Oscillation of Interfacial Tension and Spontaneous Interfacial Flow at a Water/Oil Interface Composed of Di(2-ethylhexyl)phosphoric Acid

Abstract

Oscillation of dynamic interfacial tension and spontaneous interfacial flow due to the Marangoni instability were investigated for the water/oil interface containing di(2-ethylhexyl) phosphoric acid (DEHPA) and calcium chloride. Characteristics of interfacial flow were extracted from experimental results with tracer particles. The interfacial flow appeared randomly at the interface. Movement of a small particle put on the interface was essentially a random walk. By application of the theory of random walk, the kinetic energy of the interfacial flow was evaluated. The energy evaluated decayed exponentially with time after the contact of aqueous and organic phases. Equilibrium interfacial tension was measured as a function of surfactant concentration. There was a region in which equilibrium interfacial tension increased with an increase in surfactant concentration. This concentration region agreed with the concentration range where dynamic interfacial tension oscillated. The difference in equilibrium interfacial tensions between CaCl2-containing and CaCl2-free systems was almost equal to the amplitude of the tension oscillation at the corresponding surfactant concentration. The oscillation of interfacial tension was concluded to reflect alternate changes in interfacial states between DEHPA-rich and its calcium salt rich compositions. The tension oscillation continued even when the kinetic energy of interfacial flow became extremely small. Spontaneous interfacial flow appeared only when the amplitude of the tension oscillation was large enough. Oscillation of interfacial tension did not need appreciable interfacial flow due to the Marangoni instability.

Relaxation oscillations of solutal Marangoni convection at curved interfaces

“…The situation becomes even more complex if Marangoni effects are coupled to chemical reactions. In fact, oscillations can be observed for a variety of reactive systems [29][30][31][32][33][34][35]. Additional effects like chemical interaction of the solutes in the interfacial region, multi-component diffusion of educts and products or phase changes like precipitation however require a specific interpretation for each system.…”

Autonomously Moving Colloidal Objects that Resemble Living Matter

“…Figure 7 shows an example of tension oscillation and droplet motion. Both are closely related to Marangoni instability at the interface as discussed elsewhere [36,38]. As usual, the onset of Marangoni instability is discussed in terms of hydrodynamics.…”

“…An oil phase containing DEHP spontaneously exhibits Marangoni instability when Ca 2+ is dissolved in the coexisting water phase [37,38]. However, the instability is much weaker or almost invisible when the water phase contains Ba 2+ , Sr 2+ or Mg 2+ [39,40].…”

“…However, the sensitive dependence on the ion species suggests that another type of instability may be hidden behind the hydrodynamics. Figure 8 shows the equilibrium interfacial tension of this oil/water system [38,39]. A Ca-containing interface exhibits higher interfacial tension than others when the DEHP concentration is high.…”

Electrical oscillation at a water/octanol interface in a hydrophobic container

“…Recently, simple two-phase systems have again been studied to elucidate the mechanism of oscillation. Sawada and co-workers [16,17] and Shioi et al [18,19] reported the oscillation of interfacial tension at a nitrobenzene/water interface. Their results support the view that the sudden change in oscillation is caused by surfactant adsorption at the interface and that gradual recovery is caused by desorption accompanied by the formation of a surfactant and counterion complex.…”