Featured electrode system comprising two identical half-quadrupole electrode sets in a symmetric arrangement.
(a) Observed DNA focusing and trapping in DNA solution at and 1 kHz. (b)–(d) are snapshots showing head-on focusing of DNA streams.
(a) Measured fluorescence intensity as a function of time for concentrating DNA solution at and 1 kHz. The inset reveals that there appears Faradaic erosion on the corner portions of the quadrupole electrodes, explaining the sharp decline in the fluorescence intensity after the maximum. (b) Measured relation between the DNA concentration and fluorescence intensity. (c) Measured concentration enrichment factor as a function of applied voltage for concentrating 0.1 and DNA solutions. Distinct trapping responses of these solutions imply that part of the trapping mechanisms would have to involve intermolecular interactions between DNAs. (d) The same plot as (a) when gold electrodes are used. Compared to (a), the similar maximum can still be found but the fluorescent signal can last as long as 150 s during a detection, despite a postfading after the maximum due to photobleaching. Successive detections for every few minutes reveal that the maximum can be recovered repetitively. So the trapping actually persists for as long as 10 min as a result of a constant replenishment of fresh, unbleached DNA molecules.
Schematic illustration of how DNA molecules undergo focusing and trapping. DNA molecules (indicated by blue) are first prefocused by converging streams generated by ACEO vortices (marked by red) from the corners of the sided T-shaped electrodes. The two prefocused DNA streams then undergo head-on collision to trap DNAs at the center of the system, with the additional assistance of dipole-induced association between focused DNAs and the holding of the trapped spot by the downward DEP force (indicated by a pink arrow).
Mechanisms of ACEO vortices and pumping. (a) shows how ACEO vortices (in black) are generated by Ohmic charging on coplanar electrodes. Electric field lines are indicated by blue. Arrows on the electrode surfaces indicate the directions of the induced Coulomb forces within the double layers. (b) illustrates how a net fluid pumping is generated by asymmetric ACEO vortices when the electrodes are of unequal sizes.
Schematic mechanism for the formation of ACEO funnel generated by the half-quadrupole electrode set. Because of the asymmetric electrode configuration, asymmetric polarization on the adjacent, orthogonal electrodes will create tilted ACEO rolls in which more intense microvortices take place along the edges of the larger T-shaped electrodes, producing two converging streams to drain the fluid toward the smaller T-shaped electrode in between. These two streams then soon merge into a funnel, collecting the surrounding DNA molecules to form a prefocused jet moving toward the central vertical electrode.
Schematic illustration of field-induced self association between two polarized DNA coils (in blue). The association is caused by the attraction between the neighboring dipole charges of oppositely signs.
Schematic illustration of how a focused DNA spot is held by a downward DEP force against the upward flow by the ACEO focusing.
Formation of pitchfork streaming by first carrying out the trapping of DNA at and 1 kHz and then applying an additional horizontal pumping of 0.5 ml/h toward the right while keeping the ac field on. A steady ejection of a concentrated DNA jet is observed due to continuous collection of DNA from split streams toward the converging point at the right.
Transport of trapped DNA molecules toward the right by applying a dc field of 40 V/cm toward the left after the ac trap at and 1 kHz in DNA solution. Here we use gold electrodes to carry out the experiment to prevent possible electrode erosion arising from dc Faradaic charging.
Trapping of submicrometer particles at and distinct behaviors are found when different ac frequencies are applied. The experiments are carried out using gold electrodes. For sized particles, at 1 kHz they are trapped by ACEO in a manner similar to that of DNA [Fig. 2(d)]. At 100 kHz, a similar trapping pattern is still observed but appears less compact, whereas no apparent trapping is found at 20 MHz. For larger, sized particles, the observed trapping patterns at 1 and 100 kHz appear less apparent compared to those of sized particles. At 20 MHz, particle aggregation is somewhat visible due to DEP that is more pronounced for larger particles. The number shown in the upper right corner of each panel is the DEP to ACEO velocity ratio to highlight the relative importance between DEP and ACEO.
Article metrics loading...
Full text loading...