Periodic Expansion and Contraction Phenomena in a Pendant Droplet Associated with Marangoni Effect.

Koutaro Onoda,Ben Nanzai

doi:10.3390/ma15010239

Abstract

A spontaneous oscillation between the expansion and contraction of a nitrobenzene pendant droplet containing di-(2-ethylhexyl)phosphoric acid (DEHPA) was observed in an aqueous phase under alkaline conditions. We described this phenomenon as the spontaneous oscillation of the oil–water interfacial tension. The oscillation characteristics such as the induction period and the interfacial-tension oscillation frequency were investigated under different temperatures and aqueous phase polarities. The effects of the interfacial tension of the biphasic pendant-droplet, the surface excess of the surfactant molecules, and the amount of nitrobenzene elution from the droplet to the aqueous phase on the oscillation characteristics were investigated. Consequently, the periodic expansion–contraction oscillation mechanism was explained through the adsorption–desorption cycle of DEHPA with respect to the aggregate formation of the inverted micelle of DEHPA. This study was based on a simple vibration phenomenon of interfacial tension, and is extremely important for clarifying the predominant factors that cause fluctuations in the free interface energy, which has been ambiguous.

Highlights

Phenomena in a Pendant DropletVarious rhythmic oscillating phenomena resulting from the periodic variation of interfacial tension are observable in biphasic systems with clear boundaries between aqueous and oil phase
The variation of interfacial tension was explained by the adsorption–desorption cycle of the surfactant at the oil–water interface owing to the Marangoni effect
Where C denotes the di-(2-ethylhexyl)phosphoric acid (DEHPA) concentration in the pendant-drop phase, γ represents the initial interfacial tension, R is the gas constant (8.314 J K−1 mol−1 ), and T denotes the temperature of the biphasic system. These results demonstrated that the DEHPA concentration on the oil–water interface was not the primary factor for controlling the oscillation characteristics (Figure 4)

Summary

Introduction

Various rhythmic oscillating phenomena resulting from the periodic variation of interfacial tension are observable in biphasic systems with clear boundaries between aqueous and oil phase. These periodic oscillations, caused by the surfactant adsorption onto the oil–water liquid interface, are one of the typical self-organization phenomena in systems far from equilibrium. Considerable interest exists for understanding the rhythmic phenomena in living organisms and the direct conversion of energy from chemical to mechanical or electrical energy To date, both numerical simulation and experimental investigation have been conducted to understand the interfacial-tension oscillations [6,7,8]. Elucidation of the controlling factors is indispensable when considering industrial application, in particular, for liquid/liquid extraction, stability of foams and emulsions, correct measurement of dynamic surface tension, Associated with Marangoni Effect

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