Theory of compression and expansion of hydrogels

Compression and expansion processes of cross-linked sodium polyacrylate hyd rogels under mechanical pressure were investigated. A packed spherical gel bed shows irreversible deformation when the applied pressure is decreased; the expansion behavior depends on the maximum pressure applied to the gel b ed, The time required to attain a certain degree of deformation is directly proportional to the square of the total solid volume of the gel bed; this relation is very similar to that observed in expression or expansion proces ses of ordinary solid-liquid mixtures. The driving force of the deformation is an effective osmotic pressure gradient in the gel bed, where the effect ive osmotic pressure of the gel is the difference between the swelling pres sure of the gel and the pressure applied to the gel, The flow rate of liqui d through any gel layer can be expressed by Darcy's equation. The deformati on ceases when the swelling pressure of each gel particle is equal to the a pplied pressure, Thus, the deformation of a packed gel bed can be recognize d as a process of equalizing the swelling pressure distribution in the bed.