reactions and adsorption/desorption processes, many of which are not fully understood. Further, the electrical double layer is often nanometers thick, and bulk fluid properties typically do not apply close to the wall, where the highest charge density (and therefore most of the action) resides.
Our work on the electrokinetic properties of microfluidic substrates includes (1) experimental characterization of interface properties, (2) chemical modification of interface properties, and (3) analytical and numerical modeling of double layer phenomena.
A selection of figures from relevant publications are below. Click to open a carousel view. Links to the original manuscript are in the captions.
FIG. 1. Antibody-functionalized obstacle arrays in a 2D microfluidic device can be used to engineer differential particle
transport. This paper studies the effect of DEPassisted
immunocapture, generated by applying an AC electric field to electrodes offset from the array and parallel to the
direction of fluid flow. Target cells undergoing positive DEP (pDEP) are attracted to the high electric field magnitude
regions at the obstacles leading and trailing edges (right inset), where the shear stress is low and the residence time is long
(supporting the capture of the target cells); although they are also repelled from the low field magnitude regions at the obstacle
shoulders, the locally high shear stress and short residence time minimizes the impact on overall capture. Likewise,
contaminating cells undergoing negative DEP (nDEP) are repelled from regions where capture is likely (i.e., the obstacles’
leading edge) and attracted to regions where capture is unlikely (i.e., the obstacles’ shoulder).
FIG. 1. Schematic of the Hele-Shaw flow cell and its interdigitated electrodes with lead connections to an applied voltage
(6V) and ground (GND), and elongated straight inlet channel compared to previous designs.35,37 The elongated straight
inlet channel was 500 lm wide, the smaller branching channels were 156 lm wide, and all channels were 48 lm tall. The
main chamber geometry leads to a monotonically decreasing shear stress along the device centerline, which allows for cell
capture to be measured as a function of shear stress.40–42 Inset images show fluorescently labeled PANC-1 cells (green) and
PBMCs (red) adhered to the antibody-functionalized surface with and without DEP effects. These example images show
that at an applied AC electric field frequency of 200 kHz, more PANC-1 cells and fewer PBMCs were captured with DEP
compared to without DEP. Captured cells in each pair of 1-mm2 observation windows were enumerated and compared at a
series of observation sites corresponding to a range of shear stresses found in typical immunocapture devices.4,12,39,43
Schematic of electrokinetic cell used in theoretical modeling and
experimental investigations (top). The configuration here is not drawn to
scale; typically w [ d to approximate parallel plates. (Below) a typical
velocity profile produced by an applied pressure difference. The hydrodynamic
penetration distance, lo, is also shown.
Diagram of charge-generated potential profiles at an impermeable
charged interface. Bound wall charge (here negative) generates an
immobile (Stern) layer of ions and a diffuse layer. Schematic potential
and velocity profiles, as a result of forcing by pressure and potential fields,
illustrate characteristic length scales and behaviors. The velocity profiles
at left are comparable in shape but not magnitude.
Figure 1. Scheme of the electrical double layer.
Schematic of potential and velocity profiles for a negatively
charged polymer film. Various momentum and potential decay scales are
displayed, which mitigate both pressure and electric field actuated
transport. In the velocity–position plot at left, the up and uEO plots are
not comparable in magnitude, but do indicate differences in spatial
velocity gradients; pressure-driven flow changes continuously until the
channel center, and local to the film varies linearly when the size of the
channel is large as compared to the film thickness. The E-field generated
(electroosmotic) flow establishes on scales comparable to the Debye
length.
Figure 2. (a) Scheme of uniform electroosmotic flow resulting from an applied electric field. (b) Scheme of the redistribution of ions in pressure driven flow in a microchannel, resulting in a streaming potential. Ion sizes and distributions in the schematics are not to scale.
Potential profiles for various charge distributions derived from
eqn (25). The film extends a distance 10ld into the domain from the wall
(x* ¼ 0). The inset figures (at right) show the various charge distributions,
r, considered. In all cases, the total charge is conserved across the film of
thickness d*.
FIG. 1. Diagrammatic representation of the system under consideration. (a): Geometric definition of the parallel-plate system studied; plates of width w and lengthLare separated by a distance 2h. Included are shapes of pressure-driven and electrically forced flows for (left) a channel with rigid surfaces and (right) a channel with a porous lining. In (b) and (c), magnified diagrams at the surface detail distributions of velocity and potential for a bare, rigid surface (b) and a surface with a porous layer of thickness delta(c).doipdf
Fig 2 Image sequence showing liposome concentration and elution. Microchannel edges have been drawn for clarity. The membrane has also been highlighted in (a). HV high voltage (100 V), PV pinch voltage (40 V), Gnd ground. (a) Before loading; (b) Sample concentration; (c) After concentration; (d) Sample elution. Pinch voltage is applied to minimize the diffusion of the sample away from the membrane
