Spontaneous mixing at oil/water interface

When students working with Robert Tilton and Stephen Garoff gently layered an oil-based solution of fatty acids on top of a water-based solution of a surfactant, they did not expect to see any activity at the interface. Normally, when oil and water come into contact, they are quiescent. Instead, researchers saw fluids spontaneously start flowing at the oil/water interface.

It was several years before students in the Department of Chemical Engineering were able to investigate the phenomenon as part of their work with The Procter & Gamble Company. Their goal was to understand the spontaneous mixing and learn how to control it, as part of their efforts to enhance transport at fluid interfaces.

"We hadn't observed anything like that before," says Tilton, Chevron professor of chemical engineering. "We saw waves going back and forth. It almost looked like raindrops hitting the surface of a pond, but these were little patterns of convective flow being generated all over the oil/water interface."

Ordinarily, the energy to mix oil and water in an emulsion has to be input from somewhere. The spontaneous mixing that researchers observed is driven by energy coming primarily from the oil/water interface.

In Industrial & Engineering Chemistry Research, a team led by Tilton, Garoff, and Aditya Khair explain the convective flows through the mechanism of a reaction-driven Marangoni instability. This new type of instability occurs in multiphase fluid systems if they contain the right kinds of surfactant molecules. Mixing occurs spontaneously at the interface when two fluids, like oil and water containing the right types of solutes, come into contact.

Brian McKenzie ('25), a Ph.D. student working with Tilton, Garoff, and Khair at the time, wrote a theoretical model to explain why the interfacial instability happens and what controls its intensity. From his mathematical model of transport, adsorption, and fluid flow, he created a numerical simulation. Hayley Kilgore ('25), a chemical engineering undergraduate student, and Hao-Wen Teng ('25), a chemical engineering master's student, explored how to control the intensity of the spontaneous mixing. They tested surfactant concentrations and compared experimental results with the theory.

Surfactants are chemical compounds that decrease interfacial tension between two liquids. They have hydrophobic and hydrophilic parts, a characteristic called amphiphilicity that naturally attracts them to oil/water interfaces. When surfactants attach themselves to the interface, they lower its energy. If two different types of surfactant are present that interact strongly with one another, the dynamics of their interfacial attachment, their mutual association, and their subsequent detachment from the interface can produce gradients of energy. These gradients are what "pump" the fluid flow.

Anything that increases flow and mixing at an oil/water interface has the potential to increase the rate of transport. Tilton, Garoff, and Khair's fundamental study has potential application to oil spill cleanup, cleaning products, and personal care products.

For media inquiries, please contact Lauren Smith at lsmith2@andrew.cmu.edu.