Chemotactic and electrotactic self-propelled ionic liquid droplets
Francis, Wayne and Wagner, Klaudia and Beirne, Stephen and Officer, David and Wallace, Gordon and Florea, Larisa and Diamond, Dermot (2015) Chemotactic and electrotactic self-propelled ionic liquid droplets. In: 67th Irish Universities Chemistry Research Colloquium, 25-26 June 2015, Maynooth University, Ireland.
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Herein we report the chemotactic and electrotactic self-propelled movement of droplets composed solely of an ionic liquid (IL), namely trihexyl(tetradecyl)phosphonium chloride ([P6,6,6,14][Cl]). These IL droplets move spontaneously across the liquid/air interface and are guided to specific destinations within fluidic systems along Cl- concentration gradients.
The self-propelled movement of the droplet is due to the controlled release of the [P6,6,6,14]+, a very efficient cationic surfactant, which is a constituent of the IL droplet. The rate of [P6,6,6,14]+ release depends on the solubility of the closely associated Cl- anion in the surrounding media, as the formation of free [P6,6,6,14]+ in the aqueous phase depends on the local Cl- concentration at the IL-aqueous boundary. Therefore, in Cl- gradients there is an unsymmetrical release of surfactant into the solution, which in turn results in a surface tension gradient around the droplet. This leads to Marangoni like flows which propel the droplet from areas of low surface tension to high surface tension1.
The required gradients for movement are generated both chemically, by introducing a Cl- source in the system (e.g. HCl, NaCl) and electro-chemically, through redistribution of ions after application of an electric field. Chemically generated gradients quickly come to equilibrium and therefore the droplet will cease to move unless more chemoattractant (source of Cl-) is added. In contrast, electro-chemically generated gradients have increased lifespan and allow for on demand, multi-directional, reversible droplet movement.
This type of triggered surfactant release provides a compelling mechanism for controlling droplet movement within microfluidic devices, and could form the basis of providing sophisticated functions such as detection of and transport along chemoattractant gradients, and status-diagnosis and auto detection/repair of damage.
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