Chains of colloidal platelets in a electric, light and flow fields

We have used photolithography to produce square, plate-like, PMMA colloidal disks of dimension 4.5x4.5x0.7(1.4) microns. In an AC electric field the disks form ribbon-like chains, which at low field are separate and at higher field aggregate as in other electrorheological fluids. Here we report a study on the micromechanics of single chains. We use laser tweezers to hold the midpoint of a chain that is subjected to a transverse viscous flow. The resulting parabolic deformed shape can be used to quantitatively characterize the interactions between disks. Using a dipole-dipole interaction model, we accurately predict the chain deformation and the result allows for evaluation of the effective dielectric contrast between the particles and solvent.