Design of the electrode pair under consideration.
Experimental setup and array of EWOD chips in the upper frame.
Axisymmetric flow within a sessile PBS drop in air, under oscillating EWOD as observed from (a) above with fluorescent tracers, (b) along a drop cross-section (use of a 100 μm thick laser sheet, the white dotted contour indicates the averaged half-spherical drop shape), and (c) experimental velocity field as obtained from micro-PIV along a meridian cross-section; arrows emphasize the presence of the toroidal counterrotating vortex pair. For the sake of comparison, (d) toroidal vortices as calculated along a meridian cross-section. Chip with two half-circular electrodes separated by a gap of 3 μm, actuation frequency: f = 110 Hz, RMS voltage: 60 V, wave number: k = 2.
Comparison (modes k = 2, 4, 6) for a sessile PBS drop in air between the velocities as measured at the apex of the drop or numerically predicted from Eq. (1) with θ = 0. The experimental velocity is obtained from micro-PIV measurements especially performed on the small area bounded by the red rectangle displayed in the snapshots (tracers: fluorescent beads). The streamlines as computed numerically are made visible on the RHS of the snapshots. Chip with two half-circular electrodes separated by a gap of 3 μm, RMS voltages: 60 V (k = 2), 75 V (k = 4) or 91.7 V (k = 6). Consider also Ref. 21 for imaging of the streamlines.
Frequency-dependence of the location of U373B cell agglomerates (as imaged from the side under continuous lighting along the direction of free motion, as indicated from the horizontal dotted line) and drop shape oscillations induced by capillary waves as observed under stroboscopic lighting. For the frequencies f = (300 Hz, 600 Hz, 1 KHz), the displayed amplitude of the capillary waves is the real one multiplied by a factor 5 (better readability). Chip with two half-circular electrodes separated by a gap of 100 μm. Scaling in mm is displayed on the horizontal axis of each snapshot. RMS voltage applied: 91.7 V. Oil is used as ambient phase.
Positioning of agglomerates as a function of the EWOD actuation frequency, as described by the angular deviation, θ (colatitude).
Frequency-dependence of the vertical positioning of U373B cell clusters as made evident from side and top views of a drop consisting of culture medium in oil as ambient phase. Pinch-off is made evident at the location where the contact line crosses the axis of the electrode gap (top view). Chip with two half-circular electrodes separated by a gap of 100 μm. Mean wetting radii: see also Fig. 9 , RMS voltage: 91.7 V.
Impact of drop vortical flows on different types of biological cells: (a) red blood cells, (b) white blood cells, (c) U373B cells, as demonstrated from side view along the direction of free motion (Fig. 3 ). Oil is used as ambient phase. Chip with two half-circular electrodes separated by a gap of 100 μm, RMS voltage: 91.7 V.
Dynamics of anisotropic EWOD as characterized by oscillating wetting diameters (◆) and contact angles (▲) either along the direction of free motion (left part) or along the electrode gap (right part). The symbol, , is the argument of the oscillating drop shape. The arrows connect the curves to their respective y-axis. Experimental conditions: oil is used as ambient phase; half-circular electrodes separated by a gap of 100 μm; applied RMS voltage: 91.1 V; actuation frequencies: (a) f = 150 Hz or (b) f = 1 KHz.
Wetting data in static conditions with the EWOD chip considered in this study.
Characteristics of the biological cells.
Typical values of the parameters associated to streaming theory for water drops in air (drop volume: 1.5 μl).
Eigenmodes and resonant frequencies for PBS drops in air (drop volume: 1.5 μl). Consider also Ref. 21 for viewing resonant drop shape oscillations.
Eigenmodes and resonant frequencies for PBS drops in oil (drop volume: 1.5 μl).
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