Oscillatory dynamics induced in a responsive gel by a non-oscillatory chemical reaction: experimental evidence

Abstract

We provide experimental confirmation of the numerical simulations that predicted spontaneous chemo-mechanical oscillations induced by the size-changes of a pH-responsive hydrogel immersed in the proton-autoactivated bromate-sulphite (BS) reaction. The gel swells and shrinks in a constant and uniform non-equilibrium environment, with no external stimuli. Oscillations result neither from a kinetic feedback in the reaction mechanism nor from long-range activation diffusion instability. The dynamics clearly arises from the feedback that the size responsiveness of the hydrogel exerts on the relative stability of two different pH gradient profiles inside the gel. No oscillations occur without - or with insufficient - size-change of the gel. This is opposed to systems where the mechanical changes are merely slaved to the reaction kinetics. The present progress was obtained by considerably enhancing the swelling-deswelling ratio of the pH-responsive gel by tuning its hydrophobicity. We succeeded in producing more than a dozen of chemomechanical oscillations for over 48 h. We also demonstrate that the system is capable of working under mechanical stress, repeatedly lifting an object weighing 10 times the weight of the swollen gel.