Gel dynamics in the mixture of low and high viscosity solvents: Re-entrant volume change induced by dynamical asymmetry

Quantifying the trade-off between stiffness and permeability in hydrogels

Hydrogels have a distinct combination of mechanical and water-transport behaviors. As hydrogels stiffen when they de-swell, they become less permeable. Here, we combine de Gennes' semi-dilute polymer theory with the Kozeny-Carman equation to develop a simple, succinct scaling law describing the relationship between mechanical stiffness and hydraulic permeability where permeability scales with stiffness to the -8/9 power. We find a remarkably close agreement between the scaling law and experimental results across four different polymer families with varied crosslinkings. This inverse relationship establishes a fundamental trade-off between permeability and stiffness.

Competition between Osmotic Squeezing versus Friction-Driven Swelling of Gels

Some types of hydro-gels have almost the same equilibrium swelling volume in water and in ethylene glycol (EG), a highly viscous liquid completely miscible with water. Experiments showed that when a gel fully swollen with EG is immersed into a large amount of water, it temporarily swells up and then relaxes to the equilibrium volume in water. The temporary swelling is explained by the friction force exerted on the gel network from the outward EG flux In this paper, we experimentally show that the temporary swelling is suppressed by adding linear PEG (polyethylene glycol) in the outer water. Although the suppression seems to be explained by the osmotic pressure (i.e., by the same mechanism as the conventional osmotic squeezing), our theoretical analysis reveals that the effect of PEG is much stronger than that expected from the equilibrium osmotic pressure, implying that the PEG chains are condensed on the gel surface.