Microfluidic devices are characterized by channels with diameters ranging roughly between 100 nm and 100 microns, often involving particles with diameters ranging roughly from 10 nm to 10 microns. At these length scales, the Reynolds number is low and the flow is usually laminar, but the mass transfer Peclet number is often large, leading to unique microfluidic mixing regimes. Because the diameters are small and it is difficult to generate large flow velocities with pressure, other effects can dominate. In particular, electrokinetic effects (electroosmosis and electrophoresis) can dominate and voltage can be used to manipulate fluids, molecules, and particles. A variety of chemical separations have been developed in microfluidic devices owing to the beneficial species transport often associated with microchip devices. Surface tension can also be very important, and bubbles and drops can often be manipulated with temperature and electric fields. Study of nanofluidic phenomena can occur indirectly via interface studies, nanoporous material, or nanometer-scale microdevices.
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. 6 (a) Particle tracking experiments and comparison to simulated
transfer functions for various transverse errors allow for the quantification
of error along the length of the obstacle array, shown in a planview
schematic here. doipdf
FIG. 2. DEP alters cell trajectories within the microfluidic device, leading to changes in the mean collision frequency for
cells within a given device geometry. Advection dominates DEP at the obstacles’ shoulder, but the reverse is true at the
obstacles’ leading and trailing edges, where the fluid flow stagnates; as such, a cell’s response in the high electric field magnitude
region at the leading and trailing edges has the most effect on its trajectory through the array. For medium and large
cells (e.g., diameters B and C in this figure), pDEP attracts the cells to the high field magnitude regions near the leading
and trailing edges, increasing the mean collision frequency and the time in contact (which supports capture), whereas
nDEP (fCM < 0) repels cells from these regions. Likewise, pDEP forces small diameter cells (e.g., diameter A) toward the
region of high field magnitude, increasing collision frequency compared to without DEP, but the overall collision frequency
remains low. Although nDEP does indeed repel these small cells from the high field magnitude regions, nDEP displaces
particle diameter A enough to cause a brief “grazing” cell-obstacle collision, increasing the collision frequency; these grazing
events are brief and occur where the shear stress is highest, so capture of these cells is unlikely.
Figure 1. Scheme of the electrical double layer.
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.
Fig. 5 (a) Off-design boundary conditions, such as from a clogged
inlet channel or a lump of captured cells near the inlet, can lead to a
transverse velocity error; this additional velocity
component alters trajectories within the array. doipdf
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
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*.
Figure 1. Scheme showing an interpretation of the slip length, b.
