When an assembly of microgel particles includes one particle that’s significantly larger than the rest, that oversized particle spontaneously shrinks to match the size of its smaller neighbors. This self-healing nature of the system allows the microparticles to form defect-free colloidal crystals, an unusual property not seen in systems made up of incompressible particles.
Using small-angle X-ray and neutron-scattering techniques, the researchers carefully studied the structures formed by dense concentrations of the microparticles. They also used tiny piezoelectric pressure transducers to measure osmotic pressure changes in the system. They found that in dense assemblies of microparticles, counter ions bound to the microgels by electrostatic attraction come to be shared by multiple particles, increasing the osmotic pressure which then works to shrink the oversized particle.
“When the particles are close enough together, there is a point at which the cloud of ions can no longer be associated with individual particles because they overlap other particles,” said Alberto Fernandez-Nieves, an associate professor in Georgia Tech’s School of Physics. “The ions create an imbalance between osmotic pressure inside and outside the larger particles, pushing them to de-swell — expel solvent to change size — to match the pressure of the system given by these delocalized ions. This is only possible because the microgel particles are compressible.”
The research was reported in the journal Proceedings of the National Academy of Sciences. — John Toon