Exploring novel structure formation and mechanics of linked chains of magnetic colloidal particles and their application in microfluidics

Much of current technology in microfluidics relies on pressure and electric fields to drive miniaturized processes on a chip. We present the use of an additional parameter to control fluids: a magnetic field. Paramagnetic particles aggregate into linear chains under the influence of an external magnetic field. In this research, we create a unique system of permanently linked chains by chemically linking 1 mum streptavidin coated particles with bis-biotin-polyethylene glycol (PEG) linker molecules with the objective of creating magneto-responsive chains that can be used to manipulate fluid flow. By adjusting the length of the PEG in the linker molecules, we can control the chain flexibility. These linked chains remain responsive to the magnetic field, allowing their orientation to be controlled by directing the field.; In order to design effective microstructures for microfluidic devices, we must understand the properties of chains under the influence of various forces. Using optical tweezers, we apply forces to a chain to study its dynamic response. We compress, stretch and bend chains and evaluate the resulting chain shapes to determine their flexural rigidity. We also analyze the relaxation behavior of the chains as a measure of its stiffness.; The need for efficient mixing at the micron scale remains a challenge in microanalytical systems. By using the chains in rotating magnetic fields, we can create miniature stir-bars. We characterize the dynamics of this system by modeling the chains as rotating rods. There are two opposing torques dictating the conformation of the chain: a magnetic torque and a viscous torque. The magnetic torque arises from the chain not being parallel to the external magnetic field. The viscous torque is due to the rotational friction of the chain from the surrounding fluid. The elastic torque associated with these structures determine the shape of the chain at a given rotation rate. We characterize the instabilities associated with the longer chains created when the viscous drag produces a long phase lag between the magnetic moment of the chain and the external magnetic field.