Motion of microgels in electric fields

We review existing experimental results on the motion of microgels in the presence of electric fields and find that there can be striking differences depending on whether the polymer network comprising the microgel is neutral or charged: While for neutral microgels, the electrophoretic mobility, µ, typically decreases as the particle swells, in the case of ionic microgels, µ typically increases with particle swelling. We explain this difference in behavior by recurring to electro-osmotic fluid flows inside the particles, which are relevant in the presence of electric fields when the polymer network is ionized; these flows render the particles permeable to the solvent qualitatively changing the way to think about their electrophoretic behavior. We show that this interpretation is consistent with calculations of the drag force experienced by a permeable object as it moves inside a liquid and with recent theoretical models for the electrophoresis of soft particles. The analysis emphasizes that the electrophoresis of neutral microgels can be qualitatively treated as that of charged hard spheres, irrespective on whether the particles are swollen or de-swollen. By contrast, ionic microgels behave like free-draining polyelectrolytes in the presence of electric fields.