Propulsion of zwitterionic surfactant-stabilized water-in-oil droplets by low electric fields

We show directional and controllable propulsion of zwitterionic surfactant-stabilized water-in-oil droplets driven by low electric fields. Our results suggest that the propulsion mechanism is based on stimulus-responsive on-demand interfacial phenomena.

Sample preparation.Silicone oil was saturated with zwitterionic surfactant SB3-12 by sonicating and stirring overnight.In a separate vial, a 53 mM / 4.7 mM aqueous solution of the SB3-12 was prepared.In 1 mL of surfactant-saturated silicone oil, 3 µL of aqueous surfactant solution was added.The mixture was vigorously mixed for 1 minute.A similar procedure was followed with the nonionic reference surfactant Brij ® C10.A freshly prepared water-in-oil emulsion was always used in the experiments.

Droplet characterization.
The size distribution and zeta potential were characterized with Malvern Zetasizer Nano ZS90.The droplet size distribution was also calculated manually from the experimental videos, to better correspond with the tracked droplet population.Cyclic voltammetry (CV) experiments were performed with Metrohm Autolab electrochemical workstation in the same cell where the droplet tracking experiments were performed, with a scan rate of 100 mV/s.Electrochemical impedance spectroscopy (EIS) was measured with a HF2IS Impedance spectroscope (Zurich Instruments) in a sandwich-type electrochemical cell (A35, S100A040uNOPI, Instec Inc.).A 4-terminal measurement set-up with an AC excitation of 100 mV was used.Frequency range of 10 MHz-1 Hz was applied with an averaging of 32 measurements at each point.The analysis of EIS data was performed using ZView software (Scribner Associates).
Electrochemical set-up.An electrochemical cell was made using microscope slides as bottom and top surface, separated by a silicone spacer.Copper electrodes were placed on the opposite ends of the cell.The distance between the electrodes was 4.8±0.2mm.Needles were pinched to fill and purge the cell.The electrochemical cell was connected to a Keithley Series 2400 voltage source with alligator connectors.A schematic illustration of the electrochemical cell is shown below, showing that the direction of filling the cell (green arrows) is perpendicular to the direction of electric field (between electrodes marked with red (+) and blue (-)).

Droplet propulsion experiments.
An inverted Zeiss Observer Z1 microscope on an optical table, equipped with a x20 objective, a VIS-LED light source, and a Ximea B/W camera, was used in the experiments.The electrochemical cell was placed under microscope and was left to stand for at least 1 hour to stabilize any residual flows before start of the experiment.The microscopy experiments were recorded as an image sequence of TIF-files, with a frame rate of 1 fps.The obtained image sequence was imported into Fiji/ImageJ, calibrated and contrastadjusted.The image sequences were analyzed using Fiji/ImageJ TrackMate-plug-in 2 using Hessian detector to recognize the droplets.Tracking was performed using the Simple LAP tracker.The results were exported as XML-file for further data analysis in MATLAB, using msdanalyzer-package 3 and a custom MATLAB code.

Figure S2.
Electrochemical impedance spectra with Nyquist plots and equivalent circuits (above) and Bode plots (below) for silicone oil with saturated SB3-12 (green) and aqueous droplets stabilized by SB3-12 in silicone oil (blue).For silicone oil (green lines), the straight vertical line in Nyquist plot together with the phase angle θ = 90° in Bode plot indicates that the silicone oil is an ideal capacitor.The experimental data can be fitted with an equivalent series resistance (R1 = 175 Ω), in series with silicone oil capacitance (C = 6.2* 10 -10 F).In the case with the SB3-12 stabilized droplets in silicone oil (blue), the behavior is no longer an ideal capacitor.The phase angle has decreased, having values between 64 < θ < 84° indicating that while the sample is still capacitive, there is now a resistive contribution from the added water.Now, the system can be modeled with an equivalent circuit including resistive and capacitive elements that stem from the added aqueous solution of SB3-12.Again, the system exhibits equivalent series resistance (R1 = 180 Ω) and silicone oil capacitance (C = 6.3 * 10 -10 F), but now with additional contribution from the droplets as bulk resistance (R2 ≈ 62 kΩ) and constant phase element (CPE, where C = 1.6 * 10 -8 F and n = 0.56).

Figure S1 .
Figure S1.The cyclic voltammogram (CV) curves for pure silicone oil and the studied system with aqueous droplets stabilized by SB3-12 in silicone oil.The samples were measured in the experiment cell used in the droplet tracking experiments.Within the studied electrochemical window of ±5 V, the curves show continuous switching within the current ranges (< 16 nA), which can be interpreted as the noise of leakage current, and no major electrochemical reactions take place within the studied voltage range.

Figure S5 .
Figure S5.The velocity autocorrelation function (VAF) graphs for droplets stabilized with zwitterionic surfactant SB3-12 and nonionic surfactant Brij C10.The horizontal lines suggest that the droplets do not interact with each other.

Figure S6 .
Figure S6.Repetitive measurements.The droplet velocities parallel to the E-field from all repeated experiments during the first 300 s of tracking.Individual experiments are shown with different symbols, corresponding typically to ca. n = 70-120 tracked droplets (17 < n < 482).Inset: area of applied voltage 0 … 2 V.

Figure S7 .
Figure S7.The propulsion velocity of SB3-12-stabilized droplets on the applied voltage.The relationship is v ∝ V 2 , in accordance with previous studies on electric-field-induced interfacial mechanisms.4,5