Smart fluids: Switchable viscosity

Stimuli-responsive ‘smart fluids’ reversibly change viscosity with pH.

Materials that change in some way in response to external stimuli are generating considerable interest for a range of potential industrial uses. Such ‘smart’ materials offer the possibility of creating systems that can react to light or electric current, but the smart systems developed to date typically rely on complex, costly molecules that tend to react too slowly for practical applications. Zonglin Chu and Yujun Feng at the Chengdu Institute of Organic Chemistry in China have now developed a low-cost smart material that undergoes rapid changes in viscosity in response to variations in pH.¹

Fig. 1: Photographs and transmission electron microscopy images showing the formation of a clear, highly viscous micellar liquid at lower pH (left), and the collapse of that structure to form a cloudy, watery liquid at higher pH (right).© 2011 RSC

Under acidic conditions, the smart fluid developed by the research team is a thick, viscoelastic liquid. On the addition of an alkali solution, however, the fluid begins to flow like water within just seconds. The system is based on a molecule that is cheap, simple to produce and easy to separate from the fluid for reuse.

The smart behavior was achieved by combining maleic acid with a specially structured long-chain molecule called UC₂₂AMPM. Under acidic conditions, the UC₂₂AMPM molecules aggregate into long worm-like arrays or ‘micelles’ with surface charge, making the fluid viscous. As the pH rises with the addition of alkali species to the system, the network falls apart and the fluid becomes highly liquid.

The ability for a system to change viscosity in response to pH could be of particular interest to the oil industry, says Feng. A process called ‘acidization’ is often employed in the industry to stimulate oil wells by pumping a thick acidic solution down a well to crack any carbonate rock that is blocking the flow of oil. As the acid reacts with the rock, the pH rises—the smart fluid developed by Chu and Feng would become a watery liquid that could carry any dissolved rock back to the surface.

As well as developing this fluid further for the specific application of oil well stimulation, Feng says the next step will be to make the system more adaptable. “Our current work is focused on multi-responsive micellar systems that could be triggered by pH, temperature, salinity or light—or combinations of these stimuli,” he says.