Autonomous Droplet Motion on a Surfactant Monolayer

Abstract

Self-propelling droplets have been of much interest as vessels for transporting chemi- cals without using any external energy and replicating the behaviours of living cells in a synthetic environment. In this study, the underlying mechanism for the spontaneous self-assembly of droplets (4.98 ± 0.16 μl) of water-immiscible solvents on an aqueous substrate covered in a monolayer of the ionic liquid surfactant trihexyltetradecylphos- phonium chloride, [P6,6,6,14]Cl, is investigated. The most probable explanation for the mechanism was determined to be the formation of Marangoni flows in the direction of the self-assembling droplets formed by the depletion of the surfactant concentration of the substrate surface surrounding the droplets. This depletion is theorised to occur by the dis- solution of the surfactant in the self-assembling droplets when the surfactant molecules are transferred into it along a chemical potential gradient. The amount of the surfactant in a self-assembling droplet (pure benzoyl chloride) and a droplet that does not show self-assembling behaviour (pure mesitylene) were compared after being on the surface of an aqueous substrate covered in said surfactant for 10 minutes was determined us- ing 1H Nuclear Magnetic Resonance (NMR). The amount of surfactant found inside the droplets were 17.5 ± 2.9 μg and 0 μg respectively. Factors affecting the rate of motion of self-assembling droplets were investigated. The rates of motion of the self-assembling droplets were found to be related to the size of the three phase contact angle. The reason for this was hypothesised to be the change in the speed of the convective flows inside the droplet with changing contact angle. Other observed but unexplained phenomena in this study were presented and possible explanations given.