Electrotactic ionic liquid droplets
To our knowledge, this work describes the first example of electro-guided, self-propelled droplets composed solely of an ionic liquid (IL), namely trihexyl(tetradecyl)phosphonium chloride ([P6,6,6,14][Cl]). These self-propelled droplets travel along an aqueous-air boundary to desired destinations within the fluidic network. Electrotactic movement of the droplets is due to asymmetric electro-stimulated release of a constituent of the IL droplet, the [P6,6,6,14]+ ion, which is a very efficient cationic surfactant, through electrochemically generated Cl− gradients. The direction and speed of movement can be controlled by switching the impressed voltage (typically 5 − 9 V) ON or OFF, and by changing the polarity of the electrodes in contact with the electrolyte solution. The Cl− gradients required for droplet movement are electrochemically generated using 3D printed electrodes which are embedded within the fluidic channels. On demand creation of these Cl− gradients electrochemically allows reversible droplet movement over expended periods of time, and provides a means for precise control over the droplet trajectory.