Abstract
Herein, the oscillation of an oil droplet on the surface of water is studied. The droplet contains an anionic surfactant that can react with the cations present in water. The oscillation starts after a random motion, and the oscillation pattern apparently depends on the cation species in the water phase. However, a common pattern is included. The cation species only affects the amplitude and frequency and sometimes perturbs the regular pattern owing to the instability at the oil/water interface. This common pattern is explained by a simple model that incorporates the surfactant transport from the droplet to the surrounding water surface. The dependency of the amplitude and frequency on cation species is expressed quantitatively by a single parameter, the product of the amplitude and square of frequency. This parameter depends on the cationic species and can be understood in terms of the spreading coefficient. The simple model successfully explains this dependency.
Highlights
IntroductionThe number of studies focusing on droplet dynamics under a non-equilibrium state has increased, where the non-equilibrium is caused by mass transport and/or chemical reactions (Ye et al, 1994; Shioi et al, 2003; Sumino et al, 2005; Lagzi et al, 2010; Pimienta et al, 2011; Ban et al, 2013; Hermans et al, 2013; Banno and Toyota, 2015; Seemann et al, 2016; Zwicker et al, 2016; Seyboldt and Jülicher, 2018)
A droplet on a liquid surface often shows various spatiotemporal pattern formations
The number of studies focusing on droplet dynamics under a non-equilibrium state has increased, where the non-equilibrium is caused by mass transport and/or chemical reactions (Ye et al, 1994; Shioi et al, 2003; Sumino et al, 2005; Lagzi et al, 2010; Pimienta et al, 2011; Ban et al, 2013; Hermans et al, 2013; Banno and Toyota, 2015; Seemann et al, 2016; Zwicker et al, 2016; Seyboldt and Jülicher, 2018)
Summary
The number of studies focusing on droplet dynamics under a non-equilibrium state has increased, where the non-equilibrium is caused by mass transport and/or chemical reactions (Ye et al, 1994; Shioi et al, 2003; Sumino et al, 2005; Lagzi et al, 2010; Pimienta et al, 2011; Ban et al, 2013; Hermans et al, 2013; Banno and Toyota, 2015; Seemann et al, 2016; Zwicker et al, 2016; Seyboldt and Jülicher, 2018) This is one of the simplest pattern formations in non-equilibrium open systems. This is not fully studied for droplet dynamics on a liquid surface despite it being the simplest system
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