Contraction response of a polyelectrolyte hydrogel structure to a spatially non-uniform electric field

We study the shape fluctuations of a semi-infinite polyelectrolyte hydrogel slab to transient and spatially nonuniform sinusoidal electric fields by using extensive coarse-grained molecular dynamics simulations. Our simulations show that if the electric field is applied on a small volumetric section of the hydrogel slab, the slab contracts reversibly and uniaxially in the direction perpendicular to the electric field. The hydrogel contracts almost half of its field-free initial length before retracting its original shape, and eventually its length fluctuations decay similar to an under-damped oscillator. The contraction speed and efficiency can be controlled by tuning the frequency and amplitude of the external electric field. No contraction is observed if the field is applied uniformly on the whole hydrogel structure. The contraction response of the hydrogel is robust against various backbone charge fractions and dielectric constants but weakens at high salt concentrations. Our results demonstrate the effectiveness of spatially nonuniform electric fields to manipulate hydrogels mechanically.