Contraction response of a polyelectrolyte hydrogel to nonuniformly applied electric fields

Polyelectrolyte hydrogels can deform under electric fields due to their unique nature combining polymer elasticity and electrostatics within a single structure. While the response of hydrogels to electric fields is relatively well-characterized at the macroscale, at the mesoscale-where the behaviour of the constituent chains becomes significant-the effect of external electric potentials on the hydrogel structure is poorly understood. In this study, we explored the mechanical response of a semi-infinite polyelectrolyte hydrogel slab to transient, sinusoidal electric fields using extensive coarse-grained molecular dynamics simulations with both short and long-range electrostatics. Our simulations show that when the electric field is applied to a small volumetric section of the hydrogel slab spatially nonuniformly, the entire slab contracts reversibly and in a direction perpendicular to the field. The hydrogel contracts to almost half of its initial, field-free length before retracting to its original size, with its size fluctuations eventually decaying similar to an underdamped oscillator. Contraction is maximized if the electric field is applied to the central region of the slab, away from the slab's interfaces. Additionally, tuning the electric field frequency and amplitude controls both contraction times and contraction efficiency. Further analyses using implicit solvent simulations across various electrostatic parameters and salt concentrations confirm the robustness of the phenomenon while highlighting the importance of hydrodynamics. Our results demonstrate the effectiveness of electric fields applied spatially nonuniformly on homogeneous hydrogel structures, with potential applications in electro-mechanochemistry.