Magnetic Colloidal Assembly

Superparamagnetic colloids interact with each other upon application of external magnetic fields. These interactions spontaneously assemble the particles into different structures whose properties can be externally tuned by the parameters of the magnetic field. We utilize these assembled colloidal structures as experimental models to study equilibrium and nonequilibrium phenomena. Our group also treats these colloidal assemblies as smart responsive units to be implemented in applications such as microfluidic mixing, biomedical micro-robotics, and optical materials.

DNA-linked colloidal chains

Using static magnetic fields, magnetic colloids arrange into linear structures known as chains. We create magneto-responsive elastic filaments by combining these chains with DNA linkers that connect the particles along the chain together. These DNA-linked chains bend and buckle in response to magnetic fields. They also display complex bending behaviors under rotating magnetic fields because of the interplay between magnetic, elastic, and viscous forces present in the system. The chains also serve as a bead-spring experimental model to understand the deformation of polymers and filaments under different forces, such as gravity during sedimentation. We combine our experimental models with numerical simulations to fully elucidate the effects of these forces on the deformation of these chains.

Colloidal Crystals

Under high-frequency rotating magnetic fields, the magnetic colloids assemble into clusters of particles. By changing the strength of the magnetic field, these colloidal assemblies undergo phase transitions from droplets to crystals according to interactions that resemble those of molecules, but in this case the interactions are mediated by the magnetic field. In contrast to conventional states of matter, these assembled phases made from “colloidal atoms” exist only while the rotating field is applied. This means that the colloidal clusters are nonequilibrium structures because an external energy input is mandatory to maintain the organization of the clusters. Our group combines concepts from statistical mechanics with image processing techniques to characterize the structural evolution and the material properties of these nonequilibrium clusters.