💧 Hydrogel Swelling Mechanics

About this simulation

A hydrogel is a crosslinked polymer network that can absorb many times its dry mass in water while retaining its shape. Whether it swells or shrinks is set by a balance of two opposing forces: osmotic pressure, which pulls solvent into the network to dilute the polymer chains, and elastic restoring force from the crosslinks, which resists stretching. When the two forces balance, the gel sits at an equilibrium swelling ratio described (in the classic Flory–Rehner theory) by the network's crosslink density and the polymer–solvent interaction parameter χ.

How it works

• Crosslink density — more crosslinks make a stiffer network with shorter chains between junctions, which resists swelling more strongly and lowers the equilibrium ratio.
• Solvent availability — a solvent-rich bath drives more solvent into the gel, raising the osmotic driving force and the swelling ratio.
• Temperature — this gel behaves like a thermoresponsive polymer (e.g. PNIPAM): below ~32°C the network favours mixing with water and swells, while above that lower critical solution temperature the polymer–solvent interaction turns unfavourable (χ rises) and the gel expels water and collapses.
• The lattice of spheres and rods is the crosslinked polymer network; it isotropically scales toward the animated swelling ratio Q, while the small drifting particles represent solvent molecules diffusing in or out.

Controls

• Crosslink density — network stiffness, from a loosely crosslinked to a densely crosslinked gel.
• Solvent availability — how much solvent the surrounding bath offers the network.
• Temperature — shifts the Flory χ interaction parameter, driving the LCST-type swell/collapse transition.
• Cutaway view — slices the network in half so you can see the interior mesh.
• Show solvent flux — toggles the animated solvent particles. Drag to rotate, scroll to zoom.

Did you know?

Superabsorbent hydrogels in disposable diapers can swell to over 100 times their dry weight, while thermoresponsive hydrogels like PNIPAM are used in drug-delivery capsules that release their contents only once body temperature crosses the polymer's collapse point.