We present a novel method to tune the interaction potential from 5k B T to 40k B T in situ between micronsized superparamagnetic colloids. The potential is composed of a highly tunable long-range attraction induced via a rotating magnetic field and a short-range electrostatic repulsion. Various 2-D thermodynamic phases are observed in a colloidal suspension at different field strengths. Quantitative agreement is found between theory and experiments for dipolar interactions. A theory to account for the induced dipole due to neighboring particles is also presented. The effective three-body potential of a dilute trimer system is measured to account for many-body effects in the system. These results demonstrate an ideal model to study the phase behavior in 2-D systems.

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The self-assembly of colloidal particles using DNA linker molecules has led to novel colloidal materials. This article describes the development and characterization of a new class of colloidal structures based on the directed assembly of DNA-linked paramagnetic particles. A key obstacle to assembling these structures is understanding the fundamental chemistry and physics of the assembly processes. The stability of these cross-linked chain structures is the first step toward reliable assembly and thus important for its applications; however, chain stability has yet to be systematically studied. In this paper, we investigate both theoretically and experimentally, the stability of DNA-linked paramagnetic colloidal chains as a function of externally applied magnetic field strength and surface grafted DNA length and density. A total interparticle free energy potential model is developed accounting for all major forces contributing to chain stability, and a phase diagram is obtained from experiments to illustrate linked chain phases, unstable unlinked particle phases, and their transitions, which agree well with those predicted by the model. From this study, optimized parameters for successful linking and building stable linked chains are obtained.

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Axial rotational diffusion of rodlike polymers is important in processes such as microtubule filament sliding and flagella beating. By imaging the motion of small kinks along the backbone of chains of DNA-linked colloids, we produce a direct and systematic measurement of axial rotational diffusivity of rods both in bulk solution and near a wall. The measured diffusivities decrease linearly with the chain length, irrespective of the distance from a wall, in agreement with slender-body hydrodynamics theory. Moreover, the presence of small kinks does not affect the chain's axial diffusivity. Our system and measurements provide insights into fundamental axial diffusion processes of slender objects, which encompass a wide range of entities including biological filaments and linear polymer chains.