Oscillatory motion of water droplets in kerosene above co-planar electrodes in microfluidic chips

Abstract

We experimentally observed oscillatory motion of water droplets in microfluidic systems with coplanar microelectrodes under imposed DC electric fields. Two-electrode arrangement with no bipolar electrode and eight-electrode arrangement with six bipolar microelectrodes were investigated. Kerosene was used as the continuous phase. We studied the dependences of the oscillation frequency on the electric field intensity and ionic strength of the water phase. We found that the electric field dependence is strongly nonlinear and discussed possible reasons of this phenomenon, e.g., the droplet deformation at electrode edges that affects the charge transfer between the electrode and droplet or the interplay between the Coulomb force on free charge and the dielectrophoretic force. Our experiments further revealed that the oscillation frequency decreases with growing salt concentration in the two-electrode arrangement, but increases in the eight-electrode arrangement, which was attributed to surface tension related processes and electrochemical processes on the bipolar electrodes. Finally, we analyzed the effects of the electric field on the oscillatory motion by means of a simplified mathematical model. It was shown that the electric force imposed on the droplet charge is the key factor to induce the oscillations and the dielectrophoretic force significantly contributes to the momentum transfer at the electrode edges. For the same electric field strength, the model is able to predict the same oscillation frequency as that observed in the experiments.

Crater Formation on Electrodes during Charge Transfer with Aqueous Droplets or Solid Particles

“…Analysis of the recorded video to determine the corresponding acquired charge (by balancing the electrostatic force versus the drag force) indicates that the metal ball typically receives about 70 pC to 250 pC of charge from either electrode depending on the applied field strength, while the droplet receives approximately 110 and 200 pC from the negatively and positively charged electrode respectively. The overall behavior of the droplets and metal balls, including the magnitude of charge acquired, is broadly consistent with previous studies [9,14,18,19,24]. Examination of 50-nm thick electrodes after electrically bouncing a water droplet reveals no changes visible to the naked eye.…”

“…Although similar light flashes have been previously observed when charged objects [13,14] or liquid drops [26] approached an electrode, recent workers have assumed that electrically bouncing droplets are charged via electrochemical reactions not involving dielectric breakdown [18,19]. It is known from previous work that application of an arc current on the order of 1-100 amps between stationary electrodes through vacuum can melt the electrodes via Joule heating, with the high pressure present in the plasma jet of the arc pushing the molten material to form micron-scale craters [27,28].…”

“…A scaling analysis indicates that the crater diameter increases with the cube root of the ratio of the energy exchanged during charge transfer to the melting point of the metal, i.e., d ∼ (E/T m ) 1/3 , in accord with experimental measurements. Finally, we discuss how the process of crater formation helps explain longstanding irreproducibility in precise measurements of charge acquisition, as well as the practical implications for devices that use electric fields to manipulate charged objects [16][17][18][19].…”

Modeling of conductive particle motion in viscous medium affected by an electric field considering particle-electrode interactions and microdischarge phenomenon

“…This approach has been applied previously in some models. For example, Bernanek et al 54 used the exchanged charge value as a free parameter to adopt their model predictions with the experimental data. Figure 16 indicates that the reduction of the particle charge in downward motion can significantly retard the particle downward trajectory.…”

Rotary motion of a micro-solid particle under a stationary difference of electric potential

“…3 Numerous studies [3][4][5][6][7][8][9][10][11][12][13] have reported various modes of droplet motion between electrodes using both experimental and numerical methods. A droplet behaves as an electrical charge-carrier between a pair of electrodes in an oil medium as an insulator.…”