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The authors exploit the cryosuction-driven migration of hydrogel microparticles through solid ice along a thermal gradient to extract continuous transport properties at picoliter scales. Measuring these coupled parameters—permeability, compressibility, and poroelastic diffusivity across swelling states—has historically been hindered by the limitations of macroscale testing. High-resolution measurements of poly(ethylene glycol) diacrylate (PEGDA) particles reveal that standard homogeneous-network polymer theories fail to capture real transport behavior, providing an experimental benchmark for non-ideal soft-matter dynamics.
Watching picoliter hydrogel droplets migrate through solid ice reveals that textbook homogeneous polymer models fundamentally fail to predict real hydrogel transport properties.
Strikingly, when hydrogel particles are embedded in ice in a temperature gradient, they move through the solid ice towards warmer temperatures, while swelling as they warm up. This motion comes from a flow of unfrozen water through the hydrogel mesh, driven by a process known as `cryosuction'. Here, we show how one can use this behavior to measure -- with extremely high resolution -- a range of different hydrogel transport properties, and how these change as a hydrogel deswells. These properties include permeability, compressibility, and poroelastic diffusivity: all of which are challenging to measure, but widely important for phenomena involving swelling, dehydration, transpiration and filtration. We demonstrate the measurement technique using poly(ethylene glycol) diacrylate (PEGDA) hydrogels. The technique uses picoliter-scale hydrogel volumes, and yields measurements of hydrogel properties that are consistent with existing literature data. The high resolution of our measurements also allows us to test commonly-used, classical predictions for hydrogel properties (derived assuming an idealized, homogeneous polymer network in the hydrogel). We show that these classical predictions do not work well, highlighting the need for new models that can accurately describe real hydrogels.