Condensed Matter Theory

Abstract:

We show that DNA-linked anisotropic doublets composed of paramagnetic colloidal particles can be endowed with controlled propulsion when floating above a flat plate and subjected to a magnetic field precessing around an axis parallel to the plate. The propulsion mechanism for this artificial swimmer does not involve deformations, and it makes use of the minimal two degrees of freedom needed to propel it at low Reynolds numbers. We combine experimental observations with a theoretical analysis that fully characterizes the propulsion velocity in terms of the strength and frequency of the actuating magnetic field.

Abstract:

We examine mixtures of DNA and filamentous actin (F-actin) as a model system of like-charged rigid rods and flexible chains. Confocal microscopy reveals the formation of elongated nematic F-actin domains reticulated via defect-free vertices into a network embedded in a mesh of random DNA. Synchrotron x-ray scattering results indicate that the DNA mesh squeezes the F-actin domains into a nematic state with an interactin spacing that decreases with increasing DNA concentration as d(actin) proportional, variantrho(DNA)(-1/2). Interestingly, the system changes from a counterion-controlled regime to a depletion-controlled regime with added salt, with drastic consequences for the osmotic pressure induced phase behavior.