Reconfigurable liquid pumping in electric-field-defined virtual microchannels by dielectrophoresis.

Dielectrophoresis (DEP), widely used to generate body forces on suspended particles, is investigated to provide surface forces at the liquid-medium interfaces and pump a high-permittivity liquid in a low-permittivity medium along a virtual microchannel defined by an electric field between parallel plates. Because the pumping pressure is proportional to the square of the intensity of the electric field and independent of the channel width, DEP pumping is advantageous as the dimension of the microchannel shrinks down. The absence of the channel walls simplifies the fabrication processes and further increases its feasibility in nanofluidic applications. We demonstrate water pumping in an immiscible silicone oil medium at adjustable velocities by applying voltages above the threshold value whose square is linearly proportional to the cross-sectional aspect ratio (AR), i.e., the height to width ratio, of the microchannel. With a properly designed AR, liquid valve is achieved by appropriate voltage applications. Without the barriers of channel walls, merging multiple streams and capillary filling of the spacing between electric-field-defined virtual microchannels are observed and studied. Moreover, in situ reconfigurable liquid pumping is demonstrated by a four way switching valve on a programmable crossing electrode set.

Cross-scale electric manipulations of cells and droplets by frequency-modulated dielectrophoresis and electrowetting.

Two important electric forces, dielectrophoresis (DEP) and electrowetting-on-dielectric (EWOD), are demonstrated by dielectric-coated electrodes on a single chip to manipulate objects on different scales, which results in a dielectrophoretic concentrator in an EWOD-actuated droplet. By applying appropriate electric signals with different frequencies on identical electrodes, EWOD and DEP can be selectively generated on the proposed chip. At low frequencies, the applied voltage is consumed mostly in the dielectric layer and causes EWOD to pump liquid droplets on the millimetre scale. However, high frequency signals establish electric fields in the liquid and generate DEP forces to actuate cells or particles on the micrometre scale inside the droplet. For better performance of EWOD and DEP, square and strip electrodes are designed, respectively. Mammalian cells (Neuro-2a) and polystyrene beads are successfully actuated by a 2 MHz signal in a droplet by positive DEP and negative DEP, respectively. Droplet splitting is achieved by EWOD with a 1 kHz signal after moving cells or beads to one side of the droplet. Cell concentration, measured by a cell count chamber before and after experiments, increases 1.6 times from 8.6 x 10(5) cells ml(-1) to 1.4 x 10(6) cells ml(-1) with a single cycle of positive DEP attraction. By comparing the cutoff frequency of the voltage drop in the dielectric layer and the cross-over frequency of Re(fCM) of the suspended particles, we can estimate the frequency-modulated behaviors between EWOD, positive DEP, and negative DEP. A proposed weighted Re(fCM) facilitates analysis of the DEP phenomenon on dielectric-coated electrodes.

Asymmetric electrowetting--moving droplets by a square wave.

Here droplet oscillation and continuous pumping are demonstrated by asymmetric electrowetting on an open surface with embedded electrodes powered by a square wave electrical signal without control circuits. The polarity effect of electrowetting on an SU-8 and Teflon coated electrode is investigated, and it is found that the theta-V (contact angle-applied voltage) curve is asymmetric along the V = 0 axis by sessile drop and coplanar electrode experiments. A systematic deviation of measured contact angles from the theoretical ones is observed when the electrode beneath the droplet is negatively biased. In the sessile drop experiment, up to a 10 degrees increment of contact angle is measured on a negatively biased electrode. In addition, a coplanar electrode experiment is designed to examine the contact angles at the same applied potential but opposite polarities on two sides of one droplet at the same time. The design of the coplanar electrodes is then expanded to oscillate and transport droplets on square-wave-powered symmetric (square) and asymmetric (polygon) electrodes to demonstrate manipulation capability on an open surface. The frequency of oscillation and the speed of transportation are determined by the frequency of the applied square wave and the pitch of the electrodes. Droplets with different volumes are tested by square waves of varied frequencies and amplitudes. The 1.0 microl droplet is successfully transported on a device with a loop of 24 electrodes continuously at a speed up to 23.6 mm s(-1) when a 9 Hz square wave is applied.