Permeability of Soft Microgel Particles at Electric Fields

We study the influence of the charge and ionic strength on the permeability of microgel particles under electric fields. Electrophoresis experiments with pH-ionizable poly(N-isopropylacrylamide)-co-acrylic acid microgels show that particles interestingly behave as permeable spheres when the polymer network is ionized whereas they keep non-permeably in the absence of charge. The friction coefficient strongly reduces as the polymer mesh expands following a power law g~d^-4 for fully charged networks, in contrast to that expected for non-permeable spheres, g~d. The overall behaviour arises from the motion of counter-ions driven by the external electric field, which causes hydrodynamic flow across the microgels making them permeable to the solvent. The microgel permeability also depends on the ionic strength, c; a salt-dependent local viscosity is proposed as responsible for these permeability changes. The driving mechanism is associated to the presence of strongly hydrated counterions located around fixed charges on the network that induces strengthening of the water confinement, which in turn results in an increased viscosity, h~c^0.2, and consequently reduced permeability.