Colloids — mixtures in which one substance is evenly dispersed in another — are found in a number of everyday items from foodstuffs such as milk to cosmetics and paint. The ability to control the rheology (ease of flow) of such mixtures is important for many applications. Polymer colloids are particularly attractive in this regard because polymer synthesis can be used to carefully control both the particle size and the surface properties of the polymers.

Now, Daisuke Suzuki from Shinshu University and co-workers from the University of Tokyo1, with the support of the Japan Science and Technology Agency, have demonstrated autonomous viscosity fluctuation in a microgel — a type of polymer colloid. “We were interested in creating intelligent microgels that respond to their surroundings, but wanted to go beyond the usual heat- or pH-responsive systems,” says Suzuki.

Fig. 1: The autonomously oscillating viscosity of a polymer microgel. The viscosity peaks correspond to the fully swollen polymer (green), and the troughs correspond to the deswollen gel (orange).

The system makes use of a fascinating type of chemical reaction called a Belousov–Zhabotinsky (BZ) reaction. While such reactions are more commonly known for producing patterns or their appearance as part of ‘chemical magic’, the most important feature of these reactions is that they oscillate in way that keeps the mixture of chemical reagents far from equilibrium.

The oscillation of the microgel was dependent on the incorporation of ruthenium, which is known to be an effective catalyst in BZ reactions. The researchers showed that the oscillation of the reaction is accompanied by a change in the viscosity of the microgel.

The most important effect is the change in the oxidation state of the ruthenium catalyst as the BZ reaction system oscillates. This change in oxidation increases and decreases the charge on the ruthenium ions, which essentially strengthens and weakens repulsion between the polymer chains. This results in an increase or decrease in the volume occupied by the polymer particles in the suspension, and ultimately a commensurate change in viscosity.

This oscillation is more than just a curiosity. “We hope to use this microgel to create a tiny pumping system for use in microfluidic devices — the kind of devices that can be used for medical diagnostics in resource-poor settings,” says Suzuki.