Abstract
We analyze, theoretically and by means of molecular dynamics (MD) simulations, the generation of mechanical force by a polyelectrolyte (PE) chain grafted to a plane and exposed to an external electric field; the free end of the chain is linked to a deformable target body. Varying the field, one can alter the length of the non-adsorbed (bulk) part of the chain and hence the deformation of the target body and the arising force. We focus on the impact of added salt on the magnitude of the generated force, which is especially important for applications. In particular, we develop a simple variational theory for the double layer formed near electrodes to compute the electric field acting on the bulk part of the chain. Our theoretical predictions agree well with the MD simulations. Next, we study the effectiveness of possible PE-based nano-vices, comprised of two clenching planes connected by PEs exposed to an external electric field. We analyze a novel phenomenon - two-dimensional diffusion of a nano-particle, clenched between two planes, and introduce a quantitative criterion for clenching efficiency, the clenching coefficient. It is defined as a logarithm of the ratio of the diffusion coefficients of a free and clenched particle. Using first a microscopic counterpart of the Coulomb friction model, and then a novel microscopic model based on surface phonons, with the vibration direction normal to the surface, we calculate the clenching coefficient as a function of the external electric field. Our results demonstrate a dramatic decrease of the diffusion coefficient of a clenched nano-particle for the range of parameters relevant for applications; this proves the effectiveness of the PE-based nano-vices.
Highlights
PaperIn future nano-technology which will manipulate nano-objects, such as viruses, small bacteria, cell organelles, vesicles, colloidal particles, etc., nano-actuators will play an important role
We propose an application of electro-active polymers as a working body of nanoactuators; polyelectrolytes can change their dimension in response to an external electric eld and generate a mechanical force to keep a nano-object immobile
In this subsection we develop a theory of dynamical surface roughness caused by the transversal surface phonons, with the displacement direction normal to the surface
Summary
PaperIn future nano-technology which will manipulate nano-objects, such as viruses, small bacteria, cell organelles, vesicles, colloidal particles, etc., nano-actuators will play an important role. Carbon nano-tube nano-tweezers operated by an electric eld have been proposed and tested experimentally.[1] These, have a serious limitation: the effectiveness of such nano-tweezers is questionable for aqueous solutions, especially in the presence of salt, which makes the solution conductive.[1] Another limitation of such devices is a relatively narrow range of operating distances, which implies that the carbon nanotube nano-tweezers can be exploited to operate nano-objects of a particular size, determined by the respective size of the carbon nano-tube Another set of devices, designed to keep nano-objects xed, are optical tweezers.[2] These, require speci c optical properties of an immobilized particle, which limits possible applications of the optical tweezers. We propose an application of electro-active polymers (polyelectrolytes) as a working body of nanoactuators; polyelectrolytes can change their dimension in response to an external electric eld and generate a mechanical force to keep a nano-object immobile
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