pH-Dependent Motion of Self-Propelled Droplets due to Marangoni Effect at Neutral pH

Abstract

Oil droplets loaded with surfactant propel themselves with a velocity up to 6 mm s(-1) when they are placed in an aqueous phase of NaOH solution or buffer solution. The required driving force for such motion is generated on the interface of the droplets by the change in interfacial tension, due to deprotonation of the surfactant. This force induces Marangoni convection, which gives rise to a circulating flow inside the droplets. The droplets begin to move when the axis of this circulation deviates from the vertical line. This motion depends on the pH condition of the aqueous phase. When the initial value of pH is adjusted such that the pH exceeds the threshold at the equilibrium state, the droplets move spontaneously. It was seen that the droplets were independent of the material of the solid substrates because the droplets were not directly in contact with the surface of the substrate. The condition for the onset of this spontaneous motion was verified by comparing the prediction from the linear stability analysis with experiments. The stability analysis overestimates the value of the driving force, causing instability.

Self-propelled droplets

“…Several examples of chemical reaction schemes can be found in literature [9][10][11][24][25][26][27][28][29][30][31]. The common feature of all these systems is that the used surfactants are affected by a chemical reaction and the surface tension of an interface covered with the pristine surfactant differs from the surface tension of an interface covered with surfactant after the chemical reaction.…”

Dimensionality matters in the collective behaviour of active emulsions

“…In experimental studies, various artificial systems have been reported on, each showing interesting individual features. The systems can be categorized either as interface bound surfers [26][27][28][29], micro-machines [30][31][32], Janus particles [33][34][35][36][37] or active emulsions [38][39][40][41][42][43][44][45][46][47].…”

“…Either the restriction to quasi-2D geometries is inherent to the propulsion mechanism, as in the case of surfers, or the high and non-uniform mass density of the swimmers makes them ineligible for buoyancy match-ing, as in the case of Janus particles. For most of the active emulsion systems, the active periods are short, the chemical reactions involved are complex and the dynamics of the reaction products affect the droplet propulsion in an unpredictable manner [38][39][40][42][43][44]. Nevertheless, in certain active emulsions [45,46] the propulsion mechanism is based on a solubilisation process, where the kinetics of the reaction products are diffusive and therefore analytically tractable.…”

Negative/positive chemotaxis of a droplet: Dynamic response to a stimulant gas

“…The spontaneous motion of self-propelled droplets has recently attracted considerable attention in relation to energy transduction by living organisms, i.e., chemo-mechanical energy transduction. [1][2][3][4][5][6][7][8] Various kinds of droplet motion driven by a gradient in interfacial tension have been reported, as well as the propelled motion caused by diffusiophoresis. 9,10 In a related system, an oilwater system composed of an organic phase with potassium iodide and iodine and an aqueous phase containing stearyltrimethylammonium chloride (STAC) exhibits self-agitation at the oil-water interface, accompanied by spatio-temporal nonequilibrium fluctuation of the interfacial tension.…”

“…We can reasonably assume that the velocity profile at the point at which acceleration switches from positive to negative is given under the condition f ðtÞ % 0. Thus, from curve-fitting with a single exponent over the portion of the curve with a decrease in velocity (t > 1.2 s), we evaluated the effective viscosity n by using a value of m ¼ 22 Â 10 À6 kg for 25 ll oleic acid (density: 0.89 g/cm 3 ). With this value of n, we can evaluate the time-dependent change in the driving force, as shown in Fig.…”