In the most recent issue of Physical Review Letters, a new publication from Micosystems researchers is featured on the cover: Compression and Reswelling of Microgel Particles after an Osmotic Shock. 

Surprise in the squeezing of porous particles reveals new way of measuring their properties

We encounter sponge-like soft particles every day as components of foods, cosmetics, and other common products. When an external pressure is applied, water can be squeezed out, just as for a sponge. An external pressure is often applied by osmotic pressure, adding large molecules to the surrounding fluid. The degree of the resulting compression is a measure of the soft object's mechanics.

In this paper, we show that a surprising behavior can occur in such experiments, where particles first shrink rapidly, but then slowly reswell to their initial size. We attribute this to the large molecules slowly diffusing into the particle, thereby gradually reducing the applied pressure difference. 

We exploit this understanding to gain valuable information on both the particle and the large molecules: We measure the particle's elastic modulus and its permeability, which characterises how readily water can be pressed through a porous material. Finally, we quantify the diffusion coefficient of the osmolyte within the porous particle. Each of these properties would usually be difficult to measure, requiring specialized setups. We expect our approach to be immediately valuable to the study of sponge-like particles as used for instance in advanced drug delivery systems, food materials, or stimuli responsive systems.