Signaller and seeker droplets

The ability to seek out the source of a chemical signal is vital for many life forms, from single-celled organisms, such as white blood cells (which are able seek out pathogens due to the chemical messages they release), to complex communities, such as beehives (which requires individual units of bees to respond to a chemical signal in order to perform specific tasks). Inspired by these natural examples, herein we have developed signal (A) and seeker (B) droplets that are capable of finding each other in an open intricate fluidic network. The signal droplet (A) releases a chemical signal, which creates a chemical gradient inside the channel; in response to this signal, the seeker droplet (B) is able to chemotactically find the signal droplet (A) and merge with it at the location of droplet A. The droplets in this work are composed of ionic liquids (ILs). The signal droplet is constituted of trihexyl(tetradecyl)phosphonium dicyanamide [P6,6,6,14][DCA] containing 1M cupric nitrate (Cu(NO3)2). The seeker droplet is composed of trihexyl(tetradecyl)phosphonium chloride [P6,6,6,14][Cl] and a small amount of red dye (1-(methylamino)anthraquinone), which is purely added for visualisation. The unidirectional movement of the seeker droplet is due to the triggered release of the [P6,6,6,14]+, a very efficient cationic surfactant, which is a constituent of the IL droplet. In the presence of an ionic strength gradient in the aqueous phase, an asymmetrical surface tension gradient is created, leading to Marangoni like flows, which drive the droplet from areas of low surface tension towards areas of high surface tension [1]. When the signal droplet (A) is placed onto a solution of 10-2 M sodium hydroxide (NaOH), Cu(NO3)2 diffuses from the droplet into the aqueous phase and creates an ionic strength gradient throughout the fluid network. Due to the poor solubility of the DCA anion, no [P6,6,6,14]+ is released from the signal droplet (A) and thus it remains stationary. Therefore when both the signal droplet (A) and seeker droplet (B) are placed onto a solution of 10-2 M NaOH, the seeker droplet will autonomously seek out the signal droplet