Wave Patterns Driven by Chemomechanical Instabilities in Responsive Gels

The first experimental evidence of a chemomechanical mechanism leading to morphogenetic instabilities is demonstrated experimentally. The system consists of a pH-responsive gel that swells at high pH and shrinks at low pH, and a bistable reaction system exhibiting an acid steady state (pH ≈ 2) and an alkaline steady state (pH ≈ 10). Within the gel, the steady state selection depends on the gel size. We show that in a constant and uniform nonequilibrium chemical environment, the responsive gel undergoes large amplitude dynamical deformations under the form of travelling contraction waves and complex spatio-temporal volume oscillations. These deformations are coupled to concentration patterns of protons. We present different sequences of dynamical behaviors observed under various controlled chemical conditions. A simple heuristic model is proposed to account for the observations. These experiments open a new route for pattern formation driven by chemical energy, in soft matter systems.